Well the weather forecast looked promising for another attempt at trying to capture the comet 41P before it swings further away in its path around the sun. To that end, I knew that getting a good initial alignment was going to be key to using the goto command to point toward the comet. It turns out that 41P was not listed in the hand controller comet dictionary, so it was necessary to find the comet's RA and Dec in another application and key it in manually. But the real issue was going to be getting a
good alignment when I can't make out the north star due to city lights and the comet itself is not naked eye visible nor is much of anything in its local star background at this location. So I decided to just get out the old trusty level and set the mount zero position just using gravity with the level indicating where the proper zero location of the mount for polar alignment was located. Then to get a polar alignment, all that was necessary was to manually move the mount until Polaris showed up in the camera viewfinder. I couldn't see Polaris myself, but with the camera and scope, it was quickly found. There is still the uncertainty of whether the mechanical fixtures on the mount that were used with the level were actually machined and aligned so that the telescope optical axis actually is aligned with the mount polar axis. The mount has a polar alignment scope built into the mount but I didn't believe I could spot Polaris through it. Actually, I sort of forgot to try it out, but in retrospect it still seems it would not have worked with the city lights, but the camera worked very well. As it turns out there seems to be a little misalignment because I still found so star trails and tracking error, but I am getting ahead of the story.
So, after one star initial alignment and one quick slew to Jupiter just to check out it that would be ok, it was time to go after the comet. My initial lookup of the comet parameters used RA = 15 18 10 and Dec = +63 48 36. The image below is a magnified section of the 240 second exposure. I was quite excited to see at least a little fuzzy blob of white, in the blue circle, in the camera frame even though at the time I didn't know for sure if that was the actual comet 41P.
|Is this little smudge of light in this expanded portion of a 240 second exposure the comet 41P?
All comet images taken with 80mm refractor (Source: Palmia Observatory)
|Ok, the image is indeed centered on the predicted location of comet 41P
(This image is shown rotated 90 degrees clockwise from the expanded image view. Note 3 bright stars in a line)
Anyway, the fuzzy blob looked encouraging. Comet 41P is quite dim at reported magnitude 8.6 and with the bright city lights here at the observatory, it was going to be difficult to see this dim object By the way, I wasn't quite sure if a comet would have magnitude reported as if it were a compact object like a star, if it would be reported like an extended object like a galaxy? I can make out stars dimmer than magnitude 8.6, but comets, and certainly their tails, were going to be harder to see and this particular comet has now gone past its closest approach to the sun. You also might remember when in previous blogs we reported how difficult it was to photograph in city lights the dim extended Crab Nebula (M1), which has magnitude 8.4
Now if we were to know the RA and Dec of the actual fuzzy blob of light we can see how well it matches up with the predicted location of 41P. AIP4WIN is a good tool that this resident astronomer uses to overlay catalog star locations on top of selected actual stars in the image and then let the software calculate the best match and RA and Dec of other target objects.
Check out the screen shot below which shows my arbitrary selection of seven reference stars and three target locations. The catalog stars fit right on top of the observed stars even though some of the imaged stars had some star trailing issues. Target T1 was the placed right on top of the fuzzy blob. T2 was placed at the predicted location of 41P at 11:00, just I looked up only 1.5 hours after the image was taken at 9:32PM. Darn, I forgot to keep better track of when I looked up the comet's predicted location and so I am reduced to just using some values after the fact. But we know the comet is moving across the sky and changes in its location can be seen to change quite a bit in just the course of one day. The third target, T3, represents the 41P location just 12 hours after T2.
|AIP4WIN screenshot shows catalog stars overlaid on stars in image with target locations: T1, T2 and T3
(T1 is observed comet; T2 is predicted 1.5 hours after observation; T3 is predicted location 12 hours later)
For all of you analysts out there, I have posted the AIP4WIN report for the reference stars and target objects below.
|AIP4WIN summary chart combines observed star locations with observations
(T1 is observed comet; T2 is predicted location 1.5 hours later; T3 is predicted location 12 hours after T2)
Finally, there is one more image taken as sort of a test case to see what an example of how a dim galaxy would show up in these city lights. So, this 180 second exposure of M3 Globular Cluster is shown below:
|180 second image of M3 (Source: Palmia Observaory)
(M3 magnitude is 6.2 and comet 41P magnitude is 8.6)
Boy, seeing how dim and small that image is makes we sort of long for the bigger scope that was traded in so that the smaller and more light weight 80mm scope could be picked up. Oh well, I'm just getting more and more into ease of setting up than in having a bigger aperture.
Until next time,
Resident Astronomer George