Do I need a IR filter for webcam imaging?

[stev74]

The reason I'm asking is because of this statement on the Telescope House website -

Telescope House Revelation IR Blocking Filter 1.25"

Revelation IR Blocking Filter 1.25" Ref: BC019

Telescope objective lenses are not corrected in the infrared and ultraviolet range of the spectrum (exception: reflecting telescopes). Unlike the naked eye the CCD chip of cameras is sensitive in the near IR. As a consequence contrast and sharpness would suffer. For this reason you should cut off the IR and UV light whenever there are any glass elements between the camera and the lens, The IR blocking filter is an interference filter with a steep, precisely controlled filter curve. For the naked eye the filter seems fully transparent since the human eye doesn't perceive the IR and UV light.

As I have a 200P reflector there are no glass elements between the scope and camera (SPC 880). I have never seen this advice on any other website so I just wanted to check and maybe save myself some cash.

[Revs]

Yes, you should be using an IR filter like the one you linked

[stev74]

Thanks for the reply Paul, are saying then that the advert is wrong as it states you don't need a IR filter if you have a reflector?

[Revs]

I use one when webcam imaging in both my scopes. TBH I've not tried imaging without one for two reasons. 1/ it protects the CCD from dust, and 2/ IR filters are not detrimental.

Might be worth waiting for a second opinion regarding reflectors specifically.

[stev74]

Good point about dust, I do have a screw cap for the webcam adapter but the CCD is exposed for imaging.

Also now that I think about it the couple of times I've had a chance to use the webcam, I've tried it with a barlow as well, so I suppose that adds a glass element to the equation.

Thanks again for replying, I better get ordering!!

[Revs]

Ah, very good point

FYI, I got the Astronomik one from MA. I generally avoid the cheapest filters and I know Astronomik filter are generally highly regarded for the money

Astronomik filters @ Modern Astronomy

[stev74]

Thanks for the extra info and the link.

[callump]

Well strictly speaking you don't need an IR filter on a pure reflector - though if you use a barlow (i.e. some refractive optics), it would be.

With refractive optics, IR light comes to a different focus point, so if you don't use a filter your images will be soft or unsharp.

If you have purely reflective optics, you will find that the colour comes out strange, because the webcam chip is more sensitive to IR than visible wavelengths, so the normal colour balance does not correct it.

I have done some webcam imaging with a newtonian and a non-filtered webcam, and the pics all came out overley red - so I invested in a filter to counteract the effects (and because I wanted to start using a barlow).

Callum

[stev74]

Great, thanks for the extra clarification Callum.

[Gina]

I found the same with my webcam. Better image image even with no glass in the optical path. I haven't been using the IR filter with my DSLR camera though and since I'm using a Barlow I should do. Focussing has been difficult and inaccurate.

Regards

Gina