Lab 2: Telescope Detectives

For this lab, we were given a .FITS file and told to determine what was in the image, and which telescope took the image in the file. .FITS is an image format which is the most commonly used format in astronomy. Here's what the image looked like when viewed in Aladin, a program that can open .FITS files:

One can see from the image that there is a binary star system. From the FITS header (which came with the file), I found the declination and right ascension (RA) of the system, which were 2.4968612 and 271.36566 degrees, respectively. After looking the coordinates up in Simbad (an online catalog of astronomical objects), I found out that the two stars were GJ 702A and GJ 702B.

Using Aladin, you can find the angular distance between two points. Thus, I could find the angular distance between two dark rings on one of the stars to determine the angular resolution of the telescope used (since the Airy ring due to diffraction is related to the angular resolution).

However, there was a slight complication. The angular distances given in Aladin were incorrect - I needed to use the actual coordinates in Simbad to correct this error (i.e. I found the RA and declination of the two stars, and used the Pythagorean theorem to find the angular distance between the two).The angular distance between the two stars, from Simbad, turns out to be about 2.976 arcseconds. The distance measured on Aladin is 9.168 arcseconds. So, to find the true angular distance, we would multiply the displayed angular distance by:

to find the true angular distance.

The angular distance between two dark rings on one of the stars in Aladin was about 140.8 milliarcseconds. Thus, the true angular distance is:
 on

, which is the angular resolution of the telescope. The spatial resolution is this multiplied by the distance to the star (using the small angle approximation), which is 16.8 light years. So, the spatial resolution is:

From the FITS header, I found that the wavelength of light used to capture the image was in the K band, which is around 2.2 microns (near-infrared range). Thus, I determined the diameter of the telescope like so:

It was stated that a coronograph was used, which results in an 20% reduction in effective aperture diameter, so the actual telescope mirror diameter would be about 16m, which seems slightly high, although that may be due to error in measuring the angular resolution.

Let's move on for now. From the FITS header, I found out that the Greenwich Mean Time at the time of observation was around 11:28 AM, on April 27th. That's about a month after the Vernal Equinox, which is 2 hours Local Sidereal Time. Thus, midnight at the observation location would correspond to an RA of 14 hours directly overhead. The RA of our system is about 18, which means that the local time of observation was around 4AM. Thus, the site of observation was in a timezone 7 hours after England - somewhere in the Midwest. Furthermore, the FITS header says that the air mass was 1.16. Air mass is about 1 at the zenith and increases as you go away from the zenith, so the observation must have occurred at a latitude where the system's declination of 2 degrees (basically 0) was near the zenith. So, our telescope must be located in the Midwest and not too far from the Equator. This suggests a telescope in America.

From this list of telescopes(http://astro.nineplanets.org/bigeyes.html), I looked for telescopes in the Midwest area (e.g. Texas, etc.). Candidates could be the Hobby-Eberly telescope in Texas (9.2m), and the Large Binocular Telescope in Arizona (8.4m). 

I suspect that my angular resolution is a bit lower than it should be, which is why I included the above two mirrors as primary candidates, since they are the telescopes with the two largest mirrors in the list provided.