Image Reduction
Adapted from Kalée Tock and Ryan Caputo
In this phase of your double star research, you'll learn how to process astronomical images to extract precise measurements. The process of preparing raw telescope data for scientific use is known as "image reduction."
Reduce One Image: Practice Exercise
Before working with all your images, let's practice with a single image to ensure you understand the measurement process.
Here is an image of STF1050AB that you will use to practice your measurement techniques for this assignment if your own images have not yet been returned (if they have been returned, please use one of the reduced images from your LCO request). Follow the AstroImageJ instructions linked below.
[IMAGE PLACEHOLDER: STF1050AB double star system]
Question 1: Insert a screenshot of your measurement in AstroImageJ below. Make sure to zoom in on your stars so that they are large in your screenshot.
Question 2: What aperture radius did you use to measure these stars? How did you pick it?
Question 3: What is the last reported measurement of these stars according to Stelledoppie? What is your measurement? Do you think your measurement is reasonable? Why or why not?
(Once your results from this practice exercise are verified, you can proceed to reducing your actual double star images as described below.)
Processing Your Double Star Images
For this assignment, you need to extract the Position Angle (PA) and Separation (Sep) of your stars from each of your images.
Step 1: Download and Check Your Images
First, download your images (if you are downloading directly from LCO, make sure that you get the reduced images rather than the raw images) and make sure that they are not saturated. (If you get your images directly from LCO, make sure to only get the e91 images, not the e0.)
Understanding Saturation
To be saturated means that some of the pixels were struck by so many photons that the camera could not count them accurately. This is not as big of a problem for astrometry (measuring star positions) as it is for photometry (measuring brightness, as we do for exoplanets), but it might slightly affect the astrometric measurements that we are making here as well. The reason is that the position of each star is measured as the corresponding star centroid, which is the weighted average position of the pixel brightnesses. If most of the star's pixels have the same brightness, then the weighted average position might be slightly different from the position of peak brightness in a non-saturated image.
This is not a huge effect. Most of the time, despite a small amount of saturation, the measurements will be okay. However, for purposes of reporting the measurements in a publication, it is best practice to confirm that the images used to make the measurement are not saturated. Therefore, you need to check.
Exposure Time Considerations
Exposure times are a tricky thing. The best exposure time changes from night to night based on the seeing, the presence of clouds/wind/atmospheric effects, the position of the Moon, the color of the stars, the optics of the telescope, the properties of the camera CCD (the LCO cameras saturate if there are more than about 50,000 ADU counts per pixel) and all sorts of other factors.
If we plot a 3-D histogram of photon counts as a function of x and y pixel coordinates on the CCD, we can see that the image on the left below is not saturated, while the image on the right is saturated. As you can tell you do not want to cut down the exposure time too much, because doing so might entirely lose the much-dimmer secondary star!
[FIGURE: Two 3D histogram plots showing unsaturated vs. saturated star images]
Step 2: Set Up Your Data Spreadsheet
Use File → Make a Copy to generate an editable copy of this spreadsheet. Title your spreadsheet with your name, and set the permissions so that it is "viewable by all".
Step 3: Extract Measurements Using AstroImageJ
AstroImageJ (AIJ) is a common tool to use to extract data from double star images. Instructions for double star image reduction in AstroImageJ are here.
Alternatively, you can do the same thing in Afterglow.
Be sure to clearly label which images you are reducing.
Alternative Software Options
If none of those options work, you can accomplish this task using other software, though you will have to chart your own path figuring out how to make it work. There are a bajillion free astronomical image processing packages out there that will show you the RA and Dec of a target star's centroid; you'll have to find one that works on your particular operating system. It might be harder and may involve trig to convert the pixel coordinates of an image into RA and Dec. However, as soon as you have the centroid coordinates of your primary and secondary stars from an image, you can use this Python module from an earlier assignment to calculate the PA and sep of your system.
Step 4: Document Your Results
Your deliverable for this assignment includes a link to a viewable copy of this spreadsheet, filled in with your data, images, and links. If you did not use AIJ for image reduction, you need to explain what you used. For any images with strange features (e.g. egg-shaped stars, trailed stars, satellites, more fuzz than usual), please document this with an entry in the "Notes" column (column E), and include a close-up screenshot of your stars in AIJ inserted into the spreadsheet. Be sure to exclude compromised images from your average and standard error calculation.
Additional Tips: Checking Image Metadata
Some additional background on finding the exposure time of an image: If you need to check the exposure time of the stars in a particular image, you can check the .fits header. In AIJ, that looks like this:
[FIGURE: Screenshot of how to access FITS header in AstroImageJ]
Clicking that button will result in this:
[FIGURE: Screenshot of FITS header display]
The number in the "Exposure" field is in units of seconds.
Next Steps
After successfully reducing your images and extracting measurements, you'll move on to analyzing these measurements and comparing them with historical data to understand the movement of your double star system over time.