[BAA Comets] Help please?

[Peter Carson]

Hi Mike,
By monochrome I mean black and white not colour. Imaging with no filter or through a clear luminance filter is much the same thing. Usually the clear luminance filter is put into the LRGB set to make all the filters par-focal, therefore there’s no need to refocus when moving between the luminance sub images and the RGB sub images. Having said that most luminance clear filters contain an ultra violet and infrared blocking coating which is invisible to the naked eye but will help sharpen the images if your telescope can’t focus the wavelengths just outside the visible range.

Higher end mounts sometimes do have the ability to track at custom rates in RA and Dec which can be useful for following a very fast moving comet.  Using custom tracking rates will allow you to make exposures of a longer duration than otherwise. Despite this and the fact my mount will track at custom rates I still recommend you take a number of shorter duration exposures either tracked at sidereal rate (or at a custom rate) and stack them together. Try experimenting and decide for yourself.

All the best
Peter


From: Mike Paling 
Sent: Wednesday, May 02, 2018 7:08 PM
To: BAA Comets discussion list 
Subject: Re: [BAA Comets] Help please?

Hi Peter ...
Thanks for this .. I will try these ideas out when I get the next opportunity.

By “monochrome” do you mean using my L filter ... or is it no filter???

I forgot to say in my first message that my mount “allegedly” can track comets as they move against the stars after I input the delta RA & Dec values.

Regards Mike


From: Peter Carson 
Sent: Wednesday, May 02, 2018 6:56 PM
To: BAA Comets discussion list 
Subject: Re: [BAA Comets] Help please?

Hi Mike,
Welcome to the fascinating world of comet imaging. 
Imaging comets is similar to deep sky object imaging except the target is moving! Because comets move you’ll find an exposure longer than a minute or two will smear out the comet. The actual longest exposure duration will of course depend on how bright the comet is and how fast the comet is moving. What makes comet observing fascinating is that all comets do different things including how fast they move across the sky.
Your telescope at f/6 is well suited to comet imaging so I’d start off by picking a moderately bright comet and take several consecutive exposures (say 10) of about 60 to 120 seconds duration in monochrome only. Then stack the images with the comet as the reference point. You’ll find the resulting image shows the comet fixed against a background of trailed stars.  This is the technique most comet imagers use.
Have a read of the BAA’s Comet section observing guide, section 5 deals with imaging. https://www.britastro.org/node/6817 When you mastered monochrome then LRGB colour imaging will be less difficult because you’ll have an appreciation of the likely problem areas.
Good luck
Peter Caron
From: Mike Paling 
Sent: Wednesday, May 02, 2018 5:59 PM
To: comets-disc at britastro.org 
Subject: [BAA Comets] Help please?

Hi ...
I am trying to get into imaging comets .... I am very much a beginner with this aspect of astro photography.

I have a pretty decent permanent setup here in Nottinghamshire ... my mount is capable of up to 20 minutes unguided tracking and my CCD mono camera has LRGB (and also Ha) filters. My scope is a 123 mm x 780 mm focal length f6 refractor.

Can anybody suggest suitable exposure times to use with my LRGB filters? 

Many thanks in anticipation of any help that can be given to my question :-)

Regards Mike




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