A quick note to announce new archival search images on the site. We’re gotten through about two thirds of the processed Suprime-Cam images from  9,850 exposures (or 9,850 exposures  x 10 chips on Suprime-cam = 98,500 FITs files!) from the date between 2007 Jan to 2013 Feb.  These are observations where the exposure time is less than 60 seconds. With more data, there’s new asteroids to search, giving new opportunities to find the elusive main belt comet. If you don’t find a main belt comet, your classifications will help us put limits on the frequency of these types of outbursts in the asteroid’s main belt, which is still important. Dive in today at http://www.comethunters.org

We’ve recently  uploaded brand new images on both the HSC workflow and the Archival Workflow.

The Archival images are still from Suprime-Cam and  continuing to move backward in time. These observations will help us understand the true frequency of main-belt comets. Any good candidates we’ll follow-up when the asteroids return to the same spot in their orbit as when the observation was taken to see if the activity repeats.

We’ve now uploaded June HSC observations on to the HSC workflow. These images are  hot off the telescope, giving us a chance to follow-up these asteroids now if they are active.

Both sets of  images are of brand new asteroids never viewed before on the site with  new chances to discover a  main-belt comet. You might be the first person to know a new comet is lurking in our Solar System’s asteroid belt. Search for main-belt comets today at http://www.comethunters.org!

By now, hopefully many of you have had the chance to try out the newest iteration of our Comet Hunters main-belt comet search that features data from the Hyper-SuprimeCam (HSC) Subaru Strategic Program, or the HSC Survey for short.  This survey is a ~300-night, 5-year observing program on the 8-meter Subaru telescope (which is the telescope used to obtain all of the data you’ve been previously classifying for the original Comet Hunters archival data search, though using a different camera) that aims to address a wide variety of scientific areas from cosmology to galaxy evolution to searches for distant objects in our own solar system.  While HSC survey observations are not specifically optimized for searching for main-belt comets (see below), with a few tweaks to our original classification interface, we still hope to use the data to search for previously unknown comets, with the added advantage of the data being relatively new compared to the data being used in our archival search, and also set the stage for an even bigger future expansion of Comet Hunters.

The main difference between HSC survey observations and the archival observations that we have asked you to review until now is that each HSC survey image has an exposure time of several minutes or more.  By comparison, for our archival search, we have tried to only use images with exposure times of less than 1 minute each.  Exposure time is how long the camera’s shutter remains open collecting light for a particular image.  For regular hand-held cameras that you might be used to, longer exposure times can be used to take photos in low-light conditions.  For astronomy, longer exposure times are used to study very faint objects.  The usual analogy used by astronomers is that of a bucket collecting rain drops.  The longer you leave the bucket out, the more rain drops it will collect (or for astronomy, the more photons from an object in the sky it will detect).  For a very faint and/or distant object for which only a very small number of photons reach us on Earth, we want to leave the camera shutter/bucket open for a longer time to collect more photons/rain drops.  As a side note, the size of a telescope’s primary mirror corresponds to the diameter of our hypothetical bucket, so by using a large telescope like Subaru and using longer exposure times, we can study very faint objects indeed!

The issue for us though is that long exposures work great for studying objects that don’t move in the sky (other than the steady, predictable motion caused by the Earth’s rotation), but not so much for nearby things in our solar system, like asteroids, which typically appear to move in the sky relative to background stars.  You can think of asteroids as fence posts along a road while you’re driving, while more distant objects like stars and galaxies are mountains far in the distance.  From your perspective, the nearby fence posts appear to move (whizzing past you as you drive) while the distant mountains appear to be essentially stationary.  Using the short exposure times for our archival search, the total apparent motion of asteroids compared to background stars was relatively minimal.  However, with the longer exposure times used by the HSC survey, the motion of the asteroids we are trying to study causes their images to be noticeably elongated (e.g., somewhat sausage-shaped), just as a photo of a fence post (or other nearby objects) might appear smeared out if you tried to take a picture while driving past.

What this means for Comet Hunters is that identifying comets just became a bit harder.  For the archival search, we asked you to compare the appearance of each asteroid with reference stars chosen from the same image and make a note of any differences you might see.  With the HSC survey data, every asteroid image will look different from the background stars because they will be elongated.  Furthermore, any cometary activity that might be present will also be “smeared out”, possibly making it harder to see.  Nonetheless, given the large size of the Subaru telescope, we expect to find some comets with activity bright enough to detect even when smeared out.  Then the trick is just to try to train your eyes to spot what elongated comets might look like, since they will not really look like most people normally think of when they think of what a comet looks like.  To help you out, we have generated some images of what elongated comets could look like and included them in the HSC Survey comet search tutorial (if you need a refresher, click on the “Show the project tutorial button” at the bottom of each classification page and go to the third panel).

The other major change that experienced Comet Hunters will notice is that instead of asking you to compare the appearance of a single asteroid to two comparison stars, we now ask you to compare the appearances of a single asteroid that has been imaged (at least) twice in the same night.  In part, this is due to the fact mentioned above that comparisons to “stationary” background stars are less useful when most of the asteroids we will show you appear elongated.  The other big reason we’ve made this change is to see if this helps to distinguish overlaps (or blends) from real comets.  This subject has been discussed in previous blog posts here and here, so we will not discuss them much here, except to say that background objects (mostly stars and galaxies) that might mimic cometary activity typically will not appear to move in the same way as our target asteroids.  So, by only focusing on asteroids that consistently show activity from one image to the next, we hope that you will be able to immediately identify cases where “activity” is only present in one image of an asteroid, rather than having to do a time-consuming manual check (as described here).

While it may initially take some getting used to, we hope that you will get the hang of searching for comets in HSC data before too long.  As mentioned in previous blog posts, the exciting thing here is that this data is much more recent than the archival data we have previously been having you review, meaning that if we discover any comet candidates, we may be able to trigger immediate follow-up observations to confirm the activity and study it further.  Another exciting aspect of this sub-project is that there is a lot more archival data available than we are currently using for our original SuprimeCam archival search.  As mentioned above, for this initial search, we intentionally restricted ourselves to short-exposure images, but if our attempt to use elongated asteroid images to search for comets is successful, we will be able to draw on the even larger pool of long-exposure SuprimeCam data, and then even include long-exposure data from other medium and large telescopes.  As I also mentioned above, astronomers like to use long exposures in tandem with large telescopes to allow them to study very faint objects, so being able to take advantage of such archival observations to do comet searches will greatly increase the amount of data available to us.  Since we expect that main-belt comets will be relatively rare compared to inactive asteroids, screening a large number of asteroids is key to finding the rare active ones, so the more data we can use, the better!

If you have any other questions about our new search, please comment below.  Otherwise, we thank you in advance for helping out with the new search, and wish you happy hunting!

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:

https://www.zooniverse.org/projects/mschwamb/comet-hunters/talk/subjects/1295819

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.