Star testing before Suiter

As I wrote years ago reviewing it for Coelum magazine, Harold Suiter's book, Star Testing Astronomical Telescopes Published in its first edition in 1994, it marked a turning point in amateur astronomers' approach to commonly used astronomical instruments: for the first time, publishing houses made available to observatories a manual specifically dedicated to the qualitative evaluation of astronomical optics, a topic that until then had only been addressed cursorily in practical astronomy manuals and more thoroughly only in DIY publications, the distribution of which, however, had always been limited to this specific field.

While offering everyone the ability to recognize and, where possible, correct telescope optical flaws, Suiter's book also had some downsides, most notably fueling the acute instrumentitis that afflicts so many amateur astronomers around the world: from a simple and sensitive method for qualitatively verifying astronomical optics, star testing has slowly become a sort of obsession, even a nightmare for many instrument owners, worried about even the smallest defects visible in the diffraction image provided by their beloved and often very expensive telescopes.

But how was the optical quality of a telescope evaluated before the publication of Suiter? To satisfy my curiosity, I browsed through some literature for amateur astronomers, starting with some famous guides to sky observation published in the 19th and early 20th centuries, then moving on to some more recent literature, both in English and Italian.

Below, I will report excerpts from the relevant paragraphs, without translating them so as not to alter the meaning of the texts in any way. Instead, I will add comments after the most significant points: we will see how the guidelines given over a century ago for recognizing a defective instrument are still very valid today. Obviously, this review does not claim to be exhaustive.

THE OLDEST MANUALS

The oldest text I got my hands on was volume 1 of the Celestial Objects for Common Telescopes by Rev. T.W. Webb, first published in 1859

and where we find the following:

Actual performance is the only adequate test. The image should be neat and well defined with the highest power, and should come in and out of focus sharply; that is become indistinct by a very slight motion on either side of it.

This is the so-called "snap test": the telescope must have a single, well-defined focus point, and even the slightest movement from this position should cause the image to degrade immediately. If this doesn't happen—that is, it's difficult to decide which position is the best focus—this indicates a defective instrument. If the telescope has a significant depth of focus, the best image will be found within a small range of positions, within which, however, it must maintain the same sharpness.

A proper test object must be chosen; the Moon is too easy: Venus too severe except for first rate glasses; large stars have too much glare; Jupiter and Saturn are far better; a close double star is best of all for an experienced eye;

Here, each author has his own preferences, as we'll see later. Today, double star testing is less relevant than in the past, but it's important to consider that at the time Webb was writing, measuring double stars was a major concern for astronomers, both amateur and professional. It's undeniable, however, that a less than perfect instrument will have difficulty detecting a double star separated at the Rayleigh limit.

[…] but for general purposes a moderate sized star will suffice; its image, in focus, with the highest power, should be a very small disc, almost a point, accurately round, without wings, or rays, or mistiness, or false images, or appendages, except one or two narrow rings of light, regularly circular, and concentric with the image: and in a uniformly dark field; a slight displacement of the focus either way should enlarge the disc into a luminous circle. If this circle is irregular in outline, or much brighter or fainter toward the center or much better defined on one side of the focus than the other, the telescope may be serviceable, but is not of high excellence. […] a fair judgment may be made by day from the figures on … the image of the sun on a thermometer bulb placed as far off as possible.

This description contains everything you need, even the suggestion to use an artificial star. Note the criterion according to which the telescope is perfect when the intrafocal and extrafocal images are equal, a criterion later generalized (mistakenly, however) by Suiter but certainly valid here, since in Webb's time the most popular telescope was the Newtonian reflector. Obviously, being a guide to sky observation, Webb does not provide more specific diagnostic criteria distinguishing between the effects of the various aberrations.

Another important author who has addressed star testing is William Denning. An expert planetary and comet observer, Denning was also passionate about instrumental problems; today we would call him an "instrumentaphile." In the paragraph Testing Telescopes of his most famous book (Telescopic work for stalight evenings, London, 1891)

He first writes that the best way to judge the optical quality of an instrument is to compare it side by side with another of known and proven quality. Failing that, one must make some evaluations of celestial objects oneself:

The Moon is too easy an object for the purpose of such trials; the observer should rather select Venus or Jupiter. The former is however so brilliant on a dark sky, and so much affected with glare, that the image will almost sure to be faulty even if the glass is a good one. Let the hour be either near sunrise or sunset, and if the planet has a tolerably high altitude her disk ought to be seen beautifully sharp and white.

Venus is a terrible subject for any telescope, but if you have the foresight to “tame” it by observing it when the sky is clear and the seeing is good, then it becomes a good test for any optics, especially refractors.

Various powers should be tried, increasing them each time, and it should be noticed particularly whether the greater expansion of the image ruins the definition or simply enfeebles the light. In a thoroughly good glass faintness will come on without seriously impairing the defined contour of the object viewed. […] But in a defective telescope, a press of magnifying power at once brings out a mistiness and confuses the details of the image in a very palpable manner.

This evaluation criterion is well known to those who own high-quality instruments. It is possible, if the seeing is favorable, to push the magnification up to three or even four times the objective diameter in millimeters without the image losing definition, merely becoming darker. It is a purely qualitative test but very useful when comparing instruments with similar design features. For refractors, the test is still valid, provided you select the color for which the instrument has been corrected.

Perhaps the best test of all as to the efficiency of a telescope is that of a moderately bright star, say of the 2^nd or 3^rd magnitude. With a high power the image should be very small, circular and surrounded by two or three rings of light lying perfectly concentric with each other. No rays, wings, or extraneous appearance other than the diffraction rings should appear

warning the reader, however, that a mirror telescope will show spikes due to the secondary supports.

 Let's move on to the Flammarion, Les Etoiles et les Curiosites du Ciel, Paris, 1882.

Flammarion describes the star test of a refractor as follows:

If the atmosphere is pure and calm, naturellement un étoile (pas trop brilliant) placée au foyer de la lunette armée de ces grandessements, doit se presenter sous la formé d'un petit disque lumineux, tout petit et tout ronde, bien défini sans rayons, sans ailes, sans brouillard environnant, ressortant sur a uniform shade background; You can't distinguish between the two luminous points one or two very light concentrated anneaux.

And once again we find the snap test

The foyer of darkness occupies too much of the place, and when you find the moment of absolute clarity of the image, whether it advances or retreats the eyepiece tube, the clarity immediately disappears.

Although not expressly dedicated to amateur astronomers, The adjustment and testing of telescope objectives, written by H.D. Taylor for Cooke, Troughton & Simms – highly regarded manufacturers of astronomical instruments – represents a step forward in the popularization of star testing. The first printing of the volume dates back to 1891, but it went through three further editions, the last of which in 1946, by which time Cooke had moved to Grubb, Parsons & Co. This is the first true manual on star testing, with diagnostic indications and instructions on possible interventions to obtain regular diffraction images. While the practical astronomy manuals we have seen so far were limited to verbal descriptions, Taylor enriches his volume with an outside-the-text table that already allows for a first rough interpretation of the star images:

THE SNAP TEST

We have seen that the old authors attached great importance to the so-called “snap test”, about which Denning writes: “A well-figured glass ought to come very sharply to a focus. The slightest turn of the adjusting screw should make a noticeable difference”: If this does not happen, assuming the seeing is favorable, we are therefore in the presence of some problem. In the worst cases, continues Denning, “the blurred images are thought, at the moment of its first perception, to be caused by the object to be out of focus, and the observer vainly endeavors to get a sharper image until he finds the source of error lies elsewhere.”

Just to give an example and have fun with it Aberrator, if this is Mars observed in focus in an f/8 reflector with a lambda undercorrection (a really bad telescope)

moving the focuser tube by half a millimeter, which we can take as the slightest turn what we can do with a 19th century focuser without demultiplication, the image would become like this

that is, almost identical, and we can imagine the unfortunate amateur astronomer fiddling around looking for a decent focus where in a perfect instrument the focused image would have been:

and the one blurred by the same half millimeter:

that is the sensible difference by Denning.

Suiter rightly warns against condemning a telescope solely on the basis of the snap test and that when carrying out this test it is necessary to take into account the focal ratio of the instrument under examination, since an f/15 has a depth of focus, and therefore a range of focuser positions within which the image is good, decidedly greater than that of an f/4.

From personal experience, however, I must say that I have never seen a telescope that, despite failing the snap test, turned out to be correct at the diffraction limit. Usually, a bad snap test equates to bad optics. The exceptions to this rule are more apparent than real and are due only to the specific conditions in which the test is carried out (turbulence, thermal transients, etc.).

SOME TEXTS FROM THE 20TH CENTURY

Let's move on. On the famous text by Danjon and Couderc, Lunettes et Télescopes (1935, revised and republished in 1979), otherwise very complete and still a valid reference today, I found almost nothing

and not even on Texereau (English ed., How to make a telescope, 1951) which mentions the star test only in passing, for example when dealing with astigmatism and collimation

I close this review of foreign texts by quoting James Muirden, who many of us know as the author of L’astronomy with binoculars, a little manual that was very popular about thirty years ago.

Muirden also wrote what I believe to be the best book on practical astronomy ever published, The Amateur Astronomer's Handbook (Harper & Row, 1983, with subsequent reprints) in which he describes the star test as follows:

The optical quality of a telescope can most easily be ascertained by comparing the intrafocal and extrafocal images of a star. […] If the mirror is perfect, the disk should appear identical at equal distances inside and outside the focus. […] The disadvantage [of the test] is that it is perhaps too sensitive. Many telescopes perform splendidly even when the expanded images are somewhat anomalous, and it can safely be said that if the disks are “more or less” alike, the telescope is a good one.

TEXTS IN ITALIAN

On the’Amateur astronomer by Paolo Andrenelli (2nd ed. 1977) there is practically nothing on star testing and also on Notes on Astronomical Optics by Luigi Ferioli (1987)

A once-popular booklet. This reflected the attitude of the time toward instrumental testing: it was tacitly assumed that a commercial telescope's optical quality would allow it to be used, and that optical tests were primarily for DIYers who needed to verify their work step by step.

On Knowing the stars, by Pierre Kohler (Garzanti – Vallardi, 1978) we find the following (page 98):

“Astronomical lenses essentially have two defects, called “aberrations”:

• chromatic aberration (or chromatism)
• aberration of sphericity (or coma) [sic!]

and further

“spherical aberration is less serious than chromatic aberration, because it is only a nuisance with lenses with a small f/d ratio, and is only truly annoying in astronomical photography.”.

Without the original French text, I can't say whether these statements were the author's or the translator's, but since the latter is the talented Piero Bianucci, I lean toward the former. Regarding star testing specifically, we find this diagram on page 100:

and that's enough.

Finally the publisher Il Castello publishes The Book of Telescopes by Walter Ferreri

The first manual from which we amateur astronomers truly learned the basics of how our instruments work. Despite still being a very "classical" textbook, tied to the typical amateur astronomy approach of the 1980s, the book truly had everything, and like many other amateur astronomers, I've read it dozens of times. However, even here, star testing was a bit of a Cinderella story in an otherwise comprehensive treatise: the diffraction image drawings were virtually unusable for a reliable interpretation of what was seen through the eyepiece, and the accompanying explanations were neither comprehensive nor clear, except for astigmatism. There were also inaccuracies, such as when describing the distribution of light in the rings. Following this, however, was a description of classic optical tests like the Foucault or Ronchi, and so we left the chapter on image analysis with the feeling that star testing served little or no purpose.

From the above, one cannot help but get the impression that, faced with a telescope whose performance was not excellent, the Italian user was left without a valid diagnostic tool other than turning to some very expert friend who, in most cases, would also have been a manufacturer.

The reader will forgive me if, to conclude this examination with dignity, I cite a small book that I wrote together with some friends for Edizioni Scientifiche Coelum. The book's aim was to "popularize", as they used to say, star testing by distilling some fundamental teachings from Suiter's book to make them accessible even to those who did not speak English.

The little book is still available but in the meantime the Suiter has reached its second edition

and despite the flaws of a rather unfortunate exposition based exclusively on computer simulations – which do not represent what is actually observed in the telescope eyepiece but only an idealization – I can only recommend its purchase to anyone for whom English is not a language barrier.