[QUOTE=gregbradley;1029014]
Quote:
Originally Posted by Shiraz
My understanding of why summed short subs and one long exposure can be the same is that the primary broadband noise is due to shot noise - that comes as part of the signal-plus-background and you get the same number of photons (target, background and noise) in a given period however you choose to collect them. This argument falls over if you have a camera with high read noise - then the read noise can add to the overall noise and make the SNR worse for multiple short subs. However, it is often still possible to use relatively short subs and still keep the total read noise contribution below the level where it makes any real difference.
Something is not right here. The top imagers are going for longer exposures and getting better results. If your tracking can handle it I have noticed for example you start getting dimmer, fainter details when using 15 minute subs versus 10 minutes.
Don Goldman R Jay Gabany both mentioned they are now using 30 minute subs for their imaging.
John Gleason of Ha fame uses 40 minute subs on his FSQ and Ha imaging.
The usual barrier is tracking. Can your setup handle 30 minute subs and still get tight round stars or do they become elongated or even just bloated from the bouncing around from corrections and periodic error?
I plan on going longer (I currently image at 10 or 15 minutes for LRGB and 20 minutes for narrowband) as it seems to me the really faint parts have to get above the noise floor of the system. It has the best chance of being imaged if you go longer if your skies permit it (otherwise you will get an excellent bright image of the light pollution!). I intend tweaking PEC and Polar Alignment and getting large T-point models to make long exposure with tight round stars possible every time.
Greg.
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thanks Greg - you have proved my point. I have no desire to second guess DG or RJG, but put their systems into the system model with very dark sky (21.6). The suggestion for Don's system is for 27min subs and for RJay, 21 minute subs. Since I made a number of unknowable assumptions, I think that this is pretty good validation of the model. All along, the suggestion for NB has been "as long as possible", so JG's use of 40 minutes is also some sort of validation.
The point of the original post was that just because someone like DG uses 30 minute subs, there is no valid for anyone else to immediately change over to 30 minute subs (unless they have a similar system) - sub length should be individually chosen for the characteristics of the scope, camera and sky. For example, Mike S's system under the same conditions does not need subs longer than 3 minutes, even though Mike uses longer ones - he would be throwing away a lot of dynamic range if he used 30 minute subs. Your CDK with the 16803 needs subs of at least 15 minutes under very dark sky. There is no single sub length that is required if you want quality images and there may even be some very good reasons for using shorter subs than recommended by the maths to overcome seeing variability over a scale of a few to tens of minutes.
Quote:
Originally Posted by Merlin66
All this discussion on SNR.....
Actually how DO you guys actually measure the SNR in the images you take????
(I have to use some processing software to determine the SNR in our spectra..not easy.)
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Trying to measure SNR for a single complex image is even less easy - it is not possible to separate out structure and noise. In assessing equipment, have used a flat, removed bias and then selected by eye a region where there is no obvious gradient. then used Nebulosity to get the stats for a 21x21 region in the selected part of the image.
ISO has standards/measurement techniques for image quality/SNR.
http://en.wikipedia.org/wiki/Signal-...ratio_(imaging) might be worth a read.