My solution was to use my autoguider to track the comet's nucleus. Lovejoy has a bright nucleus; if you look at it in binoculars or a small telescope the fuzzy green nucleus is all you can see (or at least it's all I have been able to see visually), and it is easy to see with binoculars so I recommend taking a look while it's around. Evidently the green color is due to emission from C<sub>2</sub> molecules coming off the comet and being excited by sunlight. Guiding on the nucleus isn't ideal, because the nucleus is pretty large. The guider has a hard time staying centered, but I wanted long exposures so I could pick up the tail.
My first attempts were on 7 Jan 2015. I had coordinates for the comet's position on 7 Jan and 8 Jan. I found the comet by synching the scope using an Astrotortilla plate solve, doing a goto to the 7 Jan position, then, after slewing around and failing to find the comet, going to the 8 Jan position. I knew the comet would be somewhere between the 2 points, and it was. I took a bunch of 120 s subframes, guiding on the comet. It was fairly easy to combine them into an animated GIF in Photoshop, making a "movie" of the comet's motion, which was fairly fast.
Date: 7 Jan 2015
Subject: Comet C/2014 Q2 Lovejoy
Scope: Orion ST80 + Antares 0.5x telereducer
Filter: Baader Fringe Killer
Mount: CG-5 (Synta motors, PicGoto Simplificado)
Guiding: 9x50 Finder/Guider + DSI Ic + PHD 2.4.2 (Win 7 ASCOM)
Camera: DSI IIc no cooling (5 °C)
Acquisition: Nebulosity 3.2.2, no dither
Exposure: 56x120 s (not stacked)
Stacking: Neb 3, bad pixel map, bias included, no flats, histogram match, debayer, trans+rot align, 1.5 SD stack.
Processing: Photoshop CC 2014 + Carboni Astronomy Tools levels, auto color, frame animate
I had a lot of trouble trying to stack the comet images. Deep Sky Stacker has a comet stacking feature that follows the comet nucleus with one set of images and stacks them, then aligns on the stars, ignoring the comet and stacks them, then combines the two. However, I just couldn't get that to work well for me (and I tried pretty hard; no doubt I've been doing something wrong). I had the best luck using Nebulosity to align with a "translation only" method, aligning on the comet nucleus. This makes the stars trail, but a rigorous standard deviation clip gets rid of a lot of them. Finally I had some dust bunnies to clean up in Photoshop. Here's a single frame, followed by the stack:
Imaged through hazy skies (it’s inversion season here). This is a single exposure as I could not figure out how to get stacking to work. At least I was able to capture a little of the tail (I deliberately overexposed the nucleus in order to do so).
Date: 7 Jan 2015
Subject: Comet C/2014 Q2 Lovejoy
Scope: Orion ST80 + Antares 0.5x telereducer
Filter: Baader Fringe Killer
Mount: CG-5 (Synta motors, PicGoto Simplificado)
Guiding: 9x50 Finder/Guider + DSI Ic + PHD 2.4.2 (Win 7 ASCOM)
Camera: DSI IIc no cooling (5 °C)
Acquisition: Nebulosity 3.2.2, no dither
Exposure: 1x120 s (not stacked)
Stacking: Neb 3, bad pixel map, bias included, no flats, histogram match, debayer, trans+rot align, 1.5 SD stack.
Processing: StarTools 1.3.5.289 Contrast; HDR:Equalize; Contrast; Color: blue bias reduce 1.72, green bias reduce 1.93, red bias reduce 1.61; Track 1.8 pix. Photoshop CC 2014 + Carboni Astronomy Tools levels, auto color, deep space noise reduction, more levels, Astroframe.
On 23 Jan 2015 I had heard about the triple shadow transit of Jupiter's moons across the planet (I'll post separately about that), so I hauled my gear out and decided to image Lovejoy again while I was waiting for Jupiter to rise. This time I had current coordinates (I found you could import the comet empheris data into Astroplanner, which then gives current coordinates). So I synched with an Astrotortilla plate solve and did the goto, and I was right on! Amazing! This time I framed the shot to get as much of the tail as possible, and guided on the nucleus again. There was a lot less moonlight, and the comet was near the zenith (in fact, my scope bumped toward the end of the run, but the guider kept the comet in frame as the camera rotated so I didn't know it until I started processing). This time I went straight to Nebulosity and stacked on the nucleus as before. I like the result.
Decided to try Lovejoy again while waiting for Jupiter to rise so I could attempt imaging the triple shadow transit. Found the comet by Astrotortilla plate solving to sync the PicGoto, followed by a goto the coordinates reported in Astroplanner. This worked great. Guided on the comet nucleus. The sky was darker than it was on 7 Jan, and I can see a lot more tail than then. The proper motion of the comet was also noticeably less than on the 7th.
Date: 23 Jan 2015
Subject: Comet C/2014 Q2 Lovejoy
Scope: Orion ST80 + Antares 0.5x telereducer
Filter: Baader Fringe Killer
Mount: CG-5 (Synta motors, PicGoto Simplificado)
Guiding: 9x50 Finder/Guider + DSI Ic + PHD 2.4.2 (Win 7 ASCOM)
Camera: DSI IIc no cooling (5 °C)
Acquisition: Nebulosity 3.2.2, no dither
Exposure: 45x120 s
Stacking: Neb 3, bad pixel map, bias included, no flats, histogram match, debayer, trans (only) align, 0.5 SD stack.
Processing: StarTools 1.3.5.289 Crop; Develop 69.89%; Color:Scientific 201%; HDR:Optimize; Life:Less=More; Track 2.0 pix. Photoshop CC 2014 + Carboni Astronomy Tools 2 stages of dust & scratches with Gaussian blurred layer masks to take out one slight dust bunny; Astroframe.
After reading some advice about how to "star freeze" a comet image (see the url below), I decided to try one more time using Deep Sky Stacker to stack my 23 Jan 2015 Lovejoy images. I reprocessed roughly following the method of Tony Cook (http://astrob.in/151422/0/) except I didn’t do any of the "sieving" described in his method; it didn’t seem to help (in retrospect, I may want to try again with courser "sieving"; that might get rid of the remaining star trailing). As a result, I still have some star trail streaking in the comet’s tail, but I think this is dramatically better than what I got earlier. Deep Sky Stacker’s “comet only” mode gave me a sharper comet than I was able to get stacking with Nebulosity. I tried darkening the overall image to remove some of the star trail streaking, but this sacrificed some of the tail, so I backed off of that. Here's the result:
After reading some advice about how to "star freeze" a comet image (see the url below), I decided to try one more time using Deep Sky Stacker to stack my 23 Jan 2015 Lovejoy images. I reprocessed roughly following the method of Tony Cook (http://astrob.in/151422/0/) except I didn’t do any of the "sieving" described in his method; it didn’t seem to help (in retrospect, I may want to try again with courser "sieving"; that might get rid of the remaining star trailing). As a result, I still have some star trail streaking in the comet’s tail, but I think this is dramatically better than what I got earlier. Deep Sky Stacker’s “comet only” mode gave me a sharper comet than I was able to get stacking with Nebulosity. I tried darkening the overall image to remove some of the star trail streaking, but this sacrificed some of the tail, so I backed off of that. Here's the result:
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