Quote:
Originally Posted by gregbradley
Thanks H and Ray for explaining what the numbers on the images meant.
Does this model assume a particular method of combining? Usual practice is a simple mean or average combine. I have stared using sigma reject combine but if I understand the statistical maths involved that method really requires a substantial number of flats to be effective as it tries to identify outliers that are outside the statistical average or norm. You can't do that with 3 subs.
I sometimes find flats that are too bright are harsh and damage the lights.
Also I assume there is no bias or flat dark subtract here? I have found no flat dark and using a separate bias gets the best results.
Greg.
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Quote:
Originally Posted by gregbradley
I just checked a few subs I have. With my Microline 8300 the background seemed to be around 3300 (I simply moved the cursor around the image in CCDstack and the ADU is displayed in the bottom right corner).
For my Proline 16803 it was more like 2100.
So if I took 20 luminance subs like that with the Microline that is 20 x 3300=66,000. 10X rule gives me 10 x 66,000 = 660,000. If my flat subs are around 30,000 ADU then I need 660,000 divided by 30,000 =22 flats.
So an approximate rule there would be 1 flat for every light. This is all 1x1 binning.
For the Proline though it would need 1/3rd as many so say 15 flats for every 20 lights.
I did approximately 12 flats in my last imaging run.
But if I increase my subexposure length to 30 minutes I'll have to measure the background ADU (not sure if it increases with exposure length). It may mean longer exposures require less number of flats?
Another argument for longer exposures so long as bright areas don't blow out?
Greg.
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Hi Greg.
The assumption is average combine with floating point representation. It is assumed that outliers are taken care of using some non-linear technique that does not interfere significantly with the averaging process - sigma rejection would fit the bill.
This analysis does not include darks - it is assumed that dark calibration is perfect.
So an approximate rule there would be 1 flat for every light. This is all 1x1 binning. For the Proline though it would need 1/3rd as many so say 15 flats for every 20 lights.
why not stick with "1 flat for every light" - that will be close enough.
If you choose to deliberately overexpose the subs by increasing the exposure length, the sky background will increase in proportion to the time - you will still need the same number of flats to get to 10x the total signal, so there is no argument in favour of longer subs. However, the simple rule of thumb that you need one flat for every light will not apply if you deliberately overexpose - you will then need more than one flat for every light. The basic idea makes intuitive sense - if you expose for longer on the lights you need to expose for longer on the flats. It doesn't matter how you go about doing it - just how long the total sums of the exposures are.
I sometimes find flats that are too bright are harsh and damage the lights I don't understand what this means Greg - what sort of damage do you see?
regards ray