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Exposure Calculator - SNR for RGB Filter
#1
Hi
The Exposure Calculator allows to set the SNR for different filters.
I would like to compare the exposure times between a cmos mono camera (ASI1600) and color osc (ASI071MC).
My standard is a SNR of 30 for the lum images. Which value is a normal range for the R,G,B filters and what's the total result for the LRGB image?
With the same SNR of 30 for the R,G,B Filters, the exposure times are to high and binning is not the solution with the asi1600mm.
Thanks for your experiences
Ed
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#2
Hello Ed,

This is the kind of question that we have not been seeing here, and I am really glad that you asked! I would love to see more of these kinds of questions and for experienced imagers to speak up. I hope others will chime in!

Here are my musings, for what its worth, and with an emphasis on explaining some of the finer points of SkyTools Imaging.

What you get in your final LRGB image is going to depend on how the LRGB images are combined to make the final color image. For true LRGB imaging, the final SNR will be dominated by the SNR of your L image, as the RGB will be used primarily for the color information. So the answer is basically to get as much SNR as you can in each filter, but especially the L.

But there is another consideration, and that is stars. Unless you are going to remove the stars from your final image, it really makes the processing go more easily if the stars have a similar point spread function in each of the color filters. Otherwise you can get things like color fringing. This is going to be determined by the sub exposure that you use rather than the total number of images in the stack. So when you set up your exposure goals, you should pay some attention to the R(stars) column in the filter table. This tells you the sub exposure time relative to the L filter required to register a typical star on the image. The L filter will be 1.0, and if the R filter is 3.5, then that means you need a sub exposure about 3.5 times longer than the L in order to record the stars on your R image similarly to how they are captured on the L image.

Let's look at how you might use this in practice, assuming an LRGB set of filters. Normally it would be best to let SkyTools determine the sub exposure automatically, based on the conditions at the time the images are captured. But in this case we will want to force the sub exposure times. Use your experience or the Exposure Calculator to select the shortest sub exposure time for the L that will still get you a high SNR when stacked. Be sure to take into account readout time unless it is very short. A good way to do this is to use the Exposure Calculator. Select a dark night when your target is high in the sky. Select the L filter. Set the total exposure time to something like an hour. Vary the sub exposure time to see how it affects the total SNR. Determine how short you can go and still get most of the SNR you would get from a more ideal sub exposure time. That will be your L sub exposure.

So back on the Exposure Goals tab of the Imaging Project, select the L filter in the table on the right, and enable "Customize exposure goal/binning for selected filter." The total SNR should be adopted from the project exposure goal on the left. For Sub exposures, select "Force exposure to." and then enter the sub exposure time you determined in the last step.

For each of the other filters do the same, except enter a sub exposure time close to the value under R(stars) multiplied by your sub exposure time in L.

Example: your L sub exposure is 1 minute.

For the R filter, R(Stars) is 4.6 (in this example). So set the sub exposure for the R filter to 1 * 4.6 = 4.6 minutes. Maybe round this to 5 minutes.

The Scheduler/Real Time should now use the sub exposure times you selected, while still working to obtain the final SNR that you want in each filter. Remember, the SNR is calculated for the part of the target object that you chose in the Exposure Goals for your project, e.g. the core of the galaxy, or the fainter parts of the nebula, etc.

There is one other consideration when it comes to stars and that is seeing and how it relates to the size of the stars on the image. If you obtain images in different filters on different nights, you will want the seeing to be roughly the same or the stars may not combine well. And don't forget that the psf of of the stars on your image is also determined by the altitude in the sky that you are imaging at, so for the best imaging across multiple nights, it is best to insist on at least an IQ of B, of not A, which will ensure it is equally high in the sky.
Clear skies,
Greg

SkyTools Developer
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#3
Hi Greg
Many thanks for your detailed informations. I think this alone give me not my answer.
Example:
Object: M108 SNR 30 and Single Exposure Time 180"
With the ASI1600MM (Scope 500mm FL) I get: L 21x3' / R 72x3' / G 50x3' / B 57x3' ... Total Imaging Time: 600'
With the ASI071MC (same Scope and conditions) with the color Filter I get: 110'

Now, why I get a SNR of 30 with the OSC in 110' against the mono with 600'?
Sorry, but I dont get the point or my mistake.

Thanks
Ed
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#4
Hi Ed,

There is no way to answer your question in detail without knowing your detailed camera specs. At first blush, assuming everything is set up correctly, the cameras have different sensitivities. Naturally you will need a longer total exposure time when doing LRGB imaging, because the R, G, and B filters pass less light. That, and obviously we can't do them all at the same time. I may not be understanding your question. I can't tell exactly what you think is a mistake because I don't know what your assumptions are. I think maybe the problem is in your assumptions, but I'm not sure.

By the way, I was very confused by your use of the " and ' symbols. I have seen this elsewhere, such as on Cloudynights, and it is very unfortunate. The " symbol means arc seconds, which is a measure of angle on the sky. Similarly, the ' symbol means arc minutes (or minutes of arc). These are not times. In order to avoid confusion between the two it is important to use seconds (or s) to denote seconds of time, and minutes or m to denote minutes of time.

I know you are in Europe, and there may be different conventions (usually astronomy conventions don't vary all that much). If you normally use " to mean seconds of time, how do you denote seconds or arc?
Clear skies,
Greg

SkyTools Developer
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#5
Hi Greg
Sorry but here in europe the " are seconds (not arcsec) and the ' are (time) minutes.

Now, what I try is to compare the total exposure time for two comparable mono and color camera (OSC) for the same SNR and the same object.
In my case this is the ASI1600MM and the ASI071MC. I think the camera datas are correct in my ST4. I use a the camera preset gain HDR.

The question could also be: How long do I have to expose with the color (OSC) camera to get the same SNR as with the comparable mono camera with (L)RGB.
Or, in which case (object, times, conditions, etc) I should prefer the color or the mono camera.

Thanks
Ed
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#6
(2019-06-05, 08:22 PM)lewis Wrote: Hi Greg
Sorry but here in europe the " are seconds (not arcsec) and the ' are (time) minutes.

Now, what I try is to compare the total exposure time for two comparable mono and color camera (OSC) for the same SNR and the same object.
In my case this is the ASI1600MM and the ASI071MC. I think the camera datas are correct in my ST4. I use a the camera preset gain HDR.

The question could also be: How long do I have to expose with the color (OSC) camera to get the same SNR as with the comparable mono camera with (L)RGB.
Or, in which case (object, times, conditions, etc) I should prefer the color or the mono camera.

Thanks
Ed

I apologize, but I'm still struggling to understand. Taking the second form of your question, the Exposure Calculator tells you the answer, does it not? As it did in your example. You do have to sum the exposure for the separate filters for the mono camera, yes. Are you asking for it to do that automatically?

Presumably you have two different imaging systems set up for your two different cameras. If so, you could use the Compare Imaging Systems tab of the Object Information window (for your target object). That will do a comparison of the L exposure time in the mono camera vs. the color camera. If you want it to compare to the total LRGB time for the mono system instead, then I'm not sure there is a good way to do that. I mean, it assumes a color camer and a mono system are being compared in order to do the calculation, and that's a special case (among many such special cases). In the end I would always assume that the LRGB is going to take much more exposure time...

Greg
Clear skies,
Greg

SkyTools Developer
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#7
(2019-06-05, 08:49 PM)theskyhound Wrote:
(2019-06-05, 08:22 PM)lewis Wrote: Hi Greg
Sorry but here in europe the " are seconds (not arcsec) and the ' are (time) minutes.

Now, what I try is to compare the total exposure time for two comparable mono and color camera (OSC) for the same SNR and the same object.
In my case this is the ASI1600MM and the ASI071MC. I think the camera datas are correct in my ST4. I use a the camera preset gain HDR.

The question could also be: How long do I have to expose with the color (OSC) camera to get the same SNR as with the comparable mono camera with (L)RGB.
Or, in which case (object, times, conditions, etc) I should prefer the color or the mono camera.

Thanks
Ed

I apologize, but I'm still struggling to understand. Taking the second form of your question, the Exposure Calculator tells you the answer, does it not? As it did in your example. You do have to sum the exposure for the separate filters for the mono camera, yes. Are you asking for it to do that automatically?

Presumably you have two different imaging systems set up for your two different cameras. If so, you could use the Compare Imaging Systems tab of the Object Information window (for your target object). That will do a comparison of the L exposure time in the mono camera vs. the color camera. If you want it to compare to the total LRGB time for the mono system instead, then I'm not sure there is a good way to do that. I mean, it assumes a color camer and a mono system are being compared in order to do the calculation, and that's a special case (among many such special cases). In the end I would always assume that the LRGB is going to take much more exposure time...

Greg

Hi Greg
In the compare imaging tab the exposure time has two values which can differ. Whats the meaning?
Thanks
Ed
Reply
#8
Hello Ed,

If you click the Action Menu in the bottom right corner of the window, and then scroll down to the "Compare Imaging Systems" topic and expand it, you will see all of the columns defined. The Exposure time column is the most essential column to planning (and what you are looking for). This is the total exposure time required to reach the target SNR selected in the filter specified.
Clear skies,
Greg

SkyTools Developer
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#9
(2019-06-07, 02:48 PM)theskyhound Wrote: Hello Ed,

If you click the Action Menu in the bottom right corner of the window, and then scroll down to the "Compare Imaging Systems" topic and expand it, you will see all of the columns defined. The Exposure time column is the most essential column to planning (and what you are looking for). This is the total exposure time required to reach the target SNR selected in the filter specified.

Yes thats OK, its the exposure time, but please tell why there are two numbers... 250s / 8min. I have not find any help for that, see my attachment.
Ed
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#10
Hi Ed,

Ok, sorry, that should be documented. The first number is the total time exposing needed to reach the target SNR. The second number is the actual total time, exposing + readout, including time spent reading out each individual image. The latter assumed the optimum sub exposure time that will maximize the SNR (which is calculated in a way that includes time spent reading out).
Clear skies,
Greg

SkyTools Developer
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