The ideal planetary telescope?

I have been pondering this problem for a decade or so. I have asked on astronomy forums, which is the best telescope for planets, and I have actually received a variety of answers! Ultimately.... however...

There is no such thing as the ideal planetary telescope. Really? Really! Yes. If we could have what we really wanted, it would be far too expensive and impractical. We would need to be rather rich and live in a mountain-top observatory.
All we really have in the astronomical world are a series of compromises based upon different focal systems which have sprung up in the last several centuries.

Assuming size is no limit, I am going to take the reader through a series of alternatives, assuming limitless size, and aperture, to see which scope would `win' and bring out the most details. The winner of course would be the reflector. No scope can get up to its enormous sizes, and it is the default for various astronomical observatories around the world as the `large scope.' the following categories also apply somewhat to price per view, with the later ones more value for money, as well as per view.

Amateurs however, have little intention of buying the biggest possible scope. The main aim is to have a lot of fun. Something I think is little known about, but always appreciated, is getting the most image out of the smallest scope. There is a real joy to think that so much can be seen with so little. This joy is not apparent with larger scopes. The larger the scope, the more a good view is `expected'. I know that people can fall in love with small telescopes. I am unsure if people can really relate to larger scopes. A small scope is more human-sized, or tool-sized. Larger scopes are more described as `leviathans' or `monsters,' hardly endearing terms. They are less personable. Let us call this the `cute factor.'

Before we go any further, let us say that all good telescopes can be used wonderfully for planetary imaging. Where you go from there, specifically entails asking what precisely it is one wishes to accomplish.

The Refractor

The ideal planetary telescope would be something like Percvial Lowell (a very rich man!) was using a hundred years ago to view planets, an enormous refractor! There are no mirrors and the light goes straight through several lenses to reach our eyes. These are the sharpest scopes for planetary observing, due to a combination of optics, as well as long focus, which brings out various details at different distances. A longer focus will always bring out more detail than a shorter focus.

The refractor takes superb pictures, yet is not very often used for planetary imaging. They get expensive fast, the bigger they are. Centuries ago, observatories were measured in terms of foot length! Now they are measured in the width of a reflector mirror. There are better alternatives. This is the great irony of the refractor. It is the best, yet not really attainable.

Schmidt Cassegrain

Some of the world's best imagers seem to use these. These telescopes are a compromise between large diameter, long focus, and large obstruction. The large obstruction is a problem, but it is corrected for by having a long f/10 focus. This brings out more of whatever details are not obscured in terms of wavelength, by the obstruction. The Cassegrain scope masters compromise to deliver a compelling package, used by amateurs everywhere.

To look at planets, one needs a narrow field of view and a high magnification. This implies some sort of tracking coupled to a stable mount. To look at deep sky, the reverse is usually the case. you need a low mag and wide view scope. The long focus scopes are therefore very useful, but using a barlow with a reflector is also a good idea.

This type of scope is a premiere scope for planetary imagers, who are seeking the best, in the 8-16 inch range.

Planetary Dob (Newtonian)

This is essentially a long-focus Dobsonian scope, usually. Most six inch Newtonians are f/8 making them rather decent for planetary work.

Here is a little rant about the benefits of long focus:

I had some ideas about starting a business to make some unique types of telescopes, but will probably not carry that one out to fruition. I am interested in the Herschellian design (a type of Newtonian) because it eliminates the need for a mirror which interferes with the light entering the scope. The image however is more skewed than normal, by an oblique reflecting angle. This can be mostly eliminated with a long focal length, which would make it ideal for viewing planets. The mirror need only be spherical. The question is that has mass production lowered the cost of parabolic mirrors sufficiently to make the production of easier spherical mirrors pointless? Made by hand, spherical mirrors are easier. However I believe most mirrors are no longer made by hand.

At the end of the day, I think the Planetary Newtonian design would work very well. I'd go wild and initially make a prototype of maybe four inches. IF that gives a good picture, I would simply scale it up. Perhaps with a custom 12"-16" mirror (pricey!) and a focus of f/10-12 (!). A 16" (40cm) diameter at f/10 means the scope is going to be four meters long! Get the ladder! A more realistic and far far far cheaper 12" long focus mirror will make a telescope of only three meters long! And that's the problem. If you want a great view in astronomy, you have to pay the penalty somehow! That's why someone can instead buy a Schmidt Cassegrain, with the same diameter and focal range, and it will fit in their car boot. In a long newtonian, however the secondary mirror is so much smaller, and the view so much better! IS it worth it? I don't know, but since many people don't have a long focus reflector more than 6" in diameter (these beginner scopes come in f/8), it would be fun to have something a bit unique! Not sure how you're supposed to take a three meter scope to a dark sky site! Fortunately you don't have to! Planets are so bright they are largely immune to the sky pollution effect. Yay!

And thus we have the major problem with planetary dobsonians. They are too long. One can simply buy an equivalent size Schmidt Cassegrain for a similar view.
I would love to make one, but it is also easier just to get a Maksutov. The best you can get are about 8 or 10 inch and are about f/10-16. Oh well.

The Maksutov

The Mak provides some of the best views available for its size, which are almost refractor-like. Surprisingly, the Maksutov is a little-used scope relative to the Schmidt Cassegrain. Unlike the Schmidt Cassegrain, this telescope fails at compromise, in several areas. Let's discuss them.

The Maksutov is a very long focal length scope. This means that it might be good for planets or galaxies, but is not much good for the wide field view. Additionally, it takes 2 hours or so to cool down sufficiently for use. Its apertures rarely go above eight inches due to construction, and possible heat-loss use difficulties.

That is a major problem since everyone knows that `aperture rules.'

An eight inch Mak will therefore be possibly considered inferior to a Schmidt Cassegrain of 10 inches, even though views may be comparable.

The typical view through a fast (short focus) dobsonian of a planet, is a tiny flattened disc with a few shades of colour. Detail increases with either aperture and longer focal length, and a stable tracking mount providing magnification. Currently most people are going for the aperture path, with 16" dobsonians now almost mainstream. The problem is the image is still not that brilliant. This can only really be achieved with a long focus scope. With such a setup, the planet becomes almost three dimensional. That's right! I imagine a long focus reflector with a brilliant wide-view lens attached such as a Panoptic/Nagler will deliver an incredible 'star-destroyer bride' view. Such a lens on a maksutov is another alternative. Scopes of either configuration maksutov/long focus reflector, become much more expensive as they get bigger, few people use them.

The three dimensional planetary effect is created by light waves not having to be bent and refocused to the extent which occurs in a regular scope.
The Maksutov is a great planetary telescope, but it is not the only one. The best sort of reflector would be an off axis telescope, where the central obstruction doesn't even get in the way. This would deliver views approaching the refractor. Such scopes exist.

What about the off-axis reflecting telescope?

A Newtonian has advantages. It's cheap and simple, but before we discuss this, let's look at some `better' alternatives. In an off-axis scope, such as William Herschell's, he considered that the low reflectivity of materials then available was a major problem. An off-axis he considered essential to capturing maximum light, even if this obscured images somewhat.

These days, no off-axis is required, for Herschell's reason, as mirrors are very reflective these days. Herschell's images had some geometric problems, which Newtonians do not have. Defocus a star and see what the image really looks like, it looks like the light coming down a tube with a central obstruction. This is the light which is being played with... an incomplete package.

there are problems caused by the central correcting mirror interrupting the wavelength, resulting in
The off axis is simply hardly obtainable commercially. This is its major problem. Very few people are interested in building scopes, so it's a bit of an issue.

Off axis have a lot of issues. Every major commercial telescope today has is an on-axis design. People who make an off-axis telescope often make it f/20 to hide the geometrical defects. A 'slow' telescope of this type is extremely forgiving, giving a great picture regardless of the mechanism involved.

Geometrical problems skewing the image in an off-axis (correcting mirror is out of the way, to the side) scope can be counteracted by having a long focal length. This would make such a scope ideal for planetary observation. I'd love to make one but am doing other stuff at the moment.

The Large Dobsonian Newtonian Reflector

The Number One Solution for Planatery Imaging?

This telescope, in large size, in the form of dobsonian, is probably best, provided the following provisos are adhered to:

* We need tracking. This is utterly essential for acheiving magnification. Fortunately this is available for Dobsonians. Equatorial tracking is not required for imaging as Registax, (stacking software) can automatically align various images, which would rotate in a circle without equatorial tracking.

* Goto would help, as would robotic focus. Focus can be acheived by looking at a laptop on ground level, with a suitable webcam attached. Who wants a huge ladder!

* We need a garage for cover, as well as a rolling platform to place the telescope. I use an old lawnmower, like Clyde Tombaugh did, for his dobsonian. Then the telescope can be permanently set up, and rolled in and out of its little garage.

* Long focal length for planetary photography is simply acheived by using a 5x barlow or a powermate.

I have not actually tried imaging with a huge dob yet, it is simply a pipedream, but from the research I have done, I would imagine it is the best solution to the planetary problem.

Conclusions

Since focal length is so important for improving the image, the best scopes for planetary observation are long focal length scopes. These are the Maksutov, probably number one at about f/15, the Schmidt Cassegrain, at about f/10+, EVEN if it has a huge central mirror! That focal length fixes a large part of that problem. The other scope which is great for planetary is a long focal length reflector. Gererally you have to make this yourself and you might need a ladder, but I want one!

The Large Dobsonian Newtonian is probably best, for very large apertures, capturing the most detail, with the proviso that the above conditions are adhered to.

Supplementary Appendix

The moon is three-dimensional during an eclipse

An amazing effect is that the moon becomes three dimensional during an eclipse! I feel this is related to the effect seen in long focus scopes. (Less light and longer wavelengths coming in to hit the back mirror). If we observe what is happening in the situation of a lunar eclipse, it also seems that a much lower wavelength is now hitting the moon. That's my suggestion.

A more orthodox idea is simply that for some reason, normally we are getting roughly equal light bouncing back to earth from all parts of the visible moon, making it appear flat. Meanwhile, during an eclipse, we have light hitting the moon from a more oblique angle, being reflected around the earth, hitting the sides. There is more of a gradient so we can see the moon's true shape. Essentially, spherical shading in an artistic sense. At Full Moon, we have the earth in between the sun and moon so we're getting less spherical shading. Indeed I think the moon looks more three dimensional during the crescent phases.

Someone email me to explain this, if they have a better explanation and I will update the site! (please!) I will go on an astronomy forum to ask when I have time.

UPDATE! My brother says that with a long focus scope, simply more of the planet is in focus, therefore three dimensions. Sounds good to me!