Image stacking basics

[shaneaust (Mick)]

Can anyone point me in the direction of a "primer" for the principles of image stacking?? EG - what does it accomplish, what does the stacking software do that mere mortals can't do as well, etc?

I downloaded/installed Registax but am totally inthe dark (nooooo pun intended) on how this application "works".

Gracias.

[iceman (Mike)]

Hi there.

I'd start with this article I wrote, Astrophotography with a Dob.

You might find it quite helpful with regards the basics of image processing, capture techniques etc.

[Dennis]

There is stacking software and there is stacking software.

If you are taking images of galaxies, clusters, nebulae, etc, typically these objects are faint and extended. The technique is to have your ‘scope/mount being auto-guided whilst you capture say, 10 exposures of 3 minutes duration each. You would then stack these 10 exposures to give you an equivalent (sort of) exposure of 30 mins which allows you to capture faint detail and to minimize the “noise” in the image. You are taking few images with long exposures.

If you are taking images of the Moon or Planets, you have the luxury of more light available, which allows very short exposures, so you typically capture several 100’s of images at a time. Using short exposures “freezes” the seeing. Bad seeing is like looking through “wobbly” heat haze on a hot summer’s day – nothing is sharp. Good seeing is like looking through still, clear, mountain air.

When you use a relatively fast shutter speed, such as 1/10 or 1/25 sec and also take many images per second, say 10 frames per second, for a period of say 90 seconds, you end up with 900 images (typically this is an “avi” or a movie file). Some of these will be sharp, where you have been lucky enough to expose the frame during a moment of good seeing. Several will be blurry and soft where the image was taken when the seeing was bad or wobbly.

Registax is a program that allows you to select a single good frame, (from the 900 frame avi), as your master reference frame. Then the program will run and align, register and grade the other 899 frames to your master frame, sort them by quality and automatically stack them. You can choose how many you want to stack.

If you stack too few, then the image is noisy i.e. the actual image data is polluted by unwanted data such as electronic noise, thermal noise, compression artifacts, etc. When you stack say, 400 or 500 images, the real data begins to add up and become very strong as it always appears in the same place on the moon or planet's disc, and therefore re-inforces itself. The noise, which tends to be random, begins to become less and less obvious as it tends not to appear in the same place and will not therefore add up like the real or actual physical details from the moon or planet.

Registax uses mathematical models to analyse the good images and then uses these models to compare the other images to a master reference and will accept or reject them if they are good or bad respectively. Your eyes would get very tired doing this and you would probably find it difficult registering several hundred images accurately, resulting in a blurred image.

Hope that helps

Cheers

Dennis

[sheeny (Al)]

Well said Dennis.

Al.

[shaneaust (Mick)]

Thanx greatly Dennis and Mike - really good suggestions! My main prob is that I really didnt know what I was doing, using RegiStax, but your help will go a long way in cracking this one.

Much obliged, gents.

[Dennis]

Just keep plugging away. I found all the options in Registax a little daunting to start with, so I just kept it simple by only Aligning, Optimising & Stacking then applying Wavelets and gradually the software began to reveal what it was doing and suddenly all the basic operations began to make sense.

Good luck

Dennis

[alandee]

Hi there shaneaust,
just a couple of links to look at for tutorials too ..

old 1.1 registax --> http://www.threebuttes.com/RegistaxTutorial.htm
new 3.x registax --> http://www.davesastro.co.uk/techniqu...stax_tutorial/

They both helped me a bit.