Lhires IR refreshed

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Kevin Gurney
Hi all,
I just thought I would let you know what I concluded after much experimentation with the Lhires on various reference stars and with/without filters
I bought a Baader orange filter (as per Robin’s suggestion) with a view to eliminating any higher order contamination and it worked well. Compared to the no-filter option, things at least seemed more consistent. But the prior inconsistency was, I thought, a clue in itself.
My backyard is subject to quite a lot of stray light, what with street lights, my drive lights on a motion sensor (the dog has to come  outside occasionally !)
I looked at some of the raw spectra in Pixinsight and used the various stretching tools to examine luminance gradients across the entire image. There was some evidence these were larger without the filter.
I then looked again at my efforts to seal the joints between the plate components with tape and there were still some possible entry points for light. In particular, there was a gap around the rear end where the grating sits. (I am talking <= 1mm of course). I used a plastic box, sprayed with matt black paint there. Then at the front there was a seam that I had left for some reason – I taped it up.

The result was that, without the filter, the ‘up tick’ at the red end was much reduced, although IRs were not quite as consistent as using the filter as well. I conclude that there probably was some light ingress and that this was reduced by the sealing at joints, and possibly further by using the filter (it’s near the sensor).