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YankeeJeff
sage
Reged: 08/11/06
Posts: 439
Loc: Brooklyn, NY
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Hi all - it's been a while since I last posted...
I’m not sure why I never noticed this before, or maybe I have but didn’t pay it any mind - the other night I noticed that defocused stars were not the doughnuts I expected from reading about collimation over the years. Instead of circles, I would get the letter C i.e. an incomplete circle. I've never tried a star test for collimation – I use the Catseye tools and in the past I've used a laser. Have I been way off on collimation all this time or is this open ring something easily explained like the shadow of the secondary or something? That night I also noticed that the defocused "doughnut" did not reveal any inner rings – why would that be? FYI -the seeing was average at best that night.
Any comments are much appreciated - thanks.
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"When I see your heavens, the work of your fingers, the moon and stars that you set in place...What are humans that you are mindful of them, mere mortals that you care for them?" (Psalm 8:3,4)
24Pan, 17T4, 13T6, 11T6, 9BTMB, 8TVpl, 7T6, 6, 5BTMBs
Z10"Dob, Paracorr, 2xBrlw, Telrad, RACI,°Circles, ⌒Spider
Floyd Bennett: 
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4stargazers
member
Reged: 07/13/08
Posts: 32
Loc: colorado
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Hey Jeff,
When your doing a star test be sure that you use a high enough power I'll say at least 150x, and when you do your pre and post focus views remember that you want to just barely be out of focus, if you go to far you'll see your secondary and spider in your eye piece and it will be centered, but you will have gone to far and will just be looking at the reflection from your primary.
Clear skies, Dan
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meade 4.5'' reflector
etx 60
orion XT8'' dobs
Sirius 40 mm plossl
Sirius 26 mm plossl
Sirius 10 mm plossl
Orion Ultrascopic 2x FMC
3-Element Barlow Lens
Telrad with dewshield plus
and the pulsing light
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YankeeJeff
sage
Reged: 08/11/06
Posts: 439
Loc: Brooklyn, NY
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Thanks - I'm glad there's a simple explanation. I'll try that next time. It wasn't something I was trying hard to do, just something I noticed on occasion as I changed eyepieces. I'll attempt in earnest next time around. Thanks again.
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"When I see your heavens, the work of your fingers, the moon and stars that you set in place...What are humans that you are mindful of them, mere mortals that you care for them?" (Psalm 8:3,4)
24Pan, 17T4, 13T6, 11T6, 9BTMB, 8TVpl, 7T6, 6, 5BTMBs
Z10"Dob, Paracorr, 2xBrlw, Telrad, RACI,°Circles, ⌒Spider
Floyd Bennett:
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smasraum
sage
Reged: 01/13/08
Posts: 492
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Yeah, in "Astronomy Hacks" (good book, BTW), they say that you should first pick a bright star and go with a mag about equal to your aperture in mm, and then go with a slightly dimmer star at double your aperture in mm. So, for my 8" scope, they recommend a bright star at 200x and then a slightly dimmer star at 400x for the defocus test.
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Steve
Houston (Friendswood), TX
 Space Center Houston
8" Zhumell Dob - Woo Hoo!!
Celestron C102 f/5 - Thanks Tim!
21mm, 13mm Hyperion
2.5x TV Powermate
Canon Rebel XT
Nikon 7x35
Bushnell 10x50
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