As you know I have recently started terrestrial imaging [photography] with a Canon 40D.  Whilst looking at all the accessories for the new camera I came across some interesting filters called R72 filters.  These filters only allow light of wavelength longer than 720 nm through, that is they are infrared filters and they block out all the shorter [visible] wavelengths.

 Now this is quite interesting for deep-sky work as well as it allows a form of infrared astronomy.  Unfortunately the Silicon-based CCD means we have a long wavelength cut-off at about one micron, but the R72 filter will allow us to image over a relatively broad band of wavelengths from 720 nm to 1,000 nm.  Although this is not narrowband imaging, it does cut down any visible background light pollution considerably, so that you should gain a lot in contrast, and you can also image with the Moon out.  In fact the accompanying image of M42 was taken with a half Moon nearby, and the subs were 2 x 1299 secs and 1 x 10 minutes!  Also, being wideband rather than narrowband means you should be able to get a lot of signal quickly, unlike using narrowband filters!  One negative aspect is that at 720 nm you will cut out the strong visible H-alpha radiation from emission nebulae, so M42 for instance will look a lot “weaker” in the infrared than with an H-alpha filter.  However, you will be able to “look through” a lot of the denser parts of the nebula and see stars that are less noticeable in H-alpha images.

A big negative aspect with my SXVF-M25C is its poor quantum efficiency in the near infrared.  However, you may have a mono camera with reasonably good quantum efficiency in the near infrared – and if you do – I urge you to give some near infrared deep-sky imaging a try – and I look forward to seeing your results.

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