Site logo, www.grelf.net
 

Comet C/2012 C2 (Bruenjes)

 

I never got a good image of this comet but the following describes my attempts.

 2012 Feb 24

I had another go at photographing this new comet but I faced the same problem as before: a thin veil of cloud gradually getting worse until I had to give up. This was another night that had been forecast to be clear. This has been a very common story this winter; we have had a lot of cloud and yet too little rain to keep the water reservoirs full.

Here is a straight stack, with the stars fixed:

Canon EOS 5D MkII ISO6400 51x32s
254mm Newtonian f=1200mm f/4.8
HEQ5 equatorial mount, driven but not guided
2012 Feb 24 19:25:49-19:58:04 UT
From Rookhope 54.8N 2.1W 330m asl. Rural, almost no light pollution (3 Bortles)

The comet is just visible as it moved, at the intersection of the white marker lines.

I thought I might improve on this by stacking with the comet fixed but because it cannot be seen in the individual frames (let alone detected by automatic segmentation), my usual batch menu option in GRIP could not be used. Instead I wrote a little extra code using the following information.

Star positions, enabling the scale of my photos to be calibrated (from the Hipparcos/Tycho catalogues via my Hopper software):

Star idRA (J2000.0)Dec (J2000.0)(V mag)
brightest star above: HIP 641301 22 22.31+16 49 32.87.01
star on left: TYC 1197 1186 101 22 38.00+16 34 03.79.67
star on right: TYC 1197 1191 101 21 36.55+16 41 30.210.20

Predicted positions of the comet at the times when my first and last frames were taken (from BAA Computing Section's "What's Observable"):

FrameTime (UT)RA (J2000.0)Dec (J2000.0)
838319:30:3901 22 46.18+16 38 51.1
843019:58:0401 22 44.31+16 38 46.0

A small detail: I discarded the first 3 frames from my original set of 51 because they were at irregular intervals while I was setting up. Discarding them made my extra code simpler. The other 48 frames were triggered by a Canon remote timer set to 35s intervals.

Note that even if the predicted absolute positions are a minute or two off (arc or time, due to inaccuracies in the orbital elements and the fact that an unperturbed orbit is calculated), the length and direction of the great circle arc forming the difference between these positions should be well within my required accuracy. (For programmers: the difference between two SkyPoints in my API is a simple method call. For mathematicians: that method does not fall into the trap of using inverse cosine on such small angles.)

My extra code used this information to apply a little extra shift to each frame according to how the comet would have moved. Neat as I thought this might be, it did not really improve the image of the comet:

Canon EOS 5D MkII ISO6400 48x32s
254mm Newtonian f=1200mm f/4.8
HEQ5 equatorial mount, driven but not guided
2012 Feb 24 19:30:39-19:58:04 UT
From Rookhope 54.8N 2.1W 330m asl. Rural, almost no light pollution (3 Bortles)

Still I think this experiment will be worth repeating with my next set of comet images, whenever that may be. Unfortunately the Moon is too bright now and the elongation of the comet from the Sun is reducing, so there may not be another chance to try with this particular comet.

I have not yet made my experimental extra code available in a released version of GRIP because I have hard-coded the data from the tables above rather than making a new dialogue. Please ask if you want such a facility.

 2012 Feb 18

 Comet C/2012 C2 Bruenjes

This comet was only discovered 6 days previously by an amateur (Mr Bruenjes). Orbital elements were available on the Minor Planet Centre site so I generated some charts for the BAA Computing Section and went out to look for it myself. I used my Hopper software (about which I will write more soon) to find the location. Nothing was visible in a wide angle eyepiece but I hooked up the camera and hoped. Unfortunately the clouds were already descending. My starting star, η Psc (magnitude 3.6), from which I found the comet location, was already not visible to the naked eye when I started to photograph. I managed to get four 30-second frames before the results were just fog. Nevertheless, when I stacked the 4 in GRIP and stretched the contrast I was able to make out the comet. This is not a great photo but the greenish blob in the bottom half of the photo is the comet.

Canon EOS 5D MkII ISO6400 4x32s
254mm Newtonian f=1200mm f/4.8
HEQ5 equatorial mount, driven but not guided
2012 Feb 18 20:26:06-20:28:47 UT
From Rookhope 54.8N 2.1W 330m asl. Rural, almost no light pollution (3 Bortles)

The image on the right is part of the view in Hopper, from which I found the predicted location (marked by the red cross). It shows stars down to about magnitude 12. The brightest star, near the top, is HIP7447, magnitude 5.95.

This does indicate that the comet should be easy to photograph if we ever get a clear sky.

The clouds did clear away completely about an hour later but by then the comet was too low behind trees for me to try again. It stayed clear, so I did photograph some other things.