Quote:
Originally Posted by naskies
I agree with Ray's original point that total integration times do not directly correlate with the same SnR across different systems and locations. A 12" scope has over twice the aperture (area) than my 8"... so all else being equal, 20 hours for someone else may be similar to 45 hours for me.
However, if you look at the subset of people who actively aim to go really deeeep, produce images that elicits "wow" reactions, gets APODs, and so on, things start to look very similar: mag 21-22 dark skies, 100 - 130 mm refractors for nebulae, 12" reflectors with appropriately matched pixel sizes for galaxies, Astrodon/Astronomik/Baader/etc filters, 11000 / 16803 / 694 chips are all pretty common.
In this case, comparing 1 vs 10 vs 50 vs 100 hours does give a rough idea of what to expect.
|
Following up on Dave's idea, I did some modelling of a few typical hi res galaxy imaging systems with the attached results. The starting point was a Mag23 galaxy against a Mag21 sky (per arc sec2) and 0.8 atmosphere transparency. The output is time taken to reach 20dB SNR, the ISO standard for "adequate" image quality - better quality will require longer. The optimum sub is based on the Starizona 5% read noise criterion - it is the shortest sub where shot noise will still overwhelm read noise and you could use longer if you wish. Of course, this only applies to visible broadband and the optimum sub length for any system in narrowband is "as long as possible".
there is lots of variability even in this smallish group - one system needs 6.5 hours , another needs 1.5 hours to get the same SNR. Sub length varies from 3 minutes to 40 minutes - and this is for the same environment and task. Actual times could be a bit different due to initial assumptions, but nothing like enough to account for such a large spread in imaging time. Seems that, even within this subset of imagers, there is no such thing as a universal best imaging time or an optimum sub length - it strongly depends on the system design. The most significant factor is the pixel scale (imaging time will depend on 1/pixelscale^2), but QE is also very important and read noise is a major factor in determining the sub length. regards Ray