Iris is a free piece of software for Microsoft Windows that allows you to do basic data processing on astronomical data files. Sometimes you won't be able to take a long exposure (for example, there might be bright stars in the field of view which would saturate), and you have to take many shorter exposure and then stack them. This guide will run you through stacking FITS images with Iris.
In this article I will use 3 red filter observations of the same object as an example. The files are called red1.fit, red2.fit, and red3.fit. When you download your files from our archive they will have different names.
Setting your Working Path
If you have already set up your path you can skip on to the next section
Before you start you will need to set up the working path in Iris. To do this, open Iris and go to File > Settings
Click on the button under the working path section. This opens a navigation window where you can browse to and select your first filter folder. In this example, red is selected first.
Select the folder and click OK. The full path of the directory will then be placed in the working path box.
Make sure FIT is selected in the File type section, then click OK
Stacking FITS Files
Open the Threshold and Command windows by clicking on the appropriate buttons on the tool bar.
In the command window, type >load red1 and press Enter.
N.B the > character is automatically entered, you do not need to type it twice.
Your first image, red1.fit will then open. This works due to the working path - Iris looks up this directory and opens the filename that you enter in the command window.
Your image will look mostly black, press the Auto button in the Threshold window to reveal some more detail.
Your images need to be aligned before they can be stacked. Go to Processing > Stellar Registration.
A window looking like the following should appear:
Input Generic Name: In this example, r is entered here as all my FITS files begin with this letter.
Output Generic Name: This is the name given to all the files that are output from this process.
Number: This represents the total number of FITS files you are using. This example showed 6 files for the red filter.
Method: Global Matching works by matching up global patterns in the images. You can choose One star methods but they are less accurate.
Transformation: Quadratic allow Iris to bend and flip your images to make them align with each other.
When you click OK, Iris will open red2 and compare it to red1, it will then align it and output it as a new file. You will be able to see circles showing the areas in the image it is using for alignment.
This process is repeated with the rest until they are all aligned. You will get 6 (in this example) new files called r_align1.fit, r_align2.fit etc... You will also see several .lst and .pol files in your directory, these are records of how much Iris has transformed your images by (they can be opened using Wordpad or Notepad).
You can now use your newly aligned images to stack.
Go to Processing > Add a sequence
Enter r_align in the Input generic name box, and again, number is the number of FITS files you have. In this example, 6.
The options at the bottom of the window are the different stacking methods. Arithmetic is a simple addition method, whereas Median takes average pixels values.
Usually, arithmetic works, but you can experiment with the other options available.
Leave Normalize ticked if it’s applicable to the method you choose.
Click OK and you will see the screen flicker as it opens and stacks your images.
You have now successfully created a single stacked image from your 3 original files. You can save this image as a JPEG or TIFF file, using File>Save, and use it with images from other filters to produce a full-color image.
