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Re: [ATM] 32" dia mirror - what Focal ratio?
Scott,
Here's one of my three favorite astrograph designs. It probably does
not suit your purpose since it uses a hyperbolic primary mirror, but
I thought I'd work up a 32" example and present it anyway. The
following URL contains a screen shot of the design details, optical
layout of the corrector, and geometric and physical spot diagrams.
_http://members.aol.com/aplanatic/HN_810_f41.jpg_
(http://members.aol.com/aplanatic/HN_810_f41.jpg)
Make sure you view the jpeg at full resolution to avoid aliasing.
The optical system consists of an 810 mm f/5 primary with a conic
constant of -1.2 (20% "overcorrected") and a two element
corrector / reducer combination. The final system f/ratio is ~4.1.
In the optical layout photons are moving from right to left. In the
spot diagrams the small grid squares are 6 microns on a side,
which is about 0.5 arcseconds. As you can see, the spot quality
is excellent across the full field of the KAF-1001 IC. At the far right
I've included the physically-correct spot diagrams at 587 nm. Not
shown is the Strehl ratio, which is >0.95 on-axis at 587 nm.
For large instruments I think this design has a lot of merit. It's simple
compared to its competition. As demonstrated, the f/ratio can be
made quite fast. The tolerances on the lenses are tight but doable,
and the compact two-element corrector is not difficult to mount and
center, IMO. The back focus is sufficient to allow an off-axis guider
in the corrected light path if care is taken in the mechanics. As with
any fast astrograph, collimation will still be a critically important chore
that must be worked out.
There is a variation of this design that uses a parabolic primary. It
does not have the spot quality that this design exhibits but it could
be just fine for your purposes. However, as I mentioned before, I think
that a parabolic f/5 primary and a Paracorr will work well with the
larger-pixel detectors. As such, I see no real need to proceed in this
direction. I present this as an interesting astrograph variant and not
necessarily the right design for you.
In case you're wondering, I wrote the ray-tracing program myself.
Should the reader question these results, the design can be
transported into a more familiar optical simulator. Yes, my sign
convention is opposite that which is commonly in use and I like it
that way. :-)
I first saw this astrograph type in an S&T article by Paul Lind.
-- Dave
In a message dated 8/3/2005 3:26:50 A.M. Pacific Daylight Time,
polaraligned@optonline.net writes:
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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