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+--- Thread: Exposure Calculator Tool Issue (/showthread.php?tid=2711)
Exposure Calculator Tool Issue - odehjas - 2022-12-31
Hello,
I'm testing the Exposure Calculator Tool, and I'm not sure if it is working correctly. I'm comparing my real results with the calculated results. So far they are not even close to each other, despite being sure that all the settings are correct.
In addition, my understanding is that if I increase the gain of my camera, then the required time to reach a certain SNR value will be shorter, but using the tool, the total exposure time does NOT change for ALL the gain values that I have! Is this reasonable? Attached is a sample for a target where the required time is 28 minutes to reach SNR value of 50 for ALL the gain values!
Also, I have a question regarding the "Read Noise" value that I should enter for my camera, is it the minimum or the maximum or the value which corresponds to my gain value? The same question applies if I'm doing BIN2, then should I keep the value for BIN1 and the tool will convert it? Or should I enter the value of BIN2?
Thank you.
RE: Exposure Calculator Tool Issue - theskyhound - 2022-12-31
Hello,
Its difficult to comment on comparing your data without some numbers to go on. What are you comparing? Signal? SNR? A single image or the stack? How is the signal being measured on the images(s)? How much difference are we talking about? Stars a a good way to test against the calculations of SkyTools, but it is best to measure the total signal of the star rather than the peak signal, as the latter depends on the seeing.
Gain does not generally affect SNR. This is because the internal gain multiplies the noise along with the signal, so the ratio between them is unchanged. As a result, the gain does not, in general, directly affect the exposure time required to meet a specific SNR. The exception is that, in some cameras with variable gain, the read noise will vary with the gain you have selected. If the sky background is not the primary source of noise, there can be a small difference in the SNR between images taken at different gains. But the difference is usually small, and if the read noise isn't the primary source of noise, it won't really matter.
For longer exposure deep sky imaging, there is usually a single gain value that will provide the optimum trade-off between lower read noise and dynamic range. For best results, set up that gain in SkyTools and set the read noise to the read noise that is associated with that gain. Then use that gain in the field and do your calculations in SkyTools for that gain. I'm adding the ability to set the read noise for each gain separately in version 4.1, but to be honest, it's not going to make much difference for most people, especially if they have light pollution.
You can read a lot about the details of gain and varying read noise, and also the so-called optimum sub exposure time formula, but the truth is that the significance of these things are way overblown in reality. There are practical considerations that are beyond control or not easily fit into a simple formula, and they are largely ignored.
RE: Exposure Calculator Tool Issue - odehjas - 2023-01-01
(2022-12-31, 04:35 PM)theskyhound Wrote: Hello,
Its difficult to comment on comparing your data without some numbers to go on. What are you comparing? Signal? SNR? A single image or the stack? How is the signal being measured on the images(s)? How much difference are we talking about? Stars a a good way to test against the calculations of SkyTools, but it is best to measure the total signal of the star rather than the peak signal, as the latter depends on the seeing.
Gain does not generally affect SNR. This is because the internal gain multiplies the noise along with the signal, so the ratio between them is unchanged. As a result, the gain does not, in general, directly affect the exposure time required to meet a specific SNR. The exception is that, in some cameras with variable gain, the read noise will vary with the gain you have selected. If the sky background is not the primary source of noise, there can be a small difference in the SNR between images taken at different gains. But the difference is usually small, and if the read noise isn't the primary source of noise, it won't really matter.
For longer exposure deep sky imaging, there is usually a single gain value that will provide the optimum trade-off between lower read noise and dynamic range. For best results, set up that gain in SkyTools and set the read noise to the read noise that is associated with that gain. Then use that gain in the field and do your calculations in SkyTools for that gain. I'm adding the ability to set the read noise for each gain separately in version 4.1, but to be honest, it's not going to make much difference for most people, especially if they have light pollution.
You can read a lot about the details of gain and varying read noise, and also the so-called optimum sub exposure time formula, but the truth is that the significance of these things are way overblown in reality. There are practical considerations that are beyond control or not easily fit into a simple formula, and they are largely ignored.
Thanks a lot, dear Greg for your reply. Let me investigate more based on your reply, and I'll get back to you.