Lunar Imaging Beyond Basics - Part 1
In the previous articles, I mentioned the essential basic elements for good lunar imaging: Simple cameras, inexpensive software, lots of frames and good post-processing. In these next two installments I want to talk about timing. Picking the time to get plenty of practice and how to pick your shots on clear nights, getting the right exposures and choosing the best nights.
Once you have all the pieces in place, it's very natural for someone to immediately start trying to snap images of the moon. Very often, people become frustrated with the common problems of lunar and planetary imaging: Focus is difficult; centering is hard; the mount is shaky; highly magnified images are soft; software is not intuitive. It's not unusual for folks to get frustrated after a few nights out. Some even give up.
The best cure for the rush to imaging is a few sessions in the good old daylight. There's something about sunlight (even on a cloudy day) that seems to remove some of the pressures of learning about this kind of imaging. I strongly recommend practicing your lunar imaging techniques during the daytime with as much of your equipment as possible. You should be able to assemble the optics, camera, mount (without using tracking), laptop and software without a lot of trouble. Then, instead of aiming at the sky, pick some terrestrial targets - distant antenna, ceramic insulator on a telephone pole or electrical tower. My absolute favorite is the rooftop vent cap (that allows heat to escape from an attic). My axiom is that if you can't get a good picture of a vent cap, you can't get a good picture of the moon.
WARNING: Always be careful with an unfiltered telescope in daylight. Be sure you can't possibly get direct sunlight into the scope. Serious eye damage and/or serious equipment damage is possible in seconds if you inadvertently aim at the sun.
Vent capping is a great way to practice because it makes just about everything simple: Centering and focusing are easy because the vent caps are on roofs and those look distinctly different from the daytime sky. So you can just seek the darkness of a roof and once you find that, you can start focusing in and out until you get that right. Then traverse the scope until you get to the vent cap. Once you have the target centered, it's a good time to adjust your finder to precisely match the view through the telescope. You can test your precision by aiming at a different vent cap with the finder and hopefully, it will put the cap in the field of view of the CCD. (You could also use a flip mirror to help with acquisition and centering, but flip mirrors are more commonly needed for dimmer, smaller objects like planets.)
With a well-focused vent cap in view you have the chance to learn how to capture images using the capture software. You can adjust the parameters of your camera for brighter and darker image, more/less contrast, more/less color. (Note: I always image the moon in monochrome. The lunar colors are so subtle that the cameras can seldom properly reproduce them well. Monochrome images tend to be smaller.) Some programs capture video streams (AVI format), others capture stills (FITS, TIFF or JPG). Some programs can capture sequences of still images, too. Find the method that works with you. You won't need thousands of frames for lunar images, but take enough so that you can pick out the best 2 or 3 dozen images. Then you can play - I mean "experiment" - with the stacking and post processing steps at your leisure.
You'll probably find that the seeing - atmospheric turbulence - is bad on vent caps. If it's sunny the roof will be absorbing and reradiating heat and that will swirl the air. If it's cold, the interior heat will escape through the roof and caps and again swirl the air. This will be more obvious as you increase the magnification of the object with a Barlow lens or a longer focal length objective. This "boiling" appearance will be something you will often see on lunar images, too, so it's good practice to encounter it during daylight. The other issue for folks with very large telescopes - especially SCT's - is the heat imbalance of the interior and air. Taking a scope out from air conditioning into a 90-degree day will cause massive air currents inside the scope as will a 72-degree telescope coming outside on a winter day. Getting the telescope near the outside temperature is important for vent capping and nighttime imaging.
Daylight operations also make it much easier to play with multiple configurations. You can try a focal reducer to get wider images, normal "prime focus" imaging for medium FOV and Barlow lenses for greater magnification. (You can get the highest magnification with "eyepiece projection" where the camera is held behind a short length eyepiece, but very often the magnifications involved will push the limits of the optics, atmosphere and mount.)
After a little bit of work at the scope, you should be able to come inside with a few sequences of images that you can practice with - selecting, aligning and tweaking the final image. The practice you get in centering, focusing, capturing and processing will help you get ready for nights under a clear sky. Getting familiar with all these "constant" mechanics will help you prepare for the variable challenges of the moon in the sky.
While it's very helpful, the daylight practice on terrestrial targets won't help you with one important aspect of lunar imaging: Exposure time. The exposure time for a lunar image is totally dynamic: It depends on the focal length, aperture, phase of the moon, area of targeting, camera sensitivity, atmosphere steadiness, and transparency. If there's a master formula that will deliver a specific answer I haven't seen it. We'll examine each factor next month in Part II.Published in the November 2005 issue of the NightTimes