Re: [ATM] More on aperature stops

["John Sherman" <atm@mail.johnspics.com>]

Hi Mike,

>but I'm not seeing the "boiling and roiling" 
>apparently typical of this in star tests. Just the 
>regular, slow moving, horizontal "shower glass" 

You haven't mentioned if an out of focus star is round. Now, when a star is way out of focus, do you see "worms" crawling slowly around the mirror? Is that what you're saying here? If so, your primary needs to be cooled down to ambient temperature. 

>which is why I'm building two scopes with a common 
>removable primary. One for deepsky viewing and one for 
>planetary off-axis viewing without need of a secondary 

Seems like a waste. You need a good secondary for deepsky. Once you have that, the planet views will be better at full aperture!!! (You have already been told this) So what are you going to do, settle for lousy deepsky views so you can save a few bucks on the flat mirror? I can assure you, most deepsky objects will benefit from a sharp instrument (PNs, globs, gals, nebs, etc). Why sacrifice your deepsky? The best thing to do is to have one scope which works at full aperture, and you will never need to stop it down. 

Please take Mike's advice and blacken the edges of your primary and secondary, both. Paint is probably best if you can apply it cleanly, but the magic marker is far better than nothing. I have noticed that he gives very good advice on a regular basis. 

>other people's scopes (mostly 6"-10" Mead Cassegrains

Please, a commercial sct is not likely to equal a newt, altho the comparison is helpful. 

>Some of us there have 20+ years of experience.

I also have 20+ years of experience. I remember once I went for a semi-truck ride with a driver with 20+ years behind the wheel. He scratched every single gear he shifted. Years of experience often means almost nothing. 

>I use a laser collimator to
>collimate before every viewing.

Have you read Nils Olof's collimation pages? Unless you have the laser beam hit the exact center of the optical axis (which is highly unlikely), the laser beam will actually miscollimate your primary. Since you have that edge on your Mars pic, it does look like your primary is out of collimation. When you center a star in the field, in focus, does it have a little tail?

When you stop down, you are changing the f/# of the scope to a much longer number, which simple eyepieces, miscollimation, etc, are much happier with. By now you've tried other people's high-grade commercial eyepieces, right? Have you tried collimating the primary with a cheshire?

I hope you get it fixed.

John








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