2018-09-28, 01:33 AM
(2018-09-27, 11:36 PM)theskyhound Wrote: Hello,
Another person has been telling me about how they use the SQM readings to plan their imaging. Now this is a little controversial to me, because I am not convinced that larger SQM values universally mean better quality images, and the mechanism of how it would do this isn't obvious to me.
Right now, SkyTools doesn't have a means of adjusting for sky "quality," for lack of a better term. Call it transparency, or SQM, or whatever. You can adjust the seeing, and the weather conditions determine how much extinction there is and affect the sky brightness to some extent, but a clear dark night is assumed. It is sort of a baseline. Crappy nights will get you less of everything. The sky brightness is part of the location because it wasn't meant to be changed.
I am starting to suspect that the way some people are treating their SQM readings, there would be some benefit to adding some sort of sky quality adjustment. I'm not really sure how to do this, however, because I suspect that in practice the higher SQM readings not only mean that there is less sky signal, but that there is more signal from the object. So maybe we are really talking about a link to the extinction with SQM? Obviously I need to look into this more.
I had a "discussion" on CN recently on just this very topic of SQM and extinction [https://www.cloudynights.com/topic/63161...in-detail/] and SQM very definitely has an impact on extinction ... if I'm imaging in skies where the limiting magnitude is 10 due to sky brightness and I'm shooting an object that is magnitude 12, I can be taking lights for the rest of my life and I'm not going to get anything ... but ST4 already takes this into account, by increasing the exposure time needed to catch something when SQM goes down, and decreasing it when SQM goes up ...
The way that I see it and have read it described, signal from all sources arrives at different rates. Camera dark current is coming in at a measurable rate which is dependent upon temperature (which is why I'm surprised that you don't have a spot for the cooling temperature of the imaging camera, as that will make a big difference in calculated exposure times ...), object signal is coming in at a measurable rate (that's the object magnitude, and as you pointed out that will vary from spot to spot on the object) and skyfog signal is coming in at a measurable rate (that's the SQM/ sky brightness). Each of these things is contributing electrons that will saturate the camera, shift the histogram to the left and the exposure is done. The trick is, how do I limit the amount of "bad" signal coming in so that I give as much time as possible for the "good" signal to come in. One source of "bad" signal is the camera dark current, I can control that with TEC cooling and postprocess it with darks. Another source of "bad" signal is from the skyfog, I can control that by going to a place that has less skyfog (a dark site) or by using a filter (LPR or broadband) ... so what I'm left with is maximizing the time window for the "good" signal to accumulate before the sum of all three end my session. To me, that's "SNR", how much "good" signal I have of the "bad" signal ... and empirically that makes sense, if I take shots with my uncooled DSLR, even though the CCD has much the same spec's as my cooled CCD, it saturates much, much faster than the same exposure with my CCD ... much less time for the constant signal to accumulate, much lower "SNR". If I take a picture with my CCD here in 19.2 SQM Belmont, it saturates much faster than my 21.5 SQM Payson ... again, much less time for the constant signal to accumulate, much lower "SNR". By cooling my camera, by imaging in Payson, I am able to let the CCD spend more time gathering the "good" signal and my "SNR" increases ...
Seeing, on the other hand, does not impact how much signal I can accumulate but rather the quality of that signal. Imagine that I was in space, floating on the HST ... my SQM will be very high and my level of detail also very high. Now, imagine that I have a fast rotating black hole between the object and me, and it's acting as a gravitational lense, bending the light from my object back and forth every few seconds ... even though my SQM is very high, because the light from the object is "wobbling" my level of detail, my "seeing", has gone down. Back to Earth, suppose that I'm looking at an object like NGC 470, which is 2.9' by 1.6' in size ... and with my setup I've got a .33"/pixel resolving power, which means this object will fit into a very few pixels but enough that I can get *some* detail out of it. "Seeing" to me means how much the light is going to wobble around due to atmospherics, so suppose that my "Seeing" is .8" .. that means that a photon from the center of the nucleus of NGC 470 is going to go +/- .8" from that center, which for an object that is 2.9' by 1.6' is probably going to stay in that central region ... high quality! Suppose now that my seeing is bad, at 3.0" ... that means the photon from the center is going to go +/- 3.0" from the center which will likely put it into an arm region ... low quality! Notice that this has nothing to do with how much of the signal I can accumulate ... that photon is still going to get collected by my CCD. This only impacts WHERE on that CCD the photon is going to end up, in the center of the galaxy where it started or in the arms somewhere ...
So, "SQM" for me relates to how fast the CCD will fill up with the "bad" skyfog noise (just like cooling affects the "bad" dark current noise), "Seeing" for me relates to how much of the detail of the object is going to get "smeared". Some objects like galaxies are more sensitive to "smearing" than others ... I will notice the effects of "Bad" seeing on NGC 470, I wouldn't notice a 5.0 seeing on my exposure of the North America nebula. Some objects have very little signal (like NGC 470 at a 12.6 mag) so it's hard to get much signal from them before the CCD fills up from skyfog, other objects have a lot of signal (like M42 at a 4.0 mag) so I could probably image them from the center of Oakland ... so when I select objects to image I look to see how sensitive they are to skyfog ("SQM"), and how sensitive they are to detail ("Seeing"), put like with like and adjust my imaging sessions so when I've got crappy skies I image the binned and HyperStar objects, when I've got clear skies I image the F/10 and F/7 objects, when I've got crappy seeing I image the nebulae and when I've got clear skies I shoot for galaxies and planetary nebulae ...
Hope this makes some sense, I'm sure I've gotten most of these concepts and terms completely wrong but that's why I'm here to learn the right concepts and right terms!