Hallingskies

I think there is too much tech maybe. I have a written checklist of stuff to turn on and the order stuff has to happen in before I start imaging, and a similar one for shut down.

I’m glad I’m not the only one who feels that life is too short for PI.....

To look and not see is not a “fail”. To not look is a fail. I bagged it through my 8x30s last Weds night, but only because I was imaging it at the time and could line the bins up along the scope. It was a soft disc of greyish light only slightly brighter towards the middle, maybe about half full moon diameter and visible only with averted vision. Not sure how the mag 3 estimate was made. When I defocused beta Ursa Minor in the bins to the same diameter, that seemed much brighter.

A lovely shot, with 46P just sneaking into the frame at the top as a bonus...

Looking back I don’t know why I didn’t go for some longer subs as it was guiding a treat on the core in PHD. The weather looks like it won’t be letting me have another go any time soon...

From the evening of December 12th, I compiled this effort from 30 x 60 second luminance frames and 20 x 60 second each for RGB, guiding on the comet nucleus (equipment as per signature). I used a 20 second delay between each exposure to try and ensure no overlap in star images for the individual comet frames, with the idea that the sigma stack would then completely eliminate star trails (which wasn't entirely successful - maybe the delay wasn't long enough). I then let the comet drift for a couple of hours and moved the scope back to retake RGB frames (another 20 x 60s RGB) for star colour. Sigma stacking the comet images in AstroArt gave a star-trail-free(ish) colour image that I could layer the star RGB stack over ((using a single comet frame as reference). It was a complete faff, to be honest. For fast moving comets, OSC or DSLR cameras are definitely the weapons of choice when it comes to colour imaging, I reckon. I've had a real battle with colour balance, gradients and noise in processing this, but I've got something I'm finally happy with. Earlier in the evening I got a reasonable result from 30s subs tracking on a nearby star, but I was hoping the longer subs guided on the comet itself might show more detail. I think I just tempted some high clouds in to cast a fuzz over everything instead. No more 46P from me, I promise...

I find the sigma stacking in AstroArt is good for ensuring no star trails when stacking on a comet nucleus. I leave a 30 second gap between each individual sub acquisition, to try and ensure that each star in the “trail” is a distinct dot, as it were. That seems to avoid or at least greatly reduce residual trails in the final stack. Works for me, anyway...

Ow. Those lights... I used to have a similar problem but a less congenial neighbour. I set up a screen that I could put up by the scope to block out the worst of it.

Different for sure, but not as good as your truly excellent earlier version with stars. I think you show the value of the sigma stack though. Mono imagers like me wouldn’t stand a chance of getting realistic colour images without this technique being available.

Love the write-up. I understand your struggles...

Nice one....

Thanks, David. LRGB with a mono CCD is definitely a bit of a faff for fast moving comets. It was the first time I had tried it and there is a lot to be said for DSLRs/OSCs....

Subframes:70 x 30s luminance, 20 x 30s each for RGB (for comet, same again for stars). In this instance I guided on a star rather than the comet, limiting exposures to 30 seconds to avoid smearing out the comet given its rapid proper motion. I tried some longer exposures with guiding on the comet nucleus, but the subs were plagued with gradients and noise: I think some high cloud had set in. Using the sigma stacking function in Astroart, and "one star" alignment gave an image of the comet with star trails virtually eliminated. The LRGB stack showed the bright green colouration of the comet itself. I layered the RGB star stack (taken when the comet had drifted out of the field of view) over it to give coloured stars in the final image. Below is an animation using the 70 luminance frames from above, showing the comet moving against the sky background. This 3 second GIF shows the comet's movement between 19.07 and 20.01 GMT on December 12th, 2018, over a span of about 9 arc-minutes (about a third of a full moon diameter). The comet appears to graze the 9.7 magnitude Hipparcos catalogue star HIP 16342.

My own hapless efforts at imaging this comet aren't really worth posting here, but I thought I'd put up this Larson Sekanina-filtered version of my raw luminance stack by way of something different The LS filter is a tool offered up in Astroart that allegedly pulls out detail from comet images, and it indeed has flagged up what appear to be traces of separate dust and ion trails from the main nucleus. Possibly... Hope it's of interest...

I think the streaks may be residues of the star trails that were otherwise largely taken out with the sigma stacking. I have since reworked this image (see my blog) with a less aggressive stretch. This is a better rendition (I think) and has revealed some slight structure to the nucleus and a bit of a tail. As is so often the case, less is more...