A
condensed version of the following appeared as the article "Searching for Supernovae
on a Shoestring" in the July 2009 issue of Sky and Telescope magazine.
Supernovae have always fascinated me. These violent events seed the universe
with the elements that go on to make planets and life, and they continue to
be important to cosmologists as standard candles that may help to explain
how the universe has evolved since the Big Bang.
And so it was that when I started to think about a science project that
would use the skills and equipment I had acquired from several years of
astrophotography the possibility of discovering supernovae was immediately
appealing. The idea that I could be the first to see a star that exploded
many millions of years ago was compelling. But could it be done, and how
much time would I need to spend to get results?
This article describes my supernova search program which,
running from February 2007 to August 2009, has discovered three supernovae .
Challenges
Supernovae are rare. A typical galaxy, such as ours, hosts a supernova
only once or twice a century. To stand a reasonable chance of discovering
one, I would have to observe hundreds of galaxies a night. This implies that
the search has to be automated, and that set-up time, particularly after
dark, must be minimised.
I do astrophotography from the backyard of my suburban home and do not
have an observatory or permanent mount for the telescope. The gear I have is
pretty modest - a Celestron 9.25” SCT, a tripod mounted Vixen GPD mount with
SkySensor 2000, and a kit built Artemis 285 CCD camera.
Given this, the main challenge to be overcome in the design of the search
program was to find a way to automatically and accurately point the
telescope at several hundred galaxies over many hours each night. The GPD
and SkySensor combination is very capable, but I know from experience that
accurate pointing requires good polar alignment, and that accuracy
deteriorates away from the point that the mount has been synched to. I
definitely could not afford to go through the time consuming process of
polar alignment each night.
The solution to this turned out to be elegant and conceptually simple. I
ensure that each galaxy on the target list will be at about the same
altitude and azimuth at the time that its image is acquired. With the target
list constructed in this way, exact polar alignment is not needed for good
pointing accuracy. Imagine a mount that is not motorised. The telescope on
this mount points at the same place in the sky as the stars wheel around. Someone observing through this telescope would see the stars drift by, but
he would always be looking at exactly the same declination albeit with
increasing right ascension (RA). This is true even if the mount is not polar
aligned. Of course in reality, some flexibility in choosing targets is
desirable so that approximate polar alignment is still needed.
Search Design and Execution
The search program consists of three parts - target list generation,
telescope pointing and image acquisition, and a means of rapidly comparing
the acquired images with references. The core functions in all three are
carried out by scripts that I wrote in the freeware
AutoIt language. GalaxyGen generates the night’s target
galaxy list, TargetPoint reads the target list and controls telescope
pointing and image acquisition, and Examiner displays a DSS reference
alongside the corresponding acquired image allowing visual comparison, and
records any result.
A typical night’s observation starts with generating a target list. As
explained above the target list (see image on right) consists of a series of galaxies all at
about the same declination, ordered by increasing RA. I set a suitable RA
for the start of the observation such that targets are at a fairly high
elevation (above 60 degrees), and select a declination angle for the night. GalaxyGen produces a target list that minimises deviation from the desired
fixed altitude and azimuth over the duration of the run. Since pointing
accuracy depends on this I spend some time tweaking GalaxyGen’s input
parameters to achieve the best balance between number of targets and
deviation from ideal target location. The GalaxyGen script
interface is shown below left.
I carry out the tripod and mount and place it on the pavement just behind
my house. Placing the tripod feet in slight indents I have made in the
pavement ensures that there is approximate polar alignment. The telescope is
then placed on the mount, and the camera installed. I use a desktop PC on a
trolley to run the camera and mount. This is wheeled out from its home in my
garage and all cables connected. It usually takes me half an hour to set all
this up.
Using Cartes du Ciel, a freeware planetarium program, I slew the
telescope to a star close to the first target on the list, centre the star
and synch the mount to this point. I then focus the telescope.
When I am happy that everything is working fine I start the TargetPoint
script. TargetPoint works with the Artemis camera software to acquire
images, and with IRIS for telescope pointing (IRIS is primarily an
astronomical image processing program but also has the ability to talk to
Meade LX200 compatible mounts). TargetPoint sets image acquisition
parameters , slews to the first target, does the required number of
exposures, waits for images to be downloaded to the PC, then slews to the
next target on the list. Images are visible on the screen as they are taken
so I usually confirm that at least the first target has been acquired
successfully. Thereafter the observation session needs no further
intervention. I cover the PC with a plastic sheet to keep dew off, and then
am free to enjoy the rest of my evening.
The next morning, I copy all images acquired onto a USB drive, dismantle
the equipment and put everything away. This usually takes only 20 minutes,
so is easily done before I leave for work.
The next bit is the most challenging and time consuming. I use the next
script, Examiner, to compare images acquired against DSS references. When GalaxyGen generated the target list, it had also prepared a separate list
suitable for submission to the
Canadian Astronomical Data Centre (CADC) DSS
interface website. These reference images are downloaded in fits format. Examiner has a very
simple interface that enables me to quickly go through the images taken
whist recording results.
Discoveries
So what happens when I do see a star that is not in the reference image? The first step is to confirm that it is indeed a star and not, say, a cosmic
ray strike, or a CCD hot pixel. For each target, I usually take 3
consecutive images. Because of tracking imperfections there will usually be
a slight image shift between images. A hot pixel will stay in the same place
and not shift with the image. A cosmic ray hit can also be identified in
that it will also only affect one of the subject images.
The first supernova that I discovered was SN2007rv on 7th Nov 2007. It
had been a windy night and since, really, my mount is too light for the
telescope, imaging is very susceptible to wind, and I had not held much hope
of many usable images. So it was only four days later that I got around to
looking at the images. With much excitement, I saw a star just east of the
nucleus of NGC 689 that was not there in the DSS image. A quick check showed
that no known minor planet was in that location at the time.
I then set up the telescope for a confirmatory observation. The Central
Bureau for Astronomical Telegrams (CBAT), which acts as a clearinghouse for
reports on transient astronomical events, requires that any suspect
supernova discovery be confirmed by a separate observation on a second
night. Yes, the star was still there. I quickly did some astrometry of the
suspect supernova in IRIS, and submitted a discovery report to CBAT. There
followed an email exchange with Dr Dan Green, Director of CBAT which showed
how little I knew about this – everything from the number of decimal places
for the position of the object, to how photometry was done. With his help my
discovery report was eventually licked into acceptable shape and I got
confirmation a few days later that the supernova had been confirmed as a
Type 1a, and given a designation. I was elated!
Since then the observation program has made two other
discoveries, SN2008ff in August 2008 (another windy night!), and SN2009gg on
13th June 2009. The screenshot above shows the discovery of
SN2008ff. Note the wind affected stellar images.
Source code for my scripts are here.
Please be aware that these were written for my specific system and software
environment so may not work for you. But they should help you to
examine how they work and perhaps customise them.
