CFHT 2013B queue update, Part 2: The End

I’ve been a bit behind posting about this, so this is the last post regarding queue updates for 2013B. But that’s good news, though! Let me explain. Our 2013B CFHT program collects data from August to January. MegaCam, the instrument we use, is on the telescope for roughly 15 nights at a time. It is then swapped out for one of the other major instruments at CFHT: ESPaDONaS or WIRCam. You can see the schedule here. It appears our program has finished data collection by the end of 8 October 2013. It could have taken all semester to get this data in, yet it’s all done now. So that’s great news! You can see the last data release I posted about here. But here’s the final completed map:

The green indicates a pointing that's been observed from our observing program. Clearly, all pointings have been observed!

The green indicates a pointing that’s been observed from our observing program. Clearly, all pointings have been observed!

CFHT 2013B queue update, Part 1

This map shows the number of pointings completed to 31 August 2013 for our CFHT 2013B data run.

This map shows the number of pointings completed to 31 August 2013 for our CFHT 2013B data run.

In the fall of 2012, I PI’d a proposal to get 675 images taken by the Canada-France-Hawaii Telescope (CFHT) and the MegaCam instrument. We were fortunate enough to have the proposal accepted, and subsequently received our data. I wrote about it many times on my blog [queue update 1, queue update 5, and serendipitous asteroid one and two, just to name a few]. Well, the fall of 2013 is looking to be equally fruitful. We have re-submitted a similarly driven proposal to re-observe the same pointings from last data run with the aim to find things that have changed in the last year. We expect to find some objects that have either gotten brighter or dimmer over the last year, which we will then submit for extra observations (known as a target of opportunity) at Gemini. The above shows the number of pointings that have been re-observed this far, one month into the observing semester. [ASIDE: ‘observing semesters’ are broken into semester A: February-July, and semester B: August-January].

CFHT queue update, Part 5: The End

The observing team at CFHT was able to sneak my last observing group in mid January, allowing me to get 100% completeness on the proposal we submitted last year. Now it’s on to this data reduction and this year’s round of proposals.

Finding Asteroid #2: Serendipity…..again!

Late last year I posted an article on finding an asteroid, serendipitously, in my data from CFHT (see the post here). Well…I found another one! Luckily, this new asteroid happen to pass by a beautiful example of an edge-on spiral galaxy.

can you see it? There are about 20 min between the first two exposures, and almost 4 hours before the third.

You may be wondering, though, why the three images above do not look like the beautiful pictographs we are used to seeing (these, for instance). Why are my images black and white? Why is that edge-on galaxy so boring looking? Let me explain.

We take images of the sky using a CCD, which is just an array of light sensitive pixels. These pixels are sensitive to a rather large range of light, extending down to the near-UV, through optical, into the near-infrared. But, the pixels do not care what kind of light they detect. When a photon hits the pixel, a photon of any wavelength,* the pixel records it. In order to make the beautiful astronomy pictures we see, it takes a few more steps:

First, you have to filter out the colours you want individually. The above three images are the same patch of sky imaged using three different filters: the g filter (390nm – 595nm), the r filter (595nm – 680nm), and the i filter (680nm – 830nm).** While all the images look black and white, they are in fact exposed using three completly different ranges of colours of light. Again, the pixels do not care what colour. Second, you have to combine the various filtered light images into one, while imposing a colour scheme that matches what the colour should be in the wavelength ranges of the filters that you used. For instance, since the above image uses g, r, and i, I would add the images together with X amount of light that is coloured near 500nm for g, Y amount of light coloured near 650nm for r, and Z amount of light coloured near 750nm for i.

A final thought: For those of you who have looked through a telescope at things like the Orion Nebula, Andromeda Galaxy, the Ring Nebula, or any other cool astronomical object, you may notice that these objects appear to be black and white to our eye as well, and not the vibrant images I linked to just now. There is no CCD involved, so why are they black and white? The answer lies in our eye’s biology. We have two different and highly specialized, light sensing cells in our eye: rods and cones. Rods are used to detect light vs. dark, contrast, brightness. They see in black and white. Cones are used to detect different colours; there are three kinds of cones for red, green, and blue (RGB). Cones are far less sensitive than rods, in fact rods can sense light up to 100x dimmer than cones. So when we look through a telescope at a distant nebula, there is, in fact, RGB light travelling through space to us, but it is not intense enough to activate our cones. The rods DO perceive the light, and give us a great contrast image of the astronomical object.

Do not take this to mean that there is no colour in space, and that we photoshop it in. No, the colour is there, we just need to tease it out!


* Wavelength is the measure of the distance from peak-to-peak of the wave light. Ultraviolet light is 10nm to 400nm, optical light is from 400nm to 800nm, and infrared is 800nm to 2500nm. These are rough numbers.
** You can see my research post on how I sorted my CFHT targets into different redshift bins using the filters here. It also has a nice graph showing the wavelength range of the u,g,r,i,z filters.

Serendipity: Finding an Asteroid

The research group that I am a part of (Pat, Paola, Laura, and myself), proposed to the Canada-France-Hawaii Telescope (or CFHT) earlier this year to get some photometry covering a rather large section of sky. We wanted this data so that we can look for variability in the colours of quasars. Our proposal was accepted, and we have been getting data starting August 2012 through to now; you can see my blog posts on the data releases here, and here, and here, and here. The point of the project is to take images of a large section of sky in three different filters. Therefore the entire area we asked for will be imaged 3 times: once in g, once in r, and once in i.

I’ve spent my time with this data measuring magnitudes of quasars in these pass bands to look for changes in colours (compared to colours measured years ago). In order to properly measure a magnitude you need to use comparison stars (this relates to how you correct for your aperture, a post on that stuff in the future). As I was looking for a good comparison star in this particular section of sky, I noticed one ‘star’ was moving! After showing my colleagues (Lianne and George) we realized quickly we were looking at an asteroid. You can see it moving across the sky here:

This is exactly how you hunt for things in our solar system (asteroids, comets, etc.): by taking multiple images of the same section of sky and seeing if anything moves. The 3 images here are separated by about 25 min each. So in about an hour’s time the asteroid moved the distance you see there, which is a few arcminutes. Thanks to Lianne’s (@liannemanzer) investigations, we found out that, sadly, this asteroid has already been discovered (so we can’t name it after ourselves!). But it was fun to find something like this in my data!