First light test of Mark I. 450mm f/12.6 cassegrain

by Richard Crisp

Last night I finally put the new 450mm f/12.6 classical cassegrain through its first tests. All tests last night were visual.

I started out with a star test to make sure collimation was good. It was.

Then I moved over to the Trapezium and enjoyed looking at it with my 35mm panoptic and my 22mm nagler.

Then I decided to take a look at Mars, which was nearly overhead. I quickly discovered why the scope fell out of focus: the moving secondary mirror's springs that retract it to "home" weren't strong enough to overcome the weight of the secondary and baffle. So I just shortened them a bit and that solved the problem.

I enjoyed examining Mars and then swung over to Saturn. It was a bit soft but I discovered that my secondary mirror was fogged over from the dew. By then it was 9:30 so I just decided to cover things up and call it quits.

Key points:

  1. the scope holds collimation very well

  2. once the finder scope and the main scope were aimed to the same point, I noticed that as the scope was moved around to different parts of the sky, that there was a small amount of mirror movement causing the stars to not be quite in the middle of the finder when they were centered in the FOV of the scope.

    That is no surprise because there's a bit of play in the nylon bushing retaining the mirror on the post. I estimate I have between 1/16 and 1/32 inch play causing the main mirror to slide laterally by that amount. I will replace the nylon bushing with one that fits correctly.

  3. the scope is more rigid than its predecessor. it points more accurately and damps out quicker once aimed.

  4. the scope weighs 101# on the mount so I was able to drop off one 20lb counterweight.

  5. Although heavy, the main part of the scope can be handled by one person to attach to the mount.

When I collimated the scope yesterday I first did so in the garage with the scope on no mount. Then I hauled it to the backyard using a hand dolly and took it apart (removed the truss tubes). Then I was able to put it on the AP1200GTO mount. Once on the mount, I added the truss tubes and the head end to finish reassembling it. I then checked the collimation. To my surprise and delight it was basically already well collimated. I checked that during the daylight by gazing down the aperture end and adjusted the tilt-tip of the primary to get the spider vanes properly aligned and then looked for the concentric circles etc. That turned out to be good enough.

here are some new photos of it sitting on the mount added to the previous ones.

http://www.narrowbandimaging.com/mark_i_classical_cassegrain_hex_page.htm

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Posted on sf-bay-tac Jan 20, 2006 11:24:28 PT
Converted by report.pm 1.2 Mar 02, 2006 21:21:17 PT