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David N
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Reged: Nov 11 2003
Posts: 80
Loc: Portland, OR
The 50 Cent 20" String Dob Project
      #1074435 - Wed Aug 02 2006 09:38 PM

 


After lurking and learning from other fearless Cloudy Night ATM'ers for the past couple of years, I figure it is payback time - so here is what I am building: a 20" F-5 string dob.

To 'get' the name of my scope, read how I came by the mirror - HERE.

As for my experience in building scopes (and astronomy in general) I am perhaps an "advanced beginner" having built exactly one scope - an 8" portable string dob primarily for the purpose of learning how to build a scope before tackling this 20" project.

I've been planning this project (mostly in my head) for some time, and began fabrication a couple of weeks ago. The push is on as I have just 3 weeks to finish it before heading to the Oregon Star Party.

Why a string dob? Belonging to a great Astronomy Club (Rose City Astronomers of Portland, Oregon) with some innovative telescope makers - Mel Bartels, Dan Gray, Matt Vartanian - I really admired Dan and Matt's handiwork and some of the features and benefits of a string scope - plus, string scopes are sort of a new and emerging design and I like to try new things.

I know pictures are popular, and I've taken a bunch, so here we go.

..........David Nemo


 
David N
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Reged: Nov 11 2003
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1074441 - Wed Aug 02 2006 09:44 PM

 


Starting at the top, the secondary is going to be pretty traditional. Two rings connected with metal struts. I'm using 3/4" Appleply (13 layers) for virtually all components.

This is my prep for making the plunge and the rings.



David N
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Reged: Nov 11 2003
Posts: 80
Loc: Portland, OR
Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1074447 - Wed Aug 02 2006 09:52 PM

 


Making the outside cut.



Note the inside of the circle screwed to the work surface. If I did it again, I would put the screws closer to the center point as this 'waste' could someday be used for a smaller ring and I wouldn't have a screw hole to fill.

And the inside cut.

 

David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1074452 - Wed Aug 02 2006 09:54 PM

 


And eventually two rings for the secondary - ready for sanding.


 

David N
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Reged: Nov 11 2003
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1074459 - Wed Aug 02 2006 10:01 PM

I picked up a tip from somewhere about a little 'detail' that makes for a nicer finish on the secondary - and that is to create a rabbet in the ring to hold the light baffle material so that the material is basically flush with the inside face of the rings.

So, how do you do this? Well a 1/8" rabbet router bit would sure make it simple, but I could'nt find one for the life of me. Which means I had to improvise with a straight router bit and the guides and clamps you see below.



Not perfect, but not too bad either.


David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1074474 - Wed Aug 02 2006 10:18 PM

 


On to the mirror box.

I've done a fair amount of woodworking, but have never done dovetails. I figured if I'm going to do all this work I want my scope to look as classy as possible. The dovetail is at the top of the list of classy - as well as providing the maximum joint strength to a mirror box corner which I am told (and believe) is critical for a big mirror box.

I did a fair amount of research and ended up purchasing the Keller system. Not the fanciest, but worked well for me. After lots of practice and creating an adequate clamping arrangement which I learned is extremely CRITICAL to success - I went for it.


 

David N
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Loc: Portland, OR
Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1074484 - Wed Aug 02 2006 10:30 PM

 


Here's a look at my clamping system. The Keller needs to be fastened to a block of wood (of some precise width that I forget at the moment). I made a groove in each end of the block to accomodate the inner screw of my clamps which allowed me to hold 100% of the work piece.

You will also notice a 1/4" piece of masonite used as 'sacrificial' layer to try and reduce tear out of the real wood. The combination of the clamps and masonite worked fairly well (I'd say 80% perfect) - but plywood is reportedly the hardest material to get perfect dovetails on which you quickly understand when you notice the outer layer of a piece of plywood is very thin - 1/16"(?) or so.



If there is an 'advantage' about dovetailing plywood it would be that 'fixing' the tear-out is not that dificult as you can cut and glue a patch piece that looks almost like new.

 

David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1074492 - Wed Aug 02 2006 10:44 PM

 


Gluing and clamping the mirror box. I had to hussle as it was about 95 degrees in "Sunny Oregon" as Tom L. boasts.

My dovetails were very tight - which I think is good, but I ended up using a file to sand some of the joints a little so I wouldn't have to beat on the ends with a mallet to get to fit. Plus you need a little room for glue.

Everything I read and understand is that having a 'square' mirror box is very critical - but boy that can be tuff. I came within 1/16" of being perfect and will keep my fingers crossed that won't be a major problem down the road.



I also learned how critical the bit depth setting can be - and useful a good sander is in erasing the excess.

 

David N
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Loc: Portland, OR
Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1074504 - Wed Aug 02 2006 11:02 PM

 


So far so good. The mirror cell fits in the mirror box like a glove.



No, I didn't make the mirror cell. I splurged and had one custom made by Nate Currier of Aurora Precision. Nate is also an RCA member who has come up with some inovative designs for various telescope parts that he is fabricating - as well as the mirror cell.

The mirror box is only 6" high as I'm going for a low to the ground mirror box and rocker - which means giant bearings.

For the most part I am following the guidance and specifications from Kreige & Berry - with obvious deviations to suit a string scope, and my personal desire to minimize the weight of the different components.

 

David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1074512 - Wed Aug 02 2006 11:25 PM

 


So how big should my bearings be? How about 35". K & B spec 26" bearings for a 20" mirror. 35" happened to be the width of my plywood after cutting other parts - so 35" they would be. That's about 35% bigger than the 26" specification - and I figure should be in the ballpark for accomplishing my low-to-the-ground design.

I suppose someone with more math and design skills than me could calculate all these things precisely (and may well reply with an answer), but I'm going with my instinct and research of other similar designs.

I made a pattern to layout the cuts and design.



And they came out really nice (after lots of jig sawing, routing and sanding).



 

Greg K.

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1074751 - Thu Aug 03 2006 06:04 AM

 


Looks great so far, and I sure wouldn't mind finding a 20" mirror bargain like that!

Starfire
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Re: The 50 Cent 20" String Dob Project [Re: Greg K.]
      #1074789 - Thu Aug 03 2006 06:30 AM

 


Excelent! I just got a 10" F5 quite cheap and going to be building a nice low dob like that.

rboeAdministrator

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Re: The 50 Cent 20" String Dob Project [Re: Starfire]
      #1074865 - Thu Aug 03 2006 07:13 AM

 


I've got the Keller jig too, still have not quite got the hang of it and wonder if the woodblock its' bolted to is not the proper thickness.

In general you want proud dovetails so they finish flush nicely.

You bugger! Nice deal on the mirror!

Just picked up an Amana 1/8" dado bit the other day. Plan on cutting a groove for the red laminate (WilsonArt) in the upper cage with it.

I like the jig you made for doing your rabbet but you could have precut the rabbet when you cut the part out. several ways to skin that cat.

How do you like that Shopsmith?

--------------------
Ron
David N
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Reged: Nov 11 2003
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Re: The 50 Cent 20" String Dob Project [Re: rboe]
      #1074971 - Thu Aug 03 2006 07:59 AM

 


Ron,

The thickness of the block you attach the Keller Jig to IS IMPORTANT, but probably not as important as making sure the one side is accurately aligned with the scribe mark on the bottom of the jig.

Where did you get the 1/8" bit (which I presume has a bearing on it) - I'd like to add that to my collection. Yes, I realized after the fact I could have done this cut when routing the circles. That's why I'm called an "Amatuer."

I got my Shopsmith in 1979 and it has served me quite well - especially since the only 'shop' I've ever had is my garage and the portability and versatility of it means I can work most anywhere. Yes, it takes a litte longer setting things up when you change tool configurations, but that gives me extra time to think through a set up or cut and usually reduces my chance for making a stupid mistake.

I'll add a photo of my 'shop' later.

rboeAdministrator

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1075009 - Thu Aug 03 2006 08:13 AM

 


I look at all my scrap and say "yup, amatuer".

No bearing on my bit, just a plunge bit - looks like a needle point. If I read you right all you needed was a dado so a fairly fat bit would have worked - if you used a 1/16" to 1/8" undersized bearing. Someones make a bit like that with a four or so bearings of different sizes. Elegant solution for problems like what you faced.

I seem to recall most of my problems were with proper indexing the flip cut. I've been considering a more conventional dovetail jig to get around that or modifing the block with a T slot to insert a moveable stop block.

We shall see.

--------------------
Ron

 

David N
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Reged: Nov 11 2003
Posts: 80
Loc: Portland, OR
Re: The 50 Cent 20" String Dob Project [Re: rboe]
      #1078271 - Fri Aug 04 2006 09:40 PM

 


Ron,

Yes, the bit I am looking for would need a bearing. I have seen the kit with different size bearings, and I think that may be the eventual answer.

I found these instructions to be quite helpful in learning how to use the Keller jig. My system was to mark more than one tail with a pencil when lining up to cut the pin. That and the eyeball check seemed to work for me.

 

rboeAdministrator

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1078278 - Fri Aug 04 2006 09:45 PM

 


Thanks Dave; I'll have to review it next time I use it.

--------------------
Ron

 

David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1078279 - Fri Aug 04 2006 09:46 PM

 


On to the rocker box.

I laid out the angles and arcs according to "The Book."


David N
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Reged: Nov 11 2003
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Loc: Portland, OR
Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1078286 - Fri Aug 04 2006 09:50 PM

 


And making the cut in the side boards.



David N
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Reged: Nov 11 2003
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1078303 - Fri Aug 04 2006 09:58 PM

 


On my small scope and on many others I've inspected, there seems to be the need for something to keep the bearings from falling off the rockers/pads. My idea here was to create a channel that is slightly wider than the bearings.

I should note this is one place I followed The Book's advice by doubling the thickness of the rocker, which made this 1.5" wide. This allowed me to make a channel that is about 1" wide which leaves a little clearance for the bearings on either side.



David N
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Reged: Nov 11 2003
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1078307 - Fri Aug 04 2006 10:01 PM

 


And here is the rocker box assembled, glued and clamped.


David N
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Reged: Nov 11 2003
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1078319 - Fri Aug 04 2006 10:16 PM

 


The Shop. Thank goodness it doesn't rain in Oregon in the summer.


 

Tom LAdministrator

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1078329 - Fri Aug 04 2006 10:28 PM

 




I'd have to move somewhere else!

--------------------
Tom
Greg K.

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1078331 - Fri Aug 04 2006 10:29 PM

 


thank goodness you (hopefully) don't have neighbors who will walk off with your tools! The rocker channels are an interesting idea but I wonder if they will survive a lot of abuse during setup/teardown.

 

 

David N
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Loc: Portland, OR
Re: The 50 Cent 20" String Dob Project [Re: Greg K.]
      #1078349 - Fri Aug 04 2006 10:50 PM

 


Greg,

Actually, none of my 'shop' is visible to anyone from off our property - but I do put everything in the garage at night anyway.

We'll see about how the bearing channels hold up.

I think though, the most 'abuse' during set-up and tear-down will be to my back lugging all these components around. It looks like the mirror, box and cell is going to come in at 70 pounds. Not light by any means, but something I should be able to handle for a few more years.

I really didn't want a wheelbarrow scope - but one that is more portable. And yes, one day I'll try to get it in that little car in the garage (not my primary astronomy vehicle).

 

rboeAdministrator

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1078650 - Sat Aug 05 2006 07:34 AM

 


You may want to consider brushing on epoxy inside the channel so it soaks in and adds some strength. Especially on the upper edge where it will be prone to chipping. If it was solid wood like maple this would be less of a concern but over time that darn plywood will want to delaminate if not kept on top of. Plus the veneers aren't all that strong by themselves.

I suppose you could also glue on a thin hardwood wear strip over the top edge to cover the veneers. May I recommend bloodwood? Inside joke here and I get points for finally bringing it up.

And full marks for a shop at least as messy as mine.

--------------------
Ron

 

David N
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Reged: Nov 11 2003
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Loc: Portland, OR
Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1079753 - Sat Aug 05 2006 08:34 PM

 


Lot's of progress today as I actually started putting things together - starting with the secondary.



 

David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1079762 - Sat Aug 05 2006 08:45 PM

 


And the bearing reinforcements.



David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1079782 - Sat Aug 05 2006 09:04 PM

 


I have never used threaded inserts before, and got a lesson in how to install them at our club's ATM workshop today.

You cut the head off of a bolt (with the same threading as the insert) and cinch up in a drill press. Screw the insert on a few turns, and place the tube directly below on the table. If the end is square this should put the tube perpendicular to the insert and ensure it goes in straight. Then just drive the insert down manually - POWER OFF - until it is slightly below the end. Worked like a charm.



 

David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1079789 - Sat Aug 05 2006 09:10 PM

 


Here is the mirror box with some reinforcing blocks added where the pole holders will be. As opposed to a conventional truss dob where the trusses basically just hold things in place - in a string dob the poles need to be 'pushing' on the secondary in order keep the strings under tension.


 

rboeAdministrator

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1079796 - Sat Aug 05 2006 09:15 PM

 


Instead of cutting bolt, you can purchase a stud. It's double treaded for one end to go into something - like an engine block, then something else is attached with a bolt over the exposed stud - say a head.

Although a bolt is probably cheaper and easier to find.

Love those bearings!

--------------------
Ron

 

David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1079810 - Sat Aug 05 2006 09:25 PM

 


Last task of the day was to cut out the ground board and a matching piece for the bottom of the rocker box - shown here after gluing and stapling.



Only 17 days to finish this monster - and I'm feeling pretty good about my progress. Also got some compliments from the master himself - Dan Gray - today on my design and construction.
LateViewer
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1080327 - Sun Aug 06 2006 10:12 AM

 


Now that is a great idea for a way to put in star nuts.

I will remember that!

 

David N
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Loc: Portland, OR
Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1081124 - Sun Aug 06 2006 07:53 PM

 


Another perfect day for building a scope in the driveway. Starting with attaching the mirror box cover. [Note my dovetail practice pieces.]


 

David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1081137 - Sun Aug 06 2006 08:03 PM

 


While the mirror box glue was drying, I laminated the Ebony Star to the bottom of the rocker box.

Love that 3M 90 spray adhesive. Took me about 15 minutes to glue, set, roll and route clean.




David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1081143 - Sun Aug 06 2006 08:05 PM

 


The bearings took a lot longer - probably about 30 minutes each.


 

rboeAdministrator

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1081156 - Sun Aug 06 2006 08:13 PM

 


Did you do the whole outside edge or just the bottom? Preheat?

--------------------
Ron

David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1081157 - Sun Aug 06 2006 08:13 PM

 


Back to the mirror box. Here I am calculating where to put the string holders.

I placed the pole holders an equal distance from each other, and will put the string holder at an equal distance between each pole. There are other options (like going to the extreme corners and enlarging the string triangles) but I'm opting for "equality" in order that all my string units will be the same length. That will make it easier to make them, and in theory they will be interchangeable - so I don't have to remember which string goes where if they ever get detached.



 

David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1081168 - Sun Aug 06 2006 08:23 PM

 


And then the inside of the mirror box got sprayed with black paint.



The rest of the day was spent filling holes with putty and finish sanding the rocker box, mirror box and bearings which are now all ready to apply my chosen finish: Minwax Helmsman Spar Urethane.

Not sure if I'll do that before or after first light. Any sage advice on that question?

 

David N
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Re: The 50 Cent 20" String Dob Project [Re: rboe]
      #1081174 - Sun Aug 06 2006 08:28 PM

 


Ron,

If you asking about the laminate on the bearings, it is just on the bottom. I trimmed with a 1/4' round so the surface came out less than the 3/4" thickness of the bearings. I'm hoping that won't be an issue later (having less surface riding on the teflon.)

Preheat? It was 90 degrees here in "Sunny Oregon" today - so everything was hot.
Tom LAdministrator

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1081197 - Sun Aug 06 2006 08:39 PM

 


Busy day, David! Looks really good. Aiming for OSP I take it. I didn't think about the Spray 90 adhesive when I attached my ebony star to the base of my mirror box...good idea! Have you used it before or any of the other RCA'ers? What will be your final focuser height?

--------------------
Tom
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rboeAdministrator

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Re: The 50 Cent 20" String Dob Project [Re: Tom L]
      #1081286 - Sun Aug 06 2006 09:49 PM

 


90?, That would be a cold front for us.

I should have finished my dob last year. I'm getting too many good idea's to steal - and they're in totally different directions than I had planned on going. Oiy!

Looking Good!

--------------------
Ron

 

David N
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Re: The 50 Cent 20" String Dob Project [Re: Tom L]
      #1081742 - Mon Aug 07 2006 07:56 AM

 


Tom,

The spray adhesive was a 'find' I made several years ago when building some cabinets. Gone are the days of inhaling contact cement fumes and waiting - and waiting - for it to dry, hoping no sawdust settled on the cement, and then fooling around with the dowels to position the laminate.

Yes, the Oregon Star Party is the goal - meaning I have just 15 days to complete.

The focal length of the mirror is 98.75" - a hair under F5. I'm guessing the focuser will end up about 88" from the mirror face. Determining that distance will hopefully be my project for this next weekend.

 

Tom LAdministrator

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1082547 - Mon Aug 07 2006 04:08 PM

 


Good height...only need a 2 rung step ladder. I've seen the Spray 90 on several occasions and wondered how it will do for attaching laminate to plywood. I was concerned about my strips on my bearings. I ended up using Titebond III glue and it was a hassle clamping the bearings...the base was easy since it was flat and I had big, heavy flat items in the garage to hold it together.

How goes the hunt for the RCA observing site? Did any of those government land offerings sound promising?

Ron, you need to just start working on your Dob and stop with all the excuses! 120 degrees is nothing! It's a dry heat anyway!!! BTW, I was asking about a specific spray adhesive called "3M 90 Spray Adhesive" or something like that...

--------------------
Tom

 

David N
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Re: The 50 Cent 20" String Dob Project [Re: Tom L]
      #1083434 - Tue Aug 08 2006 07:44 AM

 


Tom,

Yes, it's 3M High Strength 90 Spray Adhesive that I use.

No work on the scope last night as we had an RCA Board meeting. The site search continues, but can't really get too serious until we raise more money.

David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1090295 - Fri Aug 11 2006 10:13 PM

 


Lots of little things accomplished today - including about 2 hours of running around picking up parts and stuff. How come no one warned me you spend as much time hunting down parts as you do actually building a telescope?

My objective today was to complete assembly of the secondary, and I can now check that off the list - almost.



Notice my unique and revolutionary spider?



Well, the Protostar spider I have ordered isn't here yet, and I can't wait for it to determine my pole and string lengths and still meet my deadline just 11 days away. So, thanks to another fellow club member who just happened to have a 3.1" secondary mirror holder on hand, I should be able to get that major moment over this weekend - and with any luck, maybe first light.

If anyone wants specifications and instructions for making this spider, I'd be happy to oblige.

 

commonlyhunted
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1090313 - Fri Aug 11 2006 10:31 PM

 


if you look through that... i really want a picture of something like demeb, or jupiter... just to see the difraction spikes on that thing... more like i-beams, not spikes!
But it would be funny if it's not that bad!

Tom LAdministrator

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1090326 - Fri Aug 11 2006 10:41 PM

 


Looks great!!!

I remember in the final throws of getting my scope ready last summer for our Sunriver vacation I would schedule my ACE HW trips during lunch and do what was necassary that evening...making a list for the next day's trip back to ACE after completing the project du jour! The best stocked ACE I found is on TV Hwy and 185th in Aloha...

I sure hope your spider shows up soon!!! Does Bryan understand the critical nature of having it delivered on time?

--------------------
Tom
David N
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Re: The 50 Cent 20" String Dob Project [Re: Tom L]
      #1090391 - Sat Aug 12 2006 12:01 AM

 


Tom,

Ace in the Pearl is my lifesaver.

I've talked to Bryan a couple of times, and it is scheduled to ship on the 18th - EXPRESS MAIL - and should be here Tuesday at the latest. I'll have the holes pre-drilled and ready to slip it in on my way out the driveway.

If it doesn't get here, I'm seriously going with the plywood spider.
Dennis/OR
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1090414 - Sat Aug 12 2006 12:58 AM

 


How bout coat hanger wire? I'll be doing better in the morning.

--------------------
Dennis

rboeAdministrator

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Re: The 50 Cent 20" String Dob Project [Re: Dennis/OR]
      #1090747 - Sat Aug 12 2006 08:19 AM

 


For starters, it's a given that all projects need several trips to the hardware store and only one of them is ever planned.

Second, you have that wood spider in wrong, but all you need to do is rotate it so the thin side is up. You'll be good to go. Make sure the upper edges are sharp to reduce glare, taper the vertical sides a bit for the same reason (angle depends on F of scope. Fast scopes need more of a rake) then you also want to make it out of opticalwood, a dense but light tropical hardwood that is getting harder to find as coffee growers are taking over it's habitat.
--------------------
Ron
David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1092020 - Sun Aug 13 2006 12:30 AM

 


With the secondary done, the task today (Saturday, OSP -12) was to determine the distance between the secondary and mirror box so I can calculate the pole and string lengths.

This is a "mirror ramp" I configured on my deck where I have a view of Mt. Hood, about 50 miles away - a rather fine object to be the first thing seen through this mirror.


 

David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1092033 - Sun Aug 13 2006 12:51 AM

 


Hauling everything to the deck and setting it up took most of the morning. I also had my first moment of fear worrying about my mirror falling as I approached the end of my ramp and the wind started gusting, but no problem. I also had to deal with a bright Sun as you can see by my custom shading devices.

After a lunch break, the clouds cleared and I spent the better part of an hour lining things up, finding a slice of the mountain to focus on and taking measurements. Then moving everything a half inch and doing it all over again, and then again to make sure.

My first calculation was pretty close so the main chore was lining the two mirrors up, and moving the ramp to find Mt. Hood. Would have been easier if it the horizon hadn't been so hazy - a combination of forest fires and hot weather.



Task for the day completed with time for a nap before heading out to help with a public star party to watch the Perseid Meteor Shower. There were enough meteors to please the crowd, but it wasn't a spectacular show. The moon's appearance pretty much ended the event - which was fine with me, so I could get home, make these posts and get to bed and dream about first light Sunday night, an ambitous but possible goal.

 

David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1093489 - Sun Aug 13 2006 09:15 PM

 


11 days and counting until the Oregon Star Party - and another busy day on the project.

First task was to create my 'telescoping' telescope poles. Based on my calculations yesterday, my poles need to be 74" long. That's a little long to fit inside my SUV, and I didn't want to haul them on the roof, so I was determined to come up with some telescoping tubes, similar to what I used on my 8" "practice" scope - but preferably with a 'fixed' stop so they extend to the same length each time.

I looked all over the World (WW) and didn't find anything off the shelf that would work, so the solution turned out to be to make my own - thanks to McMaster Carr who stocks these telescoping tube connectors.



David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1093511 - Sun Aug 13 2006 09:38 PM

 


For my tubing I purchased locally one 12' x 1.5" (OD) and one 12' x 1 3/8" (OD) aluminum tube (6061) each cut into three 4' sections. The small tube fits inside the larger tube with only a few thousands of an inch difference and slides easily in and out, but is not "loose."

With the two tubes nested together, I pulled the inner tube out about an inch and then drilled a hole through both tubes at once, about three inches from the other end. This would be the set point with the tubes stored for storage. Then I extended the tubes to the my desired finished length and drilled the holes in just the outer tube a few inches from the opposite end. Drilling the holes exactly in the center of the tube is very important.





When fully extended, there is about 2 feet of overlap of the two tubes which help make it rigid. The spring pins work great and seem to be strong enough to take the pressure that will be put on them when extended in the scope. I am very happy with the results.


 

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1093526 - Sun Aug 13 2006 09:51 PM

 


Now to make the strings - and yes, I'm really using 'string' and not wire, rope or fishing line.

The material of choice is reportedly the bow string material with the lowest stretch/creep characteristics of the many different types.



On my 8" I created an elaborate stringing jig which was fine for the smaller sections that scope required - but was out of the question for this project. My strings sections on this scope are a little over 67" so figured I would use two nails on a long piece of wood. The frame of my garage door met the criteria - and it was in the shade.



I did 11 loops (22 strands) and used the last one to tie some half-hitches over the whole lot to keep them together, as well as secure the end loops.

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1093539 - Sun Aug 13 2006 10:03 PM

 


Now to reinforce and finish the end loops.

I stretched the end loop about three inches and secured it tight.



Then using a different, and stronger string material (I forget what the pro archers call it) I tightly wound it around the end back and forth so there was two layers. Now you have a reinforced loop where it will be connected to the secondary and the mirror box. It is at these points where the strings could get snagged or torn, so this reinforcement is very important.



Then, find the center of the loop and wind the string around the whole lot of strings as tight as you can and tie off when done.



And that's all there is to it.

 

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1093551 - Sun Aug 13 2006 10:14 PM

 


Last task of the day was to determine where to install the bearings. This involved determing the balance point of the mirror and then positioning the axis of rotation (the center of the bearings) at that balance point.

My best guess at the moment is the secondary will weigh about 13# and the mirror/cell/box about 76#. With the help of some fellow club members who know these things, we calculated my balance point to be about 13" above the mirror cell, or about 8" above the top of the mirror box.

Here's how I determined where the bearings needed to be attached:


Notice my beautiful dovetails have diappeared.

 

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1093557 - Sun Aug 13 2006 10:23 PM

 


With the bearing position determined, I decided how I was going to fasten them (removable: with threaded inserts and knobs) - and took yet another trip to the hardware store to get the parts.

Attaching each bearing in the exact same place (in relation to the opposite one) is really dificult. I'll spare you the photos of the several steps I invented to do this, but here's the result.



A test run in the rocker box revealed one side is a little off - and a little is too much, so looks like tomorrow night's task will be fixing this problem.

 

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1093569 - Sun Aug 13 2006 10:45 PM

 


Just like clockwork!

Thanks for sharing this project with us, David...

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Tom
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Re: The 50 Cent 20" String Dob Project [Re: Tom L]
      #1093804 - Mon Aug 14 2006 07:06 AM

 


Dang! That scope is going to pop when you add the finish. :wheeho: It's coming along very well.

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Ron

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1095198 - Mon Aug 14 2006 10:06 PM

 


OSP - 10

First task this evening was to fix the problem I discovered last night after attaching the bearings. When I placed in the rocker box, one of the front bottom corners of the mirror box scraped the bottom of the rocker box - while the other corner was fine. Which meant that my positioning of the bearings was not EXACTLY correct.

I reasoned that lowering the bearing attached to the low corner just a bit, would raise the box corner - but determining just how much kept me thinking for a while coming up with a plan.

I placed the mirror box with the bearings still attached on the floor and rocked it to see how far out of alignment it was - maybe a 1/4." I removed the bearing bolt closest to the low corner and left the other three in place, then rocked the box until the whole front edge was flush with the floor. I clamped the bearing in place, rocked it again, and the whole front edge touched the floor evenly. In the rocker box it no longer scraped the bottom.
Success.



If I ever were to build a scope with oversize bearings again, I think I would use this 'trick' to align the bearings in the first place. Take my best shot at positioning everything where I calculate they should be, secure the bearings in three places - and then rock and roll until I have front end alignment, and secure the loose bearing in place.

 

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1095207 - Mon Aug 14 2006 10:18 PM

 


With the bearing problem fixed, only enough time and energy left this evening to complete one final detail on my strings, and that was to thread them through a hollow cord to act as a protective cover.

I use a 3/8" "slinky cord" used by fisherman to make drifting sinkers. It is hollow and very sturdy, since it is intended to drag along the bottom of a river over rocks and trees.



David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1097003 - Tue Aug 15 2006 09:39 PM

 


Tonight (OSP - 9 days) the plan was to assemble the all the parts I've been making and see if it ressembles a telescope.

First I finished the pole/string holders that will attach to the secondary - a design that I hope works. They are made out of steel (though I may re-do in aluminum some day) with holes drilled to accomodate a fastening screw and two strings. In the middle is a tapered rubber plug which I cut in half. The end of the pole fits snugly around the plug which means that it will (hopefully) stay in place as the pole is lengthened to apply tension to the strings.



The plan is to use a caribener to connect the string to the upper mounting bracket - and a turnbuckle on the mirror box end, which will allow fine tuning of the length and tension of the string.


 

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1097032 - Tue Aug 15 2006 10:03 PM

 


With the brackets in place I hauled everything into our living room (thank goodness for vaulted ceilings) and with the help of my wife, put everything together - only to realize that I had only finished 3 of the 6 strings I needed. What a dummy. So I got some rope and bungee cords and went ahead anyway. Everything actually held in place OK, and I was able to let go with some confidence, take a photo and study how the length of the poles and strings seemed to work and what adjustments need to be made.




So, tomorrow night's task will be to finish 3 more strings and try again. (Maybe I need more sleep?)

 

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1097349 - Wed Aug 16 2006 06:59 AM

 


Don't you hate it when some simple math error googs you up?

I thought you had place the lower box in the rocker box, shimmed and leveled and let fly with that to set up the bearings.

My woodshop carries melamine plugs (they look like very thin white game pieces glued to a sheet) for repairing screw holes and the like in melamine sheets. Look like the cats' meow as thin wear plates between rocker and lower cage.

It's taller than I expected. Nice scope!

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Ron

 

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Re: The 50 Cent 20" String Dob Project [Re: rboe]
      #1097420 - Wed Aug 16 2006 08:05 AM

 


Ron,

It wasn't a math error so much as a brain freeze. Thankfully it is easily fixed - only setback is taking another couple of hours out of my 'contingency' work-time available balance. If all goes well this evening, my plan is to then take apart and apply the wood finish over the next couple of nights, and then take to the RCA Star Party at White River on Saturday night for First Light.

Could you - or anyone else - give me a clue about how big my teflon pieces should be on the bearings and the ground board? Total weight of the scope (moving parts) will be about 100 pounds (secondary = 13 and mirror/box/bearings/rocker box = 87). Diameter of the ground board and bearing is 35" if that makes any difference. I believe my teflon is 1/8" thick.

Any help with the "math" (not my strong suit) would be appreciated and save me a couple of hours.

 

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1097488 - Wed Aug 16 2006 08:36 AM

 


Dang that's a big scope. Looks nice!

David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1100656 - Thu Aug 17 2006 10:39 PM

 


Last night I finished the 3 strings I had 'forgotten' to make earlier (takes about 30 - 40 minutes each) - and once again assembled everything. And once again had another problem. The key rings I was using as connectors for the strings and the eyebolts (see earlier photo) weren't strong enough and bent when I was tightening everything up, so I was foiled again from getting the scope assembled correctly.

Another trip to ACE this morning to secure something stronger to use as a connector. I'm not sure what it's official name is, but it resembles a link from a chain with a threaded post and nut. I used these on the ends connected to the turnbuckle.


Here's a photo of how the pole and string holder works.


 

David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1100674 - Thu Aug 17 2006 10:57 PM

 


Tonight, the third time was the charm as with enough strings and sturdy connectors, everything fit together and tightened up just like it was suppose to.



I didn't have the mirror in the cell, so I couldn't test the balance. That will be my task tomorrow night.

Dennis/OR
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1100741 - Fri Aug 18 2006 12:13 AM

 


Dave, for what it's worth I think those links you used at the turnbuckle end of the strings would look better at the other end of the strings as well.

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Dennis

 

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Re: The 50 Cent 20" String Dob Project [Re: Dennis/OR]
      #1101080 - Fri Aug 18 2006 07:43 AM

 


In the Dob book there is a formula for sizing the teflon. I'll look it up when I get home if someone else has not posted it yet.

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Ron
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Re: The 50 Cent 20" String Dob Project [Re: rboe]
      #1101115 - Fri Aug 18 2006 07:59 AM

 


What is the structural stability of the scope with everything tightened down? Can you twist the UTA? Is there any lateral movement?

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Tom
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Re: The 50 Cent 20" String Dob Project [Re: Dennis/OR]
      #1101126 - Fri Aug 18 2006 08:05 AM

 


Dennis,

I actually did start out with them on the other end too, as a connecter between the carabiner and the string loop. BUT, this added too much length to the string section and I couldn't get full tension on the strings.

You might know this, but for the benefit of others; basically, the length of my poles (when extended) and the distance between the bottom of the pole and the eyebolt holding the string are the 'fixed' elements of each Tension Triangle (a term I just made up). The string - and adjusting it's length - then gives me the ability to 'fine tune' the triangle to ensure that the pole will be held in place and not move when the whole scope is moved. [See Dan Gray's explanation for more information on this concept.]

I am also using some plastic pipe fittings as pole holders on the mirror cell box which also gives me the ability to "unscrew" (and extend) the pole a half inch or so and put tension on the tension triangle. Once I get my strings all the right length, my plan is that I would leave the strings alone, and when assemblying the scope, extend the poles to bring everyting into place and tension. That is how my 8" works (and quite well).

David N
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Re: The 50 Cent 20" String Dob Project [Re: Tom L]
      #1101140 - Fri Aug 18 2006 08:09 AM

 


Tom,

With everything cinched up, it is as rigid as a tree.
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1101143 - Fri Aug 18 2006 08:10 AM

 


Cool! I hope to get the chance to see it some day...I finally got around to sending my RCA dues in again!!!

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1103735 - Sat Aug 19 2006 09:37 PM

 


I didn't quite get far enough for first light tonight as I had hoped, but I'm real close.

First task this morning was to give the parts a second coat of clear finish (I did the first coat last night). Then it was outside to make the ground board. I already had a round board cut, so I decided to whittle some of the wood away and make it lighter to carry. I glued up some scraps to make the feet. After using a band saw to get them close to round, I used this setup with a disc sander to geth them smooth and round.




A quick spreadsheet to figure out the size of the teflon pads, and the ground board was done.



A third coat of finish went on before lunch - and a few hours later when dry, I started reassembling everything.



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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1103741 - Sat Aug 19 2006 09:44 PM

 


Here's the bottom of the scope assembled. I ended up making the miror box cover out of Baltic Birch (1/2" piece I had laying aournd) which as you can see, is lighter in color than the ApplyPly.


 

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1103744 - Sat Aug 19 2006 09:47 PM

 


Ah! So this is thread asking about the teflon. Lost track of it.

Kriege says optimal resistance is at 15lbs per square inch. Weigh the beast, divide by four to get the weight each teflon piece is to carry. Divide that by 15 to get your square inches.

Don't forget the fudge.

What happened to the hocky pucks?
David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1103750 - Sat Aug 19 2006 09:55 PM

 


Between other tasks, I gave my poles two coats of paint.



Highlight of the day was the arrival of my spider from ProtoStar. Thank you Bryan and the USPS Express Mail! Putting my secondary back together and attaching the spider will be my first project tomorrow morning - and maybe all I will have time for. I promised my wife I'd help clean the house and the driveway before some friends come over for dinner.
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1105414 - Sun Aug 20 2006 10:21 PM

 


Sunday was no day of rest for me. First order of business was finishing the secondary.

Starting with the baffle. I wish I could tell you what material I used, but I can't find the invoice and don't remember what it's called. It wasn't Kydex as I've heard too much about it warping in heat, which was what the sales rep at TAP Plastics confirmed. He suggested this 'other' material which is about 1/8" thick - shiny on one side but a pebbled flat black finish on the other which is perfect. I had them rip me a 12" strip which fit perfectly. I cut to length with two swipes of a razor blade and tacked in place.

Then the spider which fit perfectly, and the 3.10" mirror - and then the Moonlite CR2 focuser (after putting in backwards first), and the secondary was done. Weighed in at 11# with no eyepiece or finder.



David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1105420 - Sun Aug 20 2006 10:25 PM

 


With the secondary (and the housework) done, there was nothing left to do but put everything together and see if it worked.

Here it is ready for lift off.


 

David N
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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1105427 - Sun Aug 20 2006 10:37 PM

 


And here is the finished scope and the Amatuer Telescope Maker.



Whew - and it works too.

Some fine tuning of the string and pole connectors to do, but we're ready for the Oregon Star Party with two days to spare.

FIRST LIGHT:
August 20, 2006
9:00 PM PDT
45* 31' N
122* 43' W


Tom LAdministrator

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1105715 - Mon Aug 21 2006 07:37 AM

 


So, will it rain at OSP this year???

Scope looks great, David! Congratulations and have a great OSP!!!

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Re: The 50 Cent 20" String Dob Project [Re: Tom L]
      #1105755 - Mon Aug 21 2006 07:57 AM

 


Nice!

Looks like you'll need a ladder at zenith. What's the eye piece height at Zenith?

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Ron
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Re: The 50 Cent 20" String Dob Project [Re: rboe]
      #1105777 - Mon Aug 21 2006 08:17 AM

 


Really really nice.

I can't get over how quickly you built it. I am in shock!

Al


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Re: The 50 Cent 20" String Dob Project [Re: LateViewer]
      #1106392 - Mon Aug 21 2006 02:00 PM

 


Amazing just how fast this beauty came together!

Congrats, and enjoy!

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Matthew

 

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Re: The 50 Cent 20" String Dob Project [Re: LateViewer]
      #1107416 - Mon Aug 21 2006 10:12 PM

 


Al,

Thanks.

For the record, I started the 'woodworking' on July 1, taking advantage of a 3-day weekend, and basically worked on it pretty solid every weekend after that, and a few (but not too many) evenings, which were generally dedicated to design and work planning thought - and ordering things off the internet. So that makes it seven weeks from start of fabrication to first light. There's nothing like a deadline to drive me to produce.

As I mentioned earlier, my 'work' on the project seemed to consist of almost equal parts of running around buying parts, tools and materials - then going back to get more, or what I forgot the first time. And I've probably got enough 'leftovers' to build another scope.

I was/am blessed that I'm only 10 minutes away from Ace Hardware, 15 minutes from my plywood and woodworking tool suppliers; don't have a yard to mow or kids to worry about; have an understanding wife and a great local astronomy Club (Rose City Astronomers) with helpful members that coached me through some technical design issues - and we've had perfect Oregon Summer weather (well it did get to be 100 a couple of days which was a little much) that I needed for my "driveway shop."

I'd guess you could say the stars were all in alignment for me to pull this off.

.........David

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1107453 - Mon Aug 21 2006 10:43 PM

 


David, that's just flat out amazing! Hope to see it at OSP!

Hey, making the mirror would've only added another week.

Best,
Mark


 

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1109208 - Tue Aug 22 2006 09:22 PM

 


And here it is packed in the Telescope Utility Vehicle all ready to head to OSP first thing in the morning.



I managed to muscle it in the TUV all by myself, but I'm hoping I'll be able to recruit a helper for all future loading and unloading.

Notice how the telescoping poles fit perfectly - and yes, that's a telrad secured with masking tape on the secondary.

 

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1109220 - Tue Aug 22 2006 09:37 PM

 


I used duct tape when I first attached my Telrad! Have a good trip, dark skies and a beer with Larry on Friday night at the Sun River Observatory booth...he's bringing a keg of IPA!

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1113919 - Fri Aug 25 2006 02:21 PM

 


Yes, of course we have wireless internet service at the Oregon Star Party in the middle of nowhere - and that is where I am at the moment with about 700 other observers.

I had to do some minor field surgery on the scope to adjust the mirror cell and get perfect collimation - but with the help of Nate who built it, and because I had brought half my tool chest - I was able to correct a 'measuring' error of mine, and everything now works great.

Here's a photo of my scope getting the THUMBS UP from Dan Gray, the 'inventor' of the string scope. I'm looking a little dazed after Dan (on the left) tells me he think's mine might be the second largest string scope in the world (next to his 28"). Are we wrong?



The view through the scope was totally awesome. I've looked through bigger scopes, but nothing compares to looking through your own big one.

Greg K.

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Re: The 50 Cent 20" String Dob Project [Re: David N]
      #1113947 - Fri Aug 25 2006 02:37 PM

 


classy shroud and counterweight system you got there!

enjoy the party!

 

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Re: The 50 Cent 20" String Dob Project [Re: Greg K.]
      #1114044 - Fri Aug 25 2006 03:57 PM

 


Love the shroud! But poor form on the counterweight. Guess you need an eye piece rack down there. Lookin' good though!

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Re: The 50 Cent 20" String Dob Project [Re: rboe]
      #1114125 - Fri Aug 25 2006 05:10 PM

 


Great picture!

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Tom