Optimal Exposure and Noise Calculator

[joseph.mcgee at sbcglobal.net]

Thanks Robert,

The process that Dr Glover presented for optimal exposure time considers 
both camera read-noise for a given gain setting, and the sky quality for 
light pollution, (I've also experimented with a simple adjustment for 
filters band-pass but that has a long way to go before it's accurate).  
So if we only consider the read-noise vs light pollution issues, it's 
sort of a balancing act. An exposure needs to be long enough to 
overwhelm the read noise, but not so long that light pollution starts 
overwhelming the target signal. So the optimal sub exposure time is 
basically an intersection of two curves.  Maybe that would be a good way 
to graphically present the calculation in KStars.

The points you raise for other factors are very good, interestingly the 
exposures I now tend to use in my fairly light polluted backyard are so 
short that guiding is generally not an issue.  But when I'm able get to 
the darker site, I can use longer exposures, and then guiding accuracy, 
(and aircraft traffic), both become a limiting factors for my 
exposures.  My guiding accuracy usually varies in a sine wave that spans 
a few minutes. I sometimes manually trigger my exposures to avoid 
running exposures at the extremes of the accuracy issues.

But I think for the short term I should work to revise my current code 
to improve it before I try to integrate it into KStars.  I just added 
data for a couple of other cameras.  But I also want to work on the 
process that Warren suggested for using bias frames to determine read noise.

Also, it would be helpful for me to have a reality check on this 
calculation with input from you more experienced folks.  If anyone wants 
to send me a few details, I can run calculations and send them back for 
review.  For this I would need your camera model (preferably a ZWO or 
QHY camera), the focal ratio of the scope, and the SQM of the site.

Thanks



On 3/5/23 10:18, Robert Lancaster wrote:
> Hey,  so maybe this is more to the point.  The person taking the 
> photos may not know how all of this stuff factors into the optimal 
> exposure time.
> There are quite a lot of factors that would affect the best length of 
> an exposure and often times a number of us just take a guess.
> For example, I often will try for 5 minute exposures as a default, but 
> sometimes I am at a different location or using a different camera and 
> that doesn’t work.
> Or maybe I have a guiding issue and have to just take a guess at what 
> might work.
> I think if this tool only looks at the read noise, then that is not as 
> useful, but if you could make it take into account a number of factors 
> and then just make a recommendation, that might be extremely valuable.
> KStars knows your geographic coordinates. It knows the camera model 
> that is connected.  It knows the filter you have selected.  It knows 
> the brightness of the target you have selected.
> If you are guiding, it might also have a sense for how much guiding 
> error there is.  It can get the temperature from the sensor and other 
> connected devices.
> Since all of these things will affect the optimal exposure time, I 
> think it would be really cool if KStars could just make a 
> recommendation for the time.
> I don’t think we would need a separate module for it, just a box right 
> next to the exposure time for the recommendation.
> You could have a button next to it where you could edit parameters 
> that affect the calculation of optimal exposure as well, but it could 
> just use some defaults to start.
>
> KStars could also use side panel of the FITSViewer to display 
> estimated noise levels in addition to the statistics info that is 
> currently displayed there.  I think this is a separate idea, but the 
> current noise levels could be used in the calculation above as well.
>
> Just some thoughts,
>
> Rob
>
>> On Mar 5, 2023, at 11:50 AM, Warren <warren.craddock at gmail.com> wrote:
>>
>> Hey Joseph, you may inadvertently be getting some light into your 
>> bias frames. Make sure you cap the camera like you’d do for dark frames.
>>
>> Here’s a more explicit, complete process for measuring read noise 
>> from bias images. I don’t have access to an astro camera at the 
>> moment (I’m snowed in at Lake Tahoe, boo hoo) to verify this process, 
>> but I can try it myself in a couple days.
>>
>> http://astro.physics.uiowa.edu/~kaaret/2013f_29c137/Lab03_noise.html#:~:text=The%20read%20noise%20of%20the,removing%20hot%20and%20dead%20pixels).
>>
>> In regards to subexposure length, I’m not personally against the 
>> calculator, but maybe it should have a disclaimer. I think it’s true 
>> that almost everyone using recent CMOS cameras should just use, say, 
>> two minutes by default. This consistency really simplifies workflow.
>>
>> If you have trouble with tracking, periodic error, tracking, fast 
>> high clouds, wind gusts, polar alignment, etc. then you can switch to 
>> 30- or 60-second subs with almost no effect other than using more 
>> disk space and more CPU time.
>>
>> There may be people using older CCD cameras with KStars / Ekos though!
>>
>> - W
>>
>> On Sat, Mar 4, 2023 at 10:38 PM Wolfgang Reissenberger 
>> <sterne-jaeger at openfuture.de> wrote:
>>
>>     Joseph,
>>     I’m not sure what type of function we are talking here. Is your
>>     intention to calculate the optimal exposure time for a single
>>     frame or for the target? If its the first one, I have the same
>>     questions as Hy. For the latter, I’m happy to learn more about it.
>>
>>     Wolfgang
>>     —
>>     Wolfgang Reissenberger
>>
>>     www.sterne-jaeger.de <http://www.sterne-jaeger.de/>
>>     TSA-120 + FSQ-85 + epsilon-160 | Avalon Linear + M-zero | ASI
>>     1600mm pro + 6200mm pro
>>
>>>     Am 05.03.2023 um 06:06 schrieb joseph.mcgee at sbcglobal.net:
>>>
>>>     Hi All,
>>>
>>>     Let me explain my reasoning for developing the optimal exposure
>>>     calculator and noise calculator.  I started fairly recently in
>>>     this A.P. hobby (mid 2019), and had no mentor. Most of the
>>>     online resources seemed to provide information and suggestions
>>>     that were geared toward imaging in very dark skies with more
>>>     advanced equipment than a beginner would own. The typical
>>>     recommended exposure times I read about were on the order of a
>>>     many minutes.
>>>
>>>     But when I was experimenting with and learning to use my gear, I
>>>     typically did so in my own backyard (SQM 19.63).  I initially
>>>     spent quite a few frustrating nights trying to find exposure
>>>     settings that would produce a decent image.  As I acquired
>>>     filters, I had to repeat the learning process. Then when I had
>>>     the opportunity to travel to a darker site 90 miles from my
>>>     home, (SQM 21.65), I again had to repeat the learning process. 
>>>     The difference in the exposure times at these two sites was
>>>     pretty shocking to me.
>>>
>>>     I fully grasp that you all have considerable experience with
>>>     A.P. but this tool is really not intended to provide benefit to
>>>     folks that have such experience.  The target audience for this
>>>     tool is the newcomer to this hobby (like me three years ago).  I
>>>     would have been thrilled to have tool that says when I'm in my
>>>     backyard shooting with gain at 100, and no filter, that my
>>>     exposure time should only be around 45 seconds.
>>>
>>>     Now, back to the topic...
>>>
>>>     Warren,
>>>
>>>     You raised a suggestion that bias frames could be used to
>>>     determine sensor read noise.  I must be missing some knowledge
>>>     in this area. I just ran a test with my planetary camera
>>>     (ASI-178), where I captured a set of bias frames incrementing
>>>     the gain from 0 to 400 in steps of 50, with an exposure time
>>>     32us, (I believe that is the lower limit for the ASI-178).  I
>>>     then used a tool that can assess noise in the image.  The noise
>>>     measured in each image increased as the gain increased; so this
>>>     did not match the downward trend I expected from the ZWO
>>>     read-noise graph.
>>>
>>>     Perhaps the tool I used for noise assessment was not the best
>>>     choice.
>>>
>>>     Can you explain further how I might be able to analyze bias
>>>     frames to determine read noise?
>>>
>>>     Thanks
>>>
>>>
>>>
>>
>
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