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Posts Made By: David McGough

March 16, 2010 06:32 PM Forum: Celestron

Orange tube C-8 adapter?

Posted By David McGough

You might try asking them for the current C8, C9.25 adapter and chase the threads out a few thousands. That worked for mine to attach to the old spec rear cell threads.

I don't know if they'd still do this, I had it done when Werner and Brigitte still owned the business. The new focusers are every bit as smooth and well built though. I have 3 of them now!


October 5, 2010 03:12 PM Forum: Equipment Talk

Home Planetarium

Posted By David McGough

The Sega Homestar is very nice and gives a credible view including the Milky Way. Some distortion near the edges as it's projecting from a flat image like a planisphere but aesthetically pleasing. I use mine sometimes when the marine layer keeps me from seeing the real sky. Lying on the couch with some Jonn Serrie music on is sublime.

The motorized rotation is some minutes vice 24 hours but that keeps it animated


January 2, 2011 09:56 AM Forum: Astro-Physics

what's being made?

Posted By David McGough

Robert Howe said:

... If the latter, how many centuries back does the waiting list extend?


Robert Howe

I think it goes back to anyone who couldn't get a new scope from Alvan Clark and Sons before they closed up shop! grin


June 5, 2011 02:22 PM Forum: Reflectors

Help needed with Cassegrain assembly

Posted By David McGough


Any equations are assuming you have exact data on the mirrors, which you don't. You don't have exact Radius of Curvature data for either mirror, just rough design points.

I just ran through this with a set of old 10.25" DK optics and built an adjustable truss for the front ring and secondary holder using telescoping tubing from with hose clamps on the slotted ends, and assembly with star nuts (tube connecting nuts) from I was then able to get everything dialed in, and actually the open tube is great for cool down!

This is probably the best way to go, and the woodwork does not need to be pretty to get spacing down.


John MacDonough said:

Good morning all,

I've about "trial and errored" myself to death, so I'm asking for some help. I purchased a classical Cassegrain mirror set hoping to assemble a telescope. What I'm told by the person who sold the mirrors to me is this:
Primary: 200 mm diameter, f/4
Secondary: M = 5
EFL: 4000 mm, f/20

My problem, assuming the above is correct, is mirror spacing. I was told to target between 628- 630 mm for a back focus of either 275 mm or 210 mm. That hasn't worked out. For one thing, with the mirror cell, focuser and diagonal I have, I need approximately 339 mm of back focus. I've also consulted the VanVenrooij book and calculated a mirror separation of 614 mm for the back focus distance I want. I've currently got the scope assembled with 611 mm separation and have experimented with separations all the way up to 635 mm without being able to achieve focus.

Can someone help me out?

October 4, 2013 05:13 PM Forum: Telescope Making

Cassegrain Assembly

Posted By David McGough

Astro Systems makes Cassegrain secondary holders that fit their spiders. For my 10.25" I got the primary cell from Optical Supports with mods to their standard design being perforation in the mirror side plate and change to the fan mounting pattern to accept a Feathertouch flat base. Hard part was the spacing so I got telescoping aluminum tubing from DX Engineering and made a truss scope to work out the mirror spacing and focuser position.