High Resolution Lunar Image Processing

A step by step workflow process by Tony Gondola

(Revised 6-12-07)

Photographing the moon, sounds so simple doesn't it? After all, Luna is the brightest object in the night sky and is often the first object observed  and photographed by beginning amateur astronomers. As easy as it might seem the truth is, the lunar surface is extremely hard to image at the highest resolution your optics will deliver. Presented here is an outline of my own workflow that has proven to be a very effective approach. This is the result of research into basic methods and a lot of trial and error.  It's certainly not the last word and much variation is possible. Much depends on aperture, seeing, optical design and quality. This work is presented as a proven baseline through which others can perfect and expand their own imaging process.

Hardware

While many types of cameras can serve for lunar imaging the best choice is a video based webcam or machine vision imager. This is because the key to high resolution lunar imaging is stacking and to do that effectively you need to gather a lot of images. Only by stacking hundreds of frames can we reverse the effects of the atmosphere and create images at full optical resolution.  This type of imager is also ideal because of the generally small pixel size and small image format of the sensor.  This is one case where covering a large field of view isn't an advantage, 640x480 is all you really need and it makes it possible to capture at high frame rates. However, beware of low dynamic range video based cameras that do not give you manual control over gain.

On the optical side virtually any telescope can be used for this work and apertures as small as 5 inches can be surprising effective. The most important qualities of a lunar imaging scope are optical quality in combination with a long focal length. The ideal imaging scope would have a long native focal length and a diffraction limited field that's large enough to fully cover the sensor used with a bit more to compensate for slight collimation or centering error. The reason long f ratios are an advantage is the requirement to image at a scale were the pixels in your camera are smaller then the diffraction limited spot size your optics can produce. This is called over sampling and is critical to capturing the finest possible detail. 2x over sampling would be considered the minimum however, some imagers are finding that going even further can yield benefits.  Most common optics will require additional magnification via a barlow lens to achieve the needed minimum image scale.  However at F ratios of F/5 or lower the barlow requirements become extreme (5x or more). You'll also start running into problems with the size of the diffraction limited field at about the same point. Good work can be done below F/5 but the difficulty increases. 

On mountings, any type with an RA drive will work with an equatorial being first choice. It is possible to image with a driven Alt/Az mount however field rotation will limit how many frames you can collect as well as complicate the generation of mosaics. If your RA drive has a lot of PE then you'll need guide rate correction control to keep the image from shifting too much during the course of exposure however there's no need for sub-arc second guiding. In fact, a bit of image shift while capturing your data is actually beneficial to the process.

Beating the odds

The first step before you even begin imaging is to stack the odds in your favor. Learn how to properly collimate your telescope and make sure it's perfect before every imaging session. Also make sure that all optical surfaces are clean, including filters if used. The second step is to pick your battles. Above all else, seeing is the enemy that must be defeated here. There's no point in shooting yourself in the foot by trying to gather data when the moon is low in the sky. In general, shooting at elevations below 45 to 50 degrees above the horizon is a bad idea. Not only will the seeing be worse but below that elevation other nasty effects such as chromatic dispersion come into play. Even when the moon is well placed there will be some nights where the seeing is just so rough it's simply not worthwhile to image, especially with larger apertures. Pick your battles and learn your limits for getting good data.

Data collection

Focus, focus, focus! This is one of the hardest aspects of shooting at long f ratios but it's absolutely critical to get it right. We're not talking about getting close but getting it nearly perfect. This is especially critical with faster F ratio instruments with their extremely small focus zone. As an example, a 6" F/8 Newtonian has a 1/8th wave focusing tolerance of plus or minus 0.04mm or 0.0016". Reduce the focal ratio to F/5 and the tolerance shrinks to plus or minus 0.016mm or 0.00059". That's a very narrow range to hit consistently, especially with an average quality rack and pinion focuser. Helical focusers are helpful but the best solution I've found is motorized focus. By giving repeatable movement and eliminating shake it has made focus just about a non-issue. The key is to make small adjustments and then wait for a steady moment to judge the result. What you're looking for is the visibility of very fine detail that only becomes visible when focus is right on. With practice you'll soon learn exactly what that looks on the screen with your system.

Once perfect focus is locked down and your target is acquired you can start recording your frames. The optimum number of frames you'll need for a good result will depend on the quality and nature of the seeing. My typical AVI capture runs 2000 frames if the seeing is reasonable. Going longer will increase the chance of getting enough usable frames if the seeing isn't as good as it could be. Unlike planetary imaging where there's a time limit due to rotation the moon imposes no such limit so you an let the imager run if you need to. Also keep in mind that the moon is a very high contrast object. Be sure and monitor your exposures to avoid blowing out the brighter areas. Using the live histogram function of your capture software is a must. Look for a highlight value of no more then 220 or so. If you're shooting near or on the terminator some clipping of sun facing crater rims is unavoidable but try and keep it to a minimum. when in doubt, under expose. If you're making mosaics then expose for the brightest areas you'll be covering and let the terminator fall where it may. 

Next page, Alignment and Stacking