Part 2
After the polish is _perfect_ remove the plate from the support and cut into squares a little larger than the size of the intended flat. Test these in the telescope on a _star_. If one plane turns out bad, the whole will most likely be so, and another plate must be worked with renewed energy and care, for a bad plane will spoil the action of the speculum however good, for there is no way of counteracting the curved surface of the flat or second reflector. The edge of the plate for about an inch should not be used.
When a good plane is found it can be edged by turning a piece of wood a little less in diameter than the minor axis of the plane required, turn the end square, and mark a line around it distant from the end equal to the diameter, and cut through to the opposite edge, and it will give the oval and will appear round at 45°, this will mount nicely in brass tubes of the proper size, and a cover should be made to fit easy.
“THE EQUATORIAL.”
Fig. 1 is a modification of the German principle, and it should be a sufficient recommendation to remark that it is the principle used by a maker of such experience as Mr. John Browning, and is, without doubt, the very best style of mounting for a fixed equatorial, especially when clockwork is employed, because clock power is applied to the polar axis itself direct from the driving worm.
The driving part, viz., the worm and wheel, which is out of the observer’s way (being between the standards), is not liable to meet with accident, and the driving wheel being near the lower end of the axis, is at the most rigid part, viz., the foundation of the instrument. As it has a long and very firm polar axis, and is connected with the foundation plate, it secures the utmost steadiness and freedom from strain.
An equatorial mounting, with two long and stiff shafts for the axes, has always the advantage of firmness; the _holding_ portions of the instrument being in masses, are not liable to receive injury from blows, and thus be put radically out of order. _No means should be resorted to to make an equatorial of light weight_; an instrument cannot be light and slim and at the same time _firm_ and _steady_—no amount of steadying rods, splines or strings, will make a slender tripod for a refractor so steady as a firmly made one, with proper size and weight in the parts.
Another important advantage of this kind of mount for a large size is, that although it has considerable weight, it is very convenient to move and set up, being built of convenient parts, which are easy of separation and removal. The uprights or standards are separate, and are bolted to the bed-plate, the upper and lower discs are readily detached from the stand. The cradle and tube are in this construction separable. The tube of the telescope being suspended over the _side_ of the stand, is in the most convenient position for observing objects at any altitude, as the stand is out of the way and clear of the telescope. The various disadvantages and objections to the driving clock being _carried_ by the telescope are here avoided. The clock, which is large and very powerful, is bolted down to the iron bed-plate, and the telescope, not having to carry the clock or weight, the balance is never altered nor the rate of the clock disturbed, and thus a strong clock, keeping regular time, and working much longer can be used. They will drive 2-1/2 hours. It will be readily understood that a driving clock to enable the instrument to do the most exact and best work must be powerful and a good time-keeper. Lord Lindsay has said that “the clock should have twice as much power as is _used_,” otherwise the spindles, &c. are pent up, and it is moreover sensible to any extra weight or work being put upon the instrument.
To give an idea of the efficiency and regularity of this clock, I may mention that a Newtonian telescope of 18 inches aperture, intended to carry a photographic camera of 112 lbs. weight, needed no additional provisions to do the extra work, the rate and power not being affected. There is a mechanism in the clock “for making up of time” and in setting the hour circle can be used with a joint handle or hooks as a fine screw-motion, and can be applied whether the clock is going or not. There is also a provision at the foundation for throwing the instrument out of the meridian, to follow the motion of the moon or planets with the clock and “maker up.” The above mounting is equally suitable for “Cassegrains” and “Newtonians,” to both of which forms it has been successfully applied up to 18 inches of aperture. It is well suited for large telescopes.
Fig. 2 is a mounting on the same principle, but with a shorter polar axis, and the column is in one casting. It is well suited for moderate sizes, and the circles, &c. are applied exactly as in Fig. 1, but when a clock is wanted there is no mounting equal to the former.
The “Educational” is a plain 6-1/2 inch, of 4 to 6 feet focus, and is made to this pattern, with revolving body and screw-motion for following. It is made as portable as it can be, and is a steady, good-working instrument, and much approved of.
THE UNIVERSAL ALT-AZIMUTH.
(FIG. 3.)
In this mounting I have seen little or no alteration needful, except that the tube is now made to balance so that the eye-tube can be reached for objects in the zenith without the observer having to elevate himself, and the elevating rod can be clamped without the lever, the legs of the stand are more curved and have more spread in them.
This “Alt-Azimuth” stand has met with much approval; and where portability is of consideration and the observer has to set up and remove the entire instrument after every night’s work, this, or the Angle-Block stand, will be found the most convenient, more so than an equatorial in any form. There is much less weight to remove, and being in three convenient parts is more readily put together and separated in the dark. If the stand and trunnions can be left in the open air a very small covering will serve to protect them, and then the tube alone has to be removed. The equatorial cannot be too highly recommended where it can be a fixture and undisturbed, as when once got into proper adjustments its advantages can then be realised, but not unless it is a permanent fixture. The circles of a portable equatorial can only be used for very rough reading, and consequently the adjustments are never in order, and the readings are of very limited use indeed.
THE IMPROVED “ANGLE-BLOCK” STAND.
(FIG. 4.)
With respect to the tube, it is mounted like the Alt-Azimuth, but with the plane of the horizontal movement corresponding to the latitude of the place of observation, and therefore following objects with one screw movement is in reality a telescope with equatorial motions. It need not be of heavier construction than the Alt-Azimuth, and there is not the double weight of counterpoising, &c. as in the equatorial, and when circles are not desired this will be the most economical, handiest, and easily managed instrument, as it partakes of the equatorial form or motions. The telescope tube is well balanced, and the declination movement is easy and free, and fitted with a clamp screw, so that when the instrument is turned on an object that object can then be followed by an endless screw.
It can be made to suit any latitude, and by the addition of foot screws on a level floor, can be got into suitable adjustments, and if it has to be removed (for the stand is very little heavier than an ordinary Alt-Azimuth), “guides” can be provided, so that it can go very approximately into the exact place again when brought out for use. They are made, when desired, with a revolving body and fine screw-motion in declination.
THE “POPULAR REFLECTOR.”
WITH ANGLE-BLOCK STAND.—(FIG. 4.)
The speculum is 5-1/4 inch diameter, and carefully figured, and is recommended as a very useful instrument. The size and power of this telescope has been adopted as that most likely to meet the means and requirements of a large number of amateurs. Many prefer to commence astronomical observations with “_something inexpensive_,” and are led to begin with the popular and well advertised 3 inch refractor at £5. These, except in a few chance cases, are sure to prove disappointing, and are perhaps the cause of their giving up any further attempts to follow up the subject of astronomy, which may otherwise be so pleasantly and profitably pursued with a reliable instrument.
It is well known that a less aperture than 4-1/2 inches is insufficient to give the observer a satisfactory idea of the varied and most interesting details of the planets, and the ever-changing outline and tone of the belts and markings of Jupiter, Saturn, and Mars. Large apertures bring out _details_ when the smaller ones can only show a _general outline_. The former also delineates more with a lower power, providing the focus is of proper length.
The planets and nebulæ cannot be seen to advantage without aperture and focal length. The field of view is then flat, the object is properly illuminated, with sharp and crisp definition, and is also _much_ less subject to annoyance from tremour, through the necessity for constant adjustments in the fields, &c., as is the case with small apertures, for the object is magnified more in proportion. A certain magnifying power is necessary in order that the object may be sufficiently large to scrutinize; and this, whether the aperture is large or small, must be from about 150 to about 300 times for the planets. But 150 on a 3 inch is a high power for the quantity of light obtained, to say nothing of the separate consideration and advantage of long focus. Aperture is a quality or function of the telescope considered separately from light or focus. For instance, suppose a 5 inch speculum is so thinly silvered that it gives the exact degree of light as a 2-1/2 inch refractor, the defining and separating power of the 5 inch aperture would be very superior to the 2-1/2 inch.
The “Popular Reflector,” with 5-1/4 inch speculum of 5, 5-1/2, to 6 feet focus, will be found very suitable, and if its illuminating power is not greater than a 4-1/2 inch refractor, its defining and dividing powers are superior. By choosing the above focal lengths, according to circumstances, the observer can reach the eye-tube, while standing erect for objects in the zenith, and it is a great mistake to suppose that the shorter the tube the more handy it becomes. These foci will not require a “Barlow” lens to flatten the field. The “Barlow” is very useful for short foci when the aperture is considerable, as it improves the imperfect correction for spherical error, but this is much better corrected in the mirror itself than by a “Barlow” lens, which cannot be used without more than one disadvantage. There must, by its insertion, be some loss of light, which can ill be spared with small instruments, and when used to obtain magnifying power there is some disturbance of colour, and this subtracts from the beauty and purity of the definition of a reflector. There is _nothing_ equal to a good eye-piece to obtain _power_, and _flatness_ of field by the _curve_ of the speculum.
With the “Popular Reflector” and an outlay of a few shillings on some popular books, such as the excellent work “Celestial Objects,” by the Rev. T. W. Webb, and some first-class publications by Mr. R. A. Proctor, especially his smaller star atlas, &c., the amateur can compare the work he is then capable of doing with a large and expensive refractor which might be beyond his reach.
THE ADJUSTMENTS OF THE EQUATORIAL.
When the inexperienced amateur purchases an equatorial with circles, he should not be without the third edition of “Chamber’s Handbook of Descriptive Astronomy,” which, besides being an excellent book in other respects, is a really practical guide to the use and application of the Equatorial, and is indispensable to the beginner. He will there find the fullest details of the adjustments to any degree of exactness. Besides many other matters he will be instructed in the use and application of apparatus to the perfect Equatorial, including all kinds of eye-pieces, micrometers, &c., &c., as well as other optical instruments and accessories.
The Equatorials described in this catalogue are provided with every means of adjustment. The cradle contains powerful screws to set the line of sight at right angles to the declination axis, and shifting screws to place the polar axis in the meridian and to the correct elevation, and with care and a few experiments with the adjustments, and by observations of some catalogue stars, the various adjustments will soon be correctly made, and the verniers set accordingly.
THE CASSEGRAIN.
This is a form of reflecting telescope but little known. This is rather strange, since it is a very much better principle than the “Gregorian,” so well known to the old observers. Herschel says it admits of a theoretically perfect telescope. Compared with the “Newtonian” it has its advantages and disadvantages. Its principal advantages are, first, the shortening of the tube, which in large telescopes is sometimes very important. Secondly, the observer has not to _ascend_ to the eye-tube, the observations being made at the lower end, as with a refractor. The “Cassegrain” has a flatter field of view, owing to the action of the curve of the second reflector causing the rays to travel twice the distance, and, adding the element of magnifying power, the eye-pieces need not be composed of small lenses. The adjustments are perhaps a little more trouble, as the line of collimation must be carefully attended to, this requiring only a little more care can soon be accomplished, and then the definition of a good “Cassegrain” is very pleasing.
Amongst its disadvantages is the necessity for the observer to gaze upwards as with refractors, which, when the object is at a considerable altitude, is distressing, this is one of the reasons why the “Newtonian” is so pleasant to use, on account of the natural and easy position of the observer. The eye-piece being a fixture, it is not quite so convenient to use some of the accessories of the telescope. But there are means to overcome these drawbacks, and so make the “Cassegrain” even more handy than the “Newtonian.”
I have mounted an 18 inch speculum of 12 feet focus in the “Cassegrain” form, so that objects at any altitude could be observed with the greatest possible ease. A plane was fixed near the large mirror to receive the rays from the convex reflector and to throw them out to the side, illuminating apparatus were fixed here, and micrometers, &c. used, as if it were a “Newtonian”; the tube was thus made shorter, and the flat field of a long focus realised, but there would be a little loss of light in consequence of an extra reflection, but this, with a large aperture (and the fact that the “Cassegrain” gives a little more light than the “Newtonian”) can better be spared, considering the convenience gained. The observer is not elevated for any altitudes, and a large telescope is actually occupying less room than a small one. It can be used with or without the diagonal.
I have, by request, fitted the “Cassegrain” with means for two observers to view the same object at the same time. A perforated plane was arranged to receive a portion of the converging rays and throw them to the side of the tube into an eye-tube, and the remainder passing on to the eye-tube at the proper place, two images are thus formed, and can be magnified at will and viewed simultaneously. The perforated plate was so arranged that it could be removed at pleasure.
[ Illustration: FIG. 1. ]
[ Illustration: FIG. 2. ]
[ Illustration: FIG. 3. ]
[ Illustration: FIG. 4. ]
THE ALT-AZIMUTH STAND.
Fig. 3.
The Telescope, being balanced on trunnions, can be moved from an elevation approaching the zenith to an almost horizontal position. In order that it may be secured anywhere between these extremes, attached to the upper part of the telescope is an iron rod, which, sliding through the end of the arm of the stand, can be there clamped. The telescope will now be clamped in _Altitude_. As the progression of celestial objects will apparently be very slow, resource must be had to the smoothness of motion obtained by a screw. The upper end of the Altitude rod is therefore tapped to receive a long screw with a large milled head, jointed to the telescope body; by revolving this head the telescope is raised or depressed accordingly as the screw is unscrewed, or the reverse. It is necessary that the screw should be withdrawn some way from the rod before clamping it, preparatory to following an object which has passed the meridian, or is setting; as perhaps, just when the clearest vision is obtained, the observer may be annoyed by the screw action being suddenly stopped by the milled head coming in contact with the top of the rod.
When viewing objects near the zenith, and the focus long, the handle attached to the clamp will be found useful, as it can thereby be reached without leaving the finder. The handle may be so placed that a downward push should clamp, and an upward pull release.
The second motion in the Alt-azimuth Stand, namely, _Azimuth_, is obtained as follows:—The strong iron disc which forms the upper fitting of the legs has its surface accurately turned. On this revolves an iron disc, rather less in diameter, to which the trunnions which support the telescope are attached. The main axis of this disc passes through the centre of the lower disc, and then through a hollow bearing tube, a continuation of it. All these fittings having been most carefully turned and ground together, great steadiness, combined with facility of horizontal movement, is ensured. In order that this motion can be communicated as evenly as possible, resource must be again had to a screw which is thus applied. Just within the circumference of the lower disc is a narrow groove, turned to such a depth that the ring which is thus separated from the main disc is still firmly held to it by the uncut portion. An iron clamp grooved to this ring holds the nut of a long screw, the plain end of which is jointed to the upper disc. When this clamp is fixed to the ring, any motion given to the screw will act on the upper disc, and cause it to revolve, and thus the whole telescope will be slowly moved in _Azimuth_. The advantages of this plan are many, the most important being the rapidity and ease with which the telescope can be shifted from one object to another, even to those in contrary directions; all that is necessary being to release the clamp and turn the telescope to the object required. The clamp being carried round with the upper disc, can be fixed directly the desired position is obtained, when the screw is at once in action. Should it happen, whilst following an object, that the screw becomes exhausted from the joint and clamp coming together, the clamp should be released, and the screw turned sufficiently in the reverse way to bring it into action, the weight on the upper disc keeping the telescope meanwhile in position. If this operation be rapidly performed, the whole length of screw can be brought in play before the object has left the field of view of the finder, and can thus be easily refound with the higher power of the telescope. Motion is applied to the screw by means of a Hook’s joint, named thus from its inventor. This joint being furnished with a long handle, enables the observer, by means of it, to move the Telescope in _Azimuth_ at any rate, and without removing his eye from the eye-piece.
It will be seen from the preceding remarks that by means of the vertical and horizontal screw motions, the telescope, when clamped, can be moved in any direction with the greatest facility, permitting a celestial object to be observed with high powers for a considerable time, and with the greatest pleasure and comfort to the observer.
[ Illustration: decorative ]
[ Illustration: decorative ]
ADJUSTMENTS.
These instruments are always sent out in correct adjustment, and with moderate care during transit, and afterwards, will remain so, but as the performance of the instrument greatly depends on the accuracy of the adjustments the following instructions will enable the observer not only to ascertain whether they are perfect, but also to render them so if found defective. These adjustments are by no means difficult, and will be easily understood by attention to the following remarks:—
Into the draw tube screw the “adjusting piece,” which is a small brass circle with a hole in its centre about 1/20 of an inch in diameter. (The draw tube should be in about the same position as when at focus with an eyepiece.) Place the large mirror in its cell in the tube or body of the telescope, taking care that the three bayonet-joint pins are correctly placed, that is, with grooves pointing downward. They will be found to drop easily into their corresponding holes; care must however be taken that the grooves have gone well home. Both the speculum and small mirror, or “flat,” must be uncovered. On looking through the aperture of the adjusting piece, if the mirrors are in correct adjustment their reflections will be seen as follows:—the small oval mirror being placed at an angle of 45° will appear circular, and reflected exactly in the centre of this circle will be seen the bright image of the large mirror with a dark round spot in its centre, as shown by Fig. 1. This dark spot is the double reflection of the “flat,” and should be concentric with both the bright reflection of the large mirror and the circular outline of the “flat.” All these should also be perfectly concentric with the circle given by the stop in the draw tube. Should these circles not be all central the adjustments are not perfect and must be rectified as follows.
To adjust the “Flat” or small diagonal Mirror.
If the bright reflection of the large mirror is seen as a perfect circle, but not exactly in the centre of the “flat,” the latter requires adjustment; for this purpose loosen the milled head screw at the middle of the back of the “flat” which in large instruments is made sufficiently heavy to act as a counterpoise to prevent vibration. This will allow the “flat” to be rotated by the hand vertically with respect to the tube of the telescope. Bring the bright circular reflection of the mirror exactly central in this direction, and fasten in position by screwing up the milled head screw or counterpoise. If the circular reflection is quite central no further adjustment is required, but if not, then, after completing the vertical adjustment, as described, proceed to make the horizontal adjustment by turning in one or other direction the milled head of the horizontal adjustment screw, situated in front of the vertical adjustment screw; this will bring the circle of light exactly into its proper horizontal and central position, and the adjustments are then completed.
If the bright reflection of the large mirror is _not_ seen as a perfect circle, and the small dark spot not in the centre, the speculum is out of adjustment, and consequently the adjustment of the “flat” is best performed by removing the large mirror or speculum with its cell and so arranging the body of the telescope that on looking through the “adjusting piece” a large sheet of white paper spread on the ground a short distance from the open end appears as a white circle of light reflected in the “flat.” Now bring this white circle exactly into the centre of the flat precisely in the way described above, and on this being accomplished replace the speculum uncovered with its cell, and proceed.
To adjust the large Mirror or Speculum.
[ Illustration: Fig. 1. Fig. 2. Fig. 3. Fig. 4.]