Infrared and reflector scopes

[Raga]

Hi all.

I was looking at the confusing world (to a newbie like myself) of filters and was looking at IR/UV CUT FILTERS when I found one at TelescopeHouse. In the description it reads.......

"telescope objective lenses are not corrected in the infrared and ultraviolet range of the spectrum (exception: reflecting telescopes)"

Does that mean that a reflector such as the SW 150PDS doesn't need an ir/uv cut filter if I'm imaging or for visual???

This is getting even more confusing with every corner I turn

Thank the heavens for SGL as I have somewhere and someone to seek the help I so clearly need lol.

[Cath]

Hi Raga. You still get the same amount of IR/UV light through the scope, just means the IR and UV won't be focused the same as the inband light. So the IR/UV light still needs filtering out.

[Raga]

Hi Cath and thank you for the reply.

Hope you have a great Xmas.

[E621Keith]

Hi Raga. You still get the same amount of IR/UV light through the scope, just means the IR and UV won't be focused the same as the inband light. So the IR/UV light still needs filtering out.

I think that only applies to refractor, UV and IR should still come to the same focus in a reflector, after all there is nothing special about UV and IR apart from their slight shorter and longer wave length than visible light. The reflector mirror will become less effective in IR than visible light, which is why IR telescopes uses gold coated mirror instead of silver/aluminium coated ones.

[alan potts]

I don't know much about filters but I would like to say welcome to SGL in any case, I hope you get as much out of the site as I have, there are some really knowledgeable people, Keith above is in my top 5.

Merry Christmas to you.

From a very cold Bulgaria, but the sky is clear, Alan.

[Astrobits]

Keith is correct. A refractor objective focusses different wavelengths at different points. The maker of the objective will arrange things so that the visible spectrum foci as close together as possible in a refractor intended for visual use. As the eye is not sensitive to UV and IR they can be ignored. However, CCD's are very sensitive to IR particularly and if this is not filtered out pictures will have poor resolution as the IR or Visible light will be out of focus.

Reflectors do not have the problem of different foci for different colours but pictures will still be affected by the different sensitivity of the CCD chip giving an odd colour balance if UV and IR are not filtered out.

Nigel

[JamesF]

Yup, agree with Keith and Nigel. The refractive index of a material (eg. a lens) depends on the wavelength of the incident light (which is why Newton's prism, and rainbows, split light into its component colours), so different wavelengths come to focus at different points unless some sort of correction is made. That doesn't happen with reflectors. Unless you're doing some narrowband imaging however you probably don't want the IR hitting the camera sensor in either case for the reason Nigel explains.

James