SIMPLER DSLR processing and photometry for epsilon Aurigae

A previously published procedure for processing DSLR images with IRIS was somehow complicated. Though it may be more accurate since all the necessary steps for processing images are given, the procedure presented here seems to give the same results without making so much trouble!
Since we have a bright target and small total exposure times the dark/flat/bias frames are not so critical. So we just need to align the images and add them. Then we perform the photometry analysis.

~ ~ ~

For easiest use download the last version of IRIS 5.58 (especially for any Linux user because the new command convertraw is essential to load raw images).
Then you can install it very easily either to Windows or Linux (using wine).

At the final image that is loaded in IRIS you may not be able to see something. That does not mean that nothing exists! You can increase the visibility of the stars by moving the upper slider in the Threshold Window to the left until stars appear with enough contrast. This is just changing the display settings, not the image itself. In most cases the AUTO selection works fine. In the whole processing procedure you don’t really care to look these images as all these are intermediate steps to make the final image that we are looking for (step 6a).

Step 1 – Initialization:

1a. Open File>Settings and choose your working path (where you have COPIES of your images) and the file type (PIC for DSLR RAW) [IRIS will convert raw images to CFA images].
1b. Open Camera Settings (the button with the campera picture) and select only the right camera model you used and raw interpolation method linear [DO NOT tick the Apply button of White Balance]. The rest are not needed (leave the default values).

Step 2 – Loading:

2a. Click Digital photo > Decode RAW files > (opens a window where you drag and drop your files) and drag & drop your DSLR raw images (e.g. for Canon 300D: *.CR2 files).

The dialog lets you specify a sequence name. then you press “–>CFA” to decode the images. Do this for star field images (e.g. use name ‘img’) and you end up with a series of .pic files named img1,img2,… in the working directory configured in step 1a.

2aa. [alternative way – needed for linux users] Open the command line (button right left from the camera button) and type :

>convertraw input_sequence output_sequence number

input_sequence are the images from your camera and
output_sequence the images that IRIS created (in CFA format).

Step 3 – Alignment of Images:

3a. Go to tab Digital photo>sequence CFA conversion and select the sequence name that you gave at step 2 (‘img’). Give an output name (‘img-conv’) and number of your star images and select Color output files type .

3b. Go to Processing tab>Stellar registration and put the sequence name of 3a (‘img-conv’), give output sequence name (e.g. ‘img-reg’) and number. Select “one matching zone” (no need to define a specific region) and go on.

3c. Stacking of images: Processing tab>Add a sequence and give sequence name of 3b (img-reg) along with the number of images and method. There are 2 options: arithmetic (select also “normalization if overflow” if not selected) or median. They give more or less the same results
[Note: the median option removes extra features that may be visible in the images -you take a clearer image- but the S/N ratio should be smaller]

3d. Save the image!

Step 4 – Selecting green channel:

4a. Go to Digital Photo Tab > RGB Separation (so as to take only the green channel in which we are interested) and enter names for the color channel files, e.g.


You should be able to see see these images at your working directory.

Note: before performing the rgb separation take a look at the image (by selecting AUTO of Threshold Window). You can actually see the colours of the stars !! Then for scientifc reasons we select only the green channel and the images become less beautiful (but more interesting!).

Step 5 – Photometry:

5a. IRIS opens an image with a different way than some other programs. So load (either by File > Load or at command line by typing >load final-g ) the final-g image (from step 4a) and take a carefull look to figure out your field (it should not be that hard for eps aur!).

You can increase the visibility of the stars by moving the upper slider in the Threshold Window to the left until stars appear with eough contrast. This is just changing the display settings, not the image itself. In most cases the AUTO selection works fine.

5b. Go to Analysis>Aperture photometry. With our sample data, you can probably use the default values for aperture, but for your own data you may need to change these sizes so you may wish to review how to size an aperture . If you place the circles to any object you will take an output at the output box with values for Intensity and Magnitude (along with other parameters). The important one is the Magnitude (or Intensity which are equivalent).

[This section adapted from citizensky’s DSLR documentation & reduction team:
> The inner circle defines the area where the star has to fit in. The pixels inside this area must contain all (or at least almost all) light from the star. It should contain a bit of extra sky but never a second bright star.

> The outer ring between the middle and the outer circle defines the area that is considered to be “sky background”. As a rule of thumb, the radius of the outer ring should be about the same as the radius of the inner circle, but on the other hand it should not be so big that stars near any of the stars that you measure will be included in this ring.

> The area between the outer ring and the inner circle is just there to separate the two areas, pixels in this area are ignored. You will set one aperture to fit all stars (variable and comparison stars) that you will measure in an image.

As long as you are working with the same camera, lens, and focus setting, you need to make this decision only once and use the setting all over again for your measurements. When doing the measuremts, you use the aperture like a reticule to “capture” the light of the star you want to measure.]

5c. Now, locate all of the comparison stars you wish to use (see the table of comparison stars). If you have already activated the photometry tool (already done at step 5b) you should see the circles at the mouse point and a tick -activated- left to Aperture photometry option at the Analysis tab). Now, check your comparison star (i.e. eta Aur) and record the output in the spreadsheet or on paper to three decimal places. Repeat for as many check stars you want to use. Finally you select epsilon Aurigae to take an instrumental value of its magnitude.

5d. Repeat step 5c for all your sets of images.

5e. Use the Excel Data Reduction spreadsheet that is available to extract the final results.

Leave a Reply

Your email address will not be published. Required fields are marked *