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Old 27-03-2013, 11:52 PM
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Shiraz (Ray)
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Shiraz is offline
Join Date: Apr 2010
Location: ardrossan south australia
Posts: 4,791
Originally Posted by SpaceNoob View Post
Suggested exposure lengths have been 20.7 minutes for Ha 5nm with the FSQ at f/5. While this has been fine for most of my targets, there would likely be issues with brighter targets, given limited well depth. Calculators can be used as a suggested guide for sub length; while respecting the rate of saturation for brighter objects. Like everything in this hobby, its about finding a balance.... only to find that there is some other variable you need to account for. You're dealing with different issues for narrowband.
thanks for the info Chris. yes, it always seems that there is another tradeoff... In fact, the whole reason for doing the modelling was to minimise the cost - I wanted to try many different tradeoffs before I decided what to buy.

Originally Posted by naskies View Post
Very interesting, thanks Ray. As an extension of this, how about predicting the overall integration time needed (with optimal exposure duration) to reach some arbitrary SnR for an object of a given magnitude, under a specific skyglow magnitude, and with a certain sensor/f-ratio scope?
model already does that Dave - bit awkward the way it is structured and needs a rewrite though. Results are attached - with bright sky, they largely reflect the differing Ha QEs of the various chips, with the 11002 having roughly half the QE of the others and requiring a bit more than twice the imaging time. Dark current also strongly influenced the 11002 and 3200 results, particularly at low sky brightness. The systems all have ~the same aperture and pixel scale and "5%noise" optimum subs were used regardless of how practical they were - systems were:
KAI11002M 254mm f8
KAF8300 254mm f4.75
KAF3200ME 254mm f6
icx694 254mm f4.

I could not find data on the Ha sky brightness in polluted skies and have used a dark sky measure (10nm BW) of 5 p/s/m2/arcsec2, with guesses as to how much extra in-band flux there might be in polluted sky. Also included 2.5p/s/m2/arcsec2 flux to represent a 5nm filter/dark sky. Looks like it will pay to get a 5nm Ha filter for my new 694 - the low dark current of this CCD (and to a lesser extent the 8300), gives a distinct advantage with very narrow filters.

also, does anyone have any feel for how much SNR is "enough" - it would be nice to be able to say what image quality SNR=30 represents

regards Ray
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Last edited by Shiraz; 30-03-2013 at 11:51 PM. Reason: update figure
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