After watching Robin's live presentation videos on Sharp Cap, specifically the Smart Histogram, I was motivated to buy it to aid my DSO astrophotography. My rig is a Celestron EdgeHD 8 / ASI Air / ASI 533 primary / ZWO OAG / ASI 174mm guide / ZWO Mini filter wheel and ZWO EAF.
I have also read the help files on the Smart Histogram, and I must admit I need to hear an interpretation of the my resulting graphs since all Im getting is the very basic suggestions of use. ( I did do the sensor analysis prior to SH.)
So far at my first light last week I imaged the Wizard Nebula with 10 - 10 minute tracked exposures (on a whim) that , with the help of PixInsight, gave me an image that feels like a good start. But as an ex-photographer (and pro video editor) I still tend to believe that for shooting galaxies / nebula I will get better detail and reduced signal to noise by shooting longer and more with exposures. I must add I live in a Bortle 8 area and like to use L Pro broadband and L Extreme narrowband filters.
What can I learn from the attached graphs to aid me in determining exposures and amount of images for capturing Bode's Galaxy, for example? Will I get more detail (and less SNR) if I image it with 180-1 minute exposures as opposed to 18 - 10 minute exposures?
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Last edited by lasvideo on Tue Nov 17, 2020 6:29 pm, edited 1 time in total.
I'll leave Robin to give an expert answer but I did notice one thing that might help? The sky brightness measurement looked high (~ 1.50 ev) and yet you also mentioned that you like to include narrow band filters in the optical train. I was wondering whether you possibly made the sky brightness measurement without the filter in place? Whenever I include my broad band UHC filter it cuts measured sky brightness about 4X - I think thet the narrowband filters cut it by 25X or so. So it might be that 1 minute should be taken as a suitable exposure only if no filter is in place?
Yes, I had the Optolong L Pro (broadband, not narrowband) in position when conducting this test.
In Robins document on the Smart Histogram, in one section he talks about sky brightness in a graph he is displaying...
"In the image above, you can see that with a 5e/pixel/s sky brightness (quite bad light pollution), the calculation is recommending a gain value of 398, an exposure of 9.4s and a black level of zero (because the sky brightness will be enough to pull the histogram clear of the left hand side).
My sky brightness in 1.59 e which is less than the level of 5e which he suggests is quite high light pollution. So Im not sure your observation that my sky brightness reading is high, is correct.
The point of the OP is to get an answer to the question posed in the final paragraph.
the smart histogram functionality essentially points you at the exposure length where you have pretty much run into the law of diminishing returns. If you are going to image for (as in your example above) three hours, and SharpCap suggests using 60s exposures, you have a number of choices for the balance between exposure length and number of exposures – for instance
The relative quality of the final image that you get when you stack all your individual exposures will be roughly as follows
1800 * 6s exposures - noticeably worse than 180 x 60s exposures
180 * 60s exposures - good
18 * 600s exposures - a tiny bit better than 180 x 60s exposures, but difficult to spot
Using the very short exposure time means paying the cost of read noise in every frame, which are large enough number of frames of short enough exposure will add to significant extra noise in the final image. Using the very long exposure time will theoretically give you an image that may be slightly better (perhaps the total noise in the image will be about 10% reduced from the 60s exposure example), but you have more issues with tracking, guiding, aeroplane trails, satellite trails and so on that are probably not worth the tiny increment in quality.
Also interested in the wider (what does it all mean) question raised in the OP and to take the opportunity to clarify something that I now realize that I don't understand.
It is correct isn't it that the shortest exposure (consistent with only a minimal loss of SNR) also offers the advantage over longer exposures of maximising dynamic range?
So what does Brain do that is actually different when we set it either for dynamic range or for unity gain? In both cases we presumably still want to minimise any loss of SNR (to within 10% or whatever is set) compared to longer exposures? Does the dynamic range setting do exactly the same as the unity gain except that the gain is allowed to float ? Can we expect the SNR outcome to be similar whether we have used the dynamic range or unity gain setting - or is there a slight SNR price to pay for choosing the dynamic range optimisation?
if you select to aim for unity gain, SharpCap will calculate the gain value corresponding to 1 e/ADU and try to select that gain value and an appropriate exposure value to go along with it. If the exposure value required for that gain lies outside the minimum/maximum range that you have allowed then SharpCap will come up with an alternative using a higher gain if that brings the exposure back in range (that's why it says that its aiming for unity gain, not that it will use it).
If you make the other choice and aim for maximum dynamic range, SharpCap will look for the combination of settings where the value of
full well depth (in electrons) * number of subs / final stack noise (in electrons)
is maximised. The first two terms in this multiply together to give you the number of electrons that you can measure without saturating during the imaging., While the denominator is the noise level in the final output. This expression therefore approximates the SNR for the final stack as it is the brightest thing that you can image without clipping divided by the dimmest thing you are likely to see. Again SharpCap will sacrifice optimising this value if it pushes the exposure time out of range, moving instead to a higher gain to allow the exposure to be within your minimum/maximum range.
If you look at the second graph tab in the brain window, you can examine the stack dynamic range prediction and how it changes with different choices of gain.
That is a super clear explanation of how Brain works and of the central equation that underpins the optimization for maximum dynamic range. It seems (to me) the most sensible parameter to go for most of the time. I am going to save your words and the equation "full well depth (in electrons) * number of subs / final stack noise (in electrons)" somewhere safe so I don't forget! (you no doubt get the same questions many times and could do with an archivist to pull the common ones together?)
I have a question on the number of exposures suggested by SharpCap. In the example below 60s or 60 exposures suggested. Is that for the dark part of the sky? I should not use that for the actual target, right? If I keep the exposure time 60s, how do I determine the number of exposure for the target that I am shooting?
I have ASI533 but WO GT81IV scope. Guided ASI120mini/50m scope.
the number of exposures isn't really an important value here - the key is the length of the exposure - just keep taking exposures until you run out of one of
* clear sky
* patience
* darkness
* awakeness
SharpCap calculates the number of exposures by taking the calculated exposure length and dividing it into the imaging time (which defaults to 1 hour). If you can take 2 hours of images on the target instead of 1 then you will always get a better image and see more detail...
