NOTE: This is not the "live" page on the Citizen Sky website for the IRIS Analysis Tutorial. Brian must update the main site, please send him an email whenever you make significant revisions to this document. Please regard the below content as a "rough draft" and feel free to edit it to make it current.
This tutorial provides a step-by-step procedure for processing digital images obtained in raw format to yield insturmental magnitudes which are used in the next step in our tutorial, processing the data using Excel. This process assumes that, in addition to a series of sequential star field images, dark frames, flat frames and flat-dark frames have been taken. This tutorial also assumes that you have already copied the images from your camera to some directory on your computer (this greatly speeds up processing).
IRIS is a powerful image processing utility with a command-line editor and a graphical interface. We attempt to highlight both methods below.
In this tutorial you will
- Initialize IRIS
- Load images
- Perform Bias, Dark and Flat subtraction
- Align images
- Select the green channel
- Perform Aperture photometry.
After these steps, the instrumental magnitudes are entered into an spreadsheet which will yield calibrated magnitudes which can then submitted to AAVSO
This tutorial was written by GMara, bikeman, and Roger Pieri with editorial changes by bkloppenborg and screenshots by bikeman.
Note: We recommend using the latest version of IRIS (5.58 as of the time this document was written) because it implements several new features that make it easier to work with raw images.
Note: When loading a series of images, IRIS will pause for some time to read, transform, calculate and save the images. Only the final result of these calculations will be displayed.
Note: For you Linux users out there, This tutorial was tested under IRIS 5.58 on a Ubuntu 8.10 machine running WINE. The only thing that does not work is the drag-and-drop of images from Windows Explorer.
You'll need to write a script to import your files. You have to use the convertraw command in order to load a series of raw images.
Step 1 - Initialization:
- Open File>Settings and choose your working path (where you have COPIES of your images), the script path (if you want to use some scripts..!
needed mostly for linux users, but that will be written later), and the file type (PIC for DSLR RAW). Note, IRIS will convert raw images to CFA images.
- Open Camera Settings (the button with the camera picture) and leave the default values except: binning 1x1, model (select appropriate) and raw interpolation method linear. If your version of IRIS has it, make sure the White balance button is NOT selected.
Step 2 - Loading:
- Click Digital photo > Decode RAW files. This will open up a window in which files should be dragged-and-dropped from Windows Explorer (the file browser). Drag and drop your raw DSLR images (e.g. for Canon: *.CR2 files) here. (If you are running IRIS on Linux, open up the command line input dialog, the button to the left of the camera button, and type "convertraw input_sequence output_sequence number", without the quotation marks, where input_seqeuence are the images from your camera and output_sequence are the images that IRIS created in CFA format from above.)
- This dialogue lets you specify a sequence name. After dropping in your images, press the "-->CFA" button to decode the images.
- Repeat this process for star field images (e.g. use the name "img"), dark frames (suggested name "dark"), flat field images (suggested name "flat") and bias frames (optional).
- Now you should have a series of images, darks, flats and optionally bias frames in the working directory you configured in step 1a.
Step 3 - Preprocessing > Bias, Dark, Flat:
Unlike CCD cameras, DSLR cameras dont provide the user with the ability to control temperature and apply a master dark while images are being taken. Because of this one should take dark shots during your imaging session which will include the bias pattern in your camera. Although the DSLR dark frames will account for the bias, IRIS requires separate bias frames. Because of this, we can either create a formal bias frame or a fake ("dummy") bias frame, both work equally well, but one of the two must be completed.
3.1.1 Creating a Formal Bias
To generate a Flat master frame go to Digital Photo Tab and select "Make an Offset..." (Bias frame). Type the appropriate sequence name (the one used in step 2, e.g. 'bias') and the number of bias images. Then select the "command line" button (the one on the left from "camera" button) and type:
> Save master-bias (or any name you would like) or use the menu "File/Save..."
3.1.2 Generate a "dummy" Bias
Some DSLR/DSC cameras use a "system offset." This offset is built into the camera's analog to digital convert (ADC). This offset will take the black level (ideally 0) and encode it as a higher value (normally 128 for 12-bit cameras, 256 or 1024 for 14-bit cameras). the result is that generated values can be less than the offset zero position or, in other words, negative brightnesses are permitted! IRIS doesn't handle this parameter, even if the camera types are identified, therefore we need to determine it for generating the artificial bias.
The easiest way to do this is to load one of the dark frames and look at its black level. To measure the black level use the command prompt window and type:
The output window will show an average value of the whole image. Use this value as the black zero-point reference. If you would rather not use the command prompt, you can select a blackened portion of the image using the mouse. To do this left click on the area, draw a box, right click and select "statistics." IRIS will show the average value in the boxed area. Use this value as the black zero-point.
Now we create the artifical bias frame. Use the command prompt and type
> fill value
Where "value" is the black zero-point from above. Now, save the master bias by typing:
> save master-bias
You are done.
3.2 Master Dark:
Now we need to create a master dark. Type in the generic name, the offset image (the master bias frame you just created), and the number of images in the dark sequence. Select either the mean or median method (they work equally well), and press OK. Then type the command:
> save master-dark
Or choose File/Save from the file dialog.
3.3 Master Flat:
The flat process is mandatory for eliminating the vignetting of the lens. Vignetting as high as 30~40% are common and could induce large differential magnitude errors depending location of the reference star and the target star.
Repeat the same process for darks except before saving type in the command:
in the command prompt to normalize the red and blue CFA-flat. Then save the file as "master-flat."
3.4 Improving dark frame subtraction:
This function automatically detects hot pixels above a threshold level. It might take some experimentation to determine the threshold value you need, but a good starting value is between 100 and 200 for 12-bit cameras and 500 for 14-bit cameras.
Open up the master dark frame (created in step 3.2) and type the following in the command prompt:
> find_hot cosme number
where "number" is the threshold value you have slected and "cosme" is the name of the file where the results are recorded (you could chose any you like). Check the output box (this should open automatically). If there is a number of hot pixels not detected, change the threshold accordingly.
3.5 Finish Preprocessing
Go to Digital Photo>Preprocessing. Input the sequence name of you data images (img) along with the offset map name (master-bias), dark map name (master-dark), flat-field map name (master-flat), and the cosmetic file name (cosme). Give an output sequence name (e.g. 'img-cal') and the nunber of files. DO NOT tick dark optimize (as it will take longer to finish).
Step 4 - Alignment of Images:
- Go to tab Digital photo>sequence CFA conversion and select the seq name that you gave at step 3e ('img-cal'). Give an output name ('img-cal-conv') and number of your star images and select Color output files type .
- Go to Processing tab>Stellar registration and put the sequence name of 4a ('img-cal-conv'), give output sequence name (e.g. 'img-reg') and number. Choose Global matching and Quadratic transformation
- Stacking of images: Processing tab>Add a sequence and give sequence name of 4b (img-reg) along with the number of images and method arithmetic
- Save the image!
Step 5 - Selecting green channel:
The recent version of IRIS (i.e. 5.57 or newer) include a quick and easy way to pull out the RGB channels:
From the Digital Photo menu, choose Sequence RGB Separation. Then just type in the names of the color channel files (i.e. final-r, final-g, final-b) and you're done. Click OK and you're done separating the channels!
Step 6 - Photometry:
IRIS is a little funny when it opens images. It places the (0,0) pixel in the top left corner of the screen instead of the bottom right. So open up the image from 5d and look around for a while. In our sample data, Capella (alf Aur) and eps Aur should be easy to spot. 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.
- 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). But in order to get an estimation we have to configure the Magnitude constant.
- Now, locate all of the comparison stars you wish to use (see the table of comparison stars). Then open the Analysis>Aperture photometry tab (or select Analysis>Magnitude Constant if you have already activated the photometry tool - 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.
- Repeat the photometry steps for each image you have taken recording the instrumental magnitudes as you go.
As long as you are using the same lens and DSLR, you don't have to re-create a master-flat and master-bias for every observation session! You can save them for re-use and then skip the steps 3a and 3c.
Now that you have obtained insturmental magnitudes, you need to compute calibrated magnitudes. The DSLR Documentation and Reduction team has setup an Excel Spreadsheet to help you finish your analysis.