Opportunity for a Summer Student to Work on Comet Hunters Data


Image credit: ASIAA

Many of the Comet Hunters science team are based at the Institute of Astronomy & Astrophysics at Academia Sinica (ASIAA)  in Taiwan. As part of the 2016 ASIAA Summer Student Program, we’re looking for an undergraduate or masters student to come to Taipei for July and August to help work on Comet Hunters. Over the summer, the student will help develop a suite of tools to help quickly vet and validate candidate Main belt comet  discoveries identified by Comet Hunters.

ASIAA operates in English, and all research will be conducted in English.  The description of the project can be found here. You can find the requirements,rules, and restriction of the program here.

Apply by March 25th. If you have questions or if you would  like to know more, you can contact me via email at  mschwamb AT asiaa.sinica.edu.tw or post in the comments below.

Suggested Comet Hunters Talk Hashtags

After classifying the asteroid image in the main interface, you’re presented with an option to discuss the image you’ve seen in Comet Hunters Talk, if you hit the ‘Talk’ button.

Screen Shot 2016-02-28 at 3.33.10 PM

Comet Hunters Talk is a place where you can discuss the images with other volunteers and with the science team. You can also label then image you’ve classified with descriptive hashtags like #tail.

Thanks to our Talk moderators, we now have a list of preferred hashtags (see below) we’d like suggest you use on Talk to help flag images in ways above and beyond what we can learn from the classification interface and the questions we ask you there.

We aim to also do a search using these preferred hashtags later on in the year to search for comet candidates and identify false positives.

#tail – see a very clear and definite tail. Currently many people use this for any sign of a tail, but we’d like you to use this for anything you’re very sure of it. If you think there’s any chance it might a faint background star or galaxy then use the #possible tag. (example)

#possible – maybe has a tail, but not sure if it’s real or if it could be an overlap with a found background star or galaxy (example)

#overlap – asteroid is overlapping another star or galaxy (example)

#elongated – asteroid appears to be elongated/elliptical compared to the reference stars (example)

#offcentercandidate – you see a tail but it’s on a source not in the center of the crosshairs

#nearbyobject – where there is an asteroid visible in the center of the crosshairs,  but there is a  nearby  detached faint object that’s not a tail (example)

#badrefstars – one or both of the reference stars are bad (looks like a galaxy or isn’t visible) or it’s hard to see one or both of the reference stars (example, example)

#poorimage – the asteroid image is of bad image (example)

These are suggestions. Talk enables flexible labeling, so if you don’t find any hashtags from the list above that matches what you see, definitely create a new one!


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.

overlap_subject1295819 overlap_subject1295842 overlap_subject1295854 overlap_subject1295866

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.

consistent_subject1295853  consistent_subject1295865consistent_subject1295818  consistent_subject1295839

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.


Image credit: David Shonfield


David Shonfield

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?

Miles Davis

What are five of the top ten most played songs on your iTunes/spotify/etc playlist?

Caruso/Lucio Dalla

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?

Negroni Sbagliato

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:

SUPA0051299X.zoomfitYou 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:

SUPA0051299X.zoom0.5Now (in the above image) you can start to see how many stars are on each of those CCDs. And if we keep zooming in…

SUPA0051299X.zoom1We 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


Meet the Team: Henry Hsieh

Today we have the next post in our Meet the Comet Hunters Team series. This time we’re focusing on principal investigator (PI) of Comet Hunters, Henry Hsieh.



Name:  Henry Hsieh

What is your current position and where/institution?

Research Scientist with the Planetary Science Institute, living in Honolulu, Hawaii

Where are you originally from/where did you grow up?

New Jersey, USA

What is your role in Comet Hunters?


Beyond Comet Hunters, what else do you work on?

Besides Comet Hunters, I also work on other main-belt comet and disrupted asteroid research including targeted observational analysis to understand their physical properties, dynamical analyses to understand their orbital evolution, and exploring different ways to discover more.

In 3 lines explain your PhD thesis?

I did the first in-depth observational analysis of the first discovered main-belt comet, 133P/Elst-Pizarro (although the term “main-belt comet” did not yet exist at the time).  I then performed a targeted observational search for more “Elst-Pizarros”, the success of which led to the recognition of main-belt comets as a new class of comets.

Why are you interested in main-belt comets?

Besides being the topic of my PhD dissertation, main-belt comet research is extremely new and so has many opportunities to make new discoveries.  It also has very interesting implications for understanding the formation of our solar system and maybe even the origin of water, and therefore life, on Earth itself.

Name one hobby of yours?

Free diving

What is the most recent tv show you have watched?

Mr. Robot

What is your favorite movie?


What’s one thing most people don’t know about you?

I have used telescopes on every continent (including Antarctica) except for Australia.

Favorite cocktail or beverage?


Meet the Team: Kiwi Zhang​

Today we have our next in our Meet the Comet Hunters Team series.

Name:  Kiwi Zhang​

What is your current position and where/institution?

​Project Support Engineer/Scientist at Academia Sinica Institute of Astronomy and Astrophysics (ASIAA)

Where are you originally from/where did you grow up?

​Taipei, Taiwan​

What is your role in Comet Hunters?

Develop the Subaru image analysis pipeline​

Beyond Comet Hunters, what else do you work on?

Work on the Trans-Neptunian Automated Occultation Survey (TAOS) project for developing control software

In 3 lines explain your PhD thesis?

Developed an analysis pipeline to process the image data and to detect the candidate events produced by the occultation by Trans-Neptunian Objects (TNOs)​

Why are you interested in main-belt comets?

They are a mystery group in the Main Belt.

Name one hobby of yours?


What is the most recent tv show you have watched?

The Wired​

What is the latest book you have read?

The Devotion of Suspect X by Keigo Higashino
Divergent trilogy by Veronica Roth

What is your favorite band/music artist?


Favorite cocktail or beverage?

Beer in general except STOUT​

What is an overlap and how can we tell?

Many of the comet candidates that have been flagged on Comet Hunters so far have turned out to be cases where an asteroid is “overlapping” a background star or galaxy, and the question has come up as to how do we identify such cases.  First, just to explain, an “overlap” is what we call a situation where an asteroid passes very close to, or even over, a background star or galaxy, as viewed in the sky (they are of course very far away from each other in physical distance!).  If the asteroid and background object are positioned just right relative to one another, it can make the asteroid look like it has cometary activity, when in fact it does not.

In terms of identifying overlap situation, we first note that in most cases, it is actually often not possible to say anything definitive from the displayed image alone, such as the one shown here, which at first glance, certainly looks like an excellent comet candidate:

overlap1Subject 1295819

What we do in these cases is go back to the original image and search for other images of the same field, ideally from the same night and same telescope (since this means the images will look similar, having been taken under similar observing conditions and probably with similar exposure times), to compare them.  Astronomers often take multiple images of the same field within the same night because they are looking to see how a target changes over short periods of time, or because they want to be able to reduce the effects of image artifacts (adding together multiple images can aid the removal of cosmic ray hits, or by moving the telescope slightly between exposures, they can remove the effects of bad pixels on the detector), so in many cases, we can take advantage of this fact to allow us to check for overlaps.

If we can identify observations from the same night, we can look for a consistent appearance of the comet candidate from one exposure to another.  If a “tail” appears to switch sides between two different exposures, this is a good sign that it is actually due to the asteroid moving across a background object (real comet tails do occasionally appear to switch directions as seen from Earth, but not in the time span of a single night), as in the example below where we have identified another image of the same asteroid:

Subject 1295819 and Subject 1295854

or if the tail appears to simply disappear from one image to the next:

Subject 1295819 and Subject 1295842

Looking for a consistent appearance between exposures also helps us distinguish real comets from asteroids that only look like they have tails because they are faint, meaning that background noise can randomly appear to mimic activity.

Finally, if we can find many images of the same field/asteroid from the same night, we can sometimes actually see the asteroid moving across a background object and perhaps even see the background object emerge completely from behind the asteroid in question:

overlapSubjects 129581912958421295854, and 1295866

Of course, if multiple images are not available of the same field/asteroid from the same night, this task becomes somewhat more complicated, where we might need to retrieve data taken on different nights or even by different telescopes besides Subaru to see if we can identify a background source at the location of the suspected activity.  This is more difficult and may be less conclusive than the method described above because smaller telescopes or shorter exposures or images taken under worse observing conditions may not be sensitive enough to detect the suspected background object, making the check inconclusive.

Another check that can be done is to search for other images of the same asteroid taken at a similar time (e.g., using the SSOIS web tool, which we will discuss in more detail in a future blog post), or if that is not possible, then perhaps at least at a similar point in its orbit.  If similar activity is seen, then there is a good chance that the activity is real.  If not though, then this raises the chances that the original suspected activity was due to an overlapping source, or perhaps just noise or poor observing conditions.

It is of course possible that none of these checks will reveal anything conclusive.  This is one of the limitations of using archival data, that we don’t have control over the way the data is/was obtained, and so sometimes we will simply lack enough information to directly confirm or reject overlap cases.  In these cases, we have to rely on our intuition.  Is the star field crowded or not?  If it is, it suggests that the likelihood of an overlap is higher since there are more potential background objects to overlap.  Is the image quality good or not?  If not, it means that it is easier for a background star to be fuzzy and indistinct enough that it could reasonably be mistaken for a tail.

Finally, we might also ask if the object’s orbit and orbit position at the time of the observations consistent with those of other known main-belt comets.  We want to try very hard to avoid resorting to this particular line of reasoning because it creates a bias against discovering main-belt comets in unusual orbits or at unusual orbit positions, but in the event that no other information is available, we may use this to make a judgement call…i.e., if the orbit and orbit position are consistent with previously known main-belt comets, the object might be considered a stronger candidate than if those are not consistent.  We will still not be able to absolutely confirm that the activity is real, but we can put the object higher on our list of candidates to observe in the future when they again reach a point in their orbits at which activity might be expected.

What do main-belt comets look like?

Main-belt comets have a wide variety of appearances, or “morphologies”, depending on the strength of their activity, nucleus size, angle at which they are viewed from the Earth, just to name a few of the factors involved. As you look through objects in Comet Hunters, you may wonder what kind of features you should be looking out for.  This can be a hard question to answer given the diversity of possible morphologies of main-belt comets though, so in this case, it is perhaps easier to show, rather than tell.

So, first, here is a gallery of representative images of nine of the main-belt comets known to date.

As you can see, main-belt comets can have long, thin tails like 133P, or broad ones like P/2013 R3 or P/2012 T1, or curved ones like 324P or 313P.  Some are strongly active, like 238P and P/2013 R3, while others are only weakly active, like 133P, 176P, and P/2012 T1.  Some may even have two dust tails, like 288P in the above images, or may be in the process of breaking apart, like P/2013 R3.  As you can see, it is not really possible to describe the “typical” appearance of a main-belt comet.

Complicating matters more, the appearance of an individual main-belt comet can vary over time, as its activity strength changes as it approaches and then passes through perihelion (its closest approach to the Sun in its orbit), and/or due to changes in observing conditions between different observations (i.e., on different nights or even different times in the same night, or at different observatories at different locations in the world under different weather conditions).  Below are several series of observations of various main-belt comets taken over periods of several weeks, months, or years using a variety of telescopes (ranging in sizes in terms of primary mirror diameter from 1.5m to 10m) to help illustrate this point:

images_133p133P/Elst-Pizarro in 2002 and 2007 (from Hsieh et al. 2010, Monthly Notices of the Royal Astronomical Society, Vol. 403, p. 363-377)

images_176p176P/LINEAR in 2005 (from Hsieh et al. 2011, Astronomical Journal, Vol. 142, article 29)

images_238p_2005238P/Read in 2005 (a-c) and 2007 (d) (from Hsieh et al. 2009, Astronomical Journal, Vol. 137, p. 157-168)

images_238p_2010238P/Read in 2010 (from Hsieh et al. 2011, Astrophysical Journal Letters, Vol. 736, article L18)

images_324p324P/La Sagra in 2010-2011 (from Hsieh et al. 2012, Astronomical Journal, Vol. 143, article 104)

288P/(300163) 2006 VW139 in 2011 (from Hsieh et al. 2012, Astrophysical Journal Letters, Vol. 748, article L15)

images_p2012t1P/2012 T1 (PANSTARRS) in 2012 (from Hsieh et al. 2012, Astrophysical Journal Letters, Vol. 771, article L1)

313P/Gibbs in 2003 (a-c) and 2014 (d-h) (from Hsieh et al. 2015, Astrophysical Journal Letters, Vol. 800, article L16)

Note: Many of these images are created by adding together several individual images to make a composite image equivalent to leaving the telescope shutter open for up to several hours in some cases.  During this time, the main-belt comet appears to move relative to the background stars.  From the comet’s perspective though, the stars appear to move, and so the series of dotted streaks you see in many of the above images are background stars or galaxies that have been imaged several times (while “moving” between exposures) and then combined together into a single image, keeping the comet at the center of the image at all times.

This rich variety in main-belt comet morphologies is a big reason why we started the Comet Hunters project, given the difficulty of creating computer algorithms capable of identifying several different types of activity.  There are still some things that computers can do better than the human eye (such as measure small differences in the profile “widths” of candidate objects as compared to nearby stars), but we hope that the combination of citizen science and modern computing, we will be able to discover many more new main-belt comets.

How the Images Were Obtained: Happenings in the Subaru Telescope Dome

I thought I’d expand more about the actual observing and operations of the Subaru telescope by sharing these videos of how the telescope moves about on the sky and tracks the target its locked on to so that it follows it (most times)  at the rotation rate of the Earth and also a video showing some of the maintenance that goes on during the day day.

The video below I believe shows a few nights of Subaru observing taken from a vantage point in the dome. For the Suprime-cam observations we show you on the site, most if not all didn’t use the laser guide star system, but you can see it as the red line emanating from the telescope in the video

Warning the video can be a little bit shaking/jumpy:

Credit: Subaru Observatory

As I mentioned in my last blog post, the day operations and engineering nights are vital to keeping the Subaru telescope and instruments like Suprime-cam healthy. The images you see on Comet Hunters are cutouts from the full Suprime-Cam field-of-view around where we think the asteroid is and reference stars. To get the light to the camera its journey starts once it hits the primary collecting primary which is 8.2-meters in diameter.   As Charles mentioned in his post, this mirror is a one piece and to clean it, means doing the whole thing at once. To remove dust and dirt that land on the mirror reducing how good it is at collecting and reflect light, the mirror gets a carbon dioxide snow shower every few weeks. Check it out below:

Credit: Subaru Observatory

The music you hear as the telescope is moved into position is the motor encoders. I love that sound in telescope domes. To me it always seems like the telescope is singing whenever I’ve been lucky enough get a trip in to the domes of telescopes like Subaru


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