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30 November 2001

Ten Secrets of Lunar and Planetary Observing

by Roger Gordon
Reprinted from The Practical Observer, vol. 6, no. 4.

The moon and planets are the only objects (with the exception of the Sun) on which a substantial amount of detail can be seen. For example, a 6" telescope reveals at least 30,000 craters on the moon. The first link in the chain is the telescope.

Whether it's a refractor or reflector or Catadioptric, the instrument must have excellent optics. Many larger aperture "deep sky" telescopes will show the major features on the moon and planets, but are not optically good enough to reveal the smallest structures and markings that their apertures should be revealing. The words "diffraction limited" have become a catch-all phrase used by some telescope manufacturers in the loosest of terms. The quality of optics in most telescopes has improved in the past decade, however, as amateur astronomers have become more knowledgeable (at least some of them!) and demanded more and more from their instruments.

In general, longer focal length instruments are better for lunar planetary work (see Rodger Gordon's article on this subject in Volume 8, Issue 2 of TPO. Copies of this article are available from TPO for $3.00)

A quality telescope can be considered one that not only reaches the theoretical limit of resolution (assuming quality seeing conditions), but also produces high contrast images as well. Assuming we have one of these, how do we make best use of it? Here we deal with the weakest link in the chain--the observer!

1. Frequency of Observation

Back in 1964 at a star party (we called them "field meets"), about 50 or 60 persons attended with instruments ranging from 2.4 inches to 12.5 inches in aperture. I distributed some drawing forms for Jupiter and asked various observers to draw what they could see. The seeing and transparency were both excellent at the time. Several hours later, the drawings were turned in and I took them home for study. In general, the deep sky observers drew the least amount of detail regardless of the aperture used.

At the risk of sounding like I'm bragging, none of the participants drew as much detail as I did using a 4" Unitron refractor. And this included several good lunar/planetary observers with greater aperture. I personally checked the telescopes myself to confirm the optical quality of their telescopes so as to rule this out as a factor. So am I some kind of "super-observer"? No. The only real difference between myself and the others was that, for various reasons, none of them had observed as frequently as I had. Thus it would appear that the frequency of observation by the individual is a major factor in this type of observing. It would also explain why the deep sky observers fared worst. They would have the least experience and frequency of observation when it came to studying Jupiter.

In another case, deep sky and lunar/planetary observers were asked to perform an experiment: for a few months, each were asked to observe in the other's discipline. At first, neither side saw much detail. But as the weeks progressed, each group saw more and more until by the end of the experiment, each group was as proficient in the other's observing expertise. It seems obvious that proficiency would improve with experience. However, it seems that an individual will temporarily lose some of their ability to see fine detail even in an area of observing they are experienced in if there is a lengthy interruption in the frequency of observation.

This seems to be additionally proven out by an incident that occurred in the observing career of A. Stanley Williams, one of the most famous and certainly the most frequent observer of Jupiter. Williams was asked by some colleagues to comment on some faint markings they had seen on Jupiter. Williams stated that he had been away on vacation for a fortnight and that his eyes were not "retrained" enough yet to form an opinion. It seems that if one runs into a long period of bad weather, poor seeing or other long distraction from observing, the eyes tend to lose some of their abilities (temporarily) for this type of observation.

2. Contrast

Improving the level of contrast is another method for seeing more detail. Here the most common practice is to use color filters. These are highly useful accessories which work by blocking certain wavelengths while letting others through. The Martian polar regions are often best seen with blue filters, while surface details are best seen with green, orange, or light red. A gold-coated star diagonal or secondary mirror will, in the latter case, serve the same purpose and may be better.

3. Orientation

The orientation a planet has in the field of view also plays a part. A belted planet like Jupiter may be best observed if the eyes are placed so that the lateral line of sight is 90 degrees to the orientation of the belts in the field of view, or even 180 degrees. If you doubt this, make a drawing of Jupiter in several orientations and note the differences! You can also do this for Mars and in fact, drawings of Mars may be noticeably different by the same observer who first uses the inverted south-at-top view compared to the view provided by a totally erect image (the latter provided by an Amici prism diagonal or a terrestrial erecting system).

4. Drifting

Allowing the image to drift in the field of view, instead of using the clock-drive, may also allow one to see more detail. The reasons for this are not well understood, but may involve the eye's tendency to "stare" at a stationary image, while the eye is forced to follow the moving image and work a bit harder. If this is the case, it may be more tiring over a long session.

5. Switching Eyes

Switching from one eye to the other frequently during a session is also another technique. This prevents the eyes from getting tired too quickly. In doing this, I've also noticed that with my right eye images are seemingly not quite as bright as with my left eye. And my right eye shows the lunar image to be more "yellow", the left eye having a more "white" image. Another observer/correspondent of mine also see the same thing [Editor's note: I find that that when looking at lit, white wall, the view through my left eye seems a slight bit . more "yellow" than my right. - G.B.].

Note that some coatings or multi-coatings can give a more "yellowish" cast to the moon or Jupiter. MGF₂ (magnesium fluoride) for example, transmits significantly less light in the blue region of the spectrum than from green through red.

6. High Magnification

Not using high enough magnification is a primary cause for not seeing as much detail as one should. A certain amount of magnification is necessary for the eye to see the smallest details. Many experiments have shown that the minimum magnification necessary to see all the resolved detail is 25x per inch of aperture. For a 4 inch telescope, this is 100x; an 8 inch would be 200x; and a 16 inch is 400x. But this may strain the eye and it is easier to see the smallest markings using 1.5x to 3x the minimum magnification (37.5x per inch to 75x per inch), depending on one's own eyesight and observing capabilities. In general, 30x per inch to 50x per inch is best.

It may also depend on what you're observing. Mars, for example, can withstand greater magnification than Jupiter because the former's brightness per unit area as seen from Earth is much greater. At a perihelic opposition, Mars is 25 seconds of arc in diameter and -2.8 in magnitude. By contrast, at its closest, Jupiter is 50 seconds of arc and about the same magnitude (-2.7).

7. Eyepieces

Using eyepieces having a minimal number of elements and air/glass surface eyepieces results in greater contrast levels than using a many element, air/glass surface eyepiece, all things being equal. Those with multiple elements are usually widefield (60 to 90 degrees) and while they provide excellent aesthetic viewing, their contrast levels are noticeably lower.

8. Culmination

Observing the moon and planets when they are at their highest in the sky (culmination) or an hour or two on either side of it, results (usually) in better seeing conditions because of greater atmospheric stability. It is very seldom that one gets good seeing within 25 degrees of the horizon.

9. Weather

Lunar and planetary observing can be frustrating. Just as with the deep sky observer, lunar planetary observers also need very clear, transparent nights to reach the limits of what the telescope can show. It has been said by some that haze improves seeing for bright objects like the moon and planets. However, in many cases, haze, fog and mist (if not too thick) prevents the air from mixing which results in turbulence.

We would all like to get the eight to ten nights per month of "diffraction limited" seeing as one noted Florida observer gets. But in more northerly climates, expect less than half this along with fewer clear nights in general. Some of the best observing nights coincide with the months from June to October. If a Bermuda High sets in for a week or two, the result is often nights when only the moon, planets and brightest stars are visible. However, the seeing is usually very still and there seems no limit to the magnification one can use.

10. Stopping Down

It should be remembered that what passes for mediocre seeing in a large scope may be good seeing in a smaller one. The larger telescope will "see" a larger cross section of the air cells that continuously pass in front of the aperture. If you have a large scope, an off-axis mask for a reflector or Catadioptric or a reduced aperture mask for a refractor can make a difference. A 10 inch telescope, for example, will work better on the moon and planets if stopped down to 4 inches. This also avoids diffraction effects from the secondary in a reflector, creating a cleaner image.

So there you have it! Ten secrets of lunar and planetary observing to help you explore other worlds without ever leaving Earth.

Copyright 2001 by Typographica Publishing, all rights reserved.