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New HSC Images On Comet Hunters

We have new HSC images available on Comet Hunters from last August now available now on the site. With more asteroids, there are more chances to identify cometary activity. If you don’t spot a tail, that’s okay too. You’re helping us figure out how frequent these cometary outbursts are.

Also today, the Hyper Suprime-Cam Survey, which we get our asteroid images from, just had their first public data release. You can read more about it, and see some stunning images from the HSC camera here.

Happy Comet Hunting!

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

A Comet Hunters Summer

Ishan has spent the last two months as an ASIAA summer student with Comet Hunters working on making simulated images of main-belt comets. Ishan has chronciled his progress here and here on the blog. The goal is that with this simulated main-belt comets we can learn about how faint of cometary activity can the project see. This will let us probe the frequencies of main-belt comets in ways we couldn’t do with just the HSC data alone. Thanks Ishan for all your hard work and effort this summer.

Ishan gave his final presentation at the end of August which he is kindly sharing here:

grouppresentation_mod-001Hello I am Ishan Mishra and my project is ‘Studying Main Belt Comets with Comet Hunters’

grouppresentation_mod-002Let’s quickly review what are the major differences b/w comets and asteroids:

  1. As you can see in this graphic, most of the comets reside in the Kuiper belt beyond the orbit of Neptune and in the Oort Cloud in the outer solar system.
  2. The also differ in size and composition. While comets typically range from 6 – 25 miles in diameter, asteroids are much larger with their diameters ranging from the size of small rocks to about 600 miles.
  3. Comets contain a lot of ice along with rock and hydrocarbons while asteroids are composed of rocks and metals

grouppresentation_mod-003So what are Main-Belt Comets? These are a recently discovered class of main belt objects which show cometary activity. Here’s an image of a main belt comet taken by the Subaru Telescope.

grouppresentation_mod-004Here we see images of the first six MBCs discovered. The plot on the right clearly shows that these objects are distinct from comets while being indistinguishable from asteroids.

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So why are we interested in MBCs?

  1. Firstly, our classic difference between asteroids and comets based on their composition is called into question. Asteroids were never known to have volatiles present in them.
  1. Present-day sublimation in the main belt is surprising, as surface ice is unsustainable over the age of the Solar System so close to the Sun. MBCs provide new constraints to the location of the so-called snow-line (the distance from the Sun beyond which primordial temperatures in the protoplanetary disk were low enough for water to condense as ice and thus be swept up into forming planetesimals)
  1. The astrobiological implication: Origin of water on Earth. This interests me the most!

The Earth is believed to have formed dry owing to its location inside the snow line, and therefore the water we see today must have been delivered from outside the snow line, perhaps by impacting asteroids and comets. There are models which suggest that water delivery from asteroid belt was more likely. MBCS provide us a way to test this.

grouppresentation_mod-006However, so far very few MBCs, about 12, have been discovered. We need to find more in order to:

  1. Determine their extent and abundance
  2. Having a large dataset will help us understand them better

The search efforts so far have been mainly of two kinds: Using computer based pattern recognition algorithms and using human pattern recognition abilities.

  1. Computer algorithms currently are not faring well because of the very low SNR of the coma/tail region. They are also unable to accurately account for the whole range of tail morphologies. Hence, there are a lot of false positives.
  1. Telescopic surveys provide the large datasets needed to increase chances of finding objects like MBCs. PAN-STARRS is one such survey which uses a 2 m telescope in Hawaii. Here is an image of the first MBC discovered using PAN-STARS dataset. There are a couple of issues with this method though:
  1. The image quality is not good as its a small telescope. In fast, most surveys use a small telescope.
  2. Since the search is conducted by eye and the team consists of just a few scientists, the process is very slow and laborious.

Here’s where Comet Hunters comes in as a possible savior.

grouppresentation_mod-007So, Comet Hunters is a Citizen Science project under the citizen science web portal called Zooniverse. The idea behind citizen science is crowdsourced scientific research, ie, a large number of volunteers, their numbers usually in thousands, actively participating to complete research tasks.

Projects at Zooniverse span Astronomy, Climate Science, Biology, Physics and even Humanities. Galaxy Zoo is a famous example of astronomy related. The volunteers are shown images of galaxies and they have to classify them according to their morphologies. Another is Planet Hunters, where you need to spot dips in light curves corresponding to planet transition.

In Comet Hunters, volunteers are shown images of asteroids and they need to check for the presence of a coma or a tail. A candidate is vetted by the research team only when about 20 volunteers spot any activity. The combined assessments of so many people are much more reliable than individual opinions.

  1. So we have a large number of searchers now. What about the data?

grouppresentation_mod-008The data for Comet Hunters comes from Subaru, an 8 m telescope at Mauna Kea, Hawaii.

Two key advantages:

  1. Large telescopes are rarely used for surveys. So we have a large amount of high quality data.
  2. Two catalogs: Suprime Cam Archive and the Hyper Suprime Cam current survey data. So we have 17 years worth of data. Easier to spot recurrent activity.

grouppresentation_mod-009Here’s a just a quick example of an image shown on the Comet Hunters website.

It can be also be seen with inverted colors for more clarity.

The two images are about 20 arcsec in width each and are from the same night.

The volunteers need to answer whether a tail is present in one image, both images or is completely absent.

  1. We have a large number of people now working on high quality data. But,

What is the detection efficiency of Comet Hunters? How good is the project in detecting MBCs? What kind of MBCs are getting detected?

This where my project comes in.

grouppresentation_mod-010My job is to create or simulate asteroid images similar to Subaru’s, with varying properties like the coma or tail strength, tail direction, etc.

The volunteers’ choices for these images will help us characterize the kind of MBCs that are being detected by Comet Hunters. For example, we can find the minimum strength of the coma/tail for which the volunteers are able to spot them.

Here are a two examples of images that I have generated till now. I will explain the process in the subsequent slides.

grouppresentation_mod-011

Before I start talking about my work, I need to give some background information.

  1. I will be using this term called ETA to denote the strength of coma activity. As you can see, a value of 0 means absence of coma.
  1. If you look closely in the Subaru images, you will find that the asteroid image is elliptical not circular. I exaggerated the stretching in the model images I displayed in the previous slide. And you can also see this in on the image on left.

So why does this happen?

Solar system objects move much faster in the plane of sky than the distant stars. And Subaru’s survey data images are usually taken for exposure times of 120-150 seconds, as they are not specifically designed for solar system observation.

So during this interval the asteroids/comets move a couple of arcsec resulting in smearing or stretching of the image in the direction of motion.

grouppresentation_mod-012Here I present a simplified pipeline for the generation of model MBC images. As you can see, there are a lot of steps involved. Let’s go through them one at a time.

grouppresentation_mod-013

  1. The first part is the generation of a simple asteroid image with or without a coma, whose intensity strength we can control.
  2. The second box says high resolution asteroid image. This means that we are starting with images at a much higher resolution than that of images obtained from Subaru. I will come to the reason shortly. (*Planning to talk about it when I explain bin and sum*)

Let’s see how a simple asteroid looks.

grouppresentation_mod-014Here is our asteroid, just a pixel wide, with no coma. So, all the pixels in this image have value zero except the central pixel which has the value 1.0.

  1. This is how the asteroid will look if we see it in space, without any optical or atmospheric effect. The ideal case.
  2. The image is 20 arcsec in width with a resolution of 0.01 arcsec per pixel (compared to Subaru’s resolution which is approx. 0.17 arcsec per pixel).
  3. Let’s zoom in to the central 1 arcsec region to see the asteroid clearly

grouppresentation_mod-015So we see a tiny dot at the center which represents the asteroid.

grouppresentation_mod-016

  1. Now let’s add a coma to the asteroid. If you remember, we defined a parameter ETA which is proportional to the intensity strength of the coma. Here you see an asteroid with a coma of intensity 1.0
  2. The coma is invisible in the default linear scale image. So I have also shown the log scale image to confirm the presence of a coma.

grouppresentation_mod-017

  1. Here are some images of asteroid with coma of different intensities. The ETA increases clockwise. Again, these images are all in log scale for clarity.

So, we have our asteroid image with a coma around it. Let’s move on to the next stage.

grouppresentation_mod-018

  1. We now trail the asteroid. As I had mentioned earlier, trailing is necessary to account for the stretched asteroid image which is due to the significant movement of asteroid in sky during the telescope exposure time.
  1. The information we need to trail the asteroid is its speed and direction of movement as seen in the plane of the sky and the exposure time of the telescope, which will control the length of the trailed image.

Let’s see an example.

grouppresentation_mod-019

  1. Here I trail an asteroid with coma of ETA value 25.0. We trail it with a speed of 50 arcsec/hour, at an angle of 45 degrees for an exposure time of 60 seconds.
  1. I have purposely shown the asteroid image in linear scale so that we can see the difference in the background intensity between the two images. Naturally, the comae from adjacent asteroids add up in the second image to make it look brighter.

Let’s zoom in.

grouppresentation_mod-0201. We zoom into the central 1 arcsec region and now we can clearly see the individual asteroid replicas.

2. The gap between successive asteroids is because we are imaging in an interval of 0.5 seconds for the duration of exposure. Hence, the asteroid is displaced by the distance it moves in this small interval.

grouppresentation_mod-021

Just a quick look at the trailed asteroid images for different coma intensities. All these images are of the central 1 arcsec region and are shown in logscale.

OK. Now that we have the trailed asteroid, we proceed to a very important step: Reducing the resolution or increasing the pixel scale to Subaru’s.

grouppresentation_mod-022A look at the trailed asteroid images for different trailing directions

grouppresentation_mod-023

  1. So in this step we go concert our high resolution images, of pixel scale 0.01 arcsec/pixel, to the lower Subaru resolution of 0.17 arcsec per pixel. But why did we start with a higher resolution?
  1. We are starting with how the image looks from space, before passing through the asteroid and being images by the CCD in the telescope. This is why we started with a single pixel wide asteroid, a point source.
  1. So, we are basically replicating how a CCD works, with all the photons falling in a square CCD pixel account for its value.

grouppresentation_mod-024

  1. So in this step we go concert our high resolution images, of pixel scale 0.01 arcsec/pixel, to the lower Subaru resolution of 0.17 arcsec per pixel. But why did we start with a higher resolution?
  1. We are starting with how the image looks from space, before passing through the asteroid and being images by the CCD in the telescope. This is why we started with a single pixel wide asteroid, a point source.
  1. So, we are basically replicating how a CCD works, with all the photons falling in a square CCD pixel account for its value.

grouppresentation_mod-025Here’s a practical example: the famous Eagle nebula.

Before we proceed, I would like to mention that prior to using this binning and summing technique to decrease resolution, we were trying to use interpolation. It didn’t work out at the end as we discovered a major flaw with the method or maybe the interpolation function we were using wasn’t working as expected. We can talk about this later if anyone’s interested.

OK. So let’s see how the low resolution images of our trailed asteroid look.

grouppresentation_mod-026Here’s a high resolution trailed image that we saw before. The input parameters are also shown. As you can see, this one has ETA = 0 , ie , no coma.grouppresentation_mod-027This is the low resolution image. It is 101 x 101 pixels wide. Please note that I have verified the total flux (in this case the sum of all pixel values) is conserved during the sum and bin process.

grouppresentation_mod-028

Here are some more low resolution image with same parameters as the previous image except ETA.grouppresentation_mod-029OK. Let’s move on now to the final stage: convolving our model asteroid with a background star. But why do we need this?grouppresentation_mod-030To account for two things that affect the ideal image: Atmosphere and Telescope optics. Convolving with a background star incorporates these effects into our ideal image and makes it looks realistic. grouppresentation_mod-031Here’s the background star which we will use for the convolution process. This was chosen by eye from a field image obtained from HSC database.

You will notice that the image on the right here and many subsequent images are shown in zscale. This is because the Subaru images fed into the CometHunters website are in zscale. This scaling provides a convenient perspective to look for any activity in the object.

grouppresentation_mod-032

Let’s see how our convolution process fares for some model images we have seen so far.

Case 1: The most basic one: untrailed asteroid with no coma.

As you see, since the model asteroid in this case is basically a delta function, the background star image is identically reproduced.

grouppresentation_mod-033

Case 2: Horizontally trailed coma asteroid with no coma.

Now here we encounter a major problem. The noisy background from the star’s image is getting trailed in the direction of asteroid’s movement. We have been trying to remedy this by playing with the convolution process but to no avail.

grouppresentation_mod-034

Recently, we came up with a neat trick. Why not generate a trailed sky background image same as in the convolved output but without the central asteroid. Then, ideally, subtracting this trailed background with our convolved output should remove the background from the latter image, leaving just the asteroid part we are interested in.

Let’s see how this works.

grouppresentation_mod-035

Firstly, to generate the sky background we need to convolve our model asteroid with the background image (with star removed). We remove the star from the background image by simply replacing a central square region containing the star (the image on slide 31) with some random background sky region. This process is crude and not the best way to remove the star, but will have to work for now.

You can see that the background sky gets trailed when convolved, as expected.

grouppresentation_mod-036

Now we subtract our convolved output (from slide 33) with our trailed background image from the previous slide and…..Voila! The background gets removed perfectly!

grouppresentation_mod-037

Now the final step is to add noise in this image. We add poisson noise to each pixel because of something called shot noise.

 

Shot noise is caused by the random arrival of photons.  This is a fundamental trait of light.  Since each photon is an independent event, the arrival of any given photon at a pixel cannot be precisely predicted; instead the probability of its arrival in a given time period is governed by a Poisson distribution.

Slide 38:

 

Here you look at our final output image. Looks good, doesn’t it?!

grouppresentation_mod-038grouppresentation_mod-039

Some sample cases

grouppresentation_mod-040

grouppresentation_mod-041grouppresentation_mod-042grouppresentation_mod-043grouppresentation_mod-044grouppresentation_mod-045grouppresentation_mod-046

Conclusion and Future work.
Although the pipeline is not completely ready to generate a the complete range of images of asteroids with tail, I quickly generated a basic asteroid with tail image – a pixel wide asteroid at center with a horizontal tail with intensity going of as 1/r – and fed it to the pipeline. The two images on the right depict asteroids with tails. Not bad, eh?grouppresentation_mod-047

The image on the left is the only recorded image in literature of a trailed asteroid with a tail – 107P/1949 W1. This is an old image, on photographic plate.

On the right hand side I am showing you something similar that I tried to generate from my pipeline. The image on right hand side top is in a linear scale image while the one on the right bottom is the inverse color version, just like the 107P/1949 W1 image on left hand side. grouppresentation_mod-048

New Asteroids to Search on Comet Hunters

New HSC images are ready for review on Comet Hunters.  These observations are from February 2016. These asteroid observations have never before been searched for cometary activity, and this is the first time human eyes have looked at them in such detail. As with new asteroids to review, there are new changes of finding a main-belt comet. Who knows what we’ll find! You might be the first person to spot a new comet.

If any good comet candidates are found in these observations we can ask for telescope time to reimage these specific asteroids the next year to see if they are still active. These telescope proposals are due in the early Fall, so classifying these observations now has much value. We’re also prepping June/July 2016 observations to put on the site ASAP to be able to try and catch asteroids in the ‘act’, at the start of cometary behavior. The classifications from Comet Hunters will enable us to further explore the frequency of the Main-belt comet (MBCs), an important result.

Contribute to the hunt for MBCs today at http://www.comethunters.org

Making Simulated Main-belt Comets Part 2

Hello everyone!

Its Ishan again, bringing you the updates of my work as a summer student at ASIAA,Taiwan. To recap quickly, I am working on creating simulated MBC images with variable attributes like the direction of motion, brightness of tail and coma, etc. My work till last week consisted of creating the coma around a one-pixel wide nucleus and trying to trail the image. I got a weird output and was trying to figure out what the problem was.

By the way, in my previous post I forgot to mention the reason we are trailing the coma. Well, when we image astronomical objects using telescopes, we usually use a large exposure time in order to collect sufficient number of photons. This is about 150 sec for imaging asteroids. Now the asteroids move significantly faster on the plane of the sky than the background stars due to their proximity to earth and hence cover a distance of a few arcsec in this interval. Hence, we see a stretched out, elliptical object instead of a circular one.

Coming back to my work, I’m glad to inform you that we figured out the issue! It was a pesky function I was using to shift the coma image to create the trailing effect (the function is ‘shift’ from the python module ‘scipy.ndimage.interpolation’). The function works as follows: It takes the image to be shifted and a 2D shift-vector as the arguments and shifts the image according to the shift-vector. It obtains the value of each pixel in the shifted image using interpolation. As it turns out, the default order of interpolation for this function is 3 which is unnecessary and the cause of all the trouble. Setting the order to 1 along with a shift vector having integer elements resolves the problem. Here is a trailed coma image with the corrected shift function. Looks better, doesn’t it?

Week 2_ Fig 1

Note: The image looks very pixelated due to the low resolution. Interestingly, this very closely resembles the actual resolution of the Subaru Telescope’s Hyper Suprime Cam from which the actual images are taken. However, we still need to figure out a way to make the image smoother and minimize this pixelation. Onto the next challenge!

We are a step closer to the final trailed image and I will be back with the updates next week!

Cheers!

Making Simulated Main-belt Comets

Hello everyone!

I am Ishan, a summer student at ASIAA, Taiwan. I started working with the Comet Hunters team, about 3 weeks ago, on creating simulated Main-Belt Comet (MBC) images. Using appropriate mathematical functions, we are trying to create asteroid images with variable attributes like the direction of motion, brightness of tail and coma, etc. When ready, these images will be fed into the Comet Hunters website intermixed with the real images. How the project as a whole performs on these images will help us gain better insight into how well Comet Hunters can find different strengths of cometary activity and thus the true number of main-belt comets. For example, we can figure out up to what minimum brightness level of the coma (with respect to the nucleus) of the asteroid do the volunteers generally detect it.

For my present work, I am considering the nucleus to be just one pixel wide. For modelling the coma around it, we are using a 1/r profile centered around the nucleus. A sample coma is shown below.  Note that the actual coma will be much fainter than the nucleus.

Fig 1

As we can see, there is ‘cross’ visible at the center. This is due to the fact that we are plotting a circular function in square pixel-grid. Now when we trail this image in a randomly chosen direction, we get a weird output.

Fig 2

As you can see, there is a skewed ‘X’ at the center of the trail. To check whether the trailing function is faulty, I fed it with a simple 2D gaussian coma. The resultant image looks pretty decent!

 

Fig 3

We are currently trying to figure out the issue with the 1/r profile. Maybe using polar coordinates will resolve this. I will get back to you with the developments!

 

Cheers!

In the Dome – A Tour of the Subaru Telescope

Earlier this month, I had the chance to observe from the summit of Mauna Kea at the Subaru Telescope using Hyper Suprime-Cam (HSC), the same camera we use in the new HSC survey search. I and my fellow observers got a great tour of the telescoped dome and also around the summit of Mauna Kea from our telescope operator Josh (thanks Josh!). Here’s the photos I took during the tour:

Do Something Great with the BBC’s Sky at Night

Screen Shot 2016-06-13 at 5.20.28 AM

Image credit: BBC/Sky at Night http://www.bbc.co.uk/programmes/p03xx5pm

A week and a half ago, during a trip to visit Oxford Zooniverse Headquarters, I traveled to Mullard Radio Astronomy Observatory outside of Cambridge to meet with the Sky at Night’s co-presenter  Dr. Maggie Aderin-Pocock. We talked about main-belt comets and how the public could get involved in Comet Hunters to  search for these elusive breed of comets residing in the Solar System’s asteroid belt. In particular, I discussed the new HSC survey data that recently went live on the project. website.

Below is a link to a clip from Maggie encouraging people to join Comet Hunters.

http://www.bbc.co.uk/programmes/p03xx5pm

This is part of the BBC’s Do Something Great Campaign, which promotes and encourages ways for everyone to get involved in volunteering and doing good. We’re thrilled to be involved in this effort with the Sky at Night.

Help astronomers find main-belt comets today at http://www.comethunters.org and if you’re based in the UK check out the Sky at Night’s latest episode on iPlayer.

An Introduction to Hyper Suprime-Cam (HSC)

The Hyper Suprime-Cam  (HSC) is the next generation camera on the Subaru Telescope, replacing its’ predecessor  Suprime-cam that had been used for 18 years. The field of view (FOV) has been dramatically increased from 0.16 to 1.5 square degree (10 times!!!). The HSC has not only a large FOV but it is also mounted on one of the biggest telescopes in the world. The camera’s ability and efficiency are the mightiest in the current observatories. HSCs physical extent is larger than three meters. Its focal plane (60 cm in diameter) is paved with 116 CCDs (including focus and tracking chips), operating in -100 degrees centigrade. The diameter of the front lens is 82 cm, the length of the lens barrel is 165 cm. The total weight is 890 kg. Every exposure includes 870 million pixels.

(c) Subaru Telescope, NAOJ

Before the HSC era, most of the cameras with large sky coverage were installed on  medium or small telescopes (48-inch-a few meter telescopes), e.g. GigaPixelCamera on Pan-STARRS 1 (1.8m), MegaCam on the  Canada-France-Hawaii Telescope (3.6m) and Magellan(3.6m), Dark Energy Camera on the Blanco Telescope (4m). Although they can cover a few square degrees, the sensitivity is  25% less than what you can achieve on an 8-meter class telescope. In some astrometry topics, like micro-lensing and large structure in the universe, both large aperture and FOV are vital. The enough sample will lead astrometers to a significant and compact conclusion.

hsccrosssection

(c) Subaru Telescope, NAOJ

The HSC Subaru Strategic Program (SSP) is most important and largest programs o the  Subaru Telescope in this decade. This survey started in 2014, finishing at 2019. This program will survey 1400 square degrees, using optical to near-infrared filters (g, r, i, z and y), reaching a 26.5 magnitudes in stacked images. The survey regions are fixed on the equator (Dec = 0) and cross with Ecliptic plane where are the locations including a lot of solar system bodies. Hence, beside cosmology science, this survey data is quite valuable for Solar System science. Now with the HSC Survey Search, you can search for cometary activity in asteroids imaged in the HSC survey on Comet Hunters.

Introducing the Hyper Suprime-Cam Survey Main-belt Comet Search

As of today, Comet Hunters has a new dataset and a new look. You’ll notice there’s now a button called ‘HSC Survey Search’ on the front page of the website.  We are thrilled to announce the incorporation of data from the Hyper Suprime-Cam Subaru Strategic Program. Hyper Suprime-Cam (HSC) is the largest field-of-view camera ever stuck on an 8-10-m class telescope. HSC covers nine times the size of the full moon in a single pointing!

What makes this so exciting is that the asteroid images you can review now on Comet Hunters from this survey are as from as close to right off the telescope as we can get. This means we can follow-up and try to catch the asteroid still in the act if Comet Hunters spots a tail. No one has ever looked at these images for main-belt comets before. Your eyes will be one of the first to view these images. Who knows what we’ll find!

The old Comet Hunters classification interface you know and love is still around. It’s now under the ‘Archival Data Search’ on the Comet Hunters front page. Looking for main-belt comets in the archival Suprime-Cam data is still extremely useful, and we’ll be keeping that classification interface going in parallel to the HSC Survey Search. You’ll notice the HSC images are a bit different than the Archival Data Search. There’s a tutorial and help buttons that will show you how find tails in the HSC images. You can also find more information on the Research and FAQ pages as well.

Dive into the HSC Survey data and help search for main-belt comets today!

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