Securing the guide telescope to the adjustable collar.
Securing the guide telescope to the fixed collar.
The guide telescope and its extendable linkages visible at the eyepiece end of the Orwell Park Refractor.
Orwell Astronomical Society (Ipswich)
A Guide Telescope for the
Tomline Refractor
When photographing faint objects such as distant nebulae and star clusters, exposure times as long as 30 minutes (or even longer if one has patience!) may be required. In order to assist with long exposure photographs (and to make for improved ease of use generally), at the end of 1981 we fitted an electric stepper motor drive to the Orwell Park Refractor. The new drive complemented a new torque-limiting clutch which we had installed in mid-1979. Preliminary commissioning tests of the new drive took place on 18 and 19 October 1981.
It is very difficult to engineer a telescope drive system that can track stellar objects with extreme accuracy for extended periods, and in fact the stepper motor drive proved to have a small wobble in right ascension of about nine arcseconds over a period of one minute. In order to facilitate accurate tracking for long exposure photographs, we first fitted the drive with a hand controller which enables its speed to be continuously varied over a small range. Then, in March 1983, the OASI committee decided to purchase a small refractor to use as a guide telescope for the Orwell Park Refractor, enabling the observer to compensate for the periodic drive wobble (and other defects in the drive) by using the hand controller to adjust the speed of the drive so as to keep a guide star centred on cross-hairs in the field of view of the guide telescope.
After several months of fruitless searching for a suitable guide telescope, we eventually made contact with a Mr Wood of Welwyn Garden City who had a 100 mm refractor for sale. After several rounds of negotiation we arrived at a price that was mutually agreeable. Mr Wood is himself a refractor enthusiast and he was very pleased to sell the the telescope to OASI, knowing that it would be used with the Orwell Park Refractor.
On 02 November 1983, Alan Smith and I left Ipswich for Mr Wood's home. After a two hour drive, much of it through thick fog, we finally found the address. The journey was worthwhile: the 100 mm refractor was in very good condition, although because of the weather conditions we could not test the object glass (OG) that night. The tube was black, stove enamelled aluminium with a fully adjustable cell for the OG. The OG itself was an air-spaced achromat (similar to that of the Orwell Park Refractor) of focal length 1.2 metres (f12). The focusing mount was a rack and pinion type made by Irving and Son, with adaptors for 1" and 1¼" eyepieces. Mr Wood also donated to OASI a 30 mm focal length orthoscopic eyepiece with cross hairs for use with the 100 mm refractor. The transaction complete, Mr Wood showed us two meteorites which he had in his possession1 then we left for home at about 10.00pm. By this time the fog had thickened, making the drive back tortuously slow, but eventually we arrived home safely with the telescope.
We spent much time planning the mounting for the guide telescope on the Orwell Park Refractor (much of it conducted in the Ship Inn!) We had to account for the fact that there is no suitable guide star within the typical field of view of many faint objects. We therefore planned to mount the guide telescope in a manner that allowed for up to approximately 1° of offset (in any direction) relative to the main refractor, giving the observer a much greater choice of potential guide stars.
After much deliberation and numerous sketches we came up with the following approach. The front mount is a circular clamp rigidly mounted to the Orwell Park Refractor, and in dimension approximately 1 cm greater in diameter than the 100 mm telescope tube. This mount is lined with rubber, which acts as the front pivot without the need for moving parts. The rear mount is similar except that instead of the clamp being rigidly mounted to the tube of the Orwell Park Refractor it is mounted via two extendable linkages. These are simply tapped cylinders with a left hand thread one end and a right hand thread the other. Rotation of the cylinders one way expands the linkages and the other way contracts them. This enables movement of the guide telescope in two directions, nominally at right angles, giving coverage of a square approximately 2x2° about the centre of the field of the main telescope. The photographs below show the guide telescope with its extendable linkages, and the process of mounting it.
With the guide telescope fitted and the hand controller operational, the Orwell Park Refractor is suitable for some serious astrophotography. Various adaptors and fittings are available (stored in the observatory) which enable cameras fitted with a standard 42 mm Pentax thread to be coupled to the Orwell Park Refractor for both prime focus and eyepiece projection modes. For cameras with other threads, adaptors may be purchased from any good photographic dealer. Suggested films are Ilford HP5 for black and white photographs (this is a 400 ASA film that can be safely uprated to 3200 ASA, helping to cut down exposure times or to record faint objects) or Kodak Tri X. For colour photographs it is worth trying the new 1000 ASA print films.
Securing the guide telescope to the adjustable collar.
Securing the guide telescope to the fixed collar.
The guide telescope and its extendable linkages visible at the eyepiece end of the Orwell Park Refractor.
[1]
Mr Wood showed us a nickel iron meteorite and a tektite which was one of 60 that a geologist friend had sent from Australia. The geologist, having been requested to send back a tektite from the Australian desert, commissioned some of the local aborigines to find some for him. In a very short space of time they had presented him with 60 of the objects! Tektites are one of the great mysteries of the solar system. They come to Earth from space but unlike normal meteorites they are made of almost pure quartz. They are also only found in specific regions on the Earth's surface, one of these being the Australian desert.
David Payne