Preparing for comet aquisition - need some advice - Forums

Topic starter

Feb 16, 2024

Hi to the community,

lately I tried for the first time to get some images of a comet with a guided sope. Although I was kind of successful, there are some questions that arised. There are a lot of processing tips out there, but the process of data aquisition is less clear to me.

First, I want to start with the camera. I am used to put my mono cameras on the scopes for deep sky imaging. But with a mono camera, I have to record the color channels seperately one after the other. I guess, this may be problematic, because of the fast movement of the comets. Stacking for the background seems to be difficult this way. Are YOU using OSC camras for your comet images, or are mono cameras usable as well? I own a Explore scientific 16MP ATR3CMOS16000KPA camera with the older Panasonic sensor as OSC-Version. This may be an option for me, although I don't really like imaging with this camera. There is also a QHY 183c Pro, but its small sensor may be suboptimal for the needed wider field of view.

I realized, that the scopes used to image comets are usually shorter focal length scopes. The size of the tail surely is the reason for that. Is this observation correct? How do you choose your scope? I find it hard to guess the tails length eapecially if the comet is still far away from the sun. Are there any techniques to prepare for this? And related to this topic: how do you find the optimal exposure time to get a sharp comet?

I usually record subs over multiple nights for my deep sky images. Because of the moving background (relative to the comet), this may not be a useful option here? When I tried my first comet, I centered the comet in the frame, because I had only a vague idea of where it may move to. I used Stellarium and SkyChart to find the tails direction for framing. But of course, the comet drifted out of the frame during the session, because I guided for the stars as suggested by other users. How do you deal with this? are you repositioning the comet to the center of the frame occasionally? I use NINA. How may I be able to do this? How long is your total integration time?

Maybe, there are some other things I missed. I would apreciate any advice, that helps to get started. My goal is, to use one of my three (kind of) permanent setups to hunt some comets in the future. This is getting more and more interesting to me. So maybe your answers may help to save some time and got on track a bit faster.

Thanks in advance

CS

Christian

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Elmiko

Feb 17, 2024 · Edited: Feb 17, 2024

You can capture a Comet in a single night. I just posted an image of P62 Tsuchinshan. I used around 60 180 sec subs. And Comet Integration in Pixinsight. I used a Samyang 135 mm lens, do wide field. No problem with the comet drifting out of the frame. You don't need a whole lot of time on comets. They are fairly bright.
Hope this helps.https://astrob.in/5uiv51/C/
Mike

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Jan Erik Vallestad

Feb 17, 2024 · Edited: Feb 17, 2024

Mono for comets is fine, as you mentioned you have to rotate the filters in a sequence. If you have filter offsets as a part of your controls then it will be a non-issue, if not then it helps to have filters that are as parfocal as possible. I use the Asiair so I trust the parfocality of my filters which seems to work well, I focus once and let it roll as long as I need it to. Adam Block claims that mono is simpler than OSC when it comes to post processing (I only own mono, so I can't comment on that).

As for scopes it really depends on several factors. Most comets will be very small and by using too short focal lengths any detail in the tail itself will be tough to spot. The very best images out there are taken with the likes of C11 RASA's or huge & fast 14" scopes - at least the best images I've seen. The key is to use fast scopes in order to take short exposures and get the most detail. The pro's also seem to use a relatively short amount of exposures (around 30 minutes or a bit more I think). Since I'm at 550mm and F/5.5 at the moment I've let it run for 1-2 hours with 60s exposures, I'm not picking up any fine details anyway.

I'm imaging 62P Tsuchinshan as we speak, it's a tiny tiny dot at 550mm. 12P Pons Brooks was a little better, I did a couple of takes on it a week ago, but not even close to C2022 E3 ZTF last year - That one will get closer for those in the southern hemisphere though. If there's a comet getting really close like C2022 E3 ZTF a fast widefield scope makes more sense.

You can collect subs for the background during several nights if you want to get NB data, galaxies or faint dust. But the comet itself should only be captured during that particular session as it's changing very rapidly. If you use Stellarium you can get all sorts of information about the approximate tail length, coma size, position, speed etc. It gives you an idea about framing as well. Just remember that the position of the tail is determined by the solar wind, not the direction of the comet.

There are two options as I see it, you guide on the stars and frame it for the length of your session. Or you guide on the comet itself, and then shoot the background separately. I've only properly used the first method though. Generally I would figure out where it's headed and how far it will move during the planned session and roughly frame it based on that.

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