Brain function Exposure /Gain vs Star size

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nexusjeep
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Brain function Exposure /Gain vs Star size

#1

Post by nexusjeep »

Hi,
As we have had a couple of good nights I have been playing with the brain function with my ASI1600MM Pro and LRGB filters, as already stated elsewhere I do not have calibration frames for everything the brain proposes so I adjust the gain to 75 as it usually recommends 64 and then adjust the exposure length down to the nearest calibration data that I have.

My first question is relating to the gain settings when I originally purchased the camera I did a lot of reading and there were several articles discussing the standard 0, 75, 139, 200, etc gain values and the reason for them was that it prevented quantization (think that is the term) errors with the cell read out as they were at the correct change over points for the cells ADU. With the brain function it choses gain values outside of these ranges does this potentially mean I will be seeing quantization noise in the images.

The second question relates to the actual sensitivity of the cell at different wavelengths, the other night I used the brain to calculate the values for the LRGB filters and the adjusted setting came out as all at 75 gain as the brain proposed 64. L exposure 15s m R 90s, G 90s and B at 30s these values are all within a few seconds of the times the brain calculated and changed slightly I am assuming as the nigh became later and the sky darkness changed.

However if I look at the frames the star sizes look to be very similar between L/G and B filters but massively different with the red as can be seen in the image below which show the same star at the same scale for each filter. Is this something other people are seeing.
Star Size.JPG
Star Size.JPG (22.96 KiB) Viewed 2073 times
What I am thinking of doing on the next clear night is to experiment and the plan will be to calculate an exposure with the filters using the brain but then reference them back to the multi FWHM value for the shot through probably the green filter and then adjust the exposure from the brain calculated length to give a similar FWHM reading between all of the shots to hopefully reduce halo around the stars as it become more pronounced the larger the star.

Has anyone else experimented with this, I also get the same effect with Ha when using narrowband filters.

Cheers
Nick
BlackWikkett
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Re: Brain function Exposure /Gain vs Star size

#2

Post by BlackWikkett »

Not really sure why there's a difference in apparent star size with the red filter but your comment on sky brightness changes throughout the night is a similar discussion that has come up time to time on NSN. Rule of thumb I've always followed is astronomical dark was about 2 hours after sunset. Have a look at https://www.timeanddate.com/astronomy/d ... light.html Not sure when you started imaging but if you imaged the red channel earlier in the evening this may have something to do with what you're observing.

When I get a clear night I usually try to take full advantage and image as long as possible. This usually means I'm starting within 30 to 40 min after sunset and image till sky glow from sunrise becomes unmanageable. Even on the clearest night I'll notice changes as frames are stacking. My assumption has always been seeing just changed or very high thin clouds not observable but enough to make an observable change to brightness and in my OSC cameras color.
nexusjeep
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Re: Brain function Exposure /Gain vs Star size

#3

Post by nexusjeep »

Hi
Yes red was the first filter up and we started the imaging run at about 22.50, then green was about 22:50 and then flip and luminance from 23:50 and finally blue at about 1:40 until 14:35. So it is possible that it could be related to astronomical dark times but would the exposure not have calculated shorter if it was not fully dark as there would have been more light pollution so I would have thought the brain would calculate a shorter exposure as the read noise would be swamped faster unless I am misunderstanding the way the calculation works.

Makes for a more interesting hobby though as who wants a hobby that is simple.

Cheers
Nick
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Re: Brain function Exposure /Gain vs Star size

#4

Post by BlackWikkett »

Did you use the brain's calculation for each filter very close to capturing your subs?

If this astronomy game were easy I think I would have quit a long time ago. Most things that are rewarding take some effort.
nexusjeep
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Re: Brain function Exposure /Gain vs Star size

#5

Post by nexusjeep »

Exactly before running a set I ran the brain then went into imaging the live stack + saving individual frames immediately after so they should be fairly representative of the sky brightness.

Cheers
Nick
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admin
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Re: Brain function Exposure /Gain vs Star size

#6

Post by admin »

Hi,

My current thinking is that worrying about quantisation errors unity gain is unnecessary. When we correctly exposed deep sky images, we try to ensure that we capture enough photon in the darkest area of the image to make sure that the shot noise (poisson noise) far exceeds the read noise generated by the sensor electronics. If you look at typical sensor analysis data, it's incredibly rare the electrons per ADU figure for a particular sensor at a particular gain to be much higher than the read noise at the same gain level. That means that if our shot noise is massively outweighing the read noise, it will similarly be outweighing any quantisation errors, making the effect of those quantisation errors in significant.

On the subject of star sizes – are you refocusing between colour channels? It's possible that your filters are not par focal, or that your optics have a little colour dispersion in them this is showing up in bigger stars at the red end of the spectrum. Also note that if you happen to be hitting the diffraction limit of your telescope optics, that would happen to give fatter stars for longer wavelength red light than for shorter wavelength blue light. You'd have to have pretty good seeing for that to be the case though!

Cheers, Robin
nexusjeep
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Re: Brain function Exposure /Gain vs Star size

#7

Post by nexusjeep »

Hi Robin,
I am refocusing between each filter even though they are allegedly par focal, the telescope is a refractor and will never be diffraction limited as it is a 130mm LZOS triplet with a strehl value of 0.98 so it will always being seeing limited never diffraction limited. I see similar effects with a HA filter so assume that is in the red band as well so not sure if the ASI1600 sensor is slightly red biased but this would not necessarily be picked up on the sensor analysis as the light source was multi spectrum for the analysis run.

At the end of the day it is not really an issue as it gives me something to investigate, I was just curious if anyone else was seeing a similar effect.

Cheers
Nick
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