WANING GIBBOUS MOON, 2 OCTOBER 2015
During the summer, I wasn’t able to get out on clear evenings to do photography. We had quite a few hazy nights, and I don’t bother to take a ‘scope out on those nights.
In the last few days, the weather has begun to turn, and we have less humidity. The night of the eclipse (27 September) was reasonably clear, and we’ve had a few good nights since then.
I took this photo on 2 October, using my Meade 127mm (five-inch) achromatic refractor). The limb of the moon appears a bit off–an artifact of the achromatic design, which does not correct perfectly for the different wavelengths of light.
A bit of technicality: A refracting telescope uses an objective lens, containing one or more elements, to gather and focus light. Sunlight–and moonlight, which is reflected sunlight–consists of a spectrum of wavelengths of light. A simple lens, consisting of a single element manufactured of one type of glass, will focus the different wavelengths of light at slightly different points: the blue image will be focused slightly closer to the lens than will the red image. This spread in focal points–called chromatic aberration–renders the image a little less crisp, as there is no single focal point. Further, depending on what focal point one chooses, a simple objective lens will create “false color”–that is, the color of the slightly out-of-focus portions of the image will be visible as a sort of halo around the object.
An achromatic lens bears that name because it reduces the color (chroma) of a refracted image. Achromatic lenses consist of two elements that are made of different glasses. Each kind of glass focuses different wavelengths of light differently. By selecting the appropriate kinds of glass, the manufacturer can reduce the amount of chromatic aberration. Some two-element lenses can achieve a high degree of color correction. Such lenses typically use an objective containing a fairly “exotic” kind of glass, which tends to be expensive.
Other lenses are designated as apochromatic. Such lenses typically use three elements, each manufactured of a different type of glass. Well-designed apochromatic lenses reduce chromatic aberration (as well as spherical aberration) to a very low level.
Some camera lenses are designated as apochromatic. But chromatic aberration is typically less important with camera lenses, as the focal length is much shorter. Chromatic aberration in a 50mm lens is obviously much less important than chromatic aberration in a 1000mm lens, where the spread in wavelengths is twenty times as great. Further, because camera lenses typically have much smaller diameters, it’s much cheaper to introduce additional elements to reduce chromatic aberration. (And most modern camera lenses consist of several elements anyway.)
So the issue of chromatic aberration generally affects telescopes rather than cameras. And the problem principally affects refracting telescopes–that is, telescopes that use a glass objective lens to collect and focus light. Chromatic aberration does not affect reflecting telescopes, which use mirrors to collect and focus light. And chromatic aberration does not generally affect hybrid telescopes–that is, telescopes using both glass lenses and mirrors. Hybrid designs include Schmidt-Cassegrain and Maksutov-Cassegrain telescopes. I have a 127mm Maksutov-Cassegrain telescope, which provides very crisp images without chromatic aberration.
My Meade 127mm refractor is an achromat: it has only two elements in its lens. So it shows a bit of chromatic aberration, which is especially visible at the limb (edge) of the moon in this photo. The defocusing at the limb resulted in an artificially bright limb. I processed the image a bit to bring out additional details in the rest of the image, and that processing bumped up the brightness of the smeary edge of the image.
As I noted above, it was reasonably clear the night of the eclipse. I did take a moon shot that night, but I didn’t use one of my telescopes. The eclipse wasn’t visible from my back yard (which is really kind of a hole). I didn’t feel like hauling one of my ‘scopes up to the street–especially because I would have had to use the Meade 127mm achromat, which is a fairly heavy beast.
Instead, I took a shot of the eclipse with a 500mm lens mounted on a simple tripod. Because the moon was so dark, the exposure was very long–three seconds. The moon moved during that time, resulting in a somewhat smeary image. It’s not a very good shot, but I’m including it here for what it’s worth:
Although this isn’t really a very good image, it does convey–at least to me–the rather moody, even spooky, quality of the moon that night.
An aside: My little girl, Grace (age seven), was asleep well before the moon reached totality. But when I saw that it was clear enough to see the moon, I woke her and carried her in her pajamas out to the street. (We live on a very, very quiet dead-end street.) One of our neighbors came out, and we stood there for a time, watching the moon and talking. I took a few photos and then took Grace back to bed.
The next afternoon, my little girl’s teacher told me that Grace had told her about my waking her up and carrying her out to the street to see the blood moon.
I’m not such a bad dad after all.