Re: [ATM] 8" Diagonal- More info

["Tom Krajci" <tom_krajci@tularosa.net>]

>From: Polaraligned <polaraligned@optonline.net>

>I thought about maybe Newport Glass, but
>I am receptive to anyone capable of making this flat with high
>quality.  Any suggestions would be welcome.

Just scan the ads for optics fabricators in Sky and Tel magazine.  Pegasus?
Intermountain Optics?  If nothing else they can refer you to someone that
specializes in larger flats.  Protostar makes diagonals...not as large as
you want, but they may provide you with some leads.

>For an 8" diagonal that only blocks an 8"
>circle, the area is about 50 sq in and linear obstruction is 28.6%.
>For an ellipse with no beveling, the area of obstruction is 62 sq in
>and linear obstruction is 31.3%.  Quite a big increase.

For a photographic instrument, I don't think that's a big increase.  I bet
you won't notice the difference in images.

>I have no idea how to space the support
>points out without elliptical PLOP.

Ouch, I didn't think of that.  I have to chew on this one a bit and see what
I can come up with.  off the cuff you could just 'stretch' an over-designed
PLOP design for a circular mirror into an ellipse...kinda like you can make
a linear stretch of an image in graphic software to make it taller.  I think
that this will preserve the balance qualities you want in a whiffle
tree...but again, I gotta think on this.

>As for the substrate, would there be any distinct advantage with
>using anything other than Pyrex?

Fused quartz is stiffer, and I think it conducts heat a little better than
Pyrex.  It's expensive, but you may be able to get a thinner/lighter/faster
cooling mirror that way.

>Holding 1/10 wave or better is the priority and I know that the
>edges are going to fade from this value.

For a prime focus imaging instrument with about 104 inch focal length, make
some estimates of what kind of atmospheric seeing you expect/hope to have at
your site, and then do some analysis of what optical tolerance you need in
the diagonal.  It may turn out that you only need a 1/4 wave diagonal...and
that sort of requirement may be easier to swallow for the optician, and for
your wallet, and also in terms of delivery time guarantees.

(If this were a visual instrument for high mag work...yes, a tighter
tolerance for the diagonal will help.  For photography...atmospheric seeing,
and maybe tracking errors, wind, local seeing/heat currents off the mirror,
etc....all lower your effective resolution.)

Tom Krajci
Cloudcroft, New Mexico
http://overton2.tamu.edu/aset/krajci/

PS.  My photometry/CCD/imaging rig is an old C-11 that star tests with about
1/3, maybe more of spherical aberration.  Yuck!  It does pretty good for
long exposure imaging, and I can often do photometry on stars with close
neighbors that many other amateurs can't.  It's surprising how bad optics
can be in imaging instruments, yet still give pretty good images.  The
atmosphere is often my biggest factor in keeping resolution low.

PPS.  You're getting an AO rig.  You may find that it is most useful in
correcting for wind/mount/tracking errors, rather than tip-tilt image motion
due to atmospheric seeing alone.  Tip-tilt image motion is more dominant for
smaller apertures, like 3 - 6 inches.  Above that you need AO correction for
defocus, astigmatism, trefoil, etc., etc.  But it will still improve your
images because it can correct for mount motions (from wind, drive errors,
etc.) faster than a 'typical' autoguider can.

PPPS.  Who makes this Research Series AO?  Where can we learn more about it?



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