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21 November 2012

Off Axis Guiders and Light Pollution Filters

Most of the time, I live in a moderately light polluted area. Like a lot of us, I use a standard light pollution filter to counteract the effects of sodium light and other artificial light when I'm doing my astro-imaging. Recently a new friend of mine explained to me that although they do a pretty good job of dealing with light pollution, they also filter out some of the light from stars and other astronomical sources, particularly in the red spectrum. After comparing some of my own astro-images with those of other astrophotographers, this became apparent, and I noticed that some of the stars in my images that should have been redder had been desaturated.

Obviously, no filter can ever replace a proper dark sky, but my friend very kindly pointed me to a certain type of specialised light pollution filter called a IDAS LPS filter. This filter, produced by Hutech, has much better colour correction. It lets all the important light through, while filtering out that unwanted light pollution, and without introducing that characteristic blue/violet tinge associated with cheap LPR filters.

Researching the IDAS LPS I noticed that many Nikon users were having trouble finding CLS versions of this filter, which clip into your DSLR before it gets attached to your imaging train. The benefits of this is that you can have your filter behind your off axis guider instead of in front. With a filter in front of your OAG it could darken already faint but usable guide stars. Now, I did find a Nikon CLS version of this filter. Only thing was, it was quite expensive and quite a bit more than the regular 2" filter. 

I took a look at my Nikon T-ring and noticed that it actually had a groove in it which would accommodate 2" filter glass. Thus, the solution presented itself.

Nikon T-ring and standard 2" LPR filter
I simply removed my LPR filter from its cell and inserted it into the T-ring. This is something I haven't seen done anywhere and thought perhaps people weren't aware of it, that being the reason for this article.

The 2" LPR filter glass inserted into the Nikon T-ring
The procedure was quite straight forward, needing only a small hex key and a small minus screw driver to unscrew the filter cell. 

In doing this I have come up with two solutions. I can now use my standard LPR filter behind my off axis guider, and when I'm ready to purchase the IDAS LPS, I can go for the cheaper 2" version and do the same modification.

So, thanks to Pawel for putting me on this train of thought.