Brain Function and Narrow Band

Somewhere to ask questions about the best way to use SharpCap
nexusjeep
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Re: Brain Function and Narrow Band

Post by nexusjeep » Thu Jan 10, 2019 10:22 pm

But surely read noise is insignificant compared to the background noise level coming from the sky glow as this will increase faster than the read / dark noise of the sensor which I believe is one of the features in the brain calculation and the purpose of measuring the sky before the calculation is done. As stated in the in depth article on the Clarkvision website about the sensor efficiency were the actual sensor read noise is the least of your worries as the image will become noise limited much more rapidly from sky glow and other pollution such as seeing / transmission.

The other advantage to shorter exposures is stars become less bloated and can help with bad visual conditions plus any minor tracking issues become less problematic, for my particular location that is 4.5 on the bortle scale if I did an L sub of over 1 minute with my setup at unity gain then the background sky glow in the image would be excessive and would mask some of the fainter details in the final stacked images from the shorter exposures. The same is not true for the narrowband image as over saturation of the becomes an issue long before sky limitations as you would need excessively long exposures in this sense even with moderate light pollution.

With modern CMOS low noise sensors the read noise is not the limiting factor as was the case with CCD that has a lot higher read noise from the sensor which meant that you had to do long exposures to ensure the sky and signal noise swamped the base sensor noise.

Regards
Nick

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oopfan
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Re: Brain Function and Narrow Band

Post by oopfan » Thu Jan 10, 2019 10:47 pm

Nick,

Experiment.

Brian

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turfpit
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Re: Brain Function and Narrow Band

Post by turfpit » Thu Jan 10, 2019 11:29 pm

Nick
As stated in the in depth article on the Clarkvision website about the sensor efficiency
Please can you provide a link to the specific article on Roger's site please? there are a lot of in-depth articles there.

Dave

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oopfan
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Re: Brain Function and Narrow Band

Post by oopfan » Thu Jan 10, 2019 11:47 pm

Nick,

Sky Glow is not noise it is signal. If you are trying to eke out every last bit of faint nebulosity you can do it as long as its signal strength is greater than the strength of the sky glow. Nebulosity that is fainter is beyond detection. Seems logical.

The problem is that you are at the mercy of noise (see attachment). The only hope you have is too reduce noise until you can separate sky glow from nebulosity. To achieve separation you need to stack and stack some more, OR you can stack the same but increase your exposure. This next part is counter-intuitive.

By increasing exposure you will in fact begin to capture sky glow photons but you will also be collecting more nebulosity photons. In my spreadsheet you saw how increasing exposure also increased the signal-noise-ratio. Since sky glow is signal and nebulosity is signal they will both benefit. When you raise the SNR enough your eye will be able to separate the two even if this means that your frame is "overexposed" and sky glow comes in at 1000 electrons or more.

Now I agree with you that the downside to longer exposures is (1) star saturation, (2) higher frame rejection due to passing clouds, airplanes, satellites, and (3) the demands it puts on your guiding. I say do it in moderation. Personally I would much rather double the exposure and keep the total integration time the same versus keeping the exposure the same and doubling the number of frames.

Brian
Attachments
SNR Steven Smith Phd.jpg
SNR Steven Smith Phd.jpg (164.08 KiB) Viewed 40 times

nexusjeep
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Re: Brain Function and Narrow Band

Post by nexusjeep » Fri Jan 11, 2019 8:56 am

Hi Brian,

The article is here
http://www.clarkvision.com/articles/ast ... .exposure/

I am not trying to be difficult or anything just trying to get to grips the differnt pro's and con's and also always happy to read new information, the only thing i find is that a lot of the information is biased around CCD and CMOS behaves differently. The other issue I tend to get with narrow band is that I suffer from the halo effect from the micro lenses on the cell in the ASI1600 due to Panasonic in there infinite wisdom deciding not to coat the cell cover with an AR coating. In narrowband I find that exposures longer that 300s really starts to show this up theoretically stacking will have the same effect over time but it appears to tame it a bit Alnitak is a prime canndidate for it when imaging the horsehead / flame.

Having a discussion on here gives me something else to do as the skies in the UK are abysmal at the moment were I am located, I am wondering if there is a forum for cloud photography at the moment. It's my own fault we brought a new telescope so will probably not see the sky properly until after the 5 year warranty expires.

The image below is 7 stacked Ha 7nm Baader filter images as calculated by the brain at 296 seconds straight out of the live stack auto stretched with no dark or flat calibration the viewing was not great that evening.

ImageStack_7frames_2074s by Nick Davis, on Flickr

Cheers
Nick

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turfpit
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Re: Brain Function and Narrow Band

Post by turfpit » Fri Jan 11, 2019 11:14 am

Nick

Thanks for the link to the Roger Clark article. To put the information in that article into context it needs to be pointed out that Roger is using a Canon EOS 7D Mark II DSLR ($1400) and a Canon EF 300mm f/2.8L IS II USM Lens ($6000). He runs off an Astrotrac or more recently a Fornax - portability being a major requirement for him. The f/2.8 lens is very fast, hence his exposures can be shorter. If you look around the gallery on his site many of the captures have less than 1 hour integration time and 30 - 60 seconds is typical of his exposure settings.

I think it is important that people reading this thread realise the above and do not try to translate techniques being used for $7500 worth of equipment (by a highly experience and skilled imager) with a more typical camera/telescope setup.

As an example see this link - Total exposure time, including the core: 31 minutes. No dark frame subtraction, no flat fields. Tracking with an astrotrac. HDR techniques were used so that the core was not overexposed.
http://www.clarkvision.com/galleries/ga ... 200vs.html

I like the Horsehead image of yours, it turned out well and I notice you are on 5m exposures. I would consider from my experiences and research that 5m is pretty much the starting point for narrowband. If you Google Sara Wager or Terry Hancock (both experienced, top class imagers) you will find that for narrowband they are using 15m+ exposures and long (hours) integration times. That should keep you busy during the cloudy nights. Like you, I am doing a lot of research/thinking and trying to make sense of it all.

If you have a look around my Astrobin at https://www.astrobin.com/users/turfpit/, you will see that I have used a variety of cameras including CMOS (183), DSLR (Canon 600D modded) and CCD (Atik 314E). I have been surprised at both the quality obtained with the DSLR and the CCD and with their ease of use - both very dated models being 8 and 10 years old respectively. The Atik 314E seemed to be prone to hitting 65535 on the histogram with quite low exposures but that was not borne out with the final results. I am finding that the better data acquisition as a result of longer exposures has significantly reduced my processing effort. Another eye-opener with the CCD has been with darks.

_M42-with-darks.jpg
_M42-with-darks.jpg (666.14 KiB) Viewed 30 times
_M42-no-darks.jpg
_M42-no-darks.jpg (664.17 KiB) Viewed 30 times

Like you, I am prepared to try things out and evaluate results.

I know what you mean about the weather, I am on the West Pennines and the last 6 weeks have been poor for imaging.

May your well always be full ;)

Dave

nexusjeep
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Re: Brain Function and Narrow Band

Post by nexusjeep » Fri Jan 11, 2019 11:59 am

Thanks for all the info Brian / Dave, I quite like reading his site although some of it gets a bit headache inducing. On the plus side I am competing with his kit in terms of price if not technical ability :D as the ASI1600mm Pro is a similar cost of camera and I have it attached to the end of a APM LZOS APO 130/780 0.98 strehl value with the M82 Massimo Riccardi reducer flattener that combined comes in at £6000 this is a bloody expensive hobby and theres always something else you can buy now debating changing the Baader narrow band to 3nm Chroma ones in the long term :shock:

Cheers
Nick

MrAstroBen
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Re: Brain Function and Narrow Band

Post by MrAstroBen » Fri Jan 11, 2019 12:04 pm

nexusjeep wrote:
Fri Jan 11, 2019 8:56 am
H
The image below is 7 stacked Ha 7nm Baader filter images as calculated by the brain at 296 seconds straight out of the live stack auto stretched with no dark or flat calibration the viewing was not great that evening.

ImageStack_7frames_2074s by Nick Davis, on Flickr

Cheers
Nick
Nick,
There is an urban myth about CCD. It suits the sellers of cmos cameras. Check out astrobins for Atil 314L , SX-694 etc. Plenty of stunning images.

People using these cameras are generally experienced. They use longer exposures not because they have to, its because if you can it improves the output.

I have used a number of cameras, 290M, 183C, 178mc, DSLR. Whatever the numbers say my CCD picks up data just as quickly. What it doesnt do is suffer from other artifacts that inflict the cmos cameras I have used.

Here is a similar image i took to yours. No calibration frames, just stacked in DSS and a mild stretch in gimp.
Level 7/8 light pollution on a night with high cloud.
240s x 8. Tracking was off. It is similar to your image.

https://flic.kr/p/PepvBH

So do not dismiss CCD. The top imagers use them for good reason.

Exposures as others said before, yes there is a sensible limit where stars will saturate, the mount may not perform well, clouds will spoil a higher % of subs. Taking these in mind its better to expose for as long as the histogram can take before clipping. Then get as many of these in as time permits.

Most of the stunning images on astrobins use longer exposures except for really bright objects like M42 or M45

Its a numbers game, the longer the exposure, the more wanted photons will be recorded. The rest are more randomised so start to cancel themselves out. SNR of the desired and constant object increases.

MrAstroBen
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Re: Brain Function and Narrow Band

Post by MrAstroBen » Fri Jan 11, 2019 12:17 pm

Also, most CCD cameras are 16bit. That gives a massive increase in the range. A very useful asset. Tiny differences in published read noise are irrelevant in comparison.

nexusjeep
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Re: Brain Function and Narrow Band

Post by nexusjeep » Fri Jan 11, 2019 12:26 pm

Don't get me wrong I am not knocking CCD but only have experience of CMOS cameras previously owned a 183C then moved to the ASI1600MM Pro and really like the ability to build a dark library as it means I can have a shorter evening. With everything integration time is going to be king so regardless of the exposure length 120 mins of integration time is always going to beat say 60 mins. Being in the UK if I am going to image something I try to get all of the filters through on the same night as you never can guarantee when you will next get out.

To this end I usually aim for a minimum of 60 mins per filter but need to do some research as I believe the total integration times may need adjusting between LRGB and Ha/Oiii/Sii as they have different transmission percentages. So my plan for the time being is to go with what the Brain function is predicting until I get more experienced and a better understanding as to what is going on this hobby after all is a never ending voyage of discovery + expense as there is always something new to be had / learned even if the learning phase turns out to be that I wont try that again :D .

Clear Skies

Nick

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