On the night following the encounter between M108, Comet 41P and the Owl Nebula, the comet was still in the field of view of my setup, so I went back for seconds. I just couldn’t pass up the opportunity to shoot a comet conjunction again!
I went with 3 minutes sub-images at ISO 200, since the 1 minute ISO 800 subs were too cooked by the LP for my taste. Unfortunately, the sky was not as transparent as the previous night and that half-stop of underexposure was needed to get around that. The trade-off was I didn’t get as much of the comet’s coma. Oh, well.
Also, at that exposure length, the comet’s pseudo-nucleus trailed a bit, since it is moving with respect to the Earth and stars and slowly picking up speed, to boot. It was not enough to notice if I carefully over-exposed it a bit in processing to make it fatter, luckily.
A Televue TV-85 w/0.8x focal reducer/field flattener, a Canon T3, my laptop running EQMOD, driving my Atlas EQ-G mount and PHD2 Guiding with an Orion StarShoot guider/Orion Ultra-Mini guidescope was some of the equipment used.
It is not uncommon for comets to pass near famous Messier objects or NGC catalog objects and put on a good show. This comet encounter is special in that there are two famous objects that the comet is having a conjunction with. One is a relatively bright galaxy called M108. The other is the Owl Nebula, one of the better planetary nebulae in the skies.
I shot this with the Canon T3 and Televue TV-85 combo. I used one minute sub-images at ISO 800 and that was about max for the skies I was under. I’m sure people with darker skies got better results. However, I think being able to pull anything out of the skies at this location is great. LP was bad and I had terrible gradients to deal with in post-processing, but I managed. 🙂
I also did a quick star-streaks version that seems to show a longer tail. Not sure, since there were some dust doughnuts left over from an apparently bad batch of flats I used and I had to clone them out. These aberrations were in the tail area, so it could be some remnant of that.
One thing I’ve found out with using electronically assisted methods to view dim objects in bad LP conditions is this: Live stacking generally improves things, but only at first. If you want something as dim as Comet 41P to show any more than just the inner coma, you have to stack with lots more subs using more advanced stacking methods than just average and additive.
So, I shot 103 subs-images, along with 30 darks and about 30 bias images for this object. I brought them into IRIS and used only the first 60 because that is all I could get to align. The usual methods of aligning based on the drift per hour in x and y coordinates doesn’t work if the sub-images are not timestamped. This is another deficiency in the low-end capture and stacking program, RisingSky (ToupSky.) Not sure if SharpCap has the same issues.
Anyway, at least I was able to obtain a basic image and considering the conditions, it is not too bad. Compare this to my previous effort with the Aptina AR0130 sensor. This would probably work lots better at a dark sky site. But, I don’t know if I would waste time on shooting with the RT224 when I could use the Canon T3, which is more sensitive when it comes to comets and captures a much larger field of view.
I was fooling around with the Toupcam from a red/white LP zone and trying to get Comet 41P to show up. I ended up using SharpCap and doing live stacking with between 10 to 23 x 8 sec exposures for each stack. Then, I combined those stacks in IRIS and aligned them on the comet. It came out to be a total of 18 minutes worth of exposure, albeit in a heavy LP zone. So, I got it to show up, but the color and faint details of the tail and coma got lost in the noise.
I was able to catch 4 comets on the night of Feb 25/26th, 2017. Comet Johnson was the last one of the night and I got some good data on it this time.
I have processed the data like usual and included both a star freeze and star streaks version. I cropped these close at 100% full size.
The full frame is below:
Update Feb 27, 2017:
I actually made a mistake on processing these and the exposure time involved. There are only two (2) sub-images that were shot for 3 minutes at ISO 3200 (the first two) and the remaining sixteen were all 5 minute subs at ISO 1600. Doh! 😮
Update Feb 28, 2017:
I had processed them with the 180 sec, ISO 3200 darks and offsets and it still came out. I wrongly stated to myself (and this blog) that I would not reprocess them with the right calibration data, since it looked good enough. However, I’m glad I changed my mind:
This was much cleaner in the reprocess. I have two layers here. One is the comet and one is the star background. I didn’t have to slice and dice 10 different processing attempts and put together an image. I only used the minimum needed. Nothing from the first processing job was used.
Images of Comets, Nebulae, Galaxies and Star Clusters