Optimal Exposure and Noise Calculator

Mon Mar 6 15:23:01 GMT 2023

Add to that this extension by Dr. Glover, which is talking about gain,
which was skipped in the youtube video below:
https://www.youtube.com/watch?v=ub1HjvlCJ5Y

Am So., 5. März 2023 um 23:50 Uhr schrieb Warren <warren.craddock at gmail.com
>:

> 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
>>>
>>>
>>>
>>>
>>>
>>
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