Image of the nova – a star that was not visible there before.
A little while back, I subscribed to the special notice bulletins from the American Association of Variable Star Observers (AAVSO). I thought that it might be interesting to hear about unusal variable stars, novas and perhaps even a supernova should one of those appear. One of these notices arrived in my inbox on Monday. There was a newly discovered nova in the constellation of Sagittarius. I wondered, could we observe this at the Harken Observatory?
Since I had never tried to do this before, I could not resist trying! The weather map showed a finger of clear sky over southeast Wisconsin so I headed to the observatory just after sunset tonight. The Starlight express camera was still on the Stellarvue D80! I grabbed some calibration frames and then pointed the scope to the indicated coordinates (by way of a star called Nunki for a more precise scope position sync). I grabbed (5) 60 second exposures. There were stars in the image, but which one was the nova? I needed to learn a few things about astrometrics and here is what happened next.
First, I needed to check a finder chart. These are available online from the AAVSO website. To be useful, the chart needed to be scaled to the telescope focal length and the CCD camera pixel size and spacing. I found a “pixel calculator” on the web. Our camera/scope comination provided a field of view of about 64 x 48 arc minutes. The finder chart requested was for field of view of 70 arc minutes and to show stars to a limiting magnitude of 11. The nova was reported to be about 9.3. The chart matched the images! There was the nova just as reported. Yeah!
Then I wondered how I could estimate the brightness of the nova. Sure there were a few stars with labelled magnitude on the finder chart. I could compare them relative to each other, but there was a better way. The Maxim DL software that we use has astrometric and photometric measurement capabilities. How do these work? After reading a little on the help screens, I was soon downloading the 320 MB of the Hubble Guide Star Catalog onto my PC. The Maxim DL program has hooks that can read data out of the GSC. After plugging in the pixel sixe and focal length information (so Maxim could compute the scale) along with the approximate position in the sky, I was able to tell the program to go “process” the image. It performed a matching of the star fields and now gave me the ability to move the cursor and see the exact position of the object displayed! The nova is at RA 18h31m32.73s, dec -16deg19m07.7s. Then by “calibrating the brightness measurement to known stars, the brightness of the nova could be read. I found it to be mag 9.4 Not bad for a first attempt! We sure have some pretty cool astronomy tools.
Right now, I would not claim to have mastered anything. I know that brightness accuracy was probably with a large tolerance, but I intend to work on those skills. Maybe we could try measuring a few periodic variable stars. How about measuring the fluctuating light from a tumbling asteroid? I think that we could do this!
Here is a screen shot of the Maxim DL measurements.
Comments and questions welcome! Enjoy!
Randy,
Very cool! I was unaware of the nova until I read your post. Great to see that the observatory can be used for science as well as pretty pictures! Was the nova very low in the sky? Do you think any stars with transiting planets are within the observatory’s reach?
Hi Tim and all – yes, there is an exoplanet that is within our capabilities. It orbits a 9th magnitude star in Ursa Major. It has a rather eccentric orbit and a transit will occur next on January 13-14th, 2010. Ingress starts around 5:30PM and ends at about 2AM. I think that it would be worth a try! The planet is identified as HD80606b. We need to hit about 1% precision in the light measurement. I think that maybe some continuous averaging of data, with many frames in series would be useful (albeit a lot of work). The “big folks” are looking for data so maybe we can help if the sky is clear!