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Posts Made By: Stuart St. Clair

March 11, 2008 11:41 AM Forum: Equipment Talk

Source for 90mm f/10 Air-Spaced Achromat w/ Cell

Posted By Stuart St. Clair

Does anybody know where I can buy an achromatic objective in a cell? I'm trying to fix a refractor that suffered a broken lens. Thanks!

March 11, 2008 09:05 PM Forum: Telescope Making

Assembling Refractor

Posted By Stuart St. Clair

I have an OTA for a 90mm refractor with a focal length of 910mm. The air-spaced achromatic objective lens assembly is broken. I'm thinking of replacing it with a 90mm objective with a focal length of 1000mm. I'm wondering whether that will give me enough in-focus for a binoviewer. I've read a recommendation for 4 inches of in-focus, but I'm wondering whether 90mm would be enough. If it is not enough, is it very difficult to cut the tube down a bit more, and end up with a straight cut? Any tips on how to make the cut would be very appreciated. Thanks!

March 28, 2008 09:11 AM Forum: Telescope Making

How to Drill Accurate Holes?

Posted By Stuart St. Clair

I'm planning to work on my 16" dob project tomorrow. I need to drill several holes in wood pieces. I have a drill press, but I'm not very knowledgeable about how to use it well. Many times I find that my holes are a bit off from the mark that I was aiming for. Does anyone have any tips on getting the hole to be centered exactly on the mark? I'm just thinking of holes from say 1/8" to 3/8" diameter drilled with regular bits. Thanks!

March 30, 2008 09:44 PM Forum: Telescope Making

Clearance on Rocker Box

Posted By Stuart St. Clair

Thanks to all of you who gave me advice on drilling accurate holes. I made a lot of progress this weekend, and most everything turned out pretty well, some parts better than others.

I have another question. I'm ready to build my rocker box. I remember reading in The Dobsonian Telescope that I should allow a certain clearance between the outside of the mirror box and the inside of the rocker box. I'm thinking it was 1/16" or 1/32". I don't have the book anymore. Can somebody please refresh my memory, or provide an alternative recommendation? Thanks!

April 11, 2008 12:29 PM Forum: Equipment Talk

Argo Navis Question

Posted By Stuart St. Clair

Will encoders from a Sky Commander work with an Argo Navis computer? Thanks.

April 12, 2008 02:48 PM Forum: Telescope Making

Focuser Baffle Question

Posted By Stuart St. Clair

Hi. I'm building a 16" f/4.5 strut dob with a single-ring upper assembly. There will be a baffle on the opposite side of the secondary from the focuser. I've read that another baffle between the focuser and secondary, just behind the bottom reach of the focuser, would be helpful. How do you figure out the hole diameter for that baffle? I'm thinking it would depend on how big of a fully illuminated field you want. Is that correct? Can anybody give me some tips on how to figure the baffle hole size out? Also, if anybody has recommendations for the size of the fully illuminated field that I should design for, please provide those also. Thanks!

April 18, 2008 05:37 PM Forum: Telescope Making

Glue for Ebony Star

Posted By Stuart St. Clair

What's the best glue for attaching Ebony Star to plywood?

Do you have any tips on the process, such as how much glue to use, or anything else I should know to make it easier? Should I put the glue all the way to the edge or stop a little short so it doesn't squeeze out and make a mess?


April 19, 2008 08:45 PM Forum: Telescope Making

Threaded Tube Inserts - Flower Shaped

Posted By Stuart St. Clair

I need to buy some of those flower-shaped inserts that go into aluminum tubes and allow you to screw a bolt into the tube. My tubes are 2" O.D. with 0.049" walls. I need seven of the inserts with threads for 1/4-20 bolts. Does anyone know where I can get these? Thanks.

April 20, 2008 09:05 PM Forum: Telescope Making

Installing Teflon

Posted By Stuart St. Clair

Thanks to all of you for your advice on gluing Ebony Star to plywood and on where to find threaded tube inserts. Now, I need some advice on Teflon. What's the best way to attach Teflon rectangles to plywood -- glue, nails, screws, or other? If you say nails, do I pre-drill holes or just pound the nails right through the Teflon? Thanks again!

April 25, 2008 10:25 AM Forum: Telescope Making

Sizing Teflon

Posted By Stuart St. Clair

I have a question about sizing Teflon. I think my Teflon pieces are a bit bigger than they need to be, but I don't want to cut them unless I have to. The 16" f/4.5 dob I'm building is the Albert Highe 3-strut design. The OTA weighs about 60 lbs and the OTA plus rocker box weighs about 80 lbs.

The Teflon pieces for the altitude bearings are just about right to provide 15 psi pressure. However, the pieces for the azimuch bearing are a little big and will only provide about 12 psi pressure. Do you think it is important for me to cut them down?