Brain Function and Narrow Band

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nexusjeep
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Brain Function and Narrow Band

Post by nexusjeep » Sun Jan 06, 2019 9:29 pm

I have been reading the manual regarding the brain function for exposure and gain and was wondering how or if it will work with narrowband filters, as I would have thought that it may have issues as the background calculation would surely only be showing noise from the ASI 1600MM Pro as theoretically there would be little to no noise in terms of background sky glow as this will not be getting through the Ha, Oii and Sii filters and at the default 10% background noise level would require a massive exposure length.

Am I misreading / misunderstanding the parameters if anyone has any experiences I would love to know about them, unfortunately the skies have been abysmal since the 13/12/18 when we narrow band imaged the soul nebula and don't look like improving anytime soon so have not had a chance to play, as for that image I just used 120s 200 gain 30 ofset for black as it appeared to give a reasonable amount of data in the individual subs. That image is in the gallery and we are happy with it just wondering if we could do better interms of subs x exposure in the same 80 minutes per filter.

Cheers
Nick

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oopfan
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Re: Brain Function and Narrow Band

Post by oopfan » Sun Jan 06, 2019 11:18 pm

Hi Nick,

Last January the Brain recommended a 46s exposure in 6nm Ha of M1 high in the sky (My kit consists of a William Optics 71mm f/5.9 and Altair 290M camera.) I stacked 30 minutes of frames but found that the signal-to-noise ratio was still too low to make a good print. With a lot of struggling I came up with this image.

I took a pause from AP to do some photometry. When I returned to it I did a little research and discovered an exposure calculator at Starizona. You could tell that it was geared towards CCD but they did add a few ZWO cameras. As a software developer I was interested in the code behind it. Thankfully their JavaScript code was not minimized so it was easy to follow. I did discover a bug in their implementation that caused CMOS cameras to report incorrectly. They made the assumption that our cameras are 16-bit which is not correct.

I implemented their code in a spreadsheet and used it the next opportunity that the sky cleared. Like the Brain you measure the sky brightness. For my location and skies I came up with an exposure of 200s. That was at the calculator's upper end of its noise scale. Less noisy images would have demanded longer exposures which is a problem for my mount since I do not actively guide. You can see my results here.

My experience shows that the longer exposure the better. I would recommend starting at 200s and working up from there if your guiding can handle it. You can probably get away with 100s but you need to increase your total integration time.

Brian

nexusjeep
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Re: Brain Function and Narrow Band

Post by nexusjeep » Mon Jan 07, 2019 6:40 pm

Hi Brian,

Thanks for the detailed reply I will have to have a bit more of a play when the skies eventually clear. I will give some longer exposures a try as we guide via an OAG so time should not be an issue although I have read that going over 6 minutes with the ASI 1600 MM Pro is not that practical as amplifier glow increases non linearly but will give 240s and 300s a shot to see what happens. I tended to go to the 120s length as this gave me 40 frames to integrate + or - the worst few frames on the Ha, Oiii and Sii in the course of one evening as with UK weather you can never guarantee consecutive clear nights.

We have a new scope which we have not had chance to use yet which has a larger aperture so that will also slightly increase the light gathering it is an APM LZOS Super ED APO 130/780 0.98 strehl and we have the Massimo Riccardi 0.75x flattener reducer for it as well as the 1.0x times flattener so have the potential of F6 or F4.5 depending on configuration so optimistically with longer exposures + aperture should gather some reasonable data.

Thanks again for the info

Cheers
Nick

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Re: Brain Function and Narrow Band

Post by oopfan » Mon Jan 07, 2019 8:45 pm

Hi Nick,

I don't follow everyone's posts so I don't know if you've got 'offset' under control. I know that ZWO, for a while, took away user control of offset but now I hear rumors that they re-enabled it. My understanding is that they were receiving a lot of questions/complaints about offset so they changed the firmware to program in the maximum offset. And then afterwards people complained about that, so they put it back in. That is my understanding at least.

Since you will be spending a lot of time acquiring images in SHO it is good to get offset right. There is a lengthy discussion about it here:
viewtopic.php?f=9&t=1209

You can save some time by not going through the process of determining the 'minimum offset' as I do. Instead select the half-maximum offset that SharpCap offers you. Maximum offset is only really necessary if you are using the highest gain setting however for SHO I'd recommend unity gain. Half-maximum offset is more than sufficient.

If I am not mistaken ZWO calls offset 'brightness'.

Brian
Last edited by oopfan on Mon Jan 07, 2019 8:56 pm, edited 1 time in total.

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Re: Brain Function and Narrow Band

Post by admin » Mon Jan 07, 2019 8:56 pm

Hi folks,

I believe that the brain suggestions really ought to work with narrowband filters as they will reduce the measured sky brightness significantly and hence give you a longer suggested exposure (you must of course use the filters while measuring the sky brightness to ensure you get the right results).

If you do get results that seem incorrect to you please feel free to post the measured sky brightness value and the model of camera in use along with the results – that data would allow me to look into the problem and see if I can work out if anything is going wrong.

Cheers, Robin

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Re: Brain Function and Narrow Band

Post by nexusjeep » Wed Jan 09, 2019 10:09 pm

Hi Robin,

Have had a bit of a play tonight with Sharpcap and the brain function with the LRGB and the Ha, Oiii and Sii filters with the L filter the brain calculated 15s gain 136 offset 10 and on the horsehead / flame this very rapidly produced an image that showed a good amount of detail.

Next we tried the narrow band Ha filter and again ran the sky calibration, this takes a bit of time with the Ha filter as it used exposures in excess of 3 minutes to measure the background sky value. Again once it completed it recommended 296s and 184 gain with an offset of 5. This again produced a very good image better than the L filter on the Horsehead / Flame but took longer as each sub was nearly 5 minutes.

These values were with my APM Super ED APO with the 0.75x reducer / flattener so at F4.5 with the ASI 1600MM Pro at -20c, the only thing I would say is that the normal talked about figures for the offset is 10 at 75 gain 20/22 at 139 (Unity) so not sure if the brain is estimating these slightly low as the histogram was very close to the left hand edge. I assume this value is calculated from the sensor analysis + measured sky brightness so will leave them as the brain calculates them for now. If we ever get any long clear periods I will have a bit more of a play.

Thanks for the excellent piece of software.

Cheers Nick

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Re: Brain Function and Narrow Band

Post by oopfan » Thu Jan 10, 2019 4:27 am

Nick,

That is very good news that the Brain recommended 296s. I reckon that Robin tweaked his code this past year!

For luminance it said 15s at unity gain? I haven't had much success with 15s exposures. I'm old school so I don't Live Stack. I imagine that if I did Live Stack it would give instant feedback of the remaining noise in the image. If it looked good to me after say one hour I would stop acquisition and move onto the next target. No Live Stacking for me so I generally put a limit on the total integration time depending on conditions and my patience. Dave and I have found that increasing exposure will get you better images over the same amount of time. Using luminance I'll take 50s-60s exposures at low gain. I've gone as long as 70s if there aren't any bright stars in the FOV that I can't crop out.

Brian

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Re: Brain Function and Narrow Band

Post by nexusjeep » Thu Jan 10, 2019 10:41 am

Hi Brian,
I use live stacking to see what we are getting but also save the individual frames and then process these through APP afterwards to generate the final integration that is then registered and combined to give me a colour image. That way I can subtract Bias / Darks / Flats / Dark Flats from the image and it processes with a lot more information visible than I see in the live stack. I don't use on the fly dark or flat subtraction as the save frames then contain this adjustment I prefer to do it inside APP. Unless I am just having a look at something and not doing a full imaging session.

Theoretically you are still capturing the same number of photons if I did 30 x 120s subs or 120 x 15s subs there is just more processing for APP due to the number of them. Also if planes / satellites etc go through I will also lose less integration time on the rejected subs.


Regards
Nick

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Re: Brain Function and Narrow Band

Post by admin » Thu Jan 10, 2019 8:20 pm

Hi Nick,

thanks for the report – it's good to hear that the brain calculation is working correctly for narrowband.

The black level is calculated by ensuring that the position of the histogram peak in a dark frame would be separated from the left-hand side by 3 standard deviations of the width of the histogram peak. This would mean that about 0.1% of pixels would be affected by the clipping to level 0 in a dark frame (and far less in the light frame with the sky brightness would move the histogram further to the right).

Cheers, Robin

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Re: Brain Function and Narrow Band

Post by oopfan » Thu Jan 10, 2019 9:15 pm

Hi Nick,

The quality of my images have increased dramatically since adopting long exposures over short.

Given a choice of exposure and one hour of total integration time (or any amount for that matter) you will always get better signal-to-noise with longer exposures. I put together a spreadsheet (screenshot is attached) to illustrate.

Brian
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Last edited by oopfan on Sat Jan 12, 2019 12:32 am, edited 1 time in total.

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