Greetings from Palmia Observatory
Well, we left off last time with one remaining mystery regarding the different values of reported RA and Dec for Pluto. What to do about these different reported values? They don't differ by much and as long as you are just interested in getting Pluto in your camera frame field of view, both seem to be acceptable. But if you want to use astrometry to verify that yes, indeed that dim object of just a few pixels in your image is Pluto, then, yes, you need accurate astrometry and the catalog location used for finding Pluto has to be right. So this is what
I found for an explanation.
The first value (identified as location T1 in the image shown in the previous post of November 2) was obtained from my old copy of USNO MICA software. The second value (identified as location T5 in the image shown in the previous post of November 2) was obtained from the website TheSkyLive. They differed by about 16 arc minutes, which is ok for visual observing and by the way, both of these locations are in the camera field of view. But I was expecting these two data points to agree within arc seconds or fractions of arc seconds, not arc minutes.
It turns out that the RA and Dec of planets, both large and mostly the small and minor planets, like asteroids and Pluto, vary quite a bit and the predicted locations needs to be updated periodically. The MICA prediction used something called "the equation of time" which is the difference between civil time and astronomical time, and the equation needs to be updated periodically. Well, my MICA software did not have the latest equation of time. I examined the "About" icon and found the latest version of the equation of time was from 2011. Well, no wonder when I calculated the position of Pluto it is off by 16 minutes. But, when I went to find the newer equation of time, I found that a newer one was not available. I found a history of updates from about 2000 up to 2011, but none after that. So, I guess I'm just stuck with the old equation of time. So from now on, I'll rely more on Megastar and several online catalogs from Lowell Observatory and the Minor Planet Center.
You may remember from my previous post of that my predicted location of the asteroids (Pallas and Ceres) was off too, but that discrepancy was fixed by downloading the latest orbit calculation for my Megastar software. Based on this whole episode, I began investigating about getting a second copy of astrometry software, just so I could compare different approaches of doing astrometry. I have been using AIP4WIN for over a year now, as has many other amateurs, and are quite happy with its performance.
I had consulted with OCA Secretary and Author, Bob, about other astrometry software packages, and then decided that I would try the MPO Canopus software. It only costs $65, which is about $30 cheaper than AIP4WIN. But AIP4WIN comes with a 600 page image processing textbook, while MPO Canopus comes with a free online PDF. Both packages do astrometry, star image analysis, and magnitude determination. Sadly MPO Canopus does not accept DSLR image files, so I will have to convert all of my images to FITS format first, so it will not be as convenient as AIP4WIN. Oh well, I'll try them out and see what the good and bad are and how it might offer better analysis for me.
Until next time,
Observing with Street Lights
Dark sky sites not always necessary to see the Milky Way (This image was taken ouside of a B&B in Julian, CA)
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