Webcam on Newtonian....IR filter in or out?

[Kropster]

When imaging planets with a webcam on a Newtonian scope, is there any advantage in using an IR filter, as presumably all wavelengths are focused together?

An IR filter would just cut down the number of photons reaching the sensor.

Is there an advantage I have overlooked?

Thanks in advance.

[Uplooker]

the red does not come to focus at the same point as all the other wave lengths, so yes, have the IR filter screwed to the front of the webcam.

Ian

[jambouk]

Are the different wavelengths split and then brought back together when reflected off a mirror? I thought this chromatic aberration was a problem for lenses!

I'd have thought the main issue would be if the target was emitting infrared which you'd block out with an IR cut filter.

I'd suggest try both methods. I suspect there wouldn't be much difference, but have no idea. I leave mine on as it protects the sensor; but maybe that's where i'm going wrong!

James

[Kropster]

Thanks for the responses.

A mirror will focus all wavelengths together, unlike a lens.... hence my question about the value of blocking some of the photons when imaging planets for instance.

[jambouk]

Sorry i mean i leave my ir cut filter on when imaging planets.

James

[JamesF]

I would leave the IR filter in place, especially for colour imaging.  Without it you may well heavily over-expose the red whilst under-exposing the green and blue.

I guess it's entirely possible that the green and blue parts of the CFA may be "leaky" in the IR part of the spectrum too, so you could end up getting IR in your green and blue channels too.  I've no evidence to support this, but it could well be the case if the CFA is designed in the knowledge that an IR filter in front of the sensor will normally be removing the IR anyhow. Filters are often leaky outside their intended operating range.

James

[hobsey]

I'm a little confused with this.

I have a modified MS lifecam which I still yet to use and followed the modding guide. It tells you to remove the IR filter.

[JamesF]

I'm a little confused with this.

I have a modified MS lifecam which I still yet to use and followed the modding guide. It tells you to remove the IR filter.

Usually, yes.  What's required of an IR filter for daytime visual use is not what's required of an astronomical IR filter.  The built-in filter in a webcam is likely to be made to a significantly lower optical standard than a proper astro IR filter.  It's not just about what wavelengths are blocked and how well, but also the filter should be "optically flat".  There are also "absorbtive" filters and "reflective" filters.  Reflective should be better, but I don't think I've seen one in the webcams I've taken apart.

James

[JamesF]

Is "absorbtive" a word?  I'm sure you know what I mean, anyhow.

James

[hobsey]

Oh I see.

Thanks for clearing that up for me.

[jambouk]

It's a good word!

For some deep space imaging it can be favourable to capture the infrared light, which is why people modify dslr cameras to remove the infrared cut filter. I've no idea if it really makes much difference for planetary when using a one shot colour camera.

James

[laser_jock99]

Often overlooked (mainly by refractor fans!!) but a Newtonian is a true apochromat- that is until you put a lens (e.g. coma corrector) into the optical path. So no- you don't need an IR filter as such as the IR should be in focus. You may want to block it for other reasons as stated.

[lensman57]

the red does not come to focus at the same point as all the other wave lengths, so yes, have the IR filter screwed to the front of the webcam.

Ian

With a well figured mirror lens all wavelengths are focused at one  point, it is not necessary to use an IR filter as it would be even for an ED or semi Apo.  You may wish to use it to keep the IR out of the imaging sensor if you so desire.

A.G

[Kropster]

Thanks fo all the comments.

Do planets reflect a lot of IR and does that make it through our atmosphere?

Presume the peak wavelengths are in the yellow part of the spectrum with a tail-off either side....matching the output from the Sun?

Removing the built=in webcam IR filter makes it a lot more sensitive... more stars visible... so good if you are using it to guide with.

Probably means the filter cuts out quite a lot of visible light too.

What is a CFA? Is it the colour separation filter on the CCD itself?

SO many questions.......

[RT65CB-SWL]

Also helps to keep 'dust' off the sensor!

[shirva]

helps to stop star bloating as well ...davy