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Exposure Calculator - SNR for RGB Filter
#11
(2019-06-07, 07:55 PM)theskyhound Wrote: 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).

Thanks Greg
Well I think that a comparison between an osc and an (L)RGB image, both with a SNR of 30, has not the same impact. The LRGB would be richer in saturation and "sharpness", because the imaging time is greater and the quantum efficiency is better with the mon camera. OK, that was my mistake, SNR is a lot but not all.

I have done the exposure calculations direct and via Comparing Systems and Actions and there is another point to ask.

I have calculated the exposure times in two different runs.
First run was: Target Selection – Exposure Calculator.
Second run was: Target Selection – Object Information – Compare Imaging Systems – Action Exposure Calculator
My surprise: The exposure times and SBr are not the same!
I have checked all the inputs (date, SNR,  ..) and cannot find the reason for this differences, but perhaps “I don’t see the wood for the trees”
Here are my steps and results:
FIRST RUN: I choose the Exposure calculator from the Target Selection and fill with these data’s:
Target Selection
25.3.2019, iTel T20, NM, L Filter, Bin 1x1
Moon Sep >60°, SNR30, MaxX 1.5, No other Filters
Object: M106 gives Img Time: 8.3 hr, Exp 40min, SBr 22.5, Mag 9.1, Res 14.9’’
Exposure Calculator (SNR30, SubExpTime 120sec)
Target Data: SBr(V) 21.6 (B-v) 0.68
Results
Img Quality: A
SBr(V) 21.6 Sub Exposure Time: 11x2 min Total Time: 31min
 
SECOND RUN: I choose the Object Information from the Target Selection, then Compare Imaging System, then Actions and again Exposure Calculator (always SNR30 and MaxX 1.5)
Catalog: Mean surface Br: 22.5 mag/arcsec2
Comparing Imaging system
Results
Exposure Time: 40min/46min
Action, Exposure Calculator
Results
Img Quality: A
SBr(V) 22.1 Sub Exposure Time: 23x2min, Total Time: 64min
Question
1.        Why this difference in the exposure times between First and Second Run?
2.        Why different SBr(V)?

Thanks
Ed
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#12
Hi,

The first question is easy. The difference is because the SBr(V) is different, which changes the calculation.

So the second question is more central. I had to try it a bunch of times, but I was able to finally reproduce the SBr(V) being wrong (22.1). Right now I am not sure what is causing this. It is definitely a bug in the initialization of the target object data for the Exposure Calculator. I'll have to dig into to the code to see what's gone wrong.
Clear skies,
Greg

SkyTools Developer
Reply
#13
Hi,

Ok, so I found the reason for the inconsistency. It only affects galaxies, and then only some of some of them. Some galaxies have a measured surface brightness. For most, it has to be estimated from the size and V magnitude. In the case of M106, when some calculations were performed for the Compare Imaging System tab, the SBr was calculated, even though it has a measured value. As a result, it would have a different value.

I have fixed the code and the fix will appear in the next update.
Clear skies,
Greg

SkyTools Developer
Reply
#14
(2019-06-09, 09:19 PM)theskyhound Wrote: Hi,

Ok, so I found the reason for the inconsistency. It only affects galaxies, and then only some of some of them. Some galaxies have a measured surface brightness. For most, it has to be estimated from the size and V magnitude. In the case of M106, when some calculations were performed for the Compare Imaging System tab, the SBr was calculated, even though it has a measured value. As a result, it would have a different value.

I have fixed the code and the fix will appear in the next update.

Thanks Greg
Not only the galaxies, also the nebulas (Example ClownFace). I think anywhere the surface brightness plays a role.
The target selection datas are not the same as the data in the exposure calculator.
Which are now the correct one, exposure calculator or target selection?
Ed
Reply
#15
Ed,

I am not seeing the reported problem for anything but galaxies, and then only for certain ones under certain conditions. Emission nebulae don't use surface brightness in the calculations, although it is often reported.

If you are seeing an inconsistency somewhere, this appears to be a different problem, and I am going to need specific info to follow it up. What data is not the same?
Clear skies,
Greg

SkyTools Developer
Reply
#16
(2019-06-11, 02:37 PM)theskyhound Wrote: Ed,

I am not seeing the reported problem for anything but galaxies, and then only for certain ones under certain conditions. Emission nebulae don't use surface brightness in the calculations, although it is often reported.

If you are seeing an inconsistency somewhere, this appears to be a different problem, and I am going to need specific info to follow it up. What data is not the same?

Hi Greg
I don’t know if you need the surface brightness for the SNR calculations, but the data are in your columns.
Anyway, the exposure times in the Target List and in the Exposure Calculator are not the same.
An example with the Eskimo Nebula:
Target Selection
12.1.2019, iTel T21, NM, AutoFilter, Bin 1x1
Moon Sep >60°, SNR30, MaxX 1.5, No other Filters
Object: Eskimo Nebula gives Img Time: 7.1 hr, Exp 60sec to reach the SNR 30, SBr 16.7, Mag 8.6
Exposure Calculator (SNR30, SubExpTime 120sec)
Target Data: SBr(V) 20.1, SBr(Total) 19.4
Results Exposure Calculator

Img Quality: A
SBr(V) 20.1 Sub Exposure Time: 1x2 min Total Time: 129sec
The Target Selection Tool computes with a SBr of 16.7 and the Exposure Calculator compotes with another SBr(V) and of course this gives different exposure times.
What is wrong in my approach or interpretation of the data?
Reply
#17
Hi Ed,

1. For an emission nebula, the Target Selection Tool does not use the surface brightness, and neither does the Exposure Calculator. So this will not affect the results.

2. The surface brightness displayed on the Target Selection column is the "mean surface brightness," which is derived from the integrated magnitude and the size of the nebula. This is not a very useful value because integrated magnitudes for emission nebulae are only estimates and the sizes are somewhat arbitrary.

3. The surface brightness displayed on the Exposure Calculator is much more accurate. It reflects the emission line strengths integrated over the V filter and employs your exposure goal selection of "Brightest Regions," "Main Extent" etc. This is what is used in all calculations, with the caveat that when you don't have a choice of exposure goal, it defaults to "Main Extent."

As for the exposure times being different, that could be due to a lot of factors. I would need to see screen captures of the Target Selection Tool and Exposure Calculator to be able to determine why one would differ from the other. In general, the Exposure Calculator has a lot more flexibility,which means that a specific setting can affect the result.
Clear skies,
Greg

SkyTools Developer
Reply
#18
(2019-06-11, 04:02 PM)theskyhound Wrote: Hi Ed,

1. For an emission nebula, the Target Selection Tool does not use the surface brightness, and neither does the Exposure Calculator. So this will not affect the results.

2. The surface brightness displayed on the Target Selection column is the "mean surface brightness," which is derived from the integrated magnitude and the size of the nebula. This is not a very useful value because integrated magnitudes for emission nebulae are only estimates and the sizes are somewhat arbitrary.

3. The surface brightness displayed on the Exposure Calculator is much more accurate. It reflects the emission line strengths integrated over the V filter and employs your exposure goal selection of "Brightest Regions," "Main Extent" etc. This is what is used in all calculations, with the caveat that when you don't have a choice of exposure goal, it defaults to "Main Extent."

As for the exposure times being different, that could be due to a lot of factors. I would need to see screen captures of the Target Selection Tool and Exposure Calculator to be able to determine why one would differ from the other. In general, the Exposure Calculator has a lot more flexibility,which means that a specific setting can affect the result.

Hi Greg
Yes, that make sense for me.
Many thanks that you have taken time to discuss this points.
But I think some corrections are usefull and some explanations also. This data on the target selector could be misleading.
I will now check your new videos on youtube to this points.
By the way: Many thanks for this very instructive videos, they are very helpful.
Ed
Reply
#19
Hi Ed,

The details of how things work is very complex and technical. I have not had the chance to fully document it, although I do intend to.
Clear skies,
Greg

SkyTools Developer
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