ISIS preparation and use, Master dark, bias and flat frames?
Posted: Tue Oct 26, 2021 11:53 pm
HI All,
I have been using ISIS to prepare master dark and bias and flat frames but would like to confirm what the program does and what should be used when reducing spectra.
Bias frames:
- ISIS master bias preparation expects a series of bias frames and i assume averages them. Ok, seems straightforward.
- in ISIS general tab, I use the master bias fits file. all good.
Dark frames:
- ISIS master dark preparation asks for master bias and a series of darks.
Can anyone confirm, that as I assume, it averages the darks, then subtracts the master bias?
So the resulting master dark would be dark current only, since bias has been subtracted.
Then in the isis general tab, if I use both a master bias and a master dark, isis appears to scale the master dark frame according to the actual target spectrum frame duration (if different to the master dark. ) and then subtracts that master dark and also subtracts the master bias from the target spectrum frames?
OR does ISIS assume that the master dark may be has NOT had the bias frame subtracted, and therefore first subtracts the nominated master bias frame first before time scaling?
Also, what happens if I do not provide a master bias frame in the general tab, and only a master dark (for example because I might have a series of dark frames of the same duration and temperature as the target spectrum frames). Will ISIS take the lack of provided bias frame as meaning no scaling of dark frame need be applied?
The latter case might be able to provide a slightly lower noise, since when calculating a master bias frame, there is still a little residual noise (since the bias frame is based on average of a finite number of bias frames), and subtraction of the master bias from the stack of master darks means the (stacking reduced) read noise gets doubled.
Does anyone have any information / details on the algorithm which ISIS applies in use of bias and dark frames for the general tab?
Similarly, master flat preparation in the masters tab asks for master bias and dark frames. I struggle to get sufficient brightness at the blue end of my flat frames, and as a result, low signal means bias frame treatment isn't negligible. Sometimes ISIS completely fails to produce a useful spectrum if I apply flat in the general tab, so usually I don't use them at all. I have seen some discussion of whether flats are even mandatory elsewhere, since problems with them can mess up the whole data reduction.
Cheers,
hamish
I have been using ISIS to prepare master dark and bias and flat frames but would like to confirm what the program does and what should be used when reducing spectra.
Bias frames:
- ISIS master bias preparation expects a series of bias frames and i assume averages them. Ok, seems straightforward.
- in ISIS general tab, I use the master bias fits file. all good.
Dark frames:
- ISIS master dark preparation asks for master bias and a series of darks.
Can anyone confirm, that as I assume, it averages the darks, then subtracts the master bias?
So the resulting master dark would be dark current only, since bias has been subtracted.
Then in the isis general tab, if I use both a master bias and a master dark, isis appears to scale the master dark frame according to the actual target spectrum frame duration (if different to the master dark. ) and then subtracts that master dark and also subtracts the master bias from the target spectrum frames?
OR does ISIS assume that the master dark may be has NOT had the bias frame subtracted, and therefore first subtracts the nominated master bias frame first before time scaling?
Also, what happens if I do not provide a master bias frame in the general tab, and only a master dark (for example because I might have a series of dark frames of the same duration and temperature as the target spectrum frames). Will ISIS take the lack of provided bias frame as meaning no scaling of dark frame need be applied?
The latter case might be able to provide a slightly lower noise, since when calculating a master bias frame, there is still a little residual noise (since the bias frame is based on average of a finite number of bias frames), and subtraction of the master bias from the stack of master darks means the (stacking reduced) read noise gets doubled.
Does anyone have any information / details on the algorithm which ISIS applies in use of bias and dark frames for the general tab?
Similarly, master flat preparation in the masters tab asks for master bias and dark frames. I struggle to get sufficient brightness at the blue end of my flat frames, and as a result, low signal means bias frame treatment isn't negligible. Sometimes ISIS completely fails to produce a useful spectrum if I apply flat in the general tab, so usually I don't use them at all. I have seen some discussion of whether flats are even mandatory elsewhere, since problems with them can mess up the whole data reduction.
Cheers,
hamish