Part 3H: Photometry and Afterglow Access


We will now use the “Photometry” tool on Afterglow Access by taking photometric measurements of the asteroid Kleopatra and a single standard star.

The final parts of Section 3 will examine and use the data collected to construct and analyze a light curve.

The 4:55 minute video below goes through the steps required to perform photometry on Afterglow Access. The same instructions are below the video.

**NOTE: Leaders/Teachers please see the SJS Resources for IDATA for help on Parts F-H (they were available in Part H).

***NOTE: Accessibility GAP: The photometry tool which marks sources on the image is not accessible at this time – specifically, sonification does not recognize the photometry circle.***

Open Afterglow Access if you do not have it open. Use either a new tab, or a new window, and arrange however is best for your vision.

Link to Afterglow Access


These are the instructions from the video:

Getting Ready

There are a few housekeeping items to do before we use the photometry tools.

1. First, you may want to clear out all the images in the File Library list on the leftmost panel.

  • To do this, you simply click the X icon with a circle around it at the top of your library (when you mouse over it it says “close all files”). A pop-up appears asking if you are sure you want to close all files. Click “Close All”. All images in your library will be deleted, but they can be found again when you click the “Open files” icon.

*Note: when you delete all files, the tools will also disappear (you must have a file open in order to see the image panel and any tools). If you want to delete each file individual, there are icons to the right of each file – an icon for “save”, an “X” for delete, or a box you can check to select the image. Realize the original, raw images are still on the skynet server even after you delete the file from your list.

2. Next, you need to open the files (images) you want. Go to the file folder icon, which is labeled “open files” if you mouse over it.  Open the “Sample” folder. This gives another list of folders.

a) Scroll down to the “Solar System” folder and open.

b) Open the folder labeled “Asteroid 216 Kleopatra”

c) There are 68 images. Simply click on an image to select it. Select the first image, the tenth, the twentieth, the thirtieth, the fortieth, fiftieth, sixtieth, and finally, the last image, number sixtyeight.

  • Open these using the “Open” button on the bottom right. These eight images will be enough for us to determine Kleopatra’s light curve.

d) Recall from previous work that you are returned to the Workbench and Display Settings when the files are opened.

3. Once the Display Settings panel opens on the Workbench, navigate to the “Photometry” tool in the list on the far right – the icon looks like a star.

  • Recall, you may use the keyboard shortcut, “/”, to enlarge the tool panel to the size of the screen, “.” to enlarge the image window, “,” to enlarge the file library and “Esc” to return to all three panels.

4. Center the open asteroid image in the middle panel by clicking on the “Zoom To Fit” icon on the bar below the image.

  • There are five icons on the lower right of the image, the “Zoom to Fit” icon is 4th from the left (or second from the right). It is also identified when it is moused over.

We are now ready to use some of the “Photometry” choices seen on the tool panel.

The first items seen at the top of the tool panel have to do with the mode. The one on the left is the Pixel Coordinate Mode, which uses the x-y coordinates of the image. The one on the right is the Sky Coordinate Mode, which uses the RA and Dec of the World Coordinate System (WCS). You will want to be in the Sky Coordinate Mode (this button will have a blue border on the top and bottom when it is chosen).

Under the modes dicussed in the previous paragraph, there is a sentence which explains how you can manually add sources by clicking on what you are interested in. (The click will leave a circle around the source you marked.) The sources can also be automatically added by clicking on “+Add Sources”. This button has settings that do not have to be changed for our purposes, but you can try it and see what happens. You will be able to delete the sources marked lower down on this tool page. For now, we will stick to the “Manual” mode.

On the same row as the “+Add Sources” button, there is a slider button labeled “Centroid clicks”. It should be on. This uses an advanced algorithm which selects the point where the values of all the surrounding pixels are balanced. There are cases where astronomers do not want to use this feature, hence the choice for turning it off or on.


1. Start by marking the asteroid with a mouse click in 2 or 3 of the images (Make sure any other sources you may have marked or auto marked for practice are deleted).

  • This step assumes you have already located the asteroid (you did this in Section 2.)
  • There is an Accessibility GAP here – Get help if needed.
  • Hint for finding the asteroid again: Look at images 10, 20, 30. You should recognize the moving asteroid, or remember where it is located from the sonification exercise we did previously.
Read through the next few paragraphs before doing step 2.

Next, there are several slider buttons. The first slider button reads “Include sources from other files”.

  • In your library, any other image (file) which contains the source (coordinates) you chose with your mouse click will also have that same source marked.
  • To see this, load in any other Kleopatra image from your library. The same source is marked even if the image orientation is different.

The next slider button in the row reads “Source Labels”. It simply numbers the sources (see the left column of the list below the slider buttons).

The most right slider button in this row is Auto-phot. When this is on it will list the RA and Dec of the source and the Magnitude and Magnitude error in the table shown below the slider buttons. These magnitudes are used by astronomers to get a ballpark figure when examining one image. It is not useful for asteroid photometry.

The “Show Apertures” slider puts larger circles around the source marked.

The table below the slider buttons contains several columns. Check boxes are to the left of the sources marked.

  • The first column is “Label”, which simply numbers each source.
  • The next column is labeled X|RA. This is the x position in the image given as right ascension. The next column is Y|DEC. Again, it is the y position in the image given as declination.
  • The other information is from the Auto-Phot being on (or off, if blank).

2. Click the box to the left of “Label” in the top row. This selects all the sources you have marked(at this point you should have marked only the asteroid in at least 2 or 3 images). Notice when sources are selected (or clicked on), the source has an orange circle instead of a blue circle around it.

3. Underneath the table is a row of 5 icons. These are the actions which can be performed on the sources in the table. When moused over, they are, from left to right, “Merge Selected Sources”, “Remove Selected Sources”, “Remove All Sources”, “Update Photometry”, and finally, “Export Data”.

4. Let’s use “Merge Selected Sources” first. This will show how the asteroid is moving across the images by using a new marker with a pointer on it. It also lets the computer know that the source marked is in a different position on each image. An algorithm takes this into account and photometers the proper position in each image.

  • With the boxes checked for the asteroid marked in at least two or three images, click the merge icon.
  • Take a look at one of the Kleopatra images you did not mark. The marker for the asteroid will be there showing you how it is moving.

We must now mark a STANDARD STAR – one that does not vary in all the images choosen.

5. To do this, click on any fairly bright source you see in any of the Kleopatra images. Don’t use the asteroid itself. This marked source should show up in all the Kleopatra images you have loaded into your library. (Again, an Accessibility GAP is present here. Get help, if needed, to mark the standard star)

We are almost ready to photometer our marked sources:

6. First, we again must select the sources we want to photometer. Click the box to the left of “Label” at the top of the table. This will select all the sources listed, specifically, the merged source (Kleopatra), and the standard star.

7. Next, we must select the files we want to photometer. This will be the eight images of Kleopatra. The drop down menu at the bottom of the tool panel is titled “Select Image(s) to Photometer”. You can check the 8 Kleopatra images OR just click the small square to the right of the menu, but only if the Kleopatra images are the only ones present in your library. It will select all the images in your file library list. The X to the right of the square removes all the images.

8. The blue icon with the light bulb will do “batch” photometry on all the images. Click that.

  • When the job is complete, a final button will appear on the bottom. This will say”Download Batch Photometry Data” and then list the number of sources and files. If you did everything right, you should have 2 sources and 8 files (images).

9. When you click this large button, it is downloaded as a csv file (comma separated values). The computer you are using will determine how you interact with ithee downloaded file.. We will do that in the next part.