Smart Histogram and The Brain

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oopfan
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Smart Histogram and The Brain

#1

Post by oopfan »

Robin,

I tried Sensor Analysis for the first time today on a new camera (Altair GP290M). I am happy to report that the results matched nearly exactly with the results published on Altair's website.

Next step is the Smart Histogram and The Brain (TM). I'll get around to asking my question in the next paragraph but first I'd like to describe how I've been doing AP until now. When I first started AP early this summer 2017 using a short refractor and the ASI120MC, I went for the bright target first: M57. With a little experimentation I discovered that a 30-second exposure at gain 50 produced a nice sub-frame. The next target was M27. There I discovered that a 30-second exposure hardly made an imprint. After studying up on Surface Brightness I calculated that I needed an exposure of 52 seconds. Sure enough, that worked -- the density of the nebulosity in the 52-second exposure of M27 was equivalent to the density of a 30-second exposure of M57. Progress is good. Next I turned my attention to a galaxy: NGC 7331. My calculations told me that I needed a 5 minute exposure. To make a long story short I decided to go with a 2 minute exposure due to guiding issues. From that day forward I decided that all of my DSO's should be captured with a 2 minute exposure. I built a nice library of dark frames for a wide range of temperatures. Now I have a new camera so I am looking for a better, more scientific approach.

The source of my knowledge of the Smart Histogram and Brain comes from this forum. I have no hands-on experience, just what I've read. What I think I am hearing is this: SharpCap is really just concerned with the Background Surface Brightness of the sky and the properties of the camera in order to calculate the optimal exposure and stack size. In my experience galaxies are very dim -- a two minute exposure shows a dim slightly fuzzy nucleus that could be easily mistaken for a star -- only through stacking does the fainter nebulosity shine through. To me it seems unlikely that SharpCap's Brain would be looking for the DSO or asking me to point to it. With regards to "ancient photons" and the need to take very long exposures in order to capture the few photons hitting the sensor per unit time, what I think you are saying is that I can tell the Brain to only consider exposure times in a range that I give it. In other words, if I really believe that the DSO needs a minimum exposure of 10 minutes, I can tell the Brain that and it will factor that into its calculation.

I hope I understand it correctly.

Thanks,
Brian (not "Brain" -- believe it or not several years ago I received a credit card with my name spelled "Brain"!)
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Re: Smart Histogram and The Brain

#2

Post by admin »

Hi Brian,

it's a good question and I'm currently writing a long series of posts in the tutorials section to try to explain how and why the 'brain' works the way it does. Dave (turfpit) is calling it my 'epic'. You can find it - currently unfinished - here viewtopic.php?f=35&t=456

To cut a long story short, the calculations that SharpCap is doing are not intended to worry about whether you can pick out a faint target in an individual sub-exposure, they are to work out (effectively) the answer to the question 'At what sub-exposure length does increasing the sub-exposure length further stop adding to the quality of the final stacked image'. So, for your configuration and sky conditions that might turn out to be 120s (for example), so in that case SharpCap is telling you that your final stacked image would be just as good from 30x120s images as it would be from 20x180s or 12x300s. Of course the individual sub images will be brighter if you take the longer subs, but the final stack will be of the same quality.

The max exposure setting is designed to be used to reflect the limitations of your mount - if you know that your mount can give you decent 5 minute subs but has problems when you try for 10 minutes then set the maximum for 5 minutes.

The min exposure setting is really there to allow you to lock out very short exposures if you decide you don't want to deal with stacking 7200 x 0.5s exposures after an hours imaging. To be honest your light pollution would have to be dreadful for SharpCap to seriously suggest 0.5s exposures though!

The overall goal of the smart histogram and brain is to point you to the exposure time that gives best results in the final stack *and no longer*. This will be shorter than a lot of people imagine for modern CMOS cameras - possibly short enough that things like guiding become superfluous.

If your goal is to see the target in each sub frame and you have calculated a minimum exposure for that, then you are really asking a different question from the one that the feature was designed to answer - you could use the min exposure time setting to lock the results to above your calculated minimum, but the basically you've then told the software your answer and it's not doing anything much to help you unless your answer turns out to be shorter than its answer.

hope this helps

Robin
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Re: Smart Histogram and The Brain

#3

Post by oopfan »

Thanks, Robin.

Dave is a wealth of knowledge and I appreciate all the help he has given. In fact he and I have had rather heated debates about the need for long exposures to capture "ancient photons". Somewhere along the way I came across another person who planted the seed in my head for the need for long exposures to capture very faint wisps of nebulosity. His argument went like this: the reason why something appears faint is due to there being only one photon reaching your sensor every minute or every hour on average, so if you are using an exposure of 15 seconds then the stacking software can't tell the difference between noise and something real. Somehow this resonated with me, and I am having a hard time giving it up -- sort of a bad meme. Perhaps what I need is a mental flossing and just realize that my 71mm aperture refractor will never see to the edge of the visible universe regardless of the length of exposure, darkness of sky, or how sensitive my sensor is -- the real determining factor is how large of an aperture I have.
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Re: Smart Histogram and The Brain

#4

Post by turfpit »

Brian

Hardly heated ... more like robust debates.

Because I have only been involved with imaging for around a year, I came into it as a clean sheet unburdened by knowledge of previous technology. As I understand it with older CCD technology, the expectation was longer exposures and higher gain. This then means:
  • guiding is needed
  • cooling is needed
A modern (and I mean 2017) CMOS camera is a game changer. This was taken with an Altair 183C, unguided with the camera fan turned off. 200 exposures x 10s with gain at 10% and 200 darks subtracted; stacked and processed in Nebulosity. The 4 Trapezium stars remained intact in the original 100Mb image. Nick @ Altair is saying use 6% for the gain with this camera and M42. Guiding and cooling are still needed but some decent images can be acquired with the shorter uncooled, unguided exposures.
M42_200x10s-g10-bin2x2-resized.png
M42_200x10s-g10-bin2x2-resized.png (79.35 KiB) Viewed 4558 times
I had started trying to systematically compare images with different exposures, gains and number stacked to see which set gave the best image. Now Smart Histogram and the Brain plays it out before my eyes and suggests settings. I am expecting that I will end up with 2 sets of guideline figures - one for the darker sky over the Pennine hills and another for the sky-glow affected targets above Manchester.

Dave
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Re: Smart Histogram and The Brain

#5

Post by oopfan »

Robin,

How would one go about determining optimal exposure settings when using a monochrome camera with LRGB broadband filters or SHO narrowband filters? Does one run the Brain for each filter in the set?

Thanks,
Brian
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Re: Smart Histogram and The Brain

#6

Post by admin »

Hi Brian,

with filters you would re-measure the sky brightness with each filter. The key value is the electrons/pixel/second rate for the sky background. With experience you may get to know typical values and just choose them from the dropdown.

cheers,

Robin
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Re: Smart Histogram and The Brain

#7

Post by oopfan »

Robin,

Thank you for the tutorial. I look forward to each new installment! And thanks for going into the math.

Quick question: When measuring the brightness of the sky, do you run star detection to help cancel out the contribution of a rich star field, or do you simply ignore the upper-end of the histogram?

Another quick question: I am eager to get outside to play with my Brain. It is MUCH easier for me to set up the scope on a fixed tripod with no tracking. Will the Brain converge without tracking?

Thank you,
Brian
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Re: Smart Histogram and The Brain

#8

Post by admin »

Hi Brian,

actually it takes no account for rich star fields - the assumption is that you can find and select a dark spot in the frame using the selection box. If that turns out to be a problem in real life then I can put some sort of sigma clipping in which would work to remove the stars as long as the dark areas were in the majority.

The sensor analysis can be done on the bench in daylight (do it for RAW8 and RAW12/16 - or MONO8 and MONO12/16).

As long as you can find a nice sized patch of non-starry sky then you can play with the brain on a stationary mount - it won't care about the drift of the image as long as the brightness isn't changing significantly.

cheers,

Robin
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Re: Smart Histogram and The Brain

#9

Post by oopfan »

Hi Robin,

It just dawned on me that The Brain is somewhat similar to the computer inside my Pentax SLR camera when I operate it in Full Auto mode. The difference is that I set the ISO rating of the emulsion and then the camera calculates the aperture and shutter speed. Similar but different.

My SLR offers two additional modes: Shutter Priority and Aperture Priority. In Shutter Priority I select the shutter speed and it calculates the aperture. In Aperture Priority I select the aperture and it calculates the shutter speed.

I was wondering if The Brain has similar flexibility? (I am speaking in the passive voice because the weather has been lousy and I haven't been able to play with The Brain yet, so my knowledge of it is incomplete.)

I have seen some absolutely stunning images of galaxies taken with low gain in order to maximize dynamic range. Can I tell The Brain to calculate the shutter speed and stack size given a fixed gain?

Thanks,
Brian
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Re: Smart Histogram and The Brain

#10

Post by admin »

Hi Brian,

I guess that's a reasonable analogy - your camera is probably considering things like depth of field, aperature, exposure time, likelihood of vibration, noise level etc to try to choose the best combination. The 'brain' is more focused on getting the exposure long enough that your subs stack without excessive noise and not going past that critical exposure length (into regions where you are just getting extra pain from needing guiding to be perfect over 5, 10, 15 minutes without gaining anything).

You currently have two options for target gain

1) Unity gain
2) Max dynamic range (this usually means minimum gain)

However, suppose you set unity gain (and for instance on an Altair 183C that is Gain=400) and also set a maximum exposure of 30s (maybe your mount only manages 30s subs without problems). If the brain works out you would need 50s exposures at gain=400, it will instead suggest turning up the gain (say to 600) and keep the exposure within your limits.

Hope this helps,

Robin
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