Infrared; friend or foe?

[Dawson]

1. When imaging the Moon or Planets with a colour sensor, say an ASI ZWO, do you want to allow IR onto the sensor, or omit it from your image?

2. Does a simple clear glass filter act as a IR-cut filter?

Thanks

James

[Bill Ward]

Hi,

When you look at the sensor can you see a blue-green coloured piece of glass mounted directly on top of the sensor? This is often seen on a lot of colour cctv type cameras.

If so, then this will be an IR cut filter so no additional filter should be needed.

The answer to your second question is no provided it really is just plain glass. “plain glass” will transmit most of the IR light in the range that silicon based sensors work. Most IR cut filters are this blue green colour I mentioned to simulated a broadly similar photopic eye response on a bare Si sensor. For example a Schott BG18 or maybe a broader pass Schott KG1 or equivalent.

cheers,

Bill.

[Dawson]

Thanks Bill. 

I think my question 1 was poorly phrased; it probably should have read: “with a colour sensor, is it desirable to block IR when imaging the moon and planets? If so, why?”

What about UV? I presume UV is blocked by clear glass.

James

[Grant Privett]

Well you certainly cannot rely on the Bayer matrix to do the filtering. On a Lumix I own theres enough leakage of light to take a photo during the day with a filter that cuts off light at a shorter wavelength than 820nm. Green lawns look like snow!

In the UK – given its dirty and humid atmosphere – hard UV certainly isnt a problem (well, it didnt seem to be for me) though the definition of where the UV starts and blue ends may vary and the big worry is how fast a refractors focus goes to pot as you move toward the blue – may get bad sooner than you think.

But even with reflective optics, the colour balance will still be off as the blue and green images will still contain a contribution from the near infrared – that can blur out fine contrast features.

Personally, if the camera already has a blocker but you have a professional quality blocker to hand its worth experimenting using the camera with both filters, your professional filter only and with only the in-camera filter and comparing. You do have to be very certain of your filter quality.

[Callum Potter]

If you use refracting optics IR will focus at a different point to the visible – so will lead to blurry images.

With a reflector this does not happen. But the IR will still be detected (silicon detectors tend to be more sensitive to IR than visible light), and cause the colour balance to be off (usually appearing much redder).
But that is something you could correct in photoshop (or whatever…)

Callum

[Robin Leadbeater]

Back in the early days of webcam planetary (and DSO) imaging. (~15 years ago) I wrote a little IR FAQ based on my experiences and those of others on the QCUIAG yahoo group.

http://www.threehillsobservatory.co.uk/astro/IR_FAQ.htm

The results achieved these days far surpass what I was getting back then though and I am out of touch with current thinking.  The basic reasoning there should still be sound. 

Of particular interest may be the test mentioned there I I made on Saturn with and without IR block which showed lower noise if IR was included, no loss of resolution and that it was possible to correct the colour caste caused by the  IR leaking past the RGB filters.

The colour camera will almost certainly already have an IR blocking filter so you would need to find out how to remove it to test this out

Cheers

Robin

[Dawson]

Thank you all.

I was attempting to image Jupiter the other night with my new camera and whilst the seeing was excellent, the results I got the next day were awful. I’ve just realised the window in the camera is just glasss and not an IR blocking filter. Doh!

James

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