Quote:
Originally Posted by rally
Hi Ray,
Always good to have another tool in the arsenal.
I am not sure that this is greatly simpler than some of the calculators around.
I spent a few minutes one day just taking the calculations out of John Smiths (CCDWare) calculator and assumptions based on some of Stan Moores work and his recommendations so I can use that in the field.
I just grab a couple of superfluous junk images while I am setting up taking a few focus tpoint mapping and images - since half the time I have no internet out there anyway - I needed a PC based calculator.
I take one of the rubbish images or a test image, whack the exposure time and the average background ADU into my little spreadsheet and it gives me the ideal minimum sub exposure time for reducing my read noise to within a fraction of the sky noise - usually about 5%
Noting that I had previously entered the other values for my cameras into a table - camera gain (available from metadata in the FITS header), pedestal, read noise and preferred % of read noise to sky flux noise (background).
The philosophy of this method is - the Noise I have control over is lost within the noise I have no control over !
But its certainly not a perfect tool for determining ideal exposure times for a given subject
It only gives me a recommended 'minimum sub exposure time' to reduce the effects of read noise, if I really want to capture deep fine detail then I need to go much, much longer and if I want to capture a high dynamic range then I will need some much shorter exposures as well so I dont have overexposed data.
So the imaging needs to be broken up into a series of exposure blocks for faint data, normal data and bright data
Maxim and CCDSoft have a pedestal value of 100 that they add to the signal so we need to remove this artificial 100 from the average background signal (its there to avoid the possibility of negative values in the calculations in final signal values)
Since 100 out 1000 is 10% its not insignificant
So just wondering if that is relevant to your calculation ?
Cheers
Rally
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Same philosophy Rally and the maths is the same as the Smith/Moore stuff only arranged for a different solution. The major difference between their approach and the suggested targetADU approach is that, with the suggested method, you will never again need to use a calculator/spreadsheet or take junk images - just do the target ADU calculation
once for your camera and use that single result forevermore with any scope/filter/sky. I found that, even though I knew the theory and could use the calculators, most of the time I was winging it - I now have a systematic methodology that only requires me to remember one number and with that I can always get the exposures right, regardless of which scope/filter/sky I am using. I can see at a glance what is going on and immediately work out the best exposure in my head - doesn't get any simpler than that. It is not going to be for everyone though and if you already have a procedure that you trust, go for it.
I don't think that you will get much more detail in the final image by using longer subs than recommended by Smith. The individual longer subs will certainly look better - but if you add a larger number of the recommended shorter subs to get to the same total exposure, the final stacked images will be identical - except that the one with shorter subs will have better dynamic range.
As I understand it, the software packages you mention apply pedestals to calibrated data after bias/dark removal. The method I am suggesting is based on raw data without any processing - you don't need to save/process anything. I guess it could be applied to pre-processed data if the software does that on the fly - in which case the Bias term in the calculation would be the pedestal value - or whatever you would normally see if there was no signal (as in posts 1 or 10).
regards Ray