[BAA Comets] Use of light pollution filters

[DENIS BUCZYNSKI]

Hi Peter/all,
Your question and comments about the use of filters for comet imaging is answered with the phrase "horses for courses". It depends on what you are trying to derive from your images. If you are using your images for astrometry then filtering probably makes no difference. If however you are wanting to derive photometry from your images then standard procedures need to be applied. Some of these allow for unfiltered  images (such as the FOCAS approach) others used to derive photometry that includes dust (Afrho) measurements require R band imaging (CARA approach). To isolate cometary emmisions, more specialised narrowband filters are required (large aperture telescopes are also needed for these type of observations).If the purpose of the imaging  is to study the changing morphology and brightness of the comet then all the types of filter observing disciplines outlined above are required.

If however there is no intention to make measurements of the comet image you are taking, then the use of light pollution supression filters is acceptable. The only reason to restrict the incoming light (with filters) from the comet onto the ccd chip is to measure and detect the flux at different wavebands. If what you want, is an impressive picture showing the comet in the least noisy ccd image, then that too is fine. You only need to look at the superb images produced by the best comet imagers to see that they use all sorts of filtering and software processing to enhance their images. They do not take those images with the intention of making measurements on them.  The best approach is to take images using a variety of filters and use the resultant frames for measurement or image processing, dependant on your required outcome. Of course this requires a number of specialised filters in computerised filter wheel to enable you to take the extra number of
 frames (which all tend to require longer exposures). Long clear periods of the night sky are also needed to obtain so many frames in different filters ( no so easy in the UK) . So if you want to get that impressive low noise image of a comet and a IDAS light suppression filter will help you acheive that end, then my advice would be to use the filter and make sure that the type of filter is recorded as part of the FITS file header. If I am talking rubbish with all the above then I will be more than pleased for someone with more expert and technical knowledge to correct what I have said.That is what a discussion group is intended for. The most important aspect is to get out there and take images however and with whatever you can. 
Denis Buczynski




>________________________________
> From: Peter Carson <petercarson100 at gmail.com>
>To: BAA Comets discussion list <comets-disc at britastro.org> 
>Sent: Thursday, 14 November 2013, 13:33
>Subject: [BAA Comets] Use of light pollution filters
> 
>
>Hi All,
>What do people think about the use of an IDAS Light pollution suppression filter for comet images?
>I do my comet imaging without any filters whatsoever to maintain any scientific value of the raw data. I also image "pretty pictures" and use all kinds of filters including IDAS LPS filters. 
>My observatory is in a heavily light polluted urban area and I know the aesthetic image quality can be improved by the use of a filter but want peoples views as to whether that compromises the RAW data.
>PS, below is a link to yesterday mornings 2013R1 image showing a gas tail over 1 degree long stretching out of the camera's field of view.
>http://www.astromania.co.uk/2013R1_20131113_0447_PCarson.jpg
>
>Regards
>Peter Carson
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