Wednesday, February 16, 2005

Battling Big Bertha

I spent most of the evening building a proper flotation mirror cell for the 17.5" mirror. The strategy was to start with a 1/2" by 24" by 24" piece of birch plywood.

1. Then I cut it down to 20.5" x 20.5" with the table saw.

2. Then I measured the exact center of it.

3. Then I measured where 17.5"/2 put me along the center lines.

4. There I marked places for four supports to hold the mirror in place.

5. The supports I cut out of a 1 1/2" x 2 1/2" piece of oak.

6. I put each block in its appropriate place, clamped it there, and then drilled one hole.

7. Screwed in a #7 x 1 1/2 wood screw.

8. Removed the clamp.

9. Drilled a second hole, to preven the supports from rotating.

10. Screwed in another #7 x 1 1/2 wood screw.

11. Repeat steps 6-10 three times.

12. Made sure the mirror actually fit before screwing down the fourth support. Then discovered that I would have to make the plywood into an octagon with the table saw, to fit inside the back of the square telescope tube assembly.

13. Then I cut four very small pieces of Delrin (the wonder material!) to make mirror clips.

14. I drilled the mirror clips and the supports on the top, and screwed them in place. The Delrin is slippery enough that they can turn, but they don't turn unless pushed.

15. Drilled three holes in the plywood, 120 degrees apart, 5.25" from the center of the mirror, a bit larger than 1/4".

16. Used a 1/2" wood boring bit to enlarge each of these holes in the top 3/16" of an inch (approximately) so that the head of the 1/4-20 machine screws would not be scraping the back of the mirror.

17. Drops the machine screws in, put lock washers on the bottom side of the panel, then tightened down nuts to keep them screws from turning.

18. Put a 12 pound spring (after clipping for length) on each screw, and then put the screws through the holes at the bottom of the telescope mirror access door.

19. Attached wing nuts to the screws on the outside of the telescope mirror acess door.

I was still a little amazed at how far off the collimation was, but with a bit of fiddling, I now have it much better than it was, and the mirror is no longer under stress. It's a little hard to tell when I have it properly collimated. I'm using the LaserMate Deluxe collimator, which projects the image back onto an aluminum screen that you can see from the back of the telescope, but the laser beam is supposed to drop into a little centered hole, and you only see a "spray" of red around that hole. By the time the beam has traversed the full focal length of this monster (about 2000mm, I think) twice, it completely overwhelms the hole.

The image isn't perfect yet--not even close. (Sad to say, not dramatically worse than many Meade and Celestron Dobsonians that I have looked through over the years.) Anything above 111x is still fuzzy. This may be that the springs are too compressed--I should probably use 4" long machine screws, not 3". I'll get those tomorrow, and try again. It may also be that the focuser isn't exactly aligned with the tube. Maybe I need to wait for the mirror to finish cooling. It could even be a lousy mirror, of which Coulter made a few.

I will say this: the phrase "light bucket" was coined for monsters like this. I am impressed, even here in suburbia, how much color the Orion Nebula has at low power, how much color lots of stars are showing (because there's enough light to excite the cones in my eyes), and how much detail the Moon shows, even at 57x. There are many satellites of Saturn visible with this beast--it takes you down a lot of magnitudes!

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