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).
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 entirely written by Nick Long, an undergraduate at the University of Denver, whose contribution we greatly appreicate.
Performing Photometry with MaximDL
Upload all DSLR raw or Jpeg files into a directory on your computer. Put your flat, dark, and bias frames into this directory as well. This can be done by using your camera’s software or by setting the camera as a USB drive. Copy the files over to the desired directory. It is recommended to name that folder with the date that the photos were taken.
Before we begin further processing you must subtract dark, flat and bias frames from each individual image. Flat and bias frames may not be necessary but it is recommended to take dark frames. The amount of dark frames recommended is the square root of all the images to take.
To dark subtract all dark frames from each image first open calibration wizard under the process menu. This is the easiest method for dark subtraction. A calibration window will open that will ask you a series of question about the parameters of the dark subtraction. These include: If your camera regulates temperature, how the program will group images, and where your dark frames are located. Once you click finish you will see all images in that folder grouped. Remove any frame that is not a dark frame. Then click add group. Notice that under type you see whether it a dark, flat or bias frame. Once all dark frame have been added click ok.
Open every image that you wish for a dark subtraction to be performed on. Then go to process and press calibrate all. This automatically performs the subtraction and every image open. Your images should appear cleaner as a result of the subtraction.
Steps 5-10 are optional depending on whether you choose to stack images or not
After Maxim DL is successfully installed, open the program on your computer.
If numerous images were taken, it can often be easier to average stack subsets of those images to ease the amount of work done later in a program such as Microsoft Excel. For example, if 50 images were taken, you could stack 10 frames into 5 sets producing 5 final frames.
On the top of the screen, go to process and then select Stack from the drop down menu.
From the newly opened stack window, the Select tab should already be open. Click Add Files. Select the first subset of files that you want to stack by highlighting them. Next click on the first frame of the subset, hold ctrl + shift, and click the last frame of the subset. Next, click open to add the files to the stack process.
Click the Align tab and under Mode, make sure Auto - star matching is selected. Click compute and wait several seconds for the software to align the selected images.
Open the Combine tab, select Average under Combine Method, and select 16-bit Int under FITS Format. To stack the images, click Go. The time required to complete the stack will take several seconds. Once the stack is complete, the stacked image will appear in the background titled group1. Close the stack window and save this stacked image into the same directory as the original frames. Naming the file (date group ##-##), where ##-## is the range of camera image numbers selected in the stack.
Repeat steps 2-5 for the remaining subsets of images until all desired images are stacked.
Now each separate image or final stacked image is ready to be photometrically analyzed. Click File, select open in the drop down menu, and open up the directory in which the images are stored. Open every image that you wish to analyze. To do this, first select the first image then hold ctrl+shift and click on the last image highlighting every image. Click open to bring up each file.
Click Analyze from the top menu and select Photometry from the drop down menu. This function allows the selection of any image opened in Maxim DL and the extraction of instrumental magnitudes. Select the first image that you would like to analyze.
If you hover the cursor over an image, you will see three green rings surrounding your cursor. Before extracting data, you must adjust these rings to their maximum potential. Right click and a menu will appear which contains adjustment capabilities for each ring. Aperture refers to the inner circle that surrounds the star. Set the Aperture Radius so that you will be completely enclosing the full diameter of the stars that you will be analyzing. Gap width is the distance between the inner and outer ring. The annulus (outer ring) thickness should be set so that there are no stars on the ring. The annulus examines the star background and subtracts it from the star to achieve accurate photometry. Set it small enough so that no star appears in the rating, but large enough to give the background subtraction maximum information. Sometimes small stars can appear right next to your selected stars do your best to eliminate them from the parameters of your aperture and annulus. A small star may not make much of an impact on accuracy.
From the information window instrumental magnitudes (magnitudes without consideration of an offset) can be extracted from the Magnitude value. Hover the cursor over your selected stars and center the star in the inner circle. Record this value from each star and then select your next image and record the instrumental Magnitude for the same stars.
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.