Calculating exposure time / Calculator?

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bluesilver
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Joined: Mon Oct 05, 2020 12:51 am

Calculating exposure time / Calculator?

#1

Post by bluesilver »

Hi, first sorry for the confusing topic there.
What i am trying to do is work out in a ball park way, what sort of exposure times i should be looking at with my equipment.
I have found the SharpCap Background calculator, but i am a bit confused at what i am suppose to do with the numbers.
For reference:
My Camera is asi2600mc ( 3.75 micron )
By bortle rating is 3.0 according to the light pollution map, ( i am in a rural area )
My scope is Skywatcher Esprit 150 ( f7 )
I am not using any filters
I have no idea about the Quantum Efficiency or even what that means, the only thing i found that related to this was a search on my camera ( asi2600mc )
It came back with:
It features a QE up to 80%, a bit rate of 16 bits, an ultra-high 14 stops dynamic range and ultra-low 1.0e readout noise

If all this is correct, when i input this data, i get the result as: Sky Electron Rate 0.21 e/pixel/s

So how do i work out exposure times with all this data?
Any help or advise would be greatly appreciated.
bluesilver
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Joined: Mon Oct 05, 2020 12:51 am

Re: Calculating exposure time / Calculator?

#2

Post by bluesilver »

Just to add a bit more,
I am using SharpCap Pro, ( so the paid version )
I have just came across what is called if i am correct: sharpcap smart histogram
I have yet to find it in the SharpCap program, will go and have another look shortly, but will this also work with my camera ( asi2600mc ) in finding out the correct exposure time?
It reads fairly straight forward, yet to find it and try it yet though.

Do i need to run what is called the sensor analysis first before doing this?
Also been reading through the very detailed (Picking the correct exposure for Deep Sky) from way back in 2017
A tad complicated for me to work out with the maths though.
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Re: Calculating exposure time / Calculator?

#3

Post by admin »

Hi,

ok, short answer : yes, use smart histogram (see https://docs.sharpcap.co.uk/4.0/#Smart%20Histogram). You should not need to run the sensor analysis yourself as SharpCap includes pre-measured analysis data for most popular cameras.

The longer answer is...

Quantum efficiency is a percentage which measures what fraction of photons landing on the pixels of the camera are converted to electrons (and therefore detected). A QE of 80% means that you are detecting 80% of the photons hitting the camera sensor. Note that when manufacturers say 'up to 80%' it usually means that the camera manages 80% for a narrow band of photon wavelengths in the green region of the spectrum, and is going to be lower for other colours of light (maybe 60-80%). Anyway, the manufacturer numbers are 'good enough'...

Your 0.21 e/pix/s sounds reasonable. A rule of thumb calculation is that your exposure length should be 10 time read noise squared divided by light pollution rate.

For your camera you might reasonably pick 100 gain, which gives a read noise of 1.5 electrons (graphs on the ZWO product page). That would require an exposure of about 110s.

On the other hand, if you picked 0 gain, you would have a read noise of 3.5 electrons, which would lead to a calculated exposure time of about 580s (nearly 10 minutes!)

cheers,

Robin
bluesilver
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Re: Calculating exposure time / Calculator?

#4

Post by bluesilver »

Hi Robin,
Thanks heaps for that reply and going over a few things there, very much appreciated.
I am keen to try out the smart histogram, I have had the Pro license now for a few years and didn't even realize this smart histogram or the sensor analysis was in there, surprising what you find when you start looking for something in particular.

I have a dedicated dimmable light box that i have been using for my flats, basically a box that fits over the end of the dew shield of the Skywatcher Esprit 150,
Will it mess up any stored reading for my camera if i do run the sensor analysis?, just like the idea of running it to see what the values come back as.
Plus if i my thinking is correct, it will then be a custom setup for my particular camera and not a run of the mill reading so to speak?

Any advice on this would be greatly appreciated.
Thanks.
Peter.
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Re: Calculating exposure time / Calculator?

#5

Post by admin »

Hi Peter,

in general with modern cameras the differences from your 2600MC to someone else's are tiny - the errors in the measurement procedure are almost certainly larger. However there is no harm in doing your own, you can always find and delete the sensor characteristics file if you want which will make SharpCap go back to using the pre-installed one.

cheers,

Robin

PS. With older CCD cameras there *was* the possibility of different read noise values between different samples of the same camera model.
bluesilver
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Re: Calculating exposure time / Calculator?

#6

Post by bluesilver »

Hi Robin,
Thanks for getting back to me on this,
One thing i did forget to ask was, Is the camera suppose to be connected to the scope like you do for flats and at correct focal distance for imaging, or can the camera be put directly onto a flat panel?

Now the only reason i am asking is that, like may others, I found out about this sensor analysis on YouTube, on there there was someone demonstrating how to do it with the camera sitting directly on a flat panel.

My thoughts are, this would be the incorrect method and it should be done as if you are taking flats, ( camera connected to scope ) in my case that is a dedicated dimmable flat panel in a dedicated box that fits snugly over the dew shield of my scope.

I am guessing the same can be done for a modified DSLR camera also.
I have a modified Cannon 550D sitting around.
Might try it out with this also, just to see how it runs with them.
Very much appreciated.
Cheers,
Peter
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Re: Calculating exposure time / Calculator?

#7

Post by admin »

Hi Peter,

it doesn't really matter how you illuminate the sensor when doing the analysis - what you are measuring is the way that the electronics side of the camera responds, so just how the photons arrived at the sensor doesn't change anything. The only thing is that getting the right initial illumination level is helpful to getting the analysis to run quickly and accurately. You may find that the illumination level is easier to adjust off the telescope (or perhaps easier on the telescope...) - pick whichever way works. I tend to run my analysis with a small (C/CS thread) lens on the camera - with an adjustable aperture on the lens you can easily get the right light levels.

I you can get your Canon camera to run stably with the ASCOM.DSLR driver to let SharpCap use it then you should be able to get a sensor analysis of it. The read noise values for low ISO (800 or less) will probably be quite scary (10e or more). It's not surprising that ISO1600+ tends to work best for DSLR cameras!

cheers,

Robin
bluesilver
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Re: Calculating exposure time / Calculator?

#8

Post by bluesilver »

Hi Robin,
Thanks for all the advice and information with this.
I will go and put it all into practice,
Very much appreciated.
Cheers,
Peter.
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