There are a number of commercial and freeware packages suitable for opening DSLR RAW files and performing aperture photometry, including MaxIM DL, AIP4WIN and IRIS. I use AIP4WIN because it came bundled with my copy of The Handbook of Astronomical Image Processing, Berry and Burnell (2009). I have had very limited experience with IRIS and none at all with other options. Therefore much of the discussion that follows is necessarily specific to AIP4WIN.
Instructions for other software packages would be welcomed. I encourage you to experiment to find your preferred software and procedure, then write it up to share with others via these web pages.
In the Bayer filter array used in DSLR cameras half the pixels record green light and one quarter of the pixels record red or blue light. RAW files contain image information from the R, G and B channels. The first task is to extract a monochrome image from each of the channels.
Software that interprets and displays these images has to estimate how much red, green and blue light would have fallen on each pixel in the image. It does this by looking at, for instance, the surrounding green pixels and interpolating how much green light should have fall on the red and blue pixels.
For normal terrestrial or astrophotography the accuracy of interpolation is not critical but for photometry it definitely is. Therefore it is essential that the software used for extracting red, green and blue channel images for photometry uses robust interpolation algorithms so that accurate raw instrumental magnitudes can be determined. Any of the specialist astronomy programs that handle DSLR raw images will carry out this process adequately.
1. RAW image conversion
In AIP4WIN this is done by selecting DSLR Conversion Settings Preferences:
Select BILIN for the De-Bayerization Algorithm.
Select DeBayer, Convert Color to Grayscale with parameters shown below for the green channel image:
Click on the Save button then click on the Done button.
The next step is to select Setup under the Calibrate menu:
Select Standard under Calibration Protocol:
Click on Select Dark Frame(s) button and navigate to the directory with your previously recorded dark frames and select them. Select the Median Combine option then click on the Process Dark Frame(s) button. When the process is finished you can save the result as a Master Dark which makes this process faster next time.
Now click on the tab labled Flat, click on Select Flat Frame(s) button and navigate to the directory with your previously recorded flat frames and select them. Select the Median Combine option then click on the Subtract Flat-Dark check box, click on the Select Flat-Darks button and navigate to the directory with your previously recorded flat-dark frames and select them. Select the Median Combine option then click on the Process Flat Frame(s) button. When the process is finished you can save the result as a Master Flat which makes this process faster next time.
Ensure the Subtract Dark Frame and Apply Flatfield Correction check boxes at the bottom are both selected. Close the Calibration Setup window.
AIP4WIN version 2.4.0 includes a very useful tool for automating the process of measuring a series of images to get a variety of statistics and produce reports in several formats. It is called the Magnitude Measurement Tool (MMT) and is selected under the Measure menu:
I use MMT to extract StarADU, maximum ADU value, Julian date and airmass information for each target and comparison star from each of my images. These are imported into Excel spreadsheets for reduction to final transformed magnitudes. I’ll now outline my procedure for using MMT. The spreadsheets will be described later.
Information entered in this tab are used in the reports and to calculate Julian date and airmass. If appropriate make sure you change the Time Zone setting to account for Daylight Savings.
Information entered in this tab define the imaging system used. I haven’t actually measured my camera’s gain, read out noise and dark current yet so these figures above are not necessarily correct.
Click on Select Disk Files then navigate to, and select, your images. Click on one file in the list window, usually the first, to open it for selection of target and comparison stars later.
This is where you select the size of the aperture and annulus for measuring star+sky ADU and sky ADU values. The aperture should be large enough to include virtually all light from the star as shown above. The annulus should not be too large or background stars may be included.
Zero point is not used when calculating standardised magnitudes and I generally leave this setting at zero. However, you may choose a Zero Point value that gives realistic magnitudes when you click on a star in the image.
This is where information about the target and comparison stars is entered. First check the Enable Star Editing box. You must be careful that RA and DEC are entered correctly so that airmass is accurately calculated. You must press the ENTER key after editing any section, otherwise an warning message will be displayed.
Once data has been entered for all target and comparison stars for a particular field click on the “Save as...” button to save a file that can be recalled next time you analyse images of this field. Uncheck the Enable Star Editing box.
Now click on the image of the Target = Variable star, three rings will be drawn around the image with the letter “V”. Now click on each of the comparison stars in the correct order.
In the example below the star labled “V” is S Cru. Eight other stars have been selected. The first six are comparison stars. Stars C7 and C8 are BL Cru and BG Cru, respectively. The Excel spreadsheet that performs standardised magnitude calculations and transformation to the standard magnitude system can handle up to six variable stars and six comparison stars. The use of this spreadsheet and others will be described later.
A number of report formats are available but I use the Raw Star Measurements report to extract the data I need for input to my Excel spreadsheets.
I have found the settings shown above work well for measuring my images. You might want to experiment to see what works best for you.
When you change any field you have to press the ENTER key on your keyboard.
Now click on a another star in the image to be the guide star. It should be well separated from other stars and reasonably bright, but not saturated.
The final step is to click on the Run Photometry button to start the measurement process which takes a couple of minutes for 10 images on my slow computer. The data is written to the AIP DataLog window and needs to be saved as a text file for importing later into the spreadsheet. Use a file name that lets you clearly identify the date of observations, target stars and which colour channel was analysed, e.g. “110603 BG BL S Cru green.txt”.
Now repeat the process for the other two colour channels. Remember to change the “DeBayer, Convert Color to Grayscale” Scale parameters appropriately for the colour channel (Step 1 above). You will end up with three text file, e.g.:
“110603 BG BL S Cru green.txt”
“110603 BG BL S Cru blue.txt”
“110603 BG BL S Cru red.txt”
Now that we have the data for the three colour channels we have finished with AIP4WIN and move on to the Excel spreadsheets for data reduction. These spreadsheets are described in the following document.