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Old 14-09-2015, 09:33 AM
glend (Glen)
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Seeing relationship to Imaging

As usual the questions we ask only serve to expose our ignorance, but I won't make progress unless I ask, and it might help someone else as well. I have been trying to wade through some of the articles on Seeing, Resolution and Aperture relationships and limitations, with a view to trying to understand the impact these have on imaging. Most recently I looked through this one:

http://www.telescope-optics.net/seeing_and_aperture.htm

As an old guy with not much real grasp of the mathematics of all this, can someone point me to a 'Dummy's Guide' to the subject of 'Seeing', Resolution and what it practically means to imaging?

For example, I often hear (or read about) that above 8" of aperture 'Seeing' is the limiting factor, and there are some good diagrams that illustrate this, but only through increasing apreture do we achieve greater resolution. At least this seems to be true visually, but is it the same for imaging. I was doing some imaging last night with my RC08, and using the scope visually, with an EP, I could see that the 'Seeing' was not great and that there was atmospheric trubulence affecting my Airy Disc. Switching over to the DSLR in Liveview the same star looked ok at the un-magnified setting, at 5X it was starting to exhibit shape change and at 10x it was dancing and swimming around. And yet, when I ran some five minute subs they looked ok. Now is this because of the resolution limits of my little RC or is this irrelevant to imaging.

I contemplate moving up to a 10" RC (chasing better resolution) but is this realistic if the 'Seeing' is thus impacting what I can produce. Will increased focal length and aperture (resolution) just give me more exposure to bad seeing?

Last edited by glend; 14-09-2015 at 10:04 AM.
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Old 14-09-2015, 11:02 AM
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Shiraz (Ray)
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the optical resolution of a scope in the lab is determined by the aperture and optic quality (eg the Rayleigh limit). For small scopes, the scope size and quality generally also determines the angular resolution that you get in the field (the ability to resolve small detail).

However, for a scope above about 6 inches, the optical resolution will be around an arcsecond or less, but the atmosphere will blur the scene by much more than that (eg 2 arcseconds or more), so, for any reasonably good optics, the resolution in the field will be determined almost entirely by the atmosphere. The seeing theory even has a term Fried parameter r0, which basically tells you the telescope size where the transition from scope-limited to seeing-limited performance occurs. For Australian conditions, this is about 80mm or less - provided you have appropriate sampling, your MN should have almost as good a resolution as the 3.9m AAT most of the time and a 10inch scope will not be any better.

from Wiki: For telescopes with diameters smaller than r0, the resolution of long-exposure images is determined primarily by diffraction and the size of the Airy pattern and thus is inversely proportional to the telescope diameter. For telescopes with diameters larger than r0, the image resolution is determined primarily by the atmosphere and is independent of telescope diameter, remaining constant at the value given by a telescope of diameter equal to r0. r0 also corresponds to the length-scale over which the turbulence becomes significant (10–20 cm at visible wavelengths at good observatories)

The above applies to long period imaging (DSOs). However, where the imaging is fast enough to take advantage of short bursts of good seeing (visual or high speed planetary imaging, or adaptive optics), bigger apertures still have an advantage.

also note that seeing measures are in angular units, so focal length does not come into the equation.

Last edited by Shiraz; 16-09-2015 at 07:36 AM.
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Old 14-09-2015, 04:39 PM
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Merlin66 (Ken)
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For visual observing the Dawes limit (based on a minimum contrast difference between two close Airy disks) is appropriate.
The seeing conditions - the distortions and movements of the stellar image are somewhat overcome by the ability of the eye to "catch" those fleeting moments when the atmosphere is steady.
Imaging generally doesn't have that luxury and just records the photon where they fall on the CCD. The quality of the imaging system defines the point spread function (PSF) - this is if you like the "translation" of the perfect Airy disk to real life conditions. The PSF always degrades the image. The seeing disk (PSF) for imaging is usually quoted as the FWHM (full width at half the peak intensity) - all unsaturated star images will have the same FWHM. The average conditions as per previous threads on the subject can vary from 1.5 to 4 arc sec. This is a much greater image size than that recorded visually.
(This is one of the reasons that the solar imagers use very fast frame rate mono cameras - to try to freeze those "good" seeing moments. It's not unusual to only end up stacking 100-200 frames from an 2000 frame AVI - those being the best quality.)
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Old 14-09-2015, 08:08 PM
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astroboof (Steve McN)
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Having some time as a visual astronomer will really help you here. You can forget about all the math and software predictive stuff if you can develop a personal feel for the sky. This extends to day and night weather. Apart from solar system influences, it is all about our local weather. There is no magic cure, other than persistent observation and learning. Not that my images bare fruit of this currently, but they have previously.
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