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73491RE: Need Advice

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  • stan_ccd
    Oct 18 9:17 AM
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      The example image shows a few reasons why you may be discouraged.  No matter how much processing is applied there are 2 shortcomings that cannot be magically fixed:  resolution and S/N (depth).


      One reason for using larger aperture is increased resolution. But that image looks like it is no better than 3 arcsec fwhm (maybe 3.5"?), which is a common value for smaller refractors and wastes the larger image scale.  So if you want to get more out of that scope then it is time to do some serious analysis. Start to routinely (and obsessively) measure FWHM and figure out the primary reasons for poor resolution. 


      The first thing is focus; precision focus is not easy. And most OTA do not stay in focus so you must continually re-focus; don't let it go from more than 30-40 minutes without at least re-testing.  Get and use a Bahtnov focusing mask to speed up the process.


      Actually, before the "first thing" is the "zero thing" of collimation. There is no such thing as perfect collimation and if you think you have it then you have not developed a critical enough eye. 


      The next thing is OTA thermal currents. Evaluate the OTA conditions via out of focus star tests, which can identify chimneys and other tube effects.  Get or make an OTA cooler. The easiest SCT approach is a back-end fan insert that circulates filtered air into the OTA.  Run it for at least an hour prior to imaging.


      After that, evaluate your immediate site/dome area.  Any thermal mass (such as  concrete floor) can generate destructive "heart waves".  Identify and fix any sources of local disturbances (e.g. if you are saddled with concrete then sprinkle it with water or cover with insulation).


      Next learn to evaluate the atmospheric seeing.  Some nights are lousy and not worth bothering with and some nights are fantastic and must be exploited.  Seeing varies at every time scale from milliseconds to annual. Learn to identify good seeing.


      AO can attenuate some of the effects of local and atmospheric seeing.  But AO is most effective in good seeing.  So if you have really lousy seeing (>3") it doesn't help much but if you have pretty good seeing it can improve the resolution by 10-30% (e.g. a non-AO of 2.4" can become 2.0" or even 1.8").


      The other shortcoming with that image is sub-optimal S/N.  Improved resolution will actually improve limiting mag and discrimination, thus improving object S/N.  But it wouldn't hurt to spend more time on a target.  Most of the better amateur pics spend at least 2-3 nights on a given target.



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