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So, what sort of performance and results can one expect from a fairly typical amateur reflecting telescope, say of 6-12 inches or so aperture and of good optical quality? Without the small investment of trouble in adjustment of the instrument and training of the eye outlined in the preceding paragraphs, the field of wide doubles is open to the observer, that is to say pairs from 1-2'' upwards. Diffraction-limited performance will not be attained by a substantial margin and such an observer will probably consider resolution of a 1'' pair something of a triumph, while subarcsecond doubles remain an unattainable holy grail. Much rewarding observation can be done in this rather undemanding way but that ingredient which gives doublestar astronomy its deepest fascination will be largely lacking: motion. Very few of these wider pairs have orbital periods of less than centuries so the observer limited to this type of observation is largely condemned to studying binaries as static showpieces, missing out thereby on the grandest gravitational ballet in the whole of celestial dynamics. Adding the dimension of time, and being able to watch these majestic systems actually in action, adds incomparably to the interest of the observations.

The representative selection of this author's observations quoted below illustrate what can be done with very ordinary amateur equipment in this dynamic, subarcsecond domain, given the attention to preliminaries described above. The instrument used is the 12.5-inch (0.32-m) Newtonian referred to earlier and shown in Figures 11.7 and 11.8. It has a plate glass primary mirror figured by George Calver in 1908 which, as discussed in the first section, was deliberately left undercorrected by its maker, with the residual

Figure 11.7. The

12.5-inch reflector

(Peter Seiden).

Figure 11.7. The

12.5-inch reflector

(Peter Seiden).

Figure 11.8.

Eyepiece end of the 12.5-inch (Christopher Taylor).

spherical aberration consequently tending to give rise to diffraction rings of largely enhanced intensity. While, as pointed out earlier, the effect of this is to make such a reflector no match for a good refractor on very unequal pairs below about 1.5'', the spurious disk remains at the ideal Airy size or even slightly smaller, so equal (Am < 1) close pairs can be resolved at least as

Figure 11.8.

Eyepiece end of the 12.5-inch (Christopher Taylor).

well as in a refractor of the same aperture. Accordingly, the results quoted are all for binaries whose components do not differ by much more than 1 magnitude in light.

Lest the reader imagines that successful observation of subarcsecond binaries requires an expensive professionally constructed instrument equipped with the latest hi-tech. conveniences, or that the author has enjoyed such advantages in making the observations reported here, a brief description of the 12.5-inch will serve as a useful counterexample. The telescope was entirely amateur-built some 60 years ago and, although standing about 9 feet (2.7 m) tall, it has never been housed in any form of building or weatherproof cover. One result of this is that while the mechanical structure of the instrument stands permanently on a concrete foundation at a good observing site in the author's garden, the entire optical system must be stored indoors and mounted anew in its various cells, etc. at the beginning of each observing session. This, of course, means that full collimation of the system is an unavoidable necessity every time it is used - the telescope simply would not work otherwise. Thanks to intelligent design, however, this entire optical assembly and collimation routine only takes five minutes or so each evening: on the general view, Figure 11.7, note:

(a) that all optics are mounted externally, and very easily accessible, on the "tube" which in reality is nothing more than a box-girder for rigidity;

(b) the linkage rods running from the inner corners of the fully adjustable main mirror cell, up the length of the tube, to the eyepiece assembly at the top; these terminate in the collimation control knobs which can be seen at the lower corners of the eyepiece turret housing in Figure 11.8 and make fine adjustment of squaring-on of the main mirror while simultaneously looking through the eyepiece or drawtube a very quick and painless affair.

The instrument weighs about 1500 pounds (680 kilograms) and is an alt-azimuth, lacking not only (therefore) setting circles or clock drive but even any form of manual slow-motion controls. It is true that the 12.5 inch moves very smoothly on its bearings and is extremely stable but it remains something of an acquired skill to follow the diurnal motion at high power simply by pulling directly on a handle at the top of the tube, to say nothing of taking PA measures of close double stars! The full force of this remark will perhaps be appreciated when it is borne in mind that an equatorial star takes rather less than ten seconds to cross the full field of view at the power most commonly used for "subarcseconders", and that the observer, perched precariously on a step-ladder some considerable height above the ground, must perform this manual tracking continuously, coordinating hand and eye to a precision of a few tens of arcseconds, at the same time leaving the mind free to concentrate on what is seen in the eyepiece. This is observing in the classic style of William Herschel, far removed from the digital conveniences of the twenty-first century.

This telescope has a primary focal ratio of 7.04, with a central obstruction equal to 16.3% of the aperture diameter. Optical quality is such that the author's standard working power for all subarcsecond double stars is x825, at which single stars appear "round as a button" whenever the seeing is II-III (Antoniadi) or better, and the instrument would comfortably bear magnifications even higher were it then possible to manage its alt-azimuth motions sufficiently well. It is clear from the observations that the smallest doublestar separation detectable with the 12.5-inch (see below) is, even so, limited by magnification, not by definition and image quality. Statistical analysis of accumulated observations of equal bright pairs at 0.4-0.9'' shows that the apparent star-disk diameter of a fifth or sixth magnitude star at x825 in good seeing is 0.311 ± 0.037''; this observed size of spurious disk is only 37% of that of the full theoretical Airy disk (out to first zero) and agrees exactly, after scaling for aperture, with the result independently determined for a 4 inch (0.102 m) refractor also used for double-star observation. This last comparison shows that the image definition of a Newtonian can be not only as good as that of a refractor of the same aperture but, after scaling, can match that of a far smaller refractor, a much more severe test. It must be emphasised once again, however, that such quality of imaging can only be expected of a Newtonian even at this f-ratio after full and accurate collimation, as detailed earlier.

The double-star results actually achieved with this 12.5-inch Newtonian are best represented by Table 11.1 which lists the typical appearance at x825 of bright, approximately equal pairs at successively smaller separations, in seeing II-III (Antoniadi) or better. Listed here are only those categories of target which can in

Table 11.1.

Separation '

Typical appearance of star disks

0.4-0.5 or so Two completely separate disks parted by a small gap, persistently split in good seeing, e.g. X Cas 1995.03 (0'.'43), ^ And 1995.80 (0'.'48), p Del 1998.72 (0'.'50), m Leo 1996.26 (0'.'52), 72 Peg. 1994.78 (0'.'53) 0.35-0.36 Two distinct disks in contact (tangent), occasionally just separating in good seeing, e.g. p Del. 1996.87 (0'.'35-6) 0.33-0.34 Disks now slightly overlapping, giving "figure 8", heavily notched but not separating, e.g. 8 Equ. 1995.79 (0'.'33-4), a Com 1996.46 (0'.'33) 0.29-0.32 Very elongated single image ("rod"), occasionally just notched at best moments; an easy elongater, the disk elongation quite obvious in even moderate seeing, e.g. 8 OriAa (Hei 42) 1998.11 (0'.'31). 0.24-0.28 A single oval disk ("olive"), the elongation still quite pronounced although noticeably less than in the last case, no hint of a notch now, e.g. p Del 1995.85 (0'.'28), a Comae 1997.35 (0'.'26), A 1377 Dra 1997.80 (0'.'25), y Per (WRH 29 Aa) 1996.881997.19 (0'.'24)

0.21-0.23 Slightly oval disk, elongation small but still quite sufficient to read PA

confidently at best moments; now becoming noticeably more difficult, the difference between 0.21" and 0.24", very obvious to the eye, e.g. k Peg. 1996.88 (0'.'21) 0.17-0.20 Very slightly oval disk, the elongation very small but in the best seeing absolutely definite, especially by comparison with a neighbouring single star as a "control"; now becoming difficult to estimate PA confidently, the detection of elongation nearing the limit for x825, e.g. Z Sge (AGC 11) 1996.77 (0'.'19) which was appreciably easier than a Com. 1998.41 (0'.'175) - the current limit for positive detection of a double star with the 12.5-inch at this power. Somewhere at, or Beyond the limit for reliable detection at x825, the star disk not clearly above, 0.13 distinct from that of a neighbouring single star even in very good seeing, e.g. k UMa (A 1585) 2000.23 (0'.'13)

any sense be considered seriously testing of the telescope's capabilities, all wider pairs always appearing on any good night as two well-separated stars divided by a large space of completely dark sky.

There is no doubt that such performance claims run heavily counter to the perceptions of the large majority of telescope users who are, perhaps, too undemanding of their instruments. To any reader inclined to be sceptical of the above results I would point out that the author had been using this same telescope on double stars and other "high resolution" targets for more than 25 years before the observations themselves forced the possibility of such subarcsecond performance on the attention of a mind not predisposed to expect it;

further that all such double-star observations are made essentially "blind", the observer having no prior information on "expected" PA, and only a rough figure for separation, on going to the eyepiece. So the relentless internal consistency of the observations with respect to separation, and their close individual agreement in virtually all cases with definitive values of PA subsequently consulted as an objective verification, are more than sufficient to establish the objective validity of these results. In the entire set of observations of pairs below 0.5 there are only two or three cases of clear contradiction with this post-observational check, none of which were in good seeing. These very few failures are, moreover, offset by a number of other instances of apparent contradiction where more authoritative data subsequently obtained have proven that the observations were correct and that it was the published information available at the time which was in error, this having occurred for X Cas, a Com, y Per, 5 Ori Aa and k UMa.

Such results should really occasion no surprise as they are actually in precise agreement with the Dawes limit (0.365' ' here) as can be seen from the first three classes in Table 11.1, as well as agreeing pretty closely with what would be expected from the previously quoted size of star disk determined quite independently from observations of much wider pairs. All of this is, in fact, entirely in line with the mainstream of historical experience in this field, from Herschel who founded subarcsecond double-star astronomy in the early 1780s with a 6.2-inch mirror (0.157 m), right down to the Hipparcos satellite observatory which made accurate measures of pairs down at least to 0.13'' in the early 1990s with a 0.29 m. mirror - rather smaller than that used by the author, although admittedly having the huge advantage of perfect seeing! The limit on detectable separation, for instance, in Table 11.1, at 0.48 of the Dawes limit, is closely comparable with the average for the closest class of new discoveries made by S.W. Burnham with a 6-inch aperture, i.e. 0.53 x the Dawes limit.

The author's observations therefore establish conclusively that double-star elongation of reasonably equal pairs is reliably detectable in a 12.5-inch mirror at x825 down to a limit somewhere about 0.17' ', as with a Com in May 1998. All such pairs down to 0.24'' inclusive are easy elongaters in good seeing, only the last two classes in Table 11.1 really presenting any significant difficulty under the best conditions. What is perhaps most remarkable about such observations is the extraordinary sensitivity of the shape of blended or partially resolved double-star images to really minute changes in separation: in the 12.5-inch at x825, a change of only 20-30 mas is quite appreciable to the eye in pairs from 0.4'' downwards, while an increase of 60-80 mas is sufficient to transform the appearance of a pair totally, from "olive" to "disks tangent", as in the case of P Del 1995-1996. It is amazing but true that a ground-based amateur telescope of unremarkable aperture and positively primitive lack of sophistication, used visually in the time-honoured fashion, can and does reveal clearly angular displacements smaller than any detail actually resolved by the Hubble Space Telescope.

Access to this subarcsecond domain opens the door on a dynamic world of binary-star astronomy usually considered the exclusive preserve of the professional using powerful instruments equipped with the latest technology and sophisticated methods such as speckle interferometry. Indeed, several of the pairs mentioned above have been used in recent years as test objects for evaluating the performance of adaptive optics systems on professional telescopes of 1.5-m aperture and above, while the entries in the third CHARA catalogue show that all are favourite targets of the speckle inter-ferometrists. It is one of the better-kept secrets of observational astronomy that it is nonetheless perfectly possible, with care and determination, to follow many of these systems' orbital motion visually with an amateur telescope of only slightly larger than average aperture, which means, almost necessarily, a reflector. This should not be a surprise to anyone: almost all of these binaries were, in fact, discovered in just this fashion, using very much this range of apertures, by e.g. Otto Struve with the Pulkova 15-inch, Burnham with 6 and 9.4-inch instruments, etc.

Among the author's more memorable experiences with the 12.5-inch telescope are several concerning some of the most legendary of the short-period visual binaries. 5 Equ (OX 535), perhaps the most famous of all such systems, was long the holder of the record for the shortest period of all visual binaries, at 5.7 years. This pair is actually quite easy on a good night in the 12.5-inch when at its widest as in 1995, appearing then as an absolutely unambiguous figure 8, only just failing to separate completely. The orbital motion is phenom

Figure 11.9.

Observations of the pair 8 Equ with the 12.5-inch reflector.

Figure 11.9.

Observations of the pair 8 Equ with the 12.5-inch reflector.

enally rapid, a total transformation in the appearance of the star occurring in a year or less, as the author witnessed in 1995 and 1996 - see Figure 11.9. This motion is actually so rapid that, if caught in very good seeing at the critical moment in the orbit, a change plainly perceptible at the eyepiece of the 12.5-inch will occur in only seven or eight weeks, 5 Equ having crossed an entire class in Table 11.1.

P Del (P 151, period 26.7 years) is another pair whose orbital advance in a single year is plainly visible in the 12.5-inch reflector even without quantitative measurement, its steady year-by-year opening out and rotation in PA having been conspicuous in that telescope in the years 1995-1998. This was first noted on 13 November 1996, the entry for which in the author's observations (obs.) book reads "P Delphini x820 showing an immediately obvious 'rod'/'figure 8'; on further scrutiny, several times glimpsed two distinct stars just touching, i.e. this pair now much easier than a year ago ... PA constantly and easily legible at 330-335°." This was a rough "by eye" estimate only, not a measurement, but very noticeably larger than it had been twelve months earlier, the seeing only fair at III-II. The definitive position at the time of this observation was subsequently found to be (0.35-0.36'', 323°). See Figure 11.10 (overleaf).

Other similar cases have been a Com (X1728, period 25.9 years); y Per (WRH 29 Aa, period 14.7 years) a beautiful system which is the brightest visual binary in the heavens also to be an eclipsing variable11, the double-star observations of which have been mostly by speckle interferometry on 3 to 4-metre class telescopes; and k Peg (P 989, period 11.6 years). The 12.5-inch followed the inward march of X1728 over the late 1990s, beginning with "figure 8" at 0.33'' in 1996, all the way down to "elongation v. slight but perfectly definite" at

Figure 11.10.

Observations of the pair ß Del with the 12.5 inch reflector.

0.175'' in 1998, the smallest separation so far detected with this telescope. The annual change in this star was quite apparent at each of these three observing seasons and, although it was much more difficult in 1998 than it had been a year earlier at 0.26'', even the limiting elongation to which it was followed was quite unmistake-able - "like a dumpy egg" - by repeated comparison with the absolutely round disk of Arcturus, then at the same zenith distance. (Given that 0.33-0.34'' appears as "figure 8", this is in fact exactly what one should expect of the same pair at 0.175'', as can easily be seen from scale drawings of spurious disks overlapping to the appropriate degrees.)

Figure 11.10.

Observations of the pair ß Del with the 12.5 inch reflector.

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