Many lunar imagers are using IR and even Hydrogen Alpha narrow band filters to image the moon. The rationale is that the longer wavelengths are less affected by the seeing, which is true and there is no doubt when the seeing is poor the IR images will be clearer. However, when the seeing is very good I wondered whether the better resolution of the shorter wavelengths gives a sharper image when the lucky imaging process is employed.

The seeing last night was very good so an opportunity to test this out. I captured 5000 frames for each filter. To be fair this means the RGB image had the benefit of 3 x 5000 frames so more data. However, I generally stacked twice the percentage of frames when imaging in IR because they were less affected by the seeing, so there were more "good" images.

The first thing to note, is that the apparently grey colour of the moon is illusory. You can clearly see the subtle colours in the RGB image against the monochrome ones.

I tried diligently to sharpen each image as much as possible and also to manage the noise. The IR images were indeed noisier and I had to do some additional denoising on them.

To my eye the RGB image is far superior in terms of resolution, noise and overall image clarity. This is most clearly seen by the myriad of small craters that come through at the top of the image on the plain of Mare Humorum.

The 'A' image is cropped in so you can see the differences in fine detail more easily.

The 'D' image shows a comparison of L, R, G & B images. This shows that the Red image has the highest definition of all the filters and I believe it has the optimal compromise between two factors. The first is overcoming the seeing, which is favoured by longer wavelengths which are less refracted by the atmosphere. The second is the superior resolution achieved at shorter wavelengths. For example the resolution of a Blue image in the absence of the atmosphere, will be twice that of an IR image....i.e. one can resolve two objects apart at half the distance.

The optimal wavelength will I believe therefore depend on the seeing. Poor seeing favours IR images as the optimum shifts to longer wavelengths. Excellent seeing will favour RGB images and exceptional seeing may even favour the Bluer end of the spectrum.

What do you think?