Hi,
I am using C6+Hyperstarm along with ZWO ASI676mc camera, in Bortel 8-9 urban area.
I did sensor analaisys (the result was simliar to the camera spec), and than a Smart Histogram, which reccomneded 7.7 sec. and gain of 88. I tried these values, but on stacking I got not enough stars for alignment. I went to 10 sec / g100 and it was so-so - 10sec g150 was fine. Than I remembered the HCG point is a gain of 180, so I used it along with 5 sec. exposure. The result looks fine.
But I am wondering - shouldn't smart histogram cosniders HCG & suffisient stars brightness for stacking?
Please advise what I should do in my case.
Smart Histogram results have issues
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Re: Smart Histogram results have issues
Hi,
currently the smart histogram only considers the noise levels of the final stacked image, not any other factors like
* Oversaturation of the background
* Lack of stars to align on
* Excessive data due to very short exposures
* Loss of frames due to satellite trails
etc, etc.
One of the main reasons for this is that most of the constraints on sub-exposure length that aren't being considered are ones that can be dealt with easily with a little trial-and-error or instinct - satellite trails in every image, go for shorter subs; not enough stars, try longer subs... The calculation that the smart histogram does for you is much harder to sort out in a trial-and-error or instinct way.
Also note that what the smart histogram recommends is a *minimum* exposure for the gain recommended - don't go below the value calculated, but if your setup will handle longer exposures, feel free to go beyond it.
I suspect that the gain of 88 was recommended because the option to 'aim for unity gain' was selected (unity gain is about 80-85 or so on these cameras). The calculations came out in such a way that the exposure for that gain was within the min/max bounds set in the Smart Histogram settings, so that was the preferred choice. If you change to 'aim for max dynamic range' then that will ignore the unity gain and possibly suggest using the HCG mode. Mind you, on this camera, the read noise is low anyway in LCG, so there is less pressure to switch to HCG.
cheers,
Robin
currently the smart histogram only considers the noise levels of the final stacked image, not any other factors like
* Oversaturation of the background
* Lack of stars to align on
* Excessive data due to very short exposures
* Loss of frames due to satellite trails
etc, etc.
One of the main reasons for this is that most of the constraints on sub-exposure length that aren't being considered are ones that can be dealt with easily with a little trial-and-error or instinct - satellite trails in every image, go for shorter subs; not enough stars, try longer subs... The calculation that the smart histogram does for you is much harder to sort out in a trial-and-error or instinct way.
Also note that what the smart histogram recommends is a *minimum* exposure for the gain recommended - don't go below the value calculated, but if your setup will handle longer exposures, feel free to go beyond it.
I suspect that the gain of 88 was recommended because the option to 'aim for unity gain' was selected (unity gain is about 80-85 or so on these cameras). The calculations came out in such a way that the exposure for that gain was within the min/max bounds set in the Smart Histogram settings, so that was the preferred choice. If you change to 'aim for max dynamic range' then that will ignore the unity gain and possibly suggest using the HCG mode. Mind you, on this camera, the read noise is low anyway in LCG, so there is less pressure to switch to HCG.
cheers,
Robin
Re: Smart Histogram results have issues
Thanks Robin.
Looking from te other side - How can I estimate when exposing more time per image or stacking beyond a certain amount, provides no extra real visual value?
Looking from te other side - How can I estimate when exposing more time per image or stacking beyond a certain amount, provides no extra real visual value?
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admin
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Re: Smart Histogram results have issues
Hi,
total exposure time *always* helps - if you go from one hour to four hours total exposure you end up with four times the signal but the noise only increases by a factor of two, so the signal-to-noise ratio doubles for collecting four times as much data.
Sub exposure lengths beyond the recommendations of the smart histogram will not noticeably improve the final stacked image - once you have reached the sub-exposure length recommended, you have ensured that any extra noise generated by the electronics of the camera has negligble effect, and the noise in the final stack is purely due to the statistical variation in the number of photons each pixel receives over the total stacking time.
One more thing to point out is that in most places, bulk of the 'signal' that the camera collects is light pollution, which means that most of the statistical noise you are trying to reduce is the noise on the light pollution. Filtering (on appropriate targets) or darker skies can help massively by reducing that light pollution level.
cheers,
Robin
total exposure time *always* helps - if you go from one hour to four hours total exposure you end up with four times the signal but the noise only increases by a factor of two, so the signal-to-noise ratio doubles for collecting four times as much data.
Sub exposure lengths beyond the recommendations of the smart histogram will not noticeably improve the final stacked image - once you have reached the sub-exposure length recommended, you have ensured that any extra noise generated by the electronics of the camera has negligble effect, and the noise in the final stack is purely due to the statistical variation in the number of photons each pixel receives over the total stacking time.
One more thing to point out is that in most places, bulk of the 'signal' that the camera collects is light pollution, which means that most of the statistical noise you are trying to reduce is the noise on the light pollution. Filtering (on appropriate targets) or darker skies can help massively by reducing that light pollution level.
cheers,
Robin