From: "Alan Friedman" <alan@greatarrow.com>

Date: October 1, 2006 12:21:50 PM MDT

To: <Astro_IIDC@yahoogroups.com>

Subject: Re: [Astro_IIDC] IR filtering for luminance

Reply-To: <alan@greatarrow.com>


Hi Ray -



---------- Original Message ----------------------------------

From: Ray Byrne <ray@in4media.co.uk>

I thought that the reason for this was to narrow down the  

wave length to Infrared because the turbulence in the atmosphere  

would be less obvious or may offer longer moments of stillness. 


Three things come together to influence the answer to your questions.


1. The potential resolution of an image captured with a filtered monochrome 

camera will increase as the wavelength admitted by the filter decreases (ie, 

resolution will increase as you move from red to green to blue.)


2. The air from my location is most stable at the longer wavelenths and becomes 

less stable as the wavelength decreases (most stable in IR, least stable in blue). 

The stability of the image is almost always improved imaging with the 

monochrome camera in a narrow band of visible light as opposed to using the chip 

unfiltered.


3. My monochrome DMK camera is most sensitive in the green wavelenths, 

followed in order of decreasing sensitivity by IR, R and B.


These factors will influence the choice of filter for a luminance image for a given 

object. In good seeing on a bright subject like the moon (or if you work with a 

large aperture on the planets), the green filter gives excellent results - you will be 

able to see a noticable difference in resolution between results through a green 

filter as opposed to IR. 


In average or poor seeing, an IR filter can produce as good or better results 

because the lower resolution of the longer wavelength is offset by the increased 

stability of the atmosphere. 


For RGB imaging it is a different story - here the influence of IR wavelengths will 

cause color shifts that will not agree with the visual appearance of a planet - hence 

it is usually the preference to block the IR wavelengths from the data recorded in 

R, G and B by using filters with IR blocking incorporated or using a separate IR 

blocking filter in front of the filterwheel.


hope this helps -

Alan