Exposure

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oopfan
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Re: Exposure

Post by oopfan » Mon Jun 04, 2018 1:00 am

I'd have to think about it more but it seems that unguided dithering is simply spreading the raining noise over a wider path and thus making it less noticeable. Interesting.

Still, I stand by my research that tells me to raise the exposure until I achieve a minimum acceptable signal-to-noise ratio for the faintest feature I am interested in. That way I don't need to perform heroic feats in post-processing, plus I need fewer frames and less data storage.

Brian

cuivenion
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Re: Exposure

Post by cuivenion » Mon Jun 04, 2018 2:32 am

Of course, I wasn't trying discount any of your earlier advice. I was just thinking dithering could work in conjunction with it. :)

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Re: Exposure

Post by cuivenion » Mon Jun 04, 2018 2:34 am

Dithering eliminates the raining noise effect. At least it did in my case when I dithered between each exposure.

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oopfan
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Re: Exposure

Post by oopfan » Mon Jun 04, 2018 1:26 pm

Sorry, I did not intend to be dismissive.

Brian

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turfpit
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Re: Exposure

Post by turfpit » Mon Jun 04, 2018 2:33 pm

Brian
stand by my research that tells me to raise the exposure until I achieve a minimum acceptable signal-to-noise ratio for the faintest feature I am interested in. That way I don't need to perform heroic feats in post-processing, plus I need fewer frames and less data storage
Given my recent systematic imaging efforts with M27 and the positive impact by looking at max/min image statistics in individual FITS frames, I certainly support your statement.

Put simply - I got better results with 30x60s than with 60x30s for that particular object.

Dave

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oopfan
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Re: Exposure

Post by oopfan » Mon Jun 04, 2018 4:54 pm

This post is entitled "The Myth of the Ultra-Short Exposure":

Three months ago I took an online course on CCD Photometry. An essential task is to determine the linearity range of my camera's sensor. I measured it and then applied it to my astrophotography.

Here is the photo that I took of M13:
https://s3.amazonaws.com/oopfan-astroph ... Morgan.jpg

I was quite happy that the core of the cluster is not overexposed and that I could resolve individual stars. Using C2A planetarium software I discovered that the faintest stars were 16th magnitude. (Actually the UCAC4 catalog shows stars in M13 down to about magnitude 16.5. My image contains stars fainter than that.)

I wondered if I could see our Sun if I magically transported it 25,100 light-years away, the distance from here to M13. I surfed the web and discovered a college lab where they determined that the Sun emits 1E+45 photons per second.

I calculated the surface area of a sphere having a radius of 25,100 light-years to be 7.0861E+47 mm^2, and then I calculated the surface area of my 71mm refractor to be 3959.2 mm^2. Dividing the smaller surface area by the larger one and then multiplying by 1E+45 yields a flux of 4.47 electrons per second after applying 0.8 for Quantum Efficiency.

As we know a star's image is affected by diffraction and atmospheric turbulence. On average my star images have FWHM of 3.2. That means that the majority of the energy is spread across 10 pixels (i.e. 3.2 squared.) So each pixel sees 0.447 electrons per second (i.e. 4.47 divided by 10.)

At a Gain of 200 my sensor's Read Noise is 1.55 electrons. A 5-second exposure is required to achieve a signal-to-noise ratio of 1. a 13-second exposure is required to achieve an SNR of 2, and a 25-second exposure is required to achieve an SNR of 3.

My M13 image is a stack of 20-second frames which means that my mythical Sun has an SNR between 2 and 3 on each frame. That tells me that the Sun would be visible in my image perhaps as one of those faint dots. M13 is relatively nearby within our galaxy. Now consider transporting the Sun to a distant galaxy like M109. You can see that the flux goes way down.

Finally, unless you are an amateur with a very large aperture telescope, you can not expect to achieve the best performance from your camera by stacking 1-second frames. A modestly sized scope will limit you to just the brightest Deep Sky Objects. You can try to go deeper by stacking more frames but post-processing will become increasingly difficult. As I have said it is more efficient to increase exposure than to stack more.

Brian

PS

Here is a website that offers formulas for calculating the apparent magnitude of the Sun at a given distance. I ran the numbers for my mythical Sun. It came in at magnitude 19. Like I said my image contains stars fainter than magnitude 16.5 so this would suggest that my flux calculation is in the ballpark:

http://www.neoprogrammics.com/stellar_magnitude_of_sun/

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Re: Exposure

Post by ve1drg » Tue Jul 24, 2018 10:35 pm

oopfan wrote:
Sat Jun 02, 2018 5:34 pm
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
Brian..

What and where do I see the 'full-well" that you speak of:
---------------------
see how your camera's "Full Well" changes with the gain setting.
----
Ted Gervais
Windsor Ontario Canada

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oopfan
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Re: Exposure

Post by oopfan » Tue Jul 24, 2018 11:35 pm

Hi Ted,

You can find Full Well in your Sensor Analysis

Brian
Attachments
Altair GP290M Sensor Analysis MONO12 2018-01-09 FULL WELL.jpg
Altair GP290M Sensor Analysis MONO12 2018-01-09 FULL WELL.jpg (165.6 KiB) Viewed 182 times

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oopfan
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Re: Exposure

Post by oopfan » Wed Jul 25, 2018 12:09 am

Ted's post reminded me to add some new information. Make certain that you match the temperature of your darks with your lights. For those of us with uncooled cameras this is vitally important especially during the warm summer nights when ambient temperatures can plunge 5F to 10F over the course of one hour. With respect to my camera, the Altair 290M, I love it but I am not happy that I cannot read the sensor temperature. This wasn't much of an issue during the winter months. My solution has been to take half of my darks upfront and the other half after my lights. During post-processing I create a master dark from the average of all of those dark frames. It helps but it is not perfect -- I still get too much thermal noise in the final stack. I am working on a solution that I will write about in detail as results roll in.

Brian

ve1drg
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Re: Exposure

Post by ve1drg » Wed Jul 25, 2018 1:25 am

oopfan wrote:
Tue Jul 24, 2018 11:35 pm
Hi Ted,

You can find Full Well in your Sensor Analysis

Brian
Thanks Brian. I will look that up right now. That is a good thing to do. Sure helps one find the right exp[osure and texture etc..

Thanks.
----
Ted Gervais
Windsor Ontario Canada

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