IR Filter

[Jim]

Just been thinking about whether I need to use an IR filter when imaging with my 127 APO. Since apo's are meant to be corrected for wavelengths between 350 - 1000 nm (ie all these wavelengths should come to the same focus) and near IR is from 700-900 it might be better not to use an IR filter as you'd capture more signal. Anyone any thoughts on this?

[Rocket Pooch]

here we go

[Ian]

Don't know, but what do they mean by corrected? Is it corrected enough given the size of your pixels in your camera?

If the correction is based on visible effects (fringeing) then extending the statement out into IR is a little meaningless as its not visible anyway.

I would suck it and see. The ir filter may also protect the ccd from dust and if your taking colour images, you might need to replace it with a plain glass to maintain focus.

[Jim]

On a mono CCD camera any IR would show as a signal (in fact CCD's are meant to be very sensitive in this region, hence the need to use an IR filter as with an Acromat the IR is out of focus and that causes star bloating). In fact I'm wondering now if using an IR filter with an ED refractor might in fact be uneccessary. You could always just use a glass filter. As you say best way is to experiment and see!

[Rocket Pooch]

Hi Jim,

All APO's are typically well corrected; however, they are corrected to visible wavelengths and therefore IR would normally be out of focus.  Give it a try if you wish, but I think you will find the images will be software than before.

Also your CCD Filters may have IR cutoff built in anyway, if they have then you don't need your current filter.

Chris

[Jim]

Hi Chris

Yes, you're probably right. Cannot find a lens test for my Meade but being a cheaper Apochromat it is probably only configured for the visual 400-700 nm range. I did find figures for the TEC140, these state " The color correction is optimized for visual use with focus shift less than 0.02% from 436nm to 1000nm". You probably do not need an IR filter with this scope, however there's an interesting article below about the sky noise in the 700-1000 nm range which suggests that even with a reflector IR cutoff might have advantages

http://www.astro.uni-bonn.de/~mischa/obstips/clearlum.html

[Rocket Pooch]

Hi Jim,

The best filter I have, although I don't use it anymore is a UV IR cutoff from baader, pictures were really crisp when I used this.  Also from you location you might think about getting a Neodium filter if you attemp RGB, this will help a lot.

Chris

P.S. Me from here no imaging for the next three months Mac has ensure that happens.

[Mac]

P.S. Me from here no imaging for the next three months Mac has ensure that happens.

in that case you free to help built the decking for the pod.

[Rocket Pooch]

I believe I have a day or two off next week!