Advanced comet hunting (Part 1)
So, for those of you who have been itching to do deeper analysis of Comet Hunters images, this blog post is for you! With the information provided here, you will be able to do many additional types of analyses if you are so inclined, but the two tasks we will focus on here are (1) checking for cases of overlapping objects, and (2) checking for consistency in an object’s appearance.
Subject information: The key to being able to perform follow-up analyses of Comet Hunters images is this table (*UPDATE* see link to updated table below) that lists the corresponding meta data for all asteroid subjects currently being shown on Comet Hunters (excluding images of known comets). The format of the file looks something like this:
SubjID ObjNum ObjName Date Time ExpT ObjRA ObjDec Filename ObsTarget 1288778 172 Baucis 2007-06-15 12:53:37 60.0 0.35268 0.08548 SUPA00547638.fits F04 1288832 172 Baucis 2007-06-15 13:01:48 60.0 0.35446 0.08665 SUPA00547678.fits F04 1288891 172 Baucis 2007-06-15 13:09:56 60.0 0.35623 0.08782 SUPA00547718.fits F04
The listed columns are subject ID (SubjID), asteroid number (ObjNum), asteroid name (ObjName), date of the original observation (Date), time of the original observation (Time), exposure time (ExpT; in seconds), Right Ascension (ObjRA; in decimal degree format), Declination (ObjDec; in decimal degree format), name of the original data file (Filename), and the original observation target (ObsTarget).
We should note that there are some duplicate images in this list where the same asteroid appears as two different subjects, one generated for the beta test (which used red crosshairs for the asteroid) and another for the official launch of the project (which uses purple crosshairs), so just be aware of this.
Overlapping objects: So, as explained in this previous blog post, determining whether something that looks like a comet tail is actually just a background star or galaxy behind our target asteroid involves looking for images of the same part of the sky, and even taken on the same night if possible, and checking to see if the activity “moves” along with the asteroid (in which case, it may be a real tail), or if it instead appears to remain stationary in the sky (in which case, it is a background object). The science team refers back to the original data files corresponding to each subject to make this determination, but it’s actually possible to do the same type of check simply using the Comet Hunters website.
This technique was actually demonstrated in our previous blog post about detecting overlaps, except that now with the individual subject data we provide you, you can search for other images of an asteroid of interest on your own. So to follow the previous example, if you found that Subject 1295819 looked like a possible comet:
You could then refer to the data table to search for the subject number “1295819” and find out that it refers to an image taken of asteroid (27473) 2000 GV78 on 2008-11-29 at 5:45:06.727 (Universal Time). The file is currently sorted by asteroid number, so you will then note that there are several other images of asteroid (27473) also taken on 2008-11-29 that have been turned into Comet Hunters subjects, namely Subjects 1295829, 1295842, 1295854, 1295866, and 1295876. To retrieve these images, simply replace the subject number at the end of the following URL:
and then by looking at the images side by side, you may be able to see the asteroid moving across the background object, revealing that the original suspected cometary activity was a case of the asteroid overlapping a background star or galaxy.
Checking for overlaps will not always be easy though. Consecutive images of other asteroids may not have been obtained close in time, so background stars may change significantly from image to image. We use software to automatically set the contrast levels of the images we display and so images of the same asteroid may sometimes be set to different levels, making the brightness of both the asteroid and background stars/galaxies appear different in different images. Image quality may also change from one image to the next, or some images may have tracking problems but not others. We are also still fixing the problem that we’ve found where objects were improperly centered by our software, so asteroids may be centered in some images but not others. In many cases though, this simple analysis should allow you to identify subjects that are clear overlap cases, which will help the science team as we work to weed those out in order to get to genuine comet candidates.
Checking for consistent appearances: Besides checking for overlaps, looking at other images of the same asteroid taken on the same night allows us to check to see if suspected cometary activity, especially faint activity, remains consistent from one image to the next. If an object shows activity in one image but not others, it is probably just caused by noise and is not what we would consider a good candidate for further follow-up.
For example, Subject 1295853 looks like it may have a faint tail at the 11 o’clock position:
However, looking at Subjects 1295818, 1295839, and 1295865, which are also images of asteroid (336184) 2008 RN107 from 2008-11-29, the same feature does not appear in any of the other images.
In this case, we would then conclude that the original suspected cometary feature is not real.
Other Analyses: In (a) future blog post(s), we will describe other types of analyses you can do with the data table we now provide, including using the raw FITS files, searching for other instances of observations of an asteroid of interest from different telescopes, retrieving and interpreting information about the orbit and viewing geometry of an asteroid of interest at the time of observations, and more.
*Update (2016-06-02): Here is the updated data table that includes new data uploaded to the Comet Hunters project since this blog post was first posted. Happy hunting!
*Update (2017-05-16): Due to a change in Dropbox’s file settings, the previously posted link to the data table no longer works. Go here instead for the updated data table posted on 2016-06-02.
Meet the Team: David Shonfield
Today we have the next post in our Meet the Comet Hunters Team series. This time we’re featuring David Shonfield (@Avanti), one of our Talk moderators.
Where are you originally from/where did you grow up?
What drew you to Comet Hunters?
Probably the Rosetta mission. Seeing a comet from close up was a revelation. Followed by the New Horizons mission, which has provided so many surprises and shown how supposedly dead worlds can be alive.
What is your role as a Comet Hunters Talk moderator?
I’m yet to find out! But I’m hoping to help build a group that works well collectively and makes an intelligent contribution to identifying these elusive objects.
Name one hobby of yours?
What is the most recent tv show you have watched?
Wolf Hall (I don’t watch much TV!)
What is your favorite movie?
Equal favourites: Trading Places and Solaris (the Tarkovsky one rather than the remake).
What is the latest book you have read?
Fighters in the Shadows, A new history of the French Resistance, by Robert Gildea
Who is your favorite singer/band/musical artist?
What are five of the top ten most played songs on your iTunes/spotify/etc playlist?
It don’t mean a thing if it ain’t got that swing/ Ella Fitzgerald
Haja o que houver/ Madredeus
Senza giacca e cravatta/Nino D’Angelo
Because the night/Patti Smith
What’s one thing most people don’t know about you?
I have a weakness for cartoons, in particular Pinky and the Brain: Pinky is my alter ego.
Favorite cocktail or beverage?
More about Suprime-Cam
Today, I thought I’d give you a different view of the camera and images that you currently see on Comet Hunters. Right now we’re showing archival images of asteroids from the Subaru Telescope with Suprime-Cam.
Suprime-Cam can image a 0.25 of a square degree patch of sky in a single observation, that’s a bit bigger than the size of the full moon as viewed from Earth. Until a few years ago with the first light of its successor instrument, Hyper-Suprime-Cam, Suprime-Cam reigned as the largest field-of-view camera on the 8-10-m class telescopes,currently the largest ground-based telescopes. Suprime-Cam is an 80-mega pixel camera weighing in at 650 pounds and located at the prime focus of the Subaru Telescope. You can learn about the camera first light and commissioning here and about the upgrade of the camera is 2008 here.
Suprime-Cam is equipped with 10 CCDs (charged coupled devices) that actually receive the photons and and are read out to produce the images we show on Comet Hunters. Subaru has two rows of 5 CCDs. You can rotate the direction of the camera as well. If you display all 10 CCDs from a single observation it looks something like this if you have the widest part of the camera along the East-West direction (for Right Ascension). If the full moon was imaged it would fill most of the imaging plane:
You can see the different CCDs and amplifiers that read out the electrons trapped in the CCD’s wells are slightly different from each other as well as the effects of how light travels through Subaru’s optics. We can take all of that out with calibration observations (flat field observations, dark and bias images). This image is pretty much a raw image off the telescope. The white lines in a grid are the small gaps/boundaries between each of the different CCDs. Let’s zoom in a bit further between the two center CCDs so you can get a better view:
Now (in the above image) you can start to see how many stars are on each of those CCDs. And if we keep zooming in…
We show a much more zoomed in image on Comet Hunters focused very close around the asteroid and reference stars.
The width and height is 20 arcseconds (100 pixels) for the bigger image on the left (the asteroid image) . On the right, two little reference star subimages are ~10 arcseconds (50 pixels) for the size. For a sense of scale each of the 10 Subaru CCDs are about 2048 x 4096 pixels