Making a Push on the Suprime-Cam Search and Waiting on New HSC Images
I wanted to give an update on both Comet Hunters Searches
HSC Search: Thanks to your help, we’ve completed all the live HSC images. We’re currently working on processing more images. We had some data processing challenges that are not solved. We hope to get new images on site by the end of February. Stay tuned for to this space for more updates.
Archival Search: We’re working towards the first paper, that will focus on the Suprime-Cam Archival Search. We’ve started to work on some of the paper text and analysis. One of the next steps is to compare to what automated analysis suggests might have a point-spread function. We think this would be an interesting comparison. We’d like to include as much completed Suprime-Cam observations in our analysis as possible. If you can spare some time, please classify an image or two on the Archival Search today at http://www.comethunters.org
Blends, Blends Blends
The science team is working on incorporating data from the Hyper Suprime-Cam (HSC) survey into Comet Hunters. We started with the archival Suprime-Cam data first to get a better understanding of what are the false positives and challenges for identifying Main-Belt Comets (MBCs) in data from 8-10-m class telescopes. We’ll continue with both datasets as there’s more Suprime-Cam asteroids, but when we have the chance we’ll move to reviewing the new HSC observations hopefully a few days after they’re taken.Most previous asteroid detection surveys are using 1-3-m class telescopes, so there are bound to be surprises that we wanted to know about before we developed the decision tree for the HSC snapshots on to the site. So we launched Comet Hunters with the archival Suprime-Cam images first. Now that things are going smoothly, we can turn our attention to the HSC data.
We combined your classifications from the first batch of Suprime-Cam images and had 125 candidates in need of further vetting. Thanks to volunteer Tadeáš Cernohous who on Talk went through our list comparing repeat images of the asteroid at slightly different positions in the same batch of subjects. What we learned that all of the candidates are unfortunately blends with stationary background sources. There are lots of faint background blobs that the asteroid moves on top of overlapping in the images creating very tail-like features. All of these images the science team would have had said has a tail.
A few examples are below (all blends with faint background sources):
There’s a lot more blends than we had anticipated given some of the team’s past experience with 2-m asteroid survey data. It’s still very much worth digging into the rest of the Suprime-Cam archive to look for MBCs. There might be many blends, but there could still be undiscovered MBCs too! Knowing that the background blends rate is much higher because of the increase in the photon collecting bucket is extremely useful. From the candidates, we could see the blends are faint blobby structures that would be likely hard to get a source extractor to pick up in all cases. Because of the quality of the HSC data and the repeat observation cadence we can try and take this into account possibly by checking the image of the asteroid and the repeat image of the same position take later on in the same night (not all Suprime-Cam images will have that and are taken in all types of sky conditions).
Now the Comet Hunters team is thinking about how best to develop a classification interface for the HSC data to include this. In the meantime, there are new Suprime-Cam images in need of review at http://www.comethunters.org if you have a minute or two to spare.
First Set of Images Complete but More Coming Soon
You might have noticed the blue banner currently on the Comet Hunters website. That’s because thanks to your help, we’ve completed the classifications needed to retire all the images that were live on the site. The team has been working to process a new batch of asteroid images. We’ve taken our time to improve on some of the data reduction issues you might have noticed in the launch images (streaked asteroids, more off center asteroids images, and some bad quality images). By having people spot and comment on these features in the images, we’ve been able to refine the data processing pipeline for this next batch of images. We will have those images live ASAP. Stay tuned to this space.
Most of the Comet Hunters science team chatted today, and we’ve decided to put on Talk our top comet candidates based on your classifications. As we’ve found thanks to your classifications and Talk comments, overlaps with background sources are a huge source of false positives for 8-m class telescope images of asteroids when you’re searching for comet-like tails. If you’re interested, we could use your help to review other images to see if the potential tail is a background galaxy or star when you view the same area after the asteroid has moved. More details here.
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:
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:
Subjects 1295819, 1295842, 1295854, 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.