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  #21  
Old 17-02-2013, 01:58 PM
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Shiraz (Ray)
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Quote:
Originally Posted by clive milne View Post
Ray,
The RCC1 was designed by Dave Rowe (not Jim Rowe)
He did indeed design the optical configuration for a number of CDK's including
those made by Planewave, Hubble optics, The Alt-Az initiative
and also one or two custom jobs, all them ex-gratis to my understanding.

As for the RCC1, it does not introduce spherical aberration to the wavefront
when used in conjunction with a paraboloid. It has something in the order of
100mm back focal length so it has significant advantages with respect to the
equipment you can fit in the imaging train.

As for the bad write up on IIS... all I can say is that the RCC1 design is
quite a bit better than the MPCC1, so if results were obtained that didn't
reflect this, then the problem is most likely the result of incorrect mechanical
assembly either due to spacing of the focal plane, or customs have
inspected it by pulling it apart and putting it back together ass-about.

Also... it is not necessary to preserve diffraction limited performance for
an imaging telescope unless you are talking about long focal ratios designed to
operate at the diffraction limit, such as the HST F/D=24.
It is only at that level of over-sampling at the focal plane that you will
notice 1/2 wave of SA or the effects of a 50% central obstruction.

There is a reason why planetary imagers use powermates.

At F5 or so, you can get away with a level of aberration that you simply wouldn't tolerate for a visual instrument.

best
~c
Thanks for the correction Clive - really annoying to get someone's name wrong.

Thanks also for the info on the RCC1 and the aberration concepts. I do not fully understand this area yet and will need to think about it a bit more. It would be nice to stop worrying about aberrations.

regards ray

Last edited by Shiraz; 17-02-2013 at 02:09 PM.
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  #22  
Old 17-02-2013, 02:29 PM
clive milne
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Quote:
Originally Posted by Shiraz View Post
Thought about AO, but it was a pretty short thought with an f4 system The RCC1 would make it a bit easier, but still just beyond the realm of the doable.
Ray,
As you shorten the focal ratio, AO actually becomes more doable.
It becomes less necessary as you shorten the focal length.


~c
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  #23  
Old 17-02-2013, 05:17 PM
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Shiraz (Ray)
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Quote:
Originally Posted by clive milne View Post
Ray,
As you shorten the focal ratio, AO actually becomes more doable.
It becomes less necessary as you shorten the focal length.


~c
although a short scope makes it easier to move the correcting element, there were two implementation problems at f4.

First off, at f4 you need a coma corrector - the Paracorrr and MPCC only provide about 55 mm of back focus, so the only coma corrector that could be used with the AO I looked at is the RCC1. As an example, the Orion AO requires about 98mm, which just fits in the 100mm or so back focus provided by the RCC1 - provided there are no adapters etc needed. Then when you look at where the light goes in, the f4 light bundle 98 mm out from the focal plane is large enough to be severely vignetted by the input aperture and the pickoff prism. Opto-mechanically it does not work well at f4, but would be OK at about f8 or more.

the other issue is that, at short fl, the pickoff prism is a fair angle away from the centre of the image - the typical isoplanatic region is not large enough to allow fast corrections, since some of the guide solution will be derived in turbulence that is not correlated with that of the image - the AO adds in new errors and even though it may still produce round stars they can be enlarged. Even at slow update rate, the AO can potentially put in slow uncorrelated turbulence corrections that add to the tracking error. This problem is also pretty well fixed if the scope is slow and the pixels big.

After thinking about it, I decided that it all seemed far too hard at f4, at least with the gear I looked at.

As an aside, the issue of deriving guidance signals from outside of the isoplanatic patch will apply to an OAG as well - not sure yet what the best guiding solution will be and may need to try both OAG and a separate guidescope aligned with the image to see which does best.

regards ray

Last edited by Shiraz; 17-02-2013 at 05:42 PM.
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  #24  
Old 17-02-2013, 07:43 PM
clive milne
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Hmmm... I wouldn't have thought that vignetting would have been serious at F4.
A KAF 3200 chip for example has a diagonal of 18mm.
The refractive element of the Orion AO has a clear aperture of 40mm.
An aperture stop of this diameter placed 80mm from the focal plane will
deliver a fully illuminated field of 20mm. At 90mm this will be reduced to
17.5mm. The shadow of the OAG pick off prism might be more noticeable
but I would be surprised if it was a deal breaker after flat fielding.

caveat... I could be wrong.
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  #25  
Old 18-02-2013, 08:54 AM
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Quote:
Originally Posted by clive milne View Post

caveat... I could be wrong.
No, you are right - the geometry is not that bad. Time to revisit the AO.

I really appreciate you taking the time to check this - I had written it off as an option some time ago, without looking at it closely enough it seems.

Do you have any feel for the problems introduced by uncorrelated turbulence along the guide and imaging paths? Much of what I found deals with big scopes with full adaptive optics - not much info to date on how big the low frequency correlated region is for a smallish scope in sea level turbulence. I guess I could go back to the theory, but I don't really want to - much better to find a useful rule of thumb. Maybe I should just image the moon for a while and see how big the regions with coherent motion are.

regards ray

Last edited by Shiraz; 18-02-2013 at 09:05 AM.
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  #26  
Old 18-02-2013, 06:47 PM
clive milne
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Ahh, no.. I don't really have a feel for it.
Orion does go in to it a little bit in the instruction manual for their AO,
but it reduces to telling you that the isoplanatic patch is variable so there is
no general rule of thumb other than it will still offer better guiding for shorter
focal lengths but the longer your focal length, the greater the advantage.

Not very helpful I know.
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  #27  
Old 18-02-2013, 07:16 PM
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btw) Just another idea to throw in to the mix:
I got the impression that you were considering the purchase of a new mount (eq8?)
An alternative to that would be a second hand AP1200. There seems to be
a constant trickle of these coming up for sale as their owners upgrade to the
new 3600. You can expect to pay $6500 on up (in the U.S.)
If you made a 16" to 20" newtonian OTA and paid attention to the opto
-mechanical support and thermal management issues, you could end up with
a world class galaxy imaging rig on a relatively modest budget. If your
primary was a 20" F3.8, it would only be marginally bigger than an RC
of similar aperture. You could use the RCC1 at its native focal length.
You could also use the Keller 1.8x corrector to image at F6.8 and have
enough BFD to use AO, off axis guiding + filter wheel for either option.

regards,
~c
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  #28  
Old 19-02-2013, 08:36 AM
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Quote:
Originally Posted by clive milne View Post
btw) Just another idea to throw in to the mix:
I got the impression that you were considering the purchase of a new mount (eq8?)
An alternative to that would be a second hand AP1200. There seems to be
a constant trickle of these coming up for sale as their owners upgrade to the
new 3600. You can expect to pay $6500 on up (in the U.S.)
If you made a 16" to 20" newtonian OTA and paid attention to the opto
-mechanical support and thermal management issues, you could end up with
a world class galaxy imaging rig on a relatively modest budget. If your
primary was a 20" F3.8, it would only be marginally bigger than an RC
of similar aperture. You could use the RCC1 at its native focal length.
You could also use the Keller 1.8x corrector to image at F6.8 and have
enough BFD to use AO, off axis guiding + filter wheel for either option.

regards,
~c
Thanks Clive.

I was thinking along similar lines, but as the replacement for what I am working on now, once I had established that I could find somewhere with good enough seeing - seeing sure is the elephant in the room when living in a sea level environment.

My vague interest for the next system is the Hubble Optics hyperbolic mirror/corrector sets, which could offer a neat solution optically and thermally. Hadn't thought about a used AP1200, but it would be a good way to go. Camera would need large pixels, so it could be the 694 if it turns out to have efficient binning. Otherwise a 3200.

regards Ray
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  #29  
Old 20-02-2013, 11:47 AM
clive milne
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Hey mate,

Yep, the Hubble optics sets do seem to represent excellent value for money.
I have heard mixed reports about them though. A few have been tested
by Wolfgang Rohr and show varying degrees of Surface roughness
astigmatism and under-correction. The testing done by Richard Berry on
the optic donated to the Cal Poly 18 project is somewhat inconclusive. The last
update I am aware of is that the secondary mirror (not supplied by HO)
was astigmatic and the overall system was highly sensitive to temperature
change so no definitive assessment of the primary could be made at this stage.
The positive is that the problem mirrors were made some time ago so perhaps
the current crop are an improvement.


More on the Cal Poly 18:
http://www.wvi.com/~rberry/calpoly18/calpoly18.htm

If you explore that link you will see that the project shows a great deal of promise.
If you can find it within yourself to look past the image defects resulting from
the secondary (and the fact that the scope itself is one seriously ugly *******)
the core of it is a sub-1m, professional observatory class research instrument.

The results obtained with the prototype so far show that the system can
track to an accuracy of 0.5 arc seconds for unguided exposures of 15 minutes
or longer, but unlike normal drive systems direct drives compensate for
loading due to wind, cables, imbalance etc. It helps that the mechanical
structure of the telescope has a resonant frequency above 10Hz.
RMS pointing accuracy at this stage is less than 10 arc seconds across the
sky just using a basic 30 star model, so this could no doubt be improved.

Dave Rowe designed the direct drives for this telescope so that they could be
replicated by amateurs albeit with access to a decent machine shop. Dan Grey
has developed a version of the Sci-Tech controller specifically for the
purpose though it hasn't been released to the public yet. I suggested to him
a little while back that the motors from Fisher & Paykel washing machines
might be a cheap option for kerb scroungers. Curiously, he was not aware
of them. F&P don't have much of a retail presence in the US but I believe
that Maytag has just acquired the motor technology so this will change.
Anyway, the upshot is that Dan has since had a Fisher & Paykel motor on
the test bench and says that it is indeed suitable for a small telescope. There
is a bit of variation between models in the gauge of wire in the windings in
these motors, some of which which require higher voltage. The last time I
talked with him he was working on a more robust output stage for the driver,
but in essence it's all pretty much just around the corner.

regards
~c

Last edited by clive milne; 20-02-2013 at 03:17 PM.
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  #30  
Old 20-02-2013, 12:19 PM
clive milne
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Some more cool stuff...

Richard Berry and the Cal Poly 18:
http://www.youtube.com/watch?v=gBbW4nzb4T4

Yes... that scope can slew at 30 degrees a second!

Dan Gray demonstrates Maxim DL driving the Cal Poly focus control/field derotator using the Si Tech controller:
http://www.youtube.com/watch?v=djMh5uAHtmU

International Space station image taken using SiTech control for acquisition & tracking:
http://siderealtechnology.com/NMSkies24/ISS-051.jpg

Dan Gray performing the initial testing of Fisher and Paykel motor with the SiTech telescope controller:
http://www.youtube.com/watch?v=9-bVYbQwEUg
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  #31  
Old 20-02-2013, 02:24 PM
clive milne
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Some test data on Hubble optics:

http://www.cloudynights.com/ubbthrea...Number/5011708
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  #32  
Old 21-02-2013, 11:39 AM
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Shiraz (Ray)
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thanks very much for all that info Clive.

the CalPoly 18 altaz scope is really impressive, particularly the drive system accuracy and resolution.

I guess that the optical quality of the big HO mirrors is likely to be quite suitable for imaging - the amosphere will totally dominate the resolution at such large apertures. The surface roughness may be an issue for image cosmetics (stars with halos for example), but it might be possible to live with that as a tradeoff for getting lots of photons.

Lots to think about - thanks again. Regards Ray
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