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I use Registar a lot. Its an awesome tool.
The choice on what combine method to do is largely dependant on quality of your data. You can never go wrong with average. Its the best all round method. Median combine is nice if you have outliers in your image. Ie a satelite in on of your frames but the signal to noise of a median combine means you get slightly less noise and also less signal....
If you dither images when autguiding then median combine is a very powerfull tool to get rid of noise. Sum is rarely used as most often you will blow the dynamic range of an image when summing. Esepecially on Orion which has a huge dynamic range. remember in a 16 bit color image you ony have 64k possible values. Summing means any bright areas will saturate very quickly causing the core to be washed out. Think of it like a bucket of water. Adding takes all the cups of water (ie your individual images) and chucks them all in the bucket. You eventualy will overflow the bucket and the water (data) is then lost. Bright areas overflow first so you lose detail and dynamic range when summing.
So stick to median, average, Sigma if you have it. Or my favourite min, max exluded average, however this is not available in all tools. Images Plus has a huge range of combination options but you can tbeat Registar for alignment.
Regards
Chris
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