Is An IR Filter Necessary?

[pbyrne]

Hi all

Having just picked up a Philips webcam I'm going to give lunar imaging a go. My question is this: Will an IR filter help in any way? If so, what way?

Looking forward to the replies.

[ollypenrice]

Not really my field but I do use one myself. However, I thnk a lot of moon specialists with mono cameras use a red filter because the long wavelengths are the least disturbed by the atmosphere. Is yours mono? If so the Baader red might be best because all their top end colour filters have built in UV/IR cut. You don't have to present the final image as red, cf course!

Olly

[Cornelius Varley]

Without the IR filter colour images will have a slight pink tinge to them. The IR block filter cuts out the IR end of the spectrum and also improves sharpness.

Peter

[brianb]

IR block - practically essential for a one-shot colour camera, because the sensor is very oversensitive to infra red (there's a IR blocking filter in the lens that you removed) and to control prismatic atmospheric dispersion.

When working with a mono camera, a deep red or IR pass filter - without IR blocking - is useful to reduce the effects of bad seeing.

[ollypenrice]

I have learned something there so thanks to Brian for that. I'm now wondering why the IR is beneficial?

Olly

[Darth Takahashi]

For terrestrial photography an IR blocking filter is necessary to stop the offending pink images; however, for astrophotography an IR PASS filter is essential for planetary and lunar photography.

As prevoisuly stated IR waves are less effected by the seeing conditions which ultimately determine image sharpness. I have a post somewhere on this site where you can see the difference between images without IR PASS and with on Jupiter.

The difference is dramatic, especailly at a focal length of 4500mm (1.5x 3000mm) Mewlon M250 with ext Q.

Some people image at F22 and beyond and its only possible with excellent weather and IR PASS filter.

[brianb]

I'm now wondering why the IR is beneficial?

Ummm ... if you look at the spectrum from a prism, the blue is very dispersed, the red end much less so - and the IR is less dispersed still. The "lumps" in the atmosphere work the same way - there is less deviation when light goes through a pocket of different density in the IR than there is in visible red, there is more still in the green & much more still in the blue. This is why the seeing is steadier in longer wavelengths.

The IR sensitivity of silicon based CCD & CMOS sensors is simply a function of the physics of the chip. The sensitivity does drop off, to zero at 1100 nm, but with no filter a large percentage of the light registered is IR ... and cool stars like Y CVn are amongst the brightest objects in the sky, even though they're on the borderline of naked eye visibility.

Because of the falloff in IR sensitivity there is a "sweet spot" for the short wave bandpass of a filter for deep red / IR imaging ... the longer you make it, the longer the exposure you need ... with my usually poor seeing I find that the Wratten 29 deep red (with a bandpass starting at about 650 nm) is very good; with better seeing the Astronomik Planet Pro 742 is better and with good seeing the Planet Pro 807 may be usable (especially on Venus which is very bright and especially with larger apertures).

Some people image at F22 and beyond and its only possible with excellent weather and IR PASS filter.

Won't argue about the "excellent weather" but people with good sites can and do make excellent images in RGB at large image scales.

[Darth Takahashi]

Won't argue about the "excellent weather" but people with good sites can and do make excellent images in RGB at large image scales.

Well I agree with you if we are discussing imaging at altitude. I would love to be at 1000m to 2500m on a mountian than in Holland at around 15m above sea level. That would most definately make a huge difference to image quality and the max resolution.

My best effort todate;

http://stargazerslounge.com/imaging-planetary/120935-jupiter-27-11-a.html

IR Spectrum

http://stargazerslounge.com/imaging-planetary/116380-nothing-special-jupiter-last-night.html

Well, using the black ends of unexposed slide film, I now have a proper Baader 685nm filter.

I love my Mewlon but it is the least used of all my telescopes because of the weather conditions, including the jet stream that moves across Holland seasonally. With a 10mm eyepiece at 300x I can already predict more or less the quality I can achieve on any one night. The IR filter helps but its definitely not a miracle worker!

But I'm open to suggestion regarding colour imaging of the planets my own plan is to use IR as Luminance and then RGB as the colour components etc...

[Patrick Gilliland]

What is best filter to buy then, anything that helps is good.

[star_chaser]

i just ordered the phillips webcam and would like to know what filter to get if i need one ?. or just a plain glass one as i don't want to leave the chip exposed

anyone have a linky to filter as i have read the above posts im still lost

star

[dph1nm]

One should also remember that the sky brightness increases dramatically at IR wavelengths, which is not good news for faint extended objects.

NigelM

[jgs001]

When I've been doing Lunar and Solar with my 5v, I've been using an IR/UV cut filter and a red filter to help with the seeing... Am I actually making things worse like this ? What would be the best filter to use for the 5v as a general protective measure on the end of the barrel ? With the SPC900 I guess I need the UV/IR filter.

[brianb]

What would be the best filter to use for the 5v as a general protective measure on the end of the barrel ?

Clear glass. You don't need a UV/IR blocker when using a deep red filter & it will interfere with IR pass filters & also if imaging in CaK (which is heavily blocked by UV/IR blockers).

[jgs001]

ok thanks Brian, better go hunting one of those... as for CaK... no danger of that ..