*Another* DSLR UV/IR filter question thread

[LAW]

Sorry if I'm covering old ground here guys but while my searching cleared up a lot of questions I still have a few...

I have a 400D and a 450D lying around here which haven't been touched since I bought my 550D and I don't really mind sacrificing one of them in the interest of science. I've read that they both have the double filter arrangement which allows me to still focus normally if I remove the main UV/IR filter. If I was to remove the filter and then take my exposures individually with Ha, Red, Blue, Yellow and Green filters in front of the lens would I get any of the abnormal effects that are associated with full spectrum exposures?

Also, is it simply a case of removing the filter if I just remove the one and not replace it with a Baader or similar?

Since this camera will be purely for astro imaging are there any other mods/tweaks that I should consider?

At the moment I'm only using a cheap F/8 1000mm lens but I'm in the process of researching a reasonable 'entry level' telescope.

Thanks in advance, sorry for being such a noob.

[citivolus]

The sensor has red, green, and blue micro-filters over individual pixels (see http://en.wikipedia.org/wiki/Color_filter_array ). As such, you wouldn't gain anything with broad-band colour filters that you couldn't just get with processing in your favourite photo app. Narrow-band imaging of the ionisation lines of Hydrogen and Sulphur are where the real value lies, as these fall at the upper end of red and beyond the range most built-in IR filters will block. The camera is still best for wavelenghts that fall within visible light, as the RGB filter is still cutting off most wavelengths outside of their designed sensitivity range, they just will not cut them off quite as sharply.

You still need a monochrome array without the bayer matrix in order to effectively image in wavelengths deeper into IR and UV.

The mods themselves are not that hard if you are handy with tools and have steady fingers, but go into it knowing that there is the real chance of damaging the camera beyond inexpensive repair. It took me a week to get the mod to my old 10D right, since the silicone I was using to hold the replacement glass in place kept getting where it was not supposed to be, and cleaning that up was a challenge. My wife also hates me for "wrecking the old camera" (her words) rather than selling it. She could care less if I can shoot in IR now :)

[citivolus]

I should add that narrow band imaging is a good use of cheaper optics, since you won't be suffering from CA, as you alluded to in your post. Shooting with individual colour filters will require re-focusing for each shot, though.

[gregbradley]

I would double check your data about simply removing the UV/IR filter.

My understanding is the filter needs to be replaced with one of various options but the key thing is same thickness glass in the replacement otherwise autofocus will no longer work.

Astrodon, Baader sell replacement UV/IR filters with Astrodon promo showing theirs may be more advanced.

Other option is to replace it with Edmunds Optics clear glass with antireflection coatings.

Then you need an external UV/IR filter on your lenses to work in the day.

Greg.

[Merlin66]

Murphy,
In the 450D there are two filter elements:
The colour balance filter (this is the one usually removed to improve the Ha response for astroimaging) then there's the front anti alias/ dust shake filter - this is infact a UV-IR cut filter (400nm-700nm).
If you only intend using the body for astro then removing the colour balance filter is the way to go.
You don't need to replace it unless you want precise focus with standard camera lenses.
I removed both filter elements from my 1000D for spectroscopy and now have access from 380nm to NIR.

[LAW]

Thanks for the info guys! Merlin, it was actually your post in an older thread which put me on that path. Thanks for clearing that up.

I was going to image in monochrome using coloured filters to avoid CA as much as possible but is it worth it considering I've got those microfilters anyway?

I won't be using that camera with normal lenses anymore since I've got the other two to keep me entertained.