Using SAO DS9 to Locate Known Asteroids


SAOImage DS9 is a free astronomical image application with a set of features that make it useful for "blinking" images to find asteroids.

DS9 does not have facilities for finding the magnitude of an unknown object nor can it accurately determine the position of an unknown object. Many software packages have such capabilities, including MPO Canopus. However, we have found that DS9 is relatively simple to use and the ability to see and blink the asteroid in one tool while performing measurements with another tool is a really efficient way to work. So we recommend using DS9 in conjunction with Canopus.

Video Tutorials

In addition to the written instructions, below, we have created a sequence of three video tutorials containing the same information. If you would like use the video tutorials, follow the following links in sequence.

Written Instructions

Step 1. Load a set of images into "SAOImage DS9".

This step assumes that you have already downloaded and installed DS9.

Procedure Sub-steps Expected Result
Invoke SAOImage DS9. The main DS9 window should appear.
From the "File" drop-down menu on the main DS9 window, select "Open". The "Open" dialog box appears.
Find the first image that you will consider and select it. Then depress the "Open" button. This is a standard Windows operation and I assume you know how to do it. The image should appear in the DS9 window.
From the "Frame" drop-down menu on the main DS9 window, select "New Frame". A blank image should appear as the DS9 image.
From the "File" drop-down menu on the main DS9 window, select "Open". The "Open" dialog box appears.
Select the next image that you will consider. Depress the "Open" button. The next image should appear in DS9.
Repeat the preceding three step until you load all the images you wish to consider. If you are working on LONEOS images, there are generally four images to load.

Step 2. Adjust DS9 to make asteroid discovery easier. In this step, we align the frames so that when the images blink, the stars remain in a fixed position.

Procedure Sub-steps Expected Result
Depress the "frame" button on the button bar just above the image. The next row of buttons change to provide "frame" options.
Depress the "next" button several times. Each time you depress the button, a new frame should appear in the viewer. If you have loaded LONEOS images, there are likely to be four frames, each displaying one of the images you just loaded. The images are likely not aligned so the stars will jump around.
Select the "Frame" drop-down menu at the top of the window. Move the pointer to "Match", then to "Frame", then to "WCS" following the sequence of cascading menus. Select "WCS". The frames should spontaneously align. Check the alignment by depressing the "next" button on the button bar. The images should obviously change but the stars should remain relatively motionless. If they move, the WCS information does not exist or is incorrect. The next step provides an alternate alignment technique when this technique does not work.

Step 2 Alternate. Align the image set manually. (NOTE: This will become an exceptionally tedious procedure if the WCS information is not in the FITS header. It would be most profitable for you to give me daily reminders to fix the images so the WCS information is correct.)

Procedure Sub-steps Expected Result
Find a star near the middle of the current image. Remeber the star because you will have to find it on each of the succeeding frames. Put the pointer in the middle of the selected star. Use the magnifier at the top of the window to help with the alignment. When the pointer is in place, depress the middle button (or wheel). The selected star should move to the middle of the image.
Depress the "next" button on the button bar. When the next frame appears, move the pointer to the center of the selected star just as in the previous step. Depress the middle button (or wheel). Repeat this step for all the remaining frames. The selected star should move to the center of the image in each frame.
Repeatedly depress the "next" button in the button bar. The frames should appear aligned.

Step 3. Alter the "stretch" so that faint asteroids are easy to find. The stretch is the way in which DS9 maps the 65536 levels of brightness in the image to the 256 level of brightness on the screen. We will alter the stretch so that we enhance objects near the background brightness.

Procedure Sub-steps Expected Result
From the "Scale" drop-down menu at the top of the DS9 window, select "Log". (Always use "Log".) The image "stretch" changes and stars may not be visible. Do not worry.
From the "Scale" drop-down menu at the top of the DS9 window, select "Zscale". (As you gain more experience, you should experiment with other options to find the scale that seems most helpful.) The image "stretch" changes again and will likely become worse.
From the "Frame" drop-down menu at the top of the DS9 window, mouse-over "Match". When the cascading sub-menu appears, select "Scale". All the frames change scale to "Log" and "Zscale". Nothing visible changes on the screen.
Move the pointer to the middle of the image. Depress and hold down the right mouse button. Then move the pointer to various locations on the screen. Attempt to find the "best" stretch for you. I find that the lower right portion of the screen is often near the best stretch. When you find a stretch that you like, release the right mouse button. Again, you are altering the stretch but you can see it happen in real time. When you release the mouse button, the stretch is no longer changes with mouse motion.
Repeat the preceding step for each of the images to be considered. Attempt to make the "look" of each frame similar. (Depress the "next" button on the button bar to go to the next frame.) All the frames should look nearly alike.

Step 4. Find an appropriate set of comparison stars.

LONEOS used no filters while obtaining images. Therefore, we have devised a "software filter" technique that allows us to compute good magnitudes for asteroids. Rather than explain all the theory, this step will just explain the sub-steps necessary to implement the software filter.

At the end of this step, you will have a printed copy of the image and all the acceptable comparison stars will be circled.

Procedure Sub-steps Expected Result
From DS9, select "Analysis" -> "Catalogs" -> "Optical" -> "Carlsberg Meridian 14" The "Catalog Tool" window will appear and will load data for many of the stars in the current frame.
The "Catalog Tool" does not initially list all the columns of the Carlsberg catalog. To get them all, check the "All Columns" checkbox and then depress the "Retrieve" button at the bottom of the tool. The "Catalog Tool" will reload the catalog for the current image and the catalog tool list will now contain all the columns in the catalog.
Depress the "Edit" button on the "Catalog Tool". A new window, "Edit", will appear.
Cut and paste the following string into the "Edit" window.

$Np>1 && $Jmag-$Ksmag<0.55 && $Jmag-$Ksmag>0.3 && ${e_r'mag}<0.05

Then depress the "Filter" button at the bottom of the page.
The "Catalog Tool" will select stars that have asteroid-like colors, that have at least two photometric measurements, and whose photometric accuracy is better than 0.05 magnitude. It will also put those stars in a list in the "Catalog Tool" and put circles around each one of the corresponding stars in the DS9 image window.
Print the picture. You can invoke printing by selecting "File" -> "Print" from the man SAOImage DS9 window.

NOTE: I have reconsidered this step. The printed picture has a scale that makes finding things very difficult. A better solution is to either re-do the preceeding steps and have DS9 handy when looking for catalog stars or save the image as a PNG or your favorate format and then use the image viewer to look at the selected stars. You can use a magnitifer on the viewer to see things better.
A printed copy of the selected stars helps us in following steps.
Close the "Catalog Tool" window. The "Catalog Tool" window disappears.

We have shown about half the steps below in second video tutorial. If you like video tutorials, please follow this link. Of course, there is another video which shows the last part of Step 5 and all of Step 6. You can find it here.

Step 5. Locate and mark all the currently known asteroids. Lowell has a web tool that will help you find all the currently known asteroids within a defined field.

Procedure Sub-steps Expected Result
Invoke a web browser and go to Your web browser should take you to the "Asteroid Plot" web site.
From the SAOImage DS9 tool, select the first image in the series. Then from the "File" drop-down menu, select "Display FITS header...". The FITS header of the first image should appear.
Note the time and the RA and DEC of the image. The corresponding keywords can change but if the image complies with the SBIG convention, the keywords will be DATE-OBS, OBJCTRA, and OBJCTDEC. Put the values for these variables in the corresponding fields of the web page. If you are using Lowell (LONEOS) images, put in the following values. Limiting V Mag: 19.5, Min Probability of Imaging: 0.3, FOV RA: 2700.0, FOV DEC: 2700.0. Take the defaults for the Plot Options. Then depress the "Build Plot" button.

Caution: The Asteroid Plot web site may be finicky about the format of date, time, RA, and Dec. If you have problems, go to the astplot help page. In general, time, RA, and Dec are in sexagesimal with a space as the delimiter. The date is in YYYY MM DD format with space as the delimiter. My personal demon is depressing the comma key rather than the period when trying to enter a decimal point. The page fails with an unhelpful cryptic message and the comma is so similar to a period that I cannot see the error.
The web site will process your request for about a minute. It will then produce a plot of the region and identify the location of each known asteroid. It will also produce a list of the asteroids and their coordinates at the bottom of the window (often, just out of sight). Note that you can get a PostScript version of this graph which is much easier to use (provided you have a PostScript viewer.)
In the previous step, we asked the web site to identify all the asteroids in a region 2700 by 2700 arcsec (45 by 45 arcmin). The image is likely to be 2700 by 10800 arcsec (45 by 180 arcmin). We could ask for the whole region at once but the server generally times out before it can process the whole asteroid data set. To get around this problem, make five total requests and change the declination by ±45 arcmin and ±90 arcmin from the FITS header value. Be sure to record all the returned asteroid positions. You should have a list of all the asteroids that were captured on the image. Make sure that each of the asteroids has coordinates that actually fit on the image. That is, two of the areas you specified are slightly larger than required so they may include a few asteroids just off the ends of the image.
For each of the asteroids in the list you just created, align (WCS) the images (in DS9) in the area where an asteroid should be. (You will have to use the panner to find the area or you will have to "scroll" to the proper area using the middle mouse button to move the image to a new center point.) Assuming the button bar is still displaying "frame" options, depress the "Blink" button. The frames should display in sequence with a frame change about every 0.5 seconds.
When you spot the asteroid, depress the "Single" button on the button bar. Then move the pointer over the asteroid and left click. A green circle should appear around the asteroid. DS9 refers to these circles as "regions".
From the button bar, depress the "next" or "previous" button and identify the asteroid in its new position. Move the pointer over the asteroid and left click to create another region. A green circle should appear around the asteroid.
Repeat the previous operation until all four LONEOS images have the asteroid marked. When blinking, the green regions should follow the asteroid track.
Repeat the previous several steps until all the known asteroids are marked in all the frames. When blinking, you should be able to see all the known asteroids moving with their green circles. When an image contains lots of asteroids, this can really be fun to watch.

Step 6. Blink the image set looking for new asteroid.

Procedure Sub-steps Expected Result
You should mark New asteroids with a region of a different color. To change the circle color for DS9, from the "Region" drop-down menu, mouse-over "Color" and select "Red". Any circle you now create will be red. Try it and see. If you click the circle again, it will grow handles. Depress the "Delete" key and it will go away.
Select a subsection of the entire image by using the "Zoom" drop-down menu. This menu is fairly intuitive so I will leave it to you to figure out. Zoom until you get an area about 512 (or more) pixels in width and height. (My monitor will not make 512 pixels in height at a magnification I like so I use about 350 pixels height.) You can drag the edge of DS9 to make the image wider or taller. Your goal is to use most of your screen to observe this sub-region of the image. Then "grab" the little green "window" in the "panner" panel at the top of the window. You grab it by putting the pointer in the green rectangle, depress and hold the left mouse button, and move the pointer to another position in the panner window. Then release the button. The goal here is to focus on a smaller section of the frame so that you can study it more carefully. You will eventually cover the entire frame so select and move the green rectangle in an order that you can remember. The main window will show a portion of the images that corresponds to the green rectangle in the panner window.
Select "Frame" -> "Match" -> "frame" -> "WCS". All the frames in the set will now be at the same zoom and centered on the same location.
Depress the "Blink" button and carefully search the blinking image for asteroids. If you find a new one, stop the blinking as before and mark the asteroid with a red circular region.

My technique is to engage in a relaxed stare at a specific small region of the screen and let DS9 blink through all four images. Then I move to an adjacent region and repeat until I cover the whole visible image. It is important to let all four images blink because an asteroid may enter your personal field of view on any of the four images.
I could give you move advice for this operation but the best thing to do is to try it and then ask specific questions. Asteroids will move in very straight lines and will look very much like stars. Defects will move with a slight wobble and are often asymmetric. Very faint asteroids will not necessarily look like stars and may, in some instances, completely disappear.

If you are unsure, seek advice.
Repeat the preceding three steps, each time moveing to a different subsection of the images, until you have blinked the entire image. Repeating these operations might take a long time. If you work on 512 by 512 pixel subsections, there will be 16 subsections. Have another cup of coffee and get back to work.