Re: ATM 6" f/4 or not?

[Robert Duvall <rduvall@pixworks.com>]

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!