Exposure calculation sky effect

[mwg3656]

When I check the exposure calculation time there is an inconsistency.

Same Imaging system for both, just date change.

Full Moon April 7th, target SNR 30 calculations
- M81 mag 7.8 60min (looks ok)
- M1   mag 8.4   8sec (not ok)

New Moon April 22, target SNR 30 calculations
- M81 mag 7.8 60min (??, time should be less with new moon)
- M1   mag 8.4  8sec  (still too short)

1. Does sky limiting brightness (19.6mag/sec) set these exposure times?

2. Why is M1 so short?

Mark

[theskyhound]

Hi Mark,

Every imaging system is different. This is why the calculations are necessary and helpful. The image scale, detector QE, filter, and the amount of light pollution for you location are critical factors, just to name a few. Without knowing the details I can't begin to answer your question. On the other hand, the integrated magnitude alone tells us very little about the target object, especially when it is an emission line object like a supernova remnant.

There is always the possibility that you have something set up incorrectly, but again, I would have to know about your setup in order to provide any insight at all.

Have you actually imaged M1 and estimated the SNR?

[mwg3656]

Hi Mark,

Every imaging system is different. This is why the calculations are necessary and helpful. The image scale, detector QE, filter, and the amount of light pollution for you location are critical factors, just to name a few. Without knowing the details I can't begin to answer your question. On the other hand, the integrated magnitude alone tells us very little about the target object, especially when it is an emission line object like a supernova remnant.

There is always the possibility that you have something set up incorrectly, but again, I would have to know about your setup in order to provide any insight at all.

Have you actually imaged M1 and estimated the SNR?

Hi Greg,

Thanks for your response.
I have a RASA 11" and ZWO 6200C camera and use a Baader UHC-S 50mm x 50mm filter, which I loaded into SkyTools4. The filter text file was easy to create and I observed a change in calculated exposure time between no filter and the UHC-s filter. Great feature. I live in a Bortle 5 area.

Q#1: I imaged both objects (M1 and M81) for 30sec to compare, but I did not check the SNR, I just looked at the the result on the monitor. M81 was clearly visible while M1 was just a fuzzy spot with no detail or color. Base on past experience looking at the images I would expect to image about 50% more exposure time on M1 versus M81 to get the same level of low light detail. I was thinking that equal SNR meant equal level of small light detail, but maybe not. If that is not the case, then how can I compare SNR with level of visible detail? I know you can always capture more detail, but I am trying to determine an optimal exposure time (or SNR?) that provides visible detail in a 19" x 13" 300dpi print, but not detail (wasted exposure time) that cannot be seen.

Q#2: My question had to deal with new moon versus full sky background light pollution. I expected a shorter exposure time for new moon (I measured 4.9e/px/sec in a dark area of the image) versus longer time with full moon (I measured 11.2e/px/sec in a dark area of the image). Do I have to change the sky background number in the location tab depending on measurement/night or does SkyTools4 auto calculate a change based on the moon height/location in the sky at the time?

thanks,
Mark

[theskyhound]

Hi Mark,

Every imaging system is different. This is why the calculations are necessary and helpful. The image scale, detector QE, filter, and the amount of light pollution for you location are critical factors, just to name a few. Without knowing the details I can't begin to answer your question. On the other hand, the integrated magnitude alone tells us very little about the target object, especially when it is an emission line object like a supernova remnant.

There is always the possibility that you have something set up incorrectly, but again, I would have to know about your setup in order to provide any insight at all.

Have you actually imaged M1 and estimated the SNR?

Hi Greg,

Thanks for your response.
I have a RASA 11" and ZWO 6200C camera and use a Baader UHC-S 50mm x 50mm filter, which I loaded into SkyTools4. The filter text file was easy to create and I observed a change in calculated exposure time between no filter and the UHC-s filter. Great feature. I live in a Bortle 5 area.

Q#1: I imaged both objects (M1 and M81) for 30sec to compare, but I did not check the SNR, I just looked at the the result on the monitor. M81 was clearly visible while M1 was just a fuzzy spot with no detail or color. Base on past experience looking at the images I would expect to image about 50% more exposure time on M1 versus M81 to get the same level of low light detail. I was thinking that equal SNR meant equal level of small light detail, but maybe not. If that is not the case, then how can I compare SNR with level of visible detail? I know you can always capture more detail, but I am trying to determine an optimal exposure time (or SNR?) that provides visible detail in a 19" x 13" 300dpi print, but not detail (wasted exposure time) that cannot be seen.

Q#2: My question had to deal with new moon versus full sky background light pollution. I expected a shorter exposure time for new moon (I measured 4.9e/px/sec in a dark area of the image) versus longer time with full moon (I measured 11.2e/px/sec in a dark area of the image). Do I have to change the sky background number in the location tab depending on measurement/night or does SkyTools4 auto calculate a change based on the moon height/location in the sky at the time?

thanks,
Mark

#1. SNR means Signal to Noise Ratio. It's not a measure of detail, but a measure of the ratio of the mean signal of the target object to the noise level. High SNR means the image will be less noisy, which is what makes images look good.

#2. The sky background is always calculated for the current conditions. The separation from the moon, altitude, etc, all matter. It is important to accurately set the amount of light pollution for the location in order to do this well. The exposure time is calculated to give you your target SNR. A brighter sky can simply be removed from the image via processing. It is the noise added by the sky background that is important. If another source of noise dominates, such as read noise, then the sky background won't matter that much and so it won't impact the exposure time required to reach the same SNR that much. But I am only speaking in general terms.