Exposure

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ve1drg
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Exposure

#1

Post by ve1drg »

Just wondering something here. It has to do with exposures taken with my zwo asi294 camera.

I am new to cmos photography but I am getting along. But slowly. I start imaging an object and wonder if it will be good enough to work on once I have it downloaded. I think that if my exposures give me a good looking image on the screen, than isn't that enough to have a good looking image in the sharpcap capture file?

If you run exposures at lets say 2 seconds a go. And use live stacking. And you take 200 exposures (or so)... wouldn't that be enough if the image I see on the scrfeen looks ok.

I look at the histogram images as well. And if the image lies about 1/4 from the left side, than I should be ok.

I just need to finally get a good image. My dslr stuff worked better for me so far. But I am learning and adapting.
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Ted Gervais
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turfpit
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Re: Exposure

#2

Post by turfpit »

You might find the information in these links useful.

https://docs.sharpcap.co.uk/2.9/#The%20Histogram

https://docs.sharpcap.co.uk/2.9/#Live%20Stacking

For faint objects, stacking alone is not enough. There is usually a need for stretching the image in software such as Photoshop or Gimp.

Dave
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oopfan
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Re: Exposure

#3

Post by oopfan »

This is what works for me. (Please know that I do not have issues with light pollution so if you do then you may have to modify this):

First off, perform sensor analysis to see how your camera's "Full Well" changes with the gain setting. Notice that at the lowest gain setting your Well is at maximum depth which means that you can keep the shutter open for a long time without causing pixels to saturate. Also notice that at the highest gain setting your Well is at minimum depth which means that you can only tolerate shorter exposures before saturation is an issue.

I attached the sensor analysis for my camera, the Altair 290M, cost $290 (Is that a coincidence or what?)

In my experience you want a combination of low gain setting and long exposure. Why? Because DSO's are very faint (most of them at least) and foreground stars are relatively bright. In this photo of NGC 6791, the foreground stars are 9th magnitude and the DSO is 16th magnitude:

https://s3.amazonaws.com/oopfan-astroph ... Morgan.jpg

I chose the lowest gain setting of 100 which gave me a Full Well of 14,839 electrons. I needed that depth in order to capture the full dynamic range of the image without saturating the foreground stars. I could have chosen a higher gain but that would have given unsatisfactory results. If I kept the exposure the same then the foreground stars would have saturated and become fat and ugly, and if I lowered the exposure to prevent saturation then the 16th magnitude stars of the DSO would have become less visible.

Here is another example of using low gain and long exposure to your benefit:

https://s3.amazonaws.com/oopfan-astroph ... Morgan.jpg

That's the Eastern Veil nebula using a 70-second exposure and gain setting of 100 (the lowest). Again, the nebula is VERY faint, and the 9th magnitude foreground stars can easily blow out on you at a higher gain.

Those two photos were taken with the Moon at 80% illumination at 20 degrees altitude.

You may ask why don't I use a much shorter exposure and increase the stack size so that the total integration time is equivalent? Two reasons: (1) you will end up with a higher signal-to-noise ratio in the final stack when you choose a longer exposure because you can never ignore noise (with short exposures your signal is at or just above the noise level -- it takes a LOT of stacking to climb out of that hole,) (2) your images will be less susceptible to "raining noise" if you use longer exposures -- this is a lesson I learned the hard way.

Second step is preparation. Use planetarium software to plan your exposure. Sometimes you will be blessed with a field-of-view having few bright foreground stars which in this case you can choose a higher gain that can shorten your exposures.

Third step is to discover the proper exposure. Take a guess at the exposure and snap a photo to a FITS file. Open the file in FITS Liberator and look at the "Max" pixel value statistic. If the value is greater than 60000 than your exposure is too long. Adjust the exposure until it is around that 60000 level. (Know your camera's hot pixels. If they are bright then you cannot use the "Max" pixel method -- instead you will have to zoom in on the brightest star and sample the disc.)

Fourth, stretch the image in SharpCap. You should easily see the DSO. If you don't then your gain setting is too high. If you still don't see it then your camera is not cut-out for capturing this DSO since it is too faint to give satisfactory results even after stacking.

I am a rebel for choosing long exposures over short ones. I found that short exposures greatly limited the number of DSO's that I could image, plus I had the problem with "raining noise". The disadvantage is that I suffer from higher read noise at lower gain settings. That is something that I have many options for mitigating in post-processing. Now, when considering a new camera my first concern is its Full Well Depth -- all else is secondary.

Brian
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Altair GP290M Sensor Analysis MONO12 2018-01-09.png
Altair GP290M Sensor Analysis MONO12 2018-01-09.png (89.14 KiB) Viewed 4817 times
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oopfan
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Re: Exposure

#4

Post by oopfan »

In my fourth step I recommended using SharpCap's histogram stretch feature. Here is what a single frame of the Eastern Veil nebula looked like:
Attachments
Eastern Veil Nebula - 1 subframe.jpg
Eastern Veil Nebula - 1 subframe.jpg (480.16 KiB) Viewed 4817 times
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oopfan
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Re: Exposure

#5

Post by oopfan »

The ASI294MC Pro looks like a fine camera having a Full Well of 63,700 electrons. You should be able to do some fine work with it. How is your mount? Do you perform active guiding with a guide scope and separate, dedicated camera, or do you do off-axis guiding with the same ASI294? In my opinion your camera is capable of 5-minute exposures without saturating 9th magnitude stars. It all depends on your aperture, of course. Anyhow, good luck! Post some images when you can.

Brian
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oopfan
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Re: Exposure

#6

Post by oopfan »

Upon reading the superb characteristics of your camera I think perhaps I can do a better job of generalizing my method that should be applicable to all CMOS cameras:

Firstly, see yourself as an artist composing a work of art. You must have a vision before you start. Frame your image using planetarium software. Decide what is in and what can be cropped out. (I say this because I see that you have a large format camera with a wide field-of-view.) In SharpCap, choose a capture area one size larger than your vision's frame. Separate yourself from your audience. Your audience wants a well-balanced composition even though you may be interested in just one element, a galaxy. If your image contains a large, bloated star your audience will perceive it as a distraction.

Secondly, use your experience to guide you but absent that choose a mid-level gain.

Thirdly, pick an exposure, any exposure. Take a snapshot and then inspect the pixel value of the brightest star. If it comes in at around 60,000 then you are done. If it is greater than 60,000 then reduce the exposure. If it is less than 60,000 then increase the exposure. (Notice that "60,000" is a matter of taste. You may decide that 50,000 or 40,000 yields a more appealing star size. It is subjective.)

Fourthly. turn on SharpCap's histogram stretch. You ought to plainly see the DSO. If you do not, or if you think it is too faint based on your experience, then you need to lower the gain and go back one step and find a new exposure.

Notice I assumed that your mount can handle any length of exposure. In reality this is seldom the case. You may find for example that you can only achieve 60-second exposures before there is significant star elongation. In that case you must reverse the procedure. Set the exposure to 60 seconds and find the gain that gives the best star size without saturating.

As a general rule I collect 2x more frames than I need to. I simply tell my stacking software to reject the worst 50%.

Also, given a chance choose a higher gain especially for super-bright DSO's like M42 and M51. This reduces your Read Noise. I know that this sounds like it is at odds with my preference for low gain/long exposure but it is not. It all comes down to this. You want to capture the DSO on a single frame so that it has a "decent" signal-to-noise ratio (SNR). You certainly don't want SNR too low but you don't want SNR so high that it affects the quality of your guiding. As always, it is a matter of experience.

Brian
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oopfan
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Re: Exposure

#7

Post by oopfan »

Here is an example of poor exposure.

This is M109, a barred spiral galaxy. Exposure was set at 9.3 seconds using a Gain setting that gave a Full Well Depth of just 4200 electrons. Please take my word for it that post-processing was very difficult mainly because I wanted to bring out the faint nebulosity in the outer arms. Processing would have been much easier if I was just interested in the core.

At the time I did not think of snapping a photo of SharpCap with histogram stretch engaged so instead I took this snapshot of a single frame viewed in FITS Liberator. Notice that the core is well exposed as is the "bar" but as you get farther from the core you have to use your imagination. This is not good.

It is quite remarkable that stacking 481 of these frames can yield so much detail but it comes at a price:
1. Total integration time of 75 minutes.
2. Noticeable "raining noise".

Raining Noise is the vertical streaks that you most prominently see in the outer arms. Primary cause is poor guiding. In my case I don't actively guide. I simply rely on the accuracy of my mount, the polar alignment, and my Periodic Error Correction. I can get excellent subframes with an exposure of 9.3 seconds but the problem is that over the course of 75 minutes the star field can shift 30 or 40 pixels due to small inaccuracies that accumulate over time. My stacking software takes care of aligning the frames but the problem is that the pixel matrix is not uniform -- some pixels run hotter and colder than their neighbors. So the result is that these hot and cold pixels form a trail across your image.

Raining Noise is most pronounced in those parts of the image that have low signal-to-noise ratio (i.e. in the outer arms.) That's the good news. The bad news is that most people choose to solve the problem by increasing complexity and cash outlays by switching to active guiding. My research suggests that simply increasing exposure can achieve similar results.

When you are trying to extract signal at the level of Read Noise you need to stack a LOT of frames. It is not linear: 1000 1-second exposures is not equivalent to 10 100-second exposures. At the time I took these images of M109 I am certain that if I quadrupled the exposure that I could have achieved the same results in 45 minutes instead of 75 minutes -- and without the Raining Noise.

There are other issues with this image that are worth discussing but I will leave it at this.

Brian
Attachments
M109 stacked 481x9.3s processed.jpg
M109 stacked 481x9.3s processed.jpg (18.83 KiB) Viewed 4776 times
M109 single frame stretched.jpg
M109 single frame stretched.jpg (240.91 KiB) Viewed 4776 times
cuivenion
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Re: Exposure

#8

Post by cuivenion »

Just to jump in, dithering will also solve the raining noise problem.
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oopfan
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Re: Exposure

#9

Post by oopfan »

Yes, that's true but dithering requires a sophisticated mount capable of executing a closed pattern of pixel movements. For example (RA+1px, DEC) -> (RA+1px, DEC+1px) -> (RA, DEC+1px) -> (RA, DEC) repeated indefinitely. You will need a guide scope or off-axis guiding to ensure that you consistently return to the starting point. Furthermore there must be coordination between the mount and the camera such that the movements occur between frames but not during them. This is possible in SharpCap but only through IronPython script, to the best of my knowledge.

My mount is 50 years old. I have a stepper motor on the RA axis but nothing on the DEC. I could spend $1000 on a new mount or I could spend nothing and use longer exposures. My old mount is capable of two-minute exposures by using SharpCap's Polar Alignment. Normally I can get within 20 arc-seconds of NCP.

I would guess that most people interested in astrophotograpy have go-to mounts these days but fewer of them commit to the additional expense and learning curve of a guide scope and camera. I am suggesting that anyone regardless of their equipment can achieve better images by simply increasing exposure. There is an added benefit to using longer exposures: lower data storage requirements.

Thanks for your reply.

Brian
cuivenion
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Re: Exposure

#10

Post by cuivenion »

I see where your coming from but you can dither with a mount such as eq3 or eq5 though. You you don't need to guide either, you can dither unguided using ASCOM. It won't be as precise of course so if you have a small FOV it might be a problem. Unfortunately dithering isn't part of of Sharpcap yet it's true, but hopefully at some point.
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