[Shiraz (Ray)]

Hi

this issue came up in Greg's thread - rather than clog that up, thought it might be a good idea to get another going. The following is a summary of where I have got to so far on the topic - be very grateful for any discussion/feedback/results/criticism. thanks for looking. regards Ray

Dither can be used to remove fixed pattern noise (FPN). FPN comes about from a number of sources:
1. pixel-pixel sensitivity variations that modulate sky and target signal.
2. residual noise introduced in calibration, including read noise in flats and darks, shot noise in flats and any dark/bias noise that is impressed on the flats. This residual noise is frozen in the master cal frames and is applied equally to all subs in the calibration process, so it has the same effect as type 1 noise.

The characteristic of FPN is that it is in a pattern that always appears at the same place in the subs – eg if you have an excess signal at a given pixel in one sub due to residual FPN in the master dark, the same excess will appear at that pixel location in all subs and consequently it will still be there after stacking. This is why FPN is so damaging – it does not stack out. However, FPN can be stacked out if you make it uncorrelated between subs and you can do that by dithering the subs, so that the star field moves around between subs. When you align subs on the stars, the FPN is randomly distributed in the subs and FPN behaves the same as any other noise - it reduces when you stack. In fact, if your camera has low pixel-pixel sensitivity variation and you dither, an argument can be made for dispensing altogether with calibration – you can stack out all noise and maybe add some extra dithered subs to compensate for the fact that stacking has to deal with extra noise.

How much do you need to dither? Since the process works best if there is no sub-to-sub correlation in FPN, it seems to make sense that you should ensure that there are more dither positions available than there are subs. For example, if you have 36 subs, it would possibly be wise to have at least a 7x7 dither matrix (49 available positions) in the main image, to ensure that not many subs share star field positions. With random dither, there will almost always be some duplicates, but possibly not to the extent that it will be a major issue unless the FPN is excessive – it may be worth experimenting to see what works best for your system and cal procedures. in particular, there will rarely be a 1:1 correspondence between guide and image sampling, so the above approach will only provide guidance, not an exact solution. And thinking about it, there are probably better ways to dither than random, but that is what current software seems to do.

If you don't have enough dither extent, you will lose some of it's effectiveness and retain a fraction of the FPN. However, any dither will be better than none (even aimpoint drift works) – the last thing you need is a guiding system that places the stars in exactly the same place, sub after sub.