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(This telescope has now been taken down and put into store, replaced on the site with a Celestron 14 SCT on an Astro-Physics 1200 mount on a concrete pier; this page remains here for historical interest. See also Journal of the British Astronomical Association, vol. 117 no. 3)
History The telescope was built by Astronomical Equipment Ltd. of Harpenden in 1970 as a specialised very long focal length lunar and planetary telescope. The mirrors were made probably by James Muirden, but were later refigured by Esmond Reid. The mounting is an AE C-type German equatorial designed by Cliff Shuttleworth, probably made by Rob Hysom. The telescope design and execution is probably by Jim Hysom in collaboration with Horace Dall. The telescope was built for an amateur astronomer then resident in Watford called Rex Hilburn. It was later aquired by Dudley Fuller, the London telescope manufacturer, who used it at his home in Borehamwood. At this stage, in 1988-89, the mirrors were refigured by Esmond Read at AE Optics (a descendant company of the original). Fuller subsequently sold the telescope to its current owner, Robert Katz. It was taken by him to an observatory in Hastings for a time, before being brought back to London for permanent loan to my observatory in Edgware. The telescope was installed here by Robert and myself in June 2004, with considerable restoration, modification and refurbishment, to be once again dedicated to lunar and planetary observing from the North London suburbs. Optical Design The telescope is a rare example of Horace Dall's "relay lens" Cassegrain reflector design, also called the Dall-Kirkham-Dall design in the 1973 AE catalogue. At least five examples of this design were made by Dall, working with the Hysom brothers at AE. The largest was Dall's own telescope, a 15.5-inch tubeless construction that he used in his observatory in Luton until his death in 1986. This telescope passed to Luton Astronomical Society, and was used from 1992 to 2002 in their observatory at Putteridge Bury, before being replaced with a 0.5m Newtonian. It is now in storage. A 12-inch D-K-D was made for Durham University, and another 10-inch one for Plymouth Polytechnic. It is not known whether these two examples remain in use. There is an 8-inch example at the Royal Masonic School in Rickmansworth, Herts, also out of use. The special feature of the D-K-D is the relay lens, which is a positive doublet positioned at the top of the Cassegrain baffle within the tube. The optical system is a variant on the Dall-Kirkham Cassegrain design (also entirely due to Dall, despite the name) which allows for even longer focal lengths and smaller secondary mirrors within a fairly short tube than the "standard" D-K Cassegrain, or the "classical" Cassegrain. Since the secondary mirror obstruction diminishes contrast and resolution perceptibly in all reflectors, the design was conceived as a means of maximising these for planetary observation in a telescope of manageable size. The diagram below illustrates the optical system, compared with other Cassegrain designs. The innovation in the D-K Cassegrain, compared to the "classical" design, is the modification of the main mirror figure to an under-corrected parabola which compensates for the use of a spherical secondary, which is far easier to make accurately than the hyperbolic figure of the classical Cassegrain secondary. This design gives more off-axis coma than the classical and is therefore unsuited to "fast" wide-field optical configurations, but highly suited to long focal lengths, and Dall developed it as an instrument for high-power work. The D-K-D variant has the secondary placed further up the tube, thus intersecting a narrower cone of light, allowing it to be much smaller than with any other Cassegrain telescope - comparable in size to the secondary in a long-focus Newtonian.
This means that focus falls within the main tube, and has to be transferred to the focuser by means of a relay lens placed afer the prime focus, which refocuses the light beyond the primary. This further increases the effective focal length of the telescope beyond the normal Cassegrain ampification, caused by the defocusing action of the secondary, by an amount that is variable, dependent on the distance between the relay lens and the eyepiece or camera. Placing the eyepiece further from the relay lens causes it to intercept a less-converging bundle of rays from the relay lens, giving a longer effective focal length. Focus is achieved at different focal lengths by racking the whole relay lens/baffle and eyepiece/camera assembly in and out with respect to the mirrors. A by-product of this arrangement is that, due to the extra ray-crossing, the telescope gives an erect image. The relay lens acts somewhat like a permanent Barlow lens with a variable-length mouning tube. The arrangement can be seen in the picture below. This arrangement gives a focal length which (without a Barlow) can be varied between about 200 and 400 inches (f20 to f40). This happens to be an ideal range for planetary webcam imaging, but is impractical for most other imaging purposes, and non-ideal for the observation of diffuse objects. It is also ideal, as originally envisaged, for visual high-power study of the planets, lunar details, and double stars. Adaptations Considerable work has been done on the telescope to get it to its current state since it was loaned to the Stag Lane Observatory, some of which are general improvements and some of which relate to its current use for webcam imaging. The original design included an optically flat BK7 glass window on the front of the telescope, to which the secondary was attached. The exact distance between the secondary and primary was adjustabgle by means of a spring-loaded arrangement, which allowed the range of available focal lengths that could be brought to focus within the focus travel to be adjusted. The optical window proved vulnerable and was damaged twice in the past. It was replaced by a spider when the telescope was first installed at Stag Lane, but in 2006 Jim Hysom re-polished the original window flat, and Es Reid re-figured it to partially compensate for the spherical aberration that the relay-lens system introduces into the image. However, this compensation can only be partial as the spherical aberration is colour-dependent - a basic drawback of the design, we have found. To speed up cool-down time of the 1.5 inch thick primary and equilibration of the tube, an arrangement of three small computer-type fans has been installed just above the primary, covered with lids when they are not in use. A sliding counterweight arrangement has been costructed for the telescope to allow balance to be attained about both axes in all positions with various combinations of cameras, eyepieces, diagonals etc. Perhaps the least satisfactory feature of the telescope as originally constructed was the difficulty of balance and the impossibility of clamping the axes to avoid the telescope "running away" when not in balance. An optical tube of this size going out of control is not much fun. The mounting has been modified to include aluminium blocks attached to springy steel brackets which can be screw-tightened onto the axes to clamp them while accessories are changed, or during partial dissasembly, which is necessary for full collimation. Improved motor control arrangements have been added for the axes, with a contol unit from Beacon Hill Telescopes, though the original 240v AC synchronous motor system has been retained. The original analogue setting circles are sometimes used, and they have been fitted with small lamps so they can actually be read. A piece of copper immersion heater tubing was adapted into a circlular grab-rail for the eyepiece end of the telescope, as seen on many observatory refractors. This invaluable addition allows the instrument to be more easily manhandled, and prevents people trying to move it by pulling on the focuser.
Home-made axis clamp modification to the dec. axis. Also shown are the AC synchronous motors, the 1+3/8 inch shafts, and the 9 inch RA gear. The relay-lens optical system, suitable as it was for planetary imaging in terms of the very long effective focal length it provided, was found to create one severe problem for webcam use: a problem of image-shift when changing the direction of focussing. This was due not to innacuracy in construction, but to a basic drawback of the design, using an amplifying lens which moved with the focus, at the top of the Cassegrain baffle, essentially on the end of a long lever arm, greatly amplifying any slight change of collimation when focusing, both optically and mechanically. This was noticeable in high-power visual work, but disasterous in webcam imaging, working at a focal length of 300-400 inches, where the image on the chip only covers about a minute of arc of sky. The image shift resulted in the object being imaged constantly leaving the chip when focus was changed. The solution to this has been to change the method of focusing for webcam imaging. An additional rack and pinion focuser is added between the original brass focuser and the filter wheel and camera. This focuser is motorised, the system being supplied by Switched Systems. Now, an initial coarse focusing adjustment, to get the right range, is made with the brass focuser, which moves the relay lens, and a fine focusing is then done with the motorised focuser, which does not move the relay lens, and hence causes zero image shift. The focus using this system is extremely relaxed and non-critical. The drawback is that the originally available range of effective focal lengths is limited by this system. With all adapters, focusers and filter wheel in place, the EFL now works out to f34. This setup is used for imaging Jupiter, Venus, Saturn and lunar features. For an even longer focal length a Telescope House Magnimax 1.6x lens can be screwed to the webcam. This arrangement has sometimes been used for imaging Mars. Barlow lenses are not used on this telescope as they are not necessary and increase the EFL too far. For visual observation, only eyepieces in the range 50-18 mm need be employed, even for the very highest practical magnifications. Hence a long eye-relief is always possible.
Arrangement for webcam imaging. A mono chip modified Philips Toucam Pro from Modern Astronomy is seen connected by the T-thread adapter to an ATK 5-position filter wheel. Further in is the motorised fine focuser and the coarse manual focuser. Connected to the refractor is a USB2 TalkCam. This is a cheap webcam which allows the refractor to act as an electronic finder, operating through the same computer as the Toucam. Also visible is the circular copper grab rail and counterwight slide, at top. 
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