Optimal Exposure and Noise Calculator

[Warren]

If you haven't seen Robin Glover's presentation that Joseph is referencing
here, please check it out at:
https://www.youtube.com/watch?v=3RH93UvP358&t=2s

And I dare you to try to watch it at 1.5x speed. 😂

On Sun, Mar 5, 2023 at 2:45 PM joseph.mcgee at sbcglobal.net <
joseph.mcgee at sbcglobal.net> wrote:

> 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>
> <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
>> 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
>>
>>
>>
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.kde.org/pipermail/kstars-devel/attachments/20230305/e7e2ab61/attachment.htm>