[ATM] 32" dia mirror - what Focal ratio?

[polaraligned <polaraligned@optonline.net>]

Thanks Dave,  I appreciate your input and experience.
You are the only one on the list to give me an actual answer
that wasn't just negative or simply  "well why not build this instead...."
I am strongly now considering doing the f/5.  I need a bit more
research into any possible disadvantage of f/5 vs f/6.
My initial reasoning for extending the focal legnth to an f/6 was:

1) With folded optics, the increase in tube legnth was not significant so:
2)  Chances of optician making better mirror increase.
3)  Less coma for the corrector to deal with.  (better images in the end)
4)  Easier collimation, especially with the folded optics.
5) Central obstruction becomes smaller.

I am not sure just how much easier the collimation would be, or
how much easier of a time a correcter would have with the coma.
That is why I have asked for some input, ie, advantages of longer
focal legnth.  I am worried on the other side that if the focal legnth is
too long, I will not be able to frame some of the larger objects, and
spend more time behind the computer doing mosaics than having fun.

I have a lot of years as an amateur machinist and have done a lot of
welding and fabricating.  This telescope is a dream of mine for the last
12 years.  I have rebuilt several Bridgeport CNC machines in the last
year with the intention of using them to help fabricate this scope, even
though I am quite competent with manual lathe and mills.
I am no stranger to large optics as I currently have a 25" f/5 Galaxy dob.

I appreciate your input, and as I head up to Stellafane this morning, I
will ponder the possibilitys.  I would like to hear more about your
corrector.

Thanks again,
Scott




----- Original Message ----- 
From: <Aplanatic@aol.com>
To: <atm@atmlist.net>
Sent: Monday, August 01, 2005 9:28 PM
Subject: Re: [ATM] 32" Focal ratio


>
> Hi Scott,
>
> I optimized a simple two-element coma corrector for a 32" f/5 parboloid 
> and
> obtained very acceptable results at the corner of a KAF 1001 CCD.  The RMS
> spot size was a tad under 10 microns.  This suggests that an f/5 mirror 
> (1.2
> arcsec/pixel) can be reasonably well corrected for coma with a very simple 
> coma
> corrector.  I'd guess that a Paracorr would do somewhat better  yet.
>
> Figuring and testing a 32" f/5 mirror is a big job and will require a 
> good
> optician to do the work.  Nonetheless, well within the  capabilities of a
> professional optician.  Mounting the beast is another  problem entirely.
>
> I see no reason not to pick your favorite focal ratio between f/4.5 and 
> f/8.
> The one thing I did not check was distortion, which may need to be
> corrected digitally during the process of making mosaics.
>
> Best wishes on this wonderful project,
>
> -- Dave
>
>
>
>
>
> In a message dated 8/1/2005 5:17:30 P.M. Pacific Daylight Time,
> polaraligned@optonline.net writes:
>
>> Hi  Scott,
>>
>> What is the largest camera you will be using with the  scope?  Are you
>> figuring the mirror or is someone else?   Will you be buying (or making) 
>> a
>> focal reducer?  How picky are  you about the images at the edge of the
>> field?  Do you plan on  doing mosaics?
>>
>
>
> Probably a KAF 1001 with a 24.6 x 24.6 mm  size.  24 um pixels.  It's
> pixel size is a good match for the  long focal legnth without binning.
> Plan on having someone else make  mirror.... Buying a coma corrector....
> Well, of course I would like the  best possible images to the edge of 
> field,
> and I can't rule out  mosaics.  Am I asking too much?  How will this scope
> compare to  an RC?  Oh, and the original question: Is a longer f ratio 
> going
> to  help me achieve better images, even with a corrector?
>
> Opinions  appreciated,
>
> Scott
>
>
>
>
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