Long vs short focal ratio for lunar & planetary observation


Rodger W. Gordon

Although usage varies, long focal ratios are generally between f/10 to f/27; intermediate f/6 to f/9 and short (fast) f/4 to f/5. In theory, short focal ratio telescopes should be equal to longer focal ratio telescopes of the same aperture, both in resolving power and image contrast. There are a number of reasons why in practice this is not so.

Achromatic refractors exhibit severe secondary spectrum below f/8 (the rule of thumb for 1/4 l secondary correction is an f/ratio 3 times per inch of aperture, i.e. 3-inch f/9; 4-inch f/12 & so on), unless additional objective elements are used e.g. the Petzval system. An example of such an objective is the Plossl form adopted for the Athens 6-inch f/11. However even three or four element apochromats work best at longer f/ratios. This is due to spherochromatism which becomes a problem below approx. f/8. Again, if a Petzval arrangement is employed, as in the TeleVue Renaissance and Genesis, spherochromatic aberration can be reduced.

Newtonian reflecting telescopes require smaller diagonals for a given fully illuminated linear field, thereby minimizing the central obstruction. When the c-o to clear aperture ratio exceeds 1/3, contrast becomes markedly reduced. Most fast f/ratio Newtonians need at least a 1/4 c - o and typically a 1/3 c - o minor axis flat.

Eyepieces with long focal lengths tend to have greater eye relief. If the telescope has a long focal length, medium and high powers can be employed using much longer and therefore more comfortable eyepieces. Yes there are eyepieces made nowadays which have 3/4-inch eye relief and yet only 1/8-inch focal lengths. But they achieve this at the cost of several additional groups of elements which reduce image contrast. I deal with this more fully in my article about eyepieces.

Errors of figure and cosmetic defects tend to be more common in short focus eyepieces because of the difficulty in working such tiny lenses, although much depends on the skill of the worker. Depth of focus is also greater for long f/ratio telescopes, making focusing less critical. While this may not seem important, it can be a problem as one grows older. The eye looses its powers of accommodation, an opthalmic condition termed "hypermetropia" due to gradual hardening of the lens. Furthermore fast f/ratio telescopes require of necessity ultra-precise focusers, because the depth of focus is typically only a few thousandths of an inch. Fast f/ratio Newtonians and apochromatic refractors are also more susceptible to defocusing due to wavefront distortions because the depth of focus is so slender. A minor tilting of the wavefront (first order distortion) that in a long focus Newtonian or refractor would merely cause the image to dither, can in a fast f/ratio Newtonian or refractor produce blurring.

When using a long focus telescope it is possible to achieve high magnifications without necessarily resorting to auxiliary optics such as a Barlow lens. Whilst Barlow lenses are nowadays mostly of excellent quality, they do add additional elements that scatter light and reduce image contrast.

The performance of eyepieces, regardless of type or design, is largely governed by the focal ratio of the telescope with which it is used. Even the much berated Huyghenian works fine at f/15, whilst almost any two lens design will give excellent results over f/20. Of course there is the portability or transportability of the telescope to be considered, but it is possible to fold the light path as in the case of the Cassegrain, or the Schiefspiegler. However it should be born in mind that a long tubed refractor places the object glass above much of the ground currents that so often spoil the seeing in Newtonians.

Long f/ratio telescopes are more forgiving of slight collimation errors. Poor collimation is a principal culprit of the indifferent performance of many short f/ratio Newtonians. The same small misalignment resulting in a virtually undetectable collimation error at f/10, could be intolerable at f/4. Long f/ratio mirrors are simpler to test, they do not require the elaborate test setups of fast f/ratio mirrors, especially those faster than f/6. It is practicable to leave a 6-inch f/10 mirror spherical, and still produce a Rayleigh limited wavefront. Mirrors with focal lengths long enough for parabolization not to be necessary for diffraction limited performance exhibit coma free fields, that are also free from off axis astigmatism. In other words they are aplanatic systems. It is hard to beat a telescope that is fully corrected for chromatic, and spherical aberration, astigmatism and coma, and possesses to all intents and purposes a flat field.

Achromatic refractors with long f/ratios do not need expensive glasses or fluorite to achieve full colour correction (less than 1/4 l between the C & F lines). Image quality of a 4-inch f/20 doublet O.G. is on a par with a 4-inch f/6 apochromat, and the f/20 O.G. does not need expensive eyepieces designed to work at f/6 or faster.

There has been a systematic shift from long to fast f/ratio telescopes since the 1980's. The chief reason commercial telescope manufacturers desire to market the low power and wide angle views possible with such 'scopes is not entirely because of the aesthetic pleasure the RFT can provide. Manufacturers can also offer a battery of expensive wide angle eyepieces, often more costly than the telescope itself. This trend is unlikely to change in the foreseeable future, and the hobbyist should be aware that if he goes down the path less frequently trodden, he will keep more green in his wallet whilst enjoying a significant advantage over his short f/ratio brethren in the field of lunar and planetary observing.

As webmaster of this site I would like to add some additional comments to Rodger W. Gordon's pertinent criticisms of the trend to ever more compact barrel assemblies.

There is a prevalent misconception that short f/ratio 'scopes because they are photographically faster, yield brighter visual images, and are therefore preferable in the field of deep sky observation. Visual image brightness is governed solely by the diameter of the exit pupil, and has nothing whatsoever to do with the objective's f/ratio. The problem with using a 6-inch f/4 Newtonian say, for wide field RFT work on deep sky objects is its lousy off-axis performance. Unless the outfield is adequately corrected for the gross coma and astigmatism exhibited by an f/4 paraboloidal mirror, that outfield is unusable. Yet it is feasible to obtain a 2 degree field of view with an 8-inch f/8, and enjoy a fov to all intents and purposes free of coma and astigmatism without employing a coma corrector. The design is less prone to veiling glare, the focal plane is flatter, and because there is little off axis coma, even the outfield is passable. An f/4 telescope requires a 28mm focal length eyepiece to yield a 7mm exit pupil, and an f/8 requires a 56mm focal length eyepiece. All else being equal, image brightness will be the same in either telescope.

As for the desirability of portable or transportable telescopes? I take issue with this. Throughout the 50 or so year period the "Gleanings for ATMs" column ran in Sky & Telescope, whilst there were many articles on compact telescope designs, it was never maintained that they could replace long focal ratio telescopes when it came to high contrast, hi-res imagery. There were countless articles all extolling the virtues of the long focal ratio telescope for visual work, in all fields including deep sky work.

Nowadays it is argued, light pollution is so bad, that the only way to get a view of faint deep sky objects is to pack the 'scope into the trunk and head for the hills. I maintain this attitude is founded on a fallacy, and if I had followed this path, I would not today be the proud owner of a 10-inch f/10.6 Calver, in a dome, at a relatively dark sky location. The answer to light pollution is not to keep heading for the hills, but to relocate to the suburbs or preferably a rural village and build a domed observatory and a heavy duty long f/ratio Newtonian, or Cassegrain. I do own a Mak-Cass., and occasionally I take it to truly dark sky sites, but the views I get are only marginally better than through the Calver. And the beauty of having a permanently mounted telescope in your back garden is it can be in use in a matter of minutes. And for those who maintain that their job in town necessitates them living in town - tough! I used to work in the aerospace industry.  I acquired my Quantum 6 whilst living in Santa Monica in 1979. I used to commute down the San Diego freeway to McDonnell Douglas' plant on Lakewood Boulevard near Long Beach. I know how hard it can be, and skies don't get much more light polluted than Los Angeles. The answer in the short term may be to head for Mt. Pinos every new moon weekend, but its no long term solution, and I speak from experience. The answer is to make your pile and get out. Instead of spending your greenbacks on fancy short f/ratio telescopes, and grotesquely overpriced eyepieces, set your priorities on a rural or semirural location, and either commute, or relocate entirely. A permanent setup should be your goal, not accepting second best and ending up a perpetual astronomical nomad!

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