Short exposures vs long?

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Re: Short exposures vs long?

#21

Post by OhNo »


I have to go back and have a better look at JT's thread. I browsed, kinda like the "h3ll or high water imaging". :lol:

I have done a lot of research on photons though. Amazing lil things. Equally amazing is how our receptor, whether it be an eyeball, camera film or a chip takes light (at various wavelengths) and stops, and interprets them. I would like to think as the earth's atmosphere as a filter. It's a filter beyond our control. Yet it has a huge influence on the receptor's perception of those photons.
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Re: Short exposures vs long?

#22

Post by STEVE333 »


Hi Larry - You have certainly received a lot of responses to your question.

I would like to answer your question using a SNR approach. I won't go through all the equations, but, will show two graphs that should allow you to choose your Exposure time. I looked up the technical data for your camera (Noise vs Gain, etc.) and put it into my analysis. The only inputs I don't have for the analysis are the measured SkyGlow signals for each channel of your camera (Red, Green, Blue) when looking high into the sky. So, I made a guess at those.

In the course of an evening (or multiple evenings) one collects images one after another. These images are then stacked to produce the Master Image ready for processing. The analysis looks at the "Quality" of the final stacked Master Image as a function of what Exposure Time was used to collect the individual images. The SNR for an individual image taken with a short Exposure Time will be relatively low, but, there are more of these images to stack. Conversely the SNR for an individual image taken with a longer Exposure Time will be higher, but, there are fewer of these images to stack. What matters is the SNR of the final stacked Master Image. The analysis takes all of this into account.

The first question is "Will the Gain setting affect the final stacked SNR?". The graph below shows the Stacked SNR (Vertical axis) vs the Sub Frame Exposure (the Exposure Time for each individual image). There are three curves shown on the graph, one for Gain = 0, one for Gain = 94 (Unity Gain), and one for Gain = 200. It is seen that there is very little difference in the three curves which means that Gain setting does not have a significant effect on final image SNR. Higher Gains produce only slightly higher stacked SNR for the stacked Master Image.

Image

Since higher Gains don't significantly improve SNR it seems desirable to use the Gain = 0 setting because it will provide the highest Dynamic Range (fewer saturated stars).


The next question is "What Exposure Time should I use?". The graph below helps answer that question.

Image

Again, the Vertical axis on the graph is an indication of the quality of the stacked Master Image, and, higher is better. Although longer exposures are better, at 35 sec you would already be achieving close to 85% of the best you could do for the R, G, and B channels. At 180 sec you are at about 98% of the best SNR you could do. The tradeoff is that longer exposures will reduce the dynamic range and cause more stars to possibly saturate. Always a tradeoff!! :confusion-confused:

Because I don't have the exact SkyGlow flux values for your camera/telescope/light pollution combination my graphs are only an estimate of your exact situation. However, I would guess the estimates won't be far off.

Hope this makes some sense and helps.

Cheers,
Steve
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Re: Short exposures vs long?

#23

Post by Larry 1969 »


Wow Steve!
Thanks for all of that!

Larry
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Re: Short exposures vs long?

#24

Post by STEVE333 »


Larry 1969 wrote: Sat Jun 20, 2020 9:04 pm Wow Steve!
Thanks for all of that!

Larry

You're welcome Larry. Hope it helps.

I did the same analysis for my setup and it indicated using 10 min exposures for my S/H/O filters. It's providing excellent images.

Steve
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Re: Short exposures vs long?

#25

Post by Larry 1969 »


STEVE333 wrote: Sat Jun 20, 2020 10:00 pm
Larry 1969 wrote: Sat Jun 20, 2020 9:04 pm Wow Steve!
Thanks for all of that!

Larry

You're welcome Larry. Hope it helps.

I did the same analysis for my setup and it indicated using 10 min exposures for my S/H/O filters. It's providing excellent images.

Steve
I just emailed your reply and charts to a friend of mine who is going through the same learning stage as I when it comes to all of this...
He just purchased a ZWO ASI 533 MC Pro and was wondering if the charts would be similar...
I'm not exactly sure what information you used to calculate mine, so I'm not sure how it would compare to his...
Do you think they would be similar?

Thanks again Steve!

Larry
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Re: Short exposures vs long?

#26

Post by STEVE333 »


Hi Larry - You are most welcome.

His charts would be different because his sensor gains are different. However, I would be glad to work up a set of charts for him.

The big unknown for him and for you is the amount of SkyGlow signal for your systems. Do you have an image when your system was aimed high in the sky. I would need an unprocessed image along with the Gain and Offset settings when the image was taken. If I can get that from you (and him) I can create graphs specific to your sensors. I'm retired, so, I have the time to do this and I love helping. This all fits in with my background when I was working, so, it's easy for me.

Steve
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Re: Short exposures vs long?

#27

Post by JayTee »


Steve,

Did you use APLab to generate those graphs? They look familiar. If so, thanks for reminding me I need to do the same thing for my new camera. (ASI 294MC Pro).

Cheers,
JT
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Re: Short exposures vs long?

#28

Post by STEVE333 »


JayTee wrote: Sun Jun 21, 2020 3:44 am Steve,

Did you use APLab to generate those graphs? They look familiar. If so, thanks for reminding me I need to do the same thing for my new camera. (ASI 294MC Pro).

Cheers,
JT

Hi JT - I first saw those graphs in APLab and really liked them. However, I've had some problems getting APLab to work, so, I redid the math in a program called MathCad that I used when I was still working. Makes it easy to create the graphs, then, I take a screenshot using the Windows "Snipping Tool", load it into Flickr, then share it from there on the forum post. sounds complicated, but only takes a few minutes.

Hopefully APLab will work for you. I still think that graph is the best tool to help choose the proper Exposure Time for a sensor. It's based on physics and that doesn't lie. It's a lot more reliable than a bunch of "I think ..." comments from individuals with likely different equipment and different conditions at their site. I don't mean to put down all the help that is provided here on the forum, because, I've benefited greatly from advice on this forum. But, it's just so difficult to answer the question "What is the best Exposure Time for my setup and location?". There are so many variables. The equations take all the guesswork out of the answer. Just my 2 cents worth :twocents: .

If you have any problems with APLab I'll be glad to help create graphs for you.

Steve
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Re: Short exposures vs long?

#29

Post by STEVE333 »


Larry - When I said an "unprocessed" image I should be clearer.

I need an image when the system was aimed well away from the horizon (don't want a huge amount of LP). The image needs to be calibrated, but, not stretched in any way. I use that image along with the camera Gain and Offset settings to determine your SkyGlow. Since you have a color sensor it will contain the SkyGlow information for the R/G/B channels. This image can be one of your recent Light frames.

Let me know if this doesn't make sense.

Steve
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Re: Short exposures vs long?

#30

Post by Larry 1969 »


Thanks Steve!
When you say "calibrated, but not stretched" would you be referring to a stacked image right out of DSS or a single raw frame?
I'll have do dig through my data. My Iris Nebula data is in the north... I think I have the Western Veil that I chased to the meridian too...

Larry
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Re: Short exposures vs long?

#31

Post by yobbo89 »


I had brief read of this thread, i don't know if it was mentioned, your sky glow will give you an indication of how long you should be exposing for aswell . steve did a nice thread with a program that will analyse your images and build you a nice graph for you showing your sqm , recommended exposure time ectt .

edit: here is the thread viewtopic.php?f=22&t=6284&hilit=sqm
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Re: Short exposures vs long?

#32

Post by STEVE333 »


Larry 1969 wrote: Sun Jun 21, 2020 12:46 pm Thanks Steve!
When you say "calibrated, but not stretched" would you be referring to a stacked image right out of DSS or a single raw frame?
I'll have do dig through my data. My Iris Nebula data is in the north... I think I have the Western Veil that I chased to the meridian too...

Larry

Hi Lary - Glad you asked.

Because I do my own calibrations using PixInsight I always have the individual calibrated Light frames. Not so with DSS where you only get the final stacked version.

I believe what I need is actually two files. First a raw Light frame and also a raw Dark frame.

Don't worry about where in the sky the Light frame was looking. The LP won't affect the results very much (unless you were looking into a spotlight!).

Cheers,
Steve
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Re: Short exposures vs long?

#33

Post by Larry 1969 »


STEVE333 wrote: Sun Jun 21, 2020 6:35 pm
Larry 1969 wrote: Sun Jun 21, 2020 12:46 pm Thanks Steve!
When you say "calibrated, but not stretched" would you be referring to a stacked image right out of DSS or a single raw frame?
I'll have do dig through my data. My Iris Nebula data is in the north... I think I have the Western Veil that I chased to the meridian too...

Larry

Hi Lary - Glad you asked.

Because I do my own calibrations using PixInsight I always have the individual calibrated Light frames. Not so with DSS where you only get the final stacked version.

I believe what I need is actually two files. First a raw Light frame and also a raw Dark frame.

Don't worry about where in the sky the Light frame was looking. The LP won't affect the results very much (unless you were looking into a spotlight!).

Cheers,
Steve
Steve,
Will the use of a filter effect these results? Just like a simple IR/UV cut filter?

Thanks again!

Larry
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Re: Short exposures vs long?

#34

Post by STEVE333 »


Larry 1969 wrote: Mon Jun 22, 2020 1:11 am Steve,
Will the use of a filter effect these results? Just like a simple IR/UV cut filter?

Thanks again!

Larry

Hi Larry - The filter will have an effect. The idea of the IR/UV filter is to block unwanted signal from getting to the sensor. If you always use that filter, then, hopefully you can get a Light frame taken with that filter in place.

Steve
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Re: Short exposures vs long?

#35

Post by Larry 1969 »


Thanks a bunch Steve!
My buddy uses one all the time. I just ordered one so I'll be using it next time out and probably from there on.
I would be curious though to see a comparison to get an idea of just exactly how the filter affects the exposure time.
I have been shooting at 0 gain and 65 offset for 180 seconds with no filter and my stacked images are pretty dark.
After reading those links JT posted, I have been tempted to lower my offset to 10 or 20 but I'm afraid DSS will no longer detect enough stars...
Then it would get even worse if I add any type of filter...

Larry
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Re: Short exposures vs long?

#36

Post by UlteriorModem »


The use of an UV/IR cut filter is to … well... cut the UV and IR spectrum of light. The reason for their use is that CCD or CMOS sensors are much more sensitive to that part of the spectrum as opposed to 'visible' light spectrum and without the filter can have noise and saturation issue.

The UV/IR filter passes all other wavelengths and will NOT reduce light pollution and or skyglow. That being said if you do use such a filter all your calibration frames should also use the filter.
Tom

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Re: Short exposures vs long?

#37

Post by SkyHiker »


You use a reflector, so a UV/IR cut filter will remove useful light and most likely deteriorate your images. Only refractors must use such filters to prevent these wavelengths from reaching the sensor in the wrong places. I presume that most of them already have a lens coating that does this I don't know of anybody who adds one for AP.
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Re: Short exposures vs long?

#38

Post by Larry 1969 »


SkyHiker wrote: Mon Jun 22, 2020 3:29 pm You use a reflector, so a UV/IR cut filter will remove useful light and most likely deteriorate your images. Only refractors must use such filters to prevent these wavelengths from reaching the sensor in the wrong places. I presume that most of them already have a lens coating that does this I don't know of anybody who adds one for AP.
I've read that too...
Reflectors are "immune" to the IR/UV issue...
However, my light is now passing through a coma corrector, which is glass and there's a clear glass cover over my sensor...
I need to try it out and see what happens...

Larry
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Re: Short exposures vs long?

#39

Post by XCalRocketMan »


I have been reading this thread with earnest and have the 0.4.0 version of APLab. However, it refuses to load my flats. Just sits there and does nothing. Anyone else having this issue.

On another note, since my camera (ASI1600mm) is a CMOS sensor, and I don't use BIAS frames at all, can APLab still be used? It insists on having a BIAS frame added.
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Re: Short exposures vs long?

#40

Post by JayTee »


Since taking a bias frame is relatively easy. I would go that route and just give APLab a bias frame as opposed to finding a work-around.

JT
∞ Primary Scopes: #1: Celestron CPC1100 #2: 8" f/7.5 Dob #3: CR150HD f/8 6" frac
∞ AP Scopes: #1: TPO 6" f/9 RC #2: ES 102 f/7 APO #3: ES 80mm f/6 APO
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∞ Mounts: iOptron CEM70AG, SW EQ6, Celestron AVX, SLT & GT (Alt-Az), Meade DS2000
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