Registar - stacking widefield images

[iceman (Mike)]

Hi guys.

Anyone use Registar? I know Bert does, hopefully he can help me.

It's a great program, so good at aligning and combining images.

I've got 3x60s shots of Orion from last night, and want to know the best method to combine them..

I've used "Average" and "Sum".. What is the difference, what do they actually do?

Is "Average" like a Registax Stacking, ie: it doesn't do any processing?

I've noticed that "Sum" seems to do some processing, like adding the weight of all the pixels in each image as it comes out much brighter, almost overprocessed.

Appreciate any help.

[davidpretorius]

can i have a play with those images in iris?

I know Iris will align and stack say 5 images with the brightness now 5x ie an add.

I usually use the other stack "median stack" the images and then multiply by say 1.5 or 2 or 2.5, depending on what looks best.

[iceman (Mike)]

Sure Dave, check the thread I just created in the DSO forum.

[cventer]

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

[Striker (Tony)]

Thanks Chris...that has explained some of the methods in Images Plus.

I only have about 472 questions left then I will be educated fully.

[iceman (Mike)]

Thanks Chris, great explanation. I love registar too, it just does wonders on rotated and misaligned images.

I'll stick with average and do post processing in something else.