From: "Duane" <macastronomer@mac.com>

Date: April 13, 2008 10:47:08 PM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: IR filter with a DMK color .AS camera?

If your camera doesn't block IR, than you should (unless you're shooting extremely dim 

nebulous objects and you absolutely need the extra photons).

The wavelength is longer which makes it impossible to bring to a point (especially the 

really long IR). If you're eyes could only see long IR, you wouldn't be able to read this text 

(even if it were printed in warm ink, unless you increased the size of the text so the edges 

could be discerned by the length of the wave).

In other words, keep blocking the IR, even in a reflector. I'm not sure why anybody would 

need to block UV in a reflector...

Duane

--- In Astro_IIDC@yahoogroups.com, Alen Salamun <alien@...> wrote:

Hey Larry!

Is it true, that this is mostly problem with refractors? What about  

SCT? Should I still use IR filter on SCT?

BR,

Alen

On 12.4.2008, at 21:42, larry.towers wrote:

The IR filter isn't just for color correction. The focal length of

lenses depends on the wavelength. Lens manufacturers compensate for

this with achromatic lenses but these only work within the visible

light spectrum. A lens focused in visible light will be somewhat

off-focus in infrared, If your ccd picks up infrared it will be out of

focus with respect to visible light. This will be especially true if

focus on the object is very critical. There will seem to be a

"glowing" aspect to the images you capture. The only reason Not to use

an IR blocking filter is when you specifically want IR capture, or you

want that subtle softening effect.