Well this time we have some discussion of the last OCA general meeting and more images and analysis for the Comet 46P/Wirtanen, but before that we should mention
the scheduled upcoming launch of a Delta IV Heavy from Vandenberg. The launch, after being scrubbed at least two times that frustrated this photographer, is now rescheduled for Tuesday, December 18, 2018 at 5:57 PM. For those living in OC, the compass reading to the launch site is right around 300 degrees azimuth. So get your cameras out and enjoy the predicted weather for clear viewing. If you want to experience the launch closer up you can check out our visit up there for a closer view of the SpaceX Falcon 9 launch as described in our October 8, 2018 blog post.
The OCA general meeting this month featured a presentation by Riccardo DeSalvo, CSULA. He discussed how the gravity waves were detected by the LIGO observations and some of the key findings just released last week. His enthusiasm for the subject, primarily that of an instrument maker, was evident and it was very worthwhile hearing about all the effort that goes into building the LIGO system so that the very minute effects of passing gravitational waves can be detected and isolated from other earth borne seismic disturbances which must be reduced by a factor of several billion times. Thanks for that great presentation, Riccardo!
While I was somewhat familiar with the instrument details, the one finding that I was intrigued by was the observation that of all observed mergers so far the masses of the binary black hole pairs all seemed to be about the same relative size. Of the ten mergers so far, when you look at each merger, the two black holes are roughly the same mass size. Mergers of a giant black hole with a tiny partner is just not observed. Why is this size pairing going on? There also appears to be a gap in the masses below about 8 solar mass size. Will the pairing data be found to be real and not just due to a limited sample size of mergers? What astrophysics associated with the formation of binary black holes could be causing this rough pairing in size? If Riccardo mentioned the answer, I missed it. We will have to look into this situation more in the future!
|Riccardo DeSalvo, CSULA, talks about gravity waves at OCA general meeting (Source: Palmia Observatory)|
Ok, time for more updates on Comet 46P. While we waited for the clear skies to take another look at the comet, we received an email from Gravity Guy, Ken, who passed on a video from his friend Joe, who took some images of the comet as seen from Mexico. Wow, this is a pretty clear image, with dark skies, taken on December 11. That is a beautiful blue/green color. Thanks for that Joe (and Ken)!
|Comet 46P/Wirtanen as seen from a location in Mexico (Source: Joe McMenamin)|
So, when the skies cleared up on Saturday night, it was time to try to get the comet again, especially now just before it is predicted to reach maximum visibility. The 120 second exposed image below was taken in bright city lights. Again, I just star hoped from Aldebaran, which I could naked eye see, to 46P, which was 10 degrees higher in elevation and about 1 degree right in azimuth. The image is full scale, 2.8 x 4.2 degrees, as stored in the camera frame. Photoshop was used to stretch the image to achieve the best contrast, but unfortunately the color detail was lost in this process. I'm not proficient enough with Photoshop to maintain the original color while at the same time enhancing the contrast above the polluted sky floor.
|Comet 46P/Wirtanen, 300mm DSLR, 120 second exposure, stretched in Photoshop (Source: Palmia Observatory)|
Let's look at this reddened image a bit more though and make estimates of the size of the comet in the image and also how well the little DSLR Sky Tracker managed to keep pace with the Earth's rotation. The apparent size of the comet is estimated to be about 33 pixels in the horizonal direction and 23 pixels in the vertical direction. Then for the 300mm lens and DSLR sensor size and pixel resolution the angular size can be estimated. The vertical dimension, in this instance is less affected by the tracking misalignment and 23 pixels corresponds to just a bit over an angular diameter of 1 arc minute. By way of comparison, many globular clusters, such as M3, have apparent sizes of 18 arc minutes in diameter.
With the camera pointed towards the sky position of the comet, the Earth's rotation rate shows up as 9 degrees/hour in the vertical direction and 12 degrees/hour in the horizonal direction. The vector sum of these two motions is 15 degrees/hour, which agrees with the Earth's rotation rate. The star tracking is about 17 pixels in length, which corresponds to about 0.8 arc minutes.So, the tracking is very good in that there is hardly any effect in the vertical direction and all of the star tracking is in the horizonal direction. So it seems that even in the horizontal direction, the tracking error is just 0.8 * 30 = 24 arc minutes per hour. Not too bad for just lining up the tripod to point north just by eyeball!
Finally, the stretched image was converted to black and white, just to see if any more detail, maybe a hint of a comet tail, for example, would show up, but not much is there. I didn't attempt to do any comet visual magnitude estimates other than to note that many of the background stars, while not being definitely identified in the image, were in the 9-13 magnitude range. So, that is about it for this comet imaging session. How did everyone else's observing session go? Anyway, for now, let's hope for clear skies for the upcoming Delta IV rocket launch!