UV/IR filters and Astrophotography

[Atmos]

I've had my Sigma 85mm out under the stars twice so far, the first time without a high quality UV/IR filter and the second with one. I usually have a high quality UV/IR filter living over the front of my lens' as a protection against the elements. A hardy piece of glass that is worth $100-150 but makes sure that nothing damages the real nice glass that comes with your high quality lens :P Bit of rain, dust, splashes of mud; remove lens, clean and put back on. No harm done.

After the 4+ hour ordeal of calibrating, debayering, registering and stacking my Rho Oph shots, most of which taken on the second night, I couldn't help but notice that the stacked image lacked the sharpness of the first night and displayed some weird star shapes.

This morning I went and stacked 8x120s from both the first night (no filter) and second night (with filter) and have since come to the conclusion that for astro work, NEVER using a filter.

I did attempt to get these two comparisons down to <200kb but by that point the quality was too bad to even tell the difference between them.

First Night (without filter) (http://www.astrobin.com/full/301763/0/)
Second Night (with filter) (http://www.astrobin.com/full/301764/0/)

Both of these are shot @ F/2.8 with a Nikon D7200 (APS-C with 3.91 micron pixels and no anti-aliasing filter)

I think for every day usage the filter is worth while, the sharpness difference is possibly not perceivable when shooting anything other than stars. Still, it is something to think about.

[ChrisV]

Why would the UV/IR filter do that? I thought it would be the other way around ...

Confused Chris

[Atmos]

An optical system is only as good as its worst piece of glass, guess which one is its worst :P
In every day photography it is a sharpness drop that is not noticeable (it is still a high quality filter) so it'll live on the lens when it isn't pointed skywards :P

[glend]

Colin, how thick is that filter? As you would know, adding glass to the light path alters the focal length and can make sharp focus impossible unless you allow for it in the setup of spacing. Most DSLRs are very sensitive to sensor spacing, and mods, or adding filters, may require changing the shims that set sensor position. Just as we do for our astro mono cameras, spacing and filter thickness have to be considered. Remember the Rule of 3Rds: for every 3mm of addition glass in the light path you will need 1mm more spacing. So a typical 2mm filter on front will lengthen the focal length by about 2/3s mm. Autofocus will usually not work.

[Merlin66]

Glen,
This doesn't apply when the filter is positioned in front of the objective.
The surface quality (flatness) could be an issue - yes some of these filters are "photographic quality" but still can give grief when imaging pinpoint star images.
The chromatic correction of the lens may be good, and not give rise to the blue/ red bloat normally associated with wavelengths beyond the usual UV/IR cut off.

[glend]

Whoops absolutely correct, sorry, maybe i was thinking it was a clipin.