Do I need IR filters for Refractor imaging?

[NickK]

I know that many sources indicate that IR filters are mandatory for refractors, of benefit to SCTs and not required for reflectors - but why?

I know CCDs are sensitive to IR but what makes a refractor transfer more IR? Is it chromatic aboration at the IR wavelength that causes soft images on the IR sensitive CCD? Or is it the amount of IR causes damage to the CCD over time?

Lots of questions

[riklaunim]

Even standard APOs aren't corrected for IR so you need IR/UV cut filter to cut the aberration caused by IR. Super APOs are corrected for 4 "colors" and they don't need it.

[robbieince]

Hi

It's a chromatic aberattions issue so even standard apos suffer from it at ir wavelengths. Ie different refractive indices for each wavelength so differing focal planes.

For reflectors with no refractive elements you can gave no chromatic problems so filters for ir are not needed.

Regards

Rob

Sent from my iPhone using Tapatalk

[Psychobilly]

Damage isnt an issue it's just as the others have said that the IR wont be brought to focus at the same plane as the visible wavelengths but the CCD will record it (unless it's fitted with a "hot mirror" ) and give rise to issues like star bloat...

if its a mono CCD the IR pass can be very useful (vissible blocked) for planets and the moon in particular...

[JamesF]

I'm surprised that IR makes that much of a difference at night. I have a webcam that I removed the IR filter from and dropped a bit of unexposed film into to make an IR camera. It works, but really only when there's a good light source (the sun, or incandescent light bulb).

Does that just mean the camera I butchered isn't very sensitive?

James

[E621Keith]

IR won't damage your CCD unless you try to do solar imaging unfiltered, but like others mentioned, super apos are the only type of refractor corrected for IR, normal apos are only good for visible spectrum. I don't image, but I guess IR lights could be useful to those doing LRGB imaging.

James, I think IR performance depends on the particular chip. CCDs are sensitive between 400 and 1000nm, but the frequency of peak sensitive depend on chip. May be your chip aren't that sensitive in IR.

IR light source is different from normal light. The Sun, halogen heater and incandescent light bulbs emits tons of IR, so they are very good source. On the other hand, energy saving bulbs emit mostly in visible spectrum, so they poor IR source, but that's why they are so efficient.