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Old 15-03-2013, 07:21 PM
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Shiraz (Ray)
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Location: ardrossan south australia
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DSO broadband exposure strategy - Narrowband added - total exposure time added

There are quite a few websites that recommend sub exposures based on optimising SNR. The general assumption seems to be that longer is always better, but that there is a practical optimum sub exposure, which is reached when the read noise is 5% of the total noise (eg the Starizona approach).
edit: here are some web references: http://starizona.com/acb/ccd/advtheoryexp.aspx http://www.starrywonders.com/snr.html http://www.hiddenloft.com/notes/SubExposures.pdf
This is perfectly reasonable if the sole objective is to minimise imaging time. However, if the objective is taking pretty pictures, then dynamic range also comes into it and long sub exposures will not be the best approach. If you wish to retain star image colour, you must minimise star saturation. This can be assisted by taking numerous short subs. For example stacking 10 x 9minute subs which each have 20,000 well depth gives 200,000 effective well depth – stacking 30 x 3 minute subs gives 600,000 effective well depth, so the signal is the same, but the stars have much more headroom. The downside is that 30 subs have more read noise than 10 subs, so dynamic range and SNR are reduced with shorter subs. SNR and good star colour are competing requirements.

The star colour issue can be quantified by modifying the dynamic range calculation to include sky noise, yielding:

Headroom = 2.51 * Log10((N * welldepth) / sqrt(N * readnoise^2 + N * skyelectrons))
where N is the number of subs and dark current is ignored. Skyelectrons is the number of sky background electrons per sub.

This parameter is essentially an indicator of how many mags above the background noise a star can be before it saturates (no account is taken of the star PSF, but the measure is fine for comparative analysis). To illustrate, the attached graphs show SNR (standard formula) and headroom plots for three different systems with ~the same aperture and pixel scale. The sky is 125 photons/s/m2/arcsec2 (fairly dark) and the extended target is 25 photons/s/m2/arcsec2 (about like NGC247). SNR and headroom are those for 3 hours of combined subs – the sub length is shown on the x axes, not the total time.

The table shows the standard recommended sub exposure (read noise 5%) and also the much shorter sub exposure for which read noise equals sky noise – this is probably a marginally useful lower bound at which total exposure time would need to be ~doubled to get the same final SNR as from "5%" subs.

With all of the systems, it is clearly possible to gain at least one mag in headroom by reducing sub exposure length and accepting a slightly lower SNR (which could be recovered by longer overall imaging time). One mag extra headroom would reduce the number of saturated stars to about 1/3 of that at the longer sub exposure. Thus, the standard "5%" approach should probably be taken as the maximum sub exposure length and imagers could consider significantly shorter subs if star colour is an issue.

The table is also very instructive WRT the wide variety of sub exposures required for the various technologies. For the same scenario, recommended KAI11002 system sub exposures are 930 seconds, whereas the icX694 only needs 110 seconds - clearly the choice of appropriate sub exposures is important to get the best out of any CCD technology.

I guess in summary then, long subs are not the way to go if you are looking for pretty broadband colour pictures, particularly when using low noise/low well depth CCDs. Haven’t looked at narrowband yet and would be very grateful for any feedback on this approach.

Thanks for taking the time to read. Regards Ray
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Last edited by Shiraz; 28-03-2013 at 07:28 AM. Reason: rewrite and condense
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