Re[2]: ATM 6" f/4 or not?

[Tom.Stock@gsbsc.gensig.com]

     >messes up viewing. Just compare Hubble to larger ground based 
     >telescope photos!
     
     Hey but this is changing now due the the "Adaptive Optics" that are 
     being developed successfully....(re-focusing or re-positioning the 
     mirror hundreds of times per second to correct burring caused by 
     atmospheric turbulance) There are larger scopes than the Hubble being 
     built with adaptive optics, which supposedly will be 17 times more 
     efficient...
     
     However, you are right, I will probably not notice the scratches...
     
     
     -tom


______________________________ Reply Separator _________________________________
Subject: Re: ATM 6" f/4 or not?
Author:  rduvall@pixworks.com at Internet2
Date:    9/17/96 10:06 AM


Tom.Stock@gsbsc.gensig.com wrote:
> 
> 
>         I recently purchased a partially completed 6" mirror from a
>      friend.. apparently this being his first mirror, he thought he was 
>      making an f/8 but actually made it f/4...it is almost completely
>      polished, but f/4 seems a little impractical for a 6" scope... 
> 
>         Does anyone have any advice as to what I should do with this 
>      mirror?
> 
>         The secondary would have to be a little over 2" for good 100" zone 
>      illumination, which then causes vignetting of the 75% zone at the
>      eyepiece...
> 
>         This seems like a big obstruction for a 6"...I was thinking it 
>      could make a good photographic scope however, except for the
>      vignetting at the eyepiece.... 
> 
>         I just hate regrinding this to f/6 or f/8 after seeing all of his 
>      polishing efforts go into it...
> 
>      Oh, and there are a couple of scratches as well... 
> 
> 
>      -tom
I just completed a 6" f4.8. That's just 5" longer fl than what you have. 
I use a low profile focuser (helical) with a 1.3" secondary which works 
just fine, even on lunar & planetary observing I don't notice any 
illumination falloff at edge of field. I really enjoyed looking at the 
Andromeda galaxy last weekend with it. Just fits in 25mm Plossl field of 
view. It used to be an f/10 mirror- I reground it. It's really a nice 
scope because it's so small and portable- I find I do much more 
observing now than I did when it was a f10 on a heavy equatorial. 
Biggest drawback is lack of power for planetary stuff- I need to use a 
barlow alot.
     
I can share observing with my kids and their friends much more, since I 
can set the dob on the ground for them to use (I put the entire DOB on a 
pedestal or table for adult viewing). It only took me about 35 hours to 
refigure the mirror, so even if you decide to make a longer fl it's not 
a huge amount of work on such a small mirror. The 12 1/2" mirror I'm 
making now is MUCH more work... 
     
The scratches probably won't hurt anything- I rushed mine at the end and 
have a few scratches - observing will not reveal them. They theortically 
scatter some light, reducing contrast. It's the atmosphere that really 
messes up viewing. Just compare Hubble to larger ground based telescope 
photos!