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

Date: October 1, 2006 2:10:25 PM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] IR filtering for luminance

Hi Alan

Thanks for a quick and informative reply. I still need to confirm my understanding of your reply so I have annotated your response:

On 1 Oct 2006, at 19:21, Alan Friedman wrote:

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.)

I understand this from the use of the Baader Solar Continuum filter for the Sun i.e. a narrow window of the spectrum that can punch through the daytime seeing in the case of Solar imaging. Am I correct here?

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. 

A green filter on the Moon is probably going to give better results on the mono camera as the camera is most sensitive in this part of the spectrum (we suffer the same weather systems I think) according to no.1 - I think I'm correct here

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. 

It might be prudent to experiment with IR also if the seeing is average to poor.

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.

When you mention RGB imaging are we talking about cameras that aren't mono i.e. "one-shot colour cameras? 

If so do the Red, Green and Blue filters need to have a block for IR? The filters I'm intending to purchase are quite expensive and are blocked for IR but I'm sure this is with DSO imaging in mind by the supplier.

hope this helps -

Alan

Thanks Alan again you're a star

ATB

Ray Byrne