From: Alen Salamun <alien@alienworld.org>

Date: April 13, 2008 2:37:48 AM MDT

To: Astro_IIDC@yahoogroups.com

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


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.