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Old 29-04-2012, 10:12 AM
clive milne
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clive milne is offline
 
Join Date: May 2011
Location: Freo WA
Posts: 1,443
Quote:
Originally Posted by Peter Ward View Post
Beg to differ with Clive's analysis.

With imaging it comes down to field correction, spot size and seeing. A well made Riccardi-Honders delivers 5 micron stars across around a 70mm flat field. Most other systems have spot sizes 2x larger on axis, and
degrade to *way more* off axis due to field curvature, astigmatism etc.

Other factors such as thermal and mechanical stability also come into play, and literally shift focus as an exposure is being taken. Exacting focusers and thermally stable materials usually come at a cost. My experience so far has been, you get what you pay for.

Peter,
I think are getting the wrong end of the stick here.
I used the Riccardi Honders as an example because it is without question as good as it gets for deep sky imaging, but let's be realistic about the factors that make it so. Even a perfectly constructed OTA (of any configuration) that has a 50% central obstruction is in reality no better than 1/2 wave once you include the effects of diffraction.

Ergo, optical quality really isn't the weakest link in most prime focus deep sky imaging chains. This is what the OP was asking. Field aberrations, mechanical construction, mount stability, seeing conditions, tracking error, operator skill, etc) are different issues and are really what separate the sheep from the goats.

fwiw) Roland does not suggest that a Riccardi-Honders delivers 5 micron images (which it cannot) What he says is that a 12" f3.8 produces an airy disk with a diameter of 5 microns, and that his OTA is diffraction limited over a 60mm field. It is left to the educated reader to fill in a couple of blanks: ie)
1) Only 50% of the encircled energy will actually fall within the airy disk, most of the rest being pushed into the first diffraction ring with a diameter of 10 - 20 microns depending on the wavelength... there is your limit (diffraction)
And 2) the point I was trying to establish.... That spec is entirely good enough for prime focus deep sky imaging at the highest level. The results speak for themselves.

best,
~c

Last edited by clive milne; 29-04-2012 at 10:41 AM.
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