Comet NEOWISE again

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Jan
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Having got an image I'm not happy with on Sunday night and sketched it on Monday night (before it disappeared behind a cloud), I had another go for an image last night, which may be my last chance as it is definitely fading (along with the forecast of clear nights here). Another night - another different approach. This was 22 10s subs, integration and most of the processing done in PixInsight.
Specs - Canon 800D, Sigma 105mm lens, f2.8, iso 1600

C2020 F3 NEOWISE by Jannyfox, on Flickr
 
Very nice Jan- may I enquire as to what shutter speed you used ?

Les c:)
 
Well done indeed.
 
Very nice Jan- may I enquire as to what shutter speed you used ?

Les c:)

10 sec, Les, but 22 10 sec frames integrated (stacked), so 220 sec (3 min 40) total exposure. 10 sec was the maximum I could use and guarantee no trailing in the stars of the comet nucleus. I also used dark frames to reduce sensor noise. I use PixInsight, a dedicated astrophotography software package, to do everything bar a few final tweeks.
 
10 sec, Les, but 22 10 sec frames integrated (stacked), so 220 sec (3 min 40) total exposure. 10 sec was the maximum I could use and guarantee no trailing in the stars of the comet nucleus.

Thank you for the pic and the technique.

May i ask what the astro software contributes to the result?
 
Sorry i meant: what does it contribute exactly?

Ah sorry. Speed reading isn't always good.
The process is -
Subtract master dark frame from each light frame to minimise sensor noise
Perform another correction that further minimises noise, hot pixels etc
Align the frames with each other
Integrate, ie add/overlay them so 22 10 sec exposures become one 220 sec exposure
Remove skyglow or unwanted light from the sky background
Colour correct
Stretch ie adjust the histogram
Finally do the usual minor levels tweeks/noise reduction/sharpening as necessary
And probably a few other steps I never remember off the top of my head. I have a written workflow as it's not that intuitive. The software was originally written solely for Linux, which probably tells you everything you need to know about it............

Hope that helps
 
Ah great, l'm on Linux Mint. Thanks for the workflow, Bookmarked.

Love the noctilucent clouds shot further on in your flicker set for this comet btw.
 
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10 sec, Les, but 22 10 sec frames integrated (stacked), so 220 sec (3 min 40) total exposure. 10 sec was the maximum I could use and guarantee no trailing in the stars of the comet nucleus. I also used dark frames to reduce sensor noise. I use PixInsight, a dedicated astrophotography software package, to do everything bar a few final tweeks.


Thank you Jan- sounds very complex, I think I'll stick to Wildlife :LOL::LOL::LOL:
 
@Jannyfox , I'm acquiring a fast wide prime next week and I'm looking to try some astrophotography which is why I'm fascinated by your project. Please excuse my ignorance but I'm unclear about one thing. From what I've read about the "500 rule" I've worked out that my shutter speed should be 20 seconds per frame to avoid star trails but you are are talking about 22 10 second frames and "integrating" (is this like focus stacking?), surely this would result in massive star trails. Maybe I've got the wrong end of the stick!

That PixInsight is very expensive isn't it?
 
@Jannyfox , I'm acquiring a fast wide prime next week and I'm looking to try some astrophotography which is why I'm fascinated by your project. Please excuse my ignorance but I'm unclear about one thing. From what I've read about the "500 rule" I've worked out that my shutter speed should be 20 seconds per frame to avoid star trails but you are are talking about 22 10 second frames and "integrating" (is this like focus stacking?), surely this would result in massive star trails. Maybe I've got the wrong end of the stick!

That PixInsight is very expensive isn't it?

Wrong end of the stick - yes, partly. The term 'astrophotography' is open to interpretation. I'm an astronomer first, photographer second, so my idea of astrophotography is imaging what I see through a telescope. For others, astrophotography means imaging a night time landscape with the Milky Way in the background, often with a bit of light painting to make everything look pretty. While I do enjoy looking at those images, it's not what I do.
Two important things to remember, both having to do with the fact that the Earth is rotating so the stars move relative to us or our cameras. First - the shorter your focal length the longer you have before you'll see unacceptable trailing in the stars. Note unacceptable. If you have to zoom right in to the image to see it, it's not going to be a problem for normal viewing. Second - The further your camera is pointed away from the celestial pole (the Pole Star in the northern hemisphere) the faster that movement.
Back to the comet. I was using a 105mm f2.8 lens. If I'd used my 17-50mm f2.8 lens at the 17mm end I'd have got much longer before trailing, but a much smaller image of the comet. It's all a trade off. The comet was also below the Plough and a fair way from the celestial pole. I'd done some experimenting with single frames a couple of nights before this shot to determine the maximum exposure length I could get.
Integrating is not focus stacking. Essentially you take exactly the same image multiple times and stack them one on top of the other, but by overlaying the stars, hence no trails. The software does the clever calcs for you. You build up the exposure but with a better signal to noise ratio than a single, long exposure. I would normally do this working off a driven mount as I would normally be imaging much smaller objects than a large comet, with either a 600mm lens (on a crop sensor camera) or telescope. At that sort of focal length time before star trailing = zero. I could have hauled the driven mount out for the comet, but I didn't really need to so I saved myself the hard labour. By driven mount I mean one that moves at the same rate as the stars. You may ask if you use a driven mount why then not take one long exposure. That's what I did in the pre digital days and it was so frustrating when a 45 min exposure was ruined as the telescope drive jumped for no reason, a cloud sneaked up out of nowhere or someone wandered past with a torch. Apart from the previously mentioned signal/noise, shorter exposures further limit sensor noise, you can stop the camera if a cloud sneaks up and carry on when it's gone, and any other random duff exposures can be dropped so you only integrate the best subs. Dark frames are also all about further reducing noise by 'mapping' the sensor.
PixInsight is expensive, but then it's a dedicated astrophotography package. As is Maxim DL, which I believe is even more expensive but will also control your telescope if you have a compatible mount. None of it's really designed for people who want the occasional pretty picture of the Milky Way. There is free software called Deep Sky Stacker which I would recommend for a beginner. I used it for quite a while until I made the investment in PI.
Sorry to go on a bit but hopefully it's clarified a few things.
 
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