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Lunar Imaging with a RED filter (Why)?

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Lunar Imaging is notably absent from my "CV" [I jest!]. But, with
my NEW ED80 being only partly used (as a Solar Frankenscope) I
am trying to think of *good excuses* for this latest addition... :p

Seriously though, I begin to think of Lunar & Solar imaging being
of close kindred technically? Which brings me to my question:

WHY do Lunar imagers (I believe!) favour use of a RED filter?
I am guessing it's down to "seeing", rather than wavelength?

In passing, I casually wondered if my Baader ERF filter might be
of some use in this? But we're back to the "Good excuse" thing! :D

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I tend to use my Baader IR pass filter (685nm long-pass), as it allows quite a bit more light to hit the sensor compared to the ZWO IR850 (850 nm long-pass). Both have the effect of stabilizing seeing because the changes in index of refraction as a function of temperature of the air are smaller at longer wavelengths. When capturing colour images (planetary or lunar) with monochrome cameras, I always see a distinct gradient in image quality when using R, G, and B filters consecutively, with the image quality dropping quite sharply in the blue band. Some people have used 35nm H-alpha filters for lunar successfully, so using the ERF should work

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Dispersion thing ...

Lookup dispersion curve on google - refractive index is higher for shorter wavelengths then it is for longer.

If you want to use filter for Moon, I would suggest going for narrow (ish) band filter. There are two kinds of seeing distortions, don't know what they are technically called - but I refer to them as shimmer and blur. One distorts image but sort of "preserves" edges / contrast - other lowers contrast and acts as blur. By using narrow band filter you are ensuring that shimmer is main distortion - something that can better be handled in stacking then blur. If you go for wide band, different wavelengths will bend by different amounts - creating blur thing.

Then again there is refracting index vs airy disk size thing - shorter wavelengths are more impacted by seeing but produce sharper image in telescope optics, longer wavelengths - the opposite.

Third thing that you need to be aware of is scope type that you are going to use for imaging. Refractors (even achromats) are well suited for this kind of filtered (narrowish - and by that I mean filters with FWHM around 30nm no need to go wild and get 3nm astrodons unless you already have them), but Strehl ratio in Refractors change with wavelength! Most refractors are optimized for green light - around 500-550 nm, and have best Strehl there.

If you already do solar, you might have Baader Continuum filter for white light (green, around 520nm) - give that a go, I think you will be impressed by results.

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Thanks ALL for the above! Yes @andyboy1970I am intrigued by an IR solution?
(As a *modest* collector, I don't have any filters beyond the visible (UV / IR) :p

Thanks @michael.h.f.wilkinsonfor your thoughts re. refractive index of air etc. :)

Ah, @vlaiv I've Been thinking quite a bit re. scope *objective* performance -- 
for both Solar & Lunar imaging! So I moved from a (fast) ST120 to ED80. etc.
Many scopes do perform "best in the green". I have a solar continuum filter.
I have a few more filters besides. lol. Maybe time / the weather will be kind? ;)

The above hardly does justice to the answers, but my thanks
(as ever printed). "Food for thought" about mutual concerns! :happy11:

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Found an interesting article on this! In German, so Google translate helped! lol :D

Basically showing: "deeper IR produces better images"... But eventually you may
lose out to decreased sensitivity of your camera CHIP! On the Baader blog, so the
Baader Filter wins! But then most of my other (dabbling) filters are by Baader! ;)

So I think I will likely invest in a Baader IR-Pass 685nm? :)

Some pretty colour lunar images by Roger Hutchinson
using an IR-pass as the luminance channel (via Twitter)! :cool:


Edited by Macavity
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