My just posted try at Lovejoy has a glaringly obvious artefact in the centre of the comet. This artefact is obvious in the DSS autosave image.
Thinking that this must be a problem with DSS I have done another stack
of 15 subs from the original 24. If you increase screen mag to 175% you
can see the shape and shading of the artefact amidst the glare of the core. Is this genuine, or a stacking/ processing problem? I have done
almost nothing to the image other than a bit of saturation. No contrast
or sharpening adjustments, just resized for posting.
Any ideas anyone?
raymo
I did a capture yesterday for me, today for you, with ISO 400, ISO 800 and ISO 1600. The core of comet is normal. There isn't fragmentation or breaking up.
DSS, last vesion, has some problem with stack comets. What seems is that you have some frame(s) not well stacked. You have two comets in the result of stacking.
Calm down. You will see this comet for a long time !
note: I am processing the photos. When ready I will post here. My goal is stack by stars and comet and creat an animation gif with its movement. When ready I will post here.
You must try to find what frame(s) isn't (aren't) stacking well. It's a long and hard task. Use first 2, first 3, first 4 ... up to identify the frames that aren't working well.
You can check each frame and see who are with big misalignment, also.
I can see that you would get elongation of the comet when DSS is using the stars for stacking; I had it with comet Lemmon, but this artefact is
not just elongation, it is ragged in outline like a cloud, and there is a smallish green hole visible in the lower part of the core. Still not sure
what it is, although you are quite possibly right.
raymo
Thankyou everyone for your insights and comments. When I did Lemmon
I had the expected elongation, but it was absolutely straight, not ragged
like this one, which is why I wondered what was going on. No I didn't
set the comet centre for each frame. As the subs were short, I didn't expect any problems.
As I had no problems with Lemmon, I didn't see the necessity. I will play with the frames as Jorge suggested.
raymo
Last edited by raymo; 10-01-2015 at 10:48 PM.
Reason: more text
I find good results with guiding on the comet and keeping the exposure short enough that you don't streak the stars too much - after all its the comet were interested in right now.
Thank you everyone. The problem has been identified as being within
DSS. Apparently DSS has a comet tracking function, so I will stack all
24 frames again. The individual frames are all good, so I shouldn't
have any further problems. I was mislead by the beautiful straight
elongation I got with Lemmon as opposed to the raggedy effect I got this time.
Kevin, I'm not particularly into comet imaging, just thought I'd get a shot, as it is such a large bright one. My last comet image before Lemmon was Halley.
raymo
You need to open the stacking options and set it to stack on "comet" or "comet and stars". You then need to register the frames as normal. Then go through each frame with the "comet tool" selected and select the comet out of the star field (if it doesn't recognise the comet then you'll need to shift-click to select it). Then you stack.
Stacking on the comet will give trailed stars. I like this effect because it gives the comet a sense of motion. But if you want both comet and stars frozen, it gets complicated. One easy way is to simply limit the number of frames so that the comet doesn't trail too much. In your example I would suggest 6 or 7 frames using a median combine in DSS to freeze both. Basically this is what I do with mine. Works for me anyway.
I don't like doing dual stacks. They are very fiddly to get right.
