How to Image a Comet?

[Matty P (Matt)]

Seeing that there has been a couple of great images of Comet Boattini C/2007 W1 that have been posted in the imaging forum. I am thinking to have a go at imaging it.

The only problem is that I am bit unsure on how to go about it. Is it just like imaging any other celestial object? …or is there a special procedure for imaging a comet?

Seeing that my setup can only handle fairly short exposures I am wondering if I will I be able to capture any kind of detail?


I'm a little confused.

[Ian Robinson]

Most my comet imaging has been with telephotos.

Comets move against the starry backgound , so unless you track the comet , it will be burred somewhat unless the exposure is too short for it trail against the stars.

But then if it is too short an exposure , you not record much or as detail as you might hope to see.

It is best to keep the exposure short unless you are guiding on the comet's head manually , or autoguiding on it (not sure how you would do that with fuzzy object) ,or you can program the mount to track it from the comet's orbital elements.

My 10" newt in currently in pieces while I prepare to rebuild it (with a better tube) and while I wait for my Atlux to arrive. So wont be trying C/2007 W1 for a few months.

The trick is to experiment , and keep on trying , it'll be there for a little while.

BTW : Comets always show more detail in the negative image .... than in real colour positive image ....

[Matty P (Matt)]

Thanks for your reply Ian,

I will using my C8 on a Alt-Az mount so will a 30 second exposure be enough to capture any kind of detail?

[Ian Robinson]

Originally Posted by Matty P

Thanks for your reply Ian,

I will using my C8 on a Alt-Az mount so will a 30 second exposure be enough to capture any kind of detail?

It's an 8" , sure unless it's faint (and faint comets don't show as much more than a fuzzy blob anyway) , but unless you are precisely tracking it (guiding) , with the focal length of those things , it will trail and with long exposures there is field rotation to deal with.

An equatorial , or a wedge for your altaz would be a good investment.

I got some nice photos on film (400 ISO) of McNaught last year with exposures as short as 15s but more detail with 5 and 10 minute exposures with my telephotos. Some were even with a camera just sitting on a fixed tripod (been a long time since I did that ... they worked out better than I expected even if there was some star trailing).

[rogerg (Roger)]

The main difference I have found for photography of comets through a telescope (quite different to wide field fixed tripod stuff like McNaught last year), is processing. You need to align the images on the comet not on the stars. Not all software will do that. I do it using CCDSoft and it's centroid alignment capabilities.

Other than that, I agree with keeping the exposures short, for simplicity. The exposure time depends on the rate of movement of the comet, but I often find 30-60 seconds isn't a problem. Also if you're just wanting a pretty picture you can push the exposure time a little longer than if you need a dead steady comet for analysis, as a slight bit of trailing in the comet head isn't usually noticable.

There is software out there which will tell your telescope to track at the correct rate for the comet. I asked in this forum about it a while back (maybe 6 months or so) and got some replies but have forgotten it's name - I'd suggest searching here. I haven't tried it, but even if it's half good, it'd mean an increase in exposure time before trailing of the comet head which would be beneficial.

As Ian said, equatorial is much better. I'm not sure how you'll go Alt-Az. Alt-Az short exposures of general starfields is OK because you can use programs like RegiStar to align the images can cater for field rotation in doing so. But for a comet, where you're aligning on just the head of the comet, I'm not sure how you'd cater for field rotation unless the comet is moving slow enough that you can do a normal alignment on the whole starfield.

Roger.