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Observatory News August 2006

Written by Chris Hopper, Observatory Director July 2006

On the night of the New Moon, 24th July, three of us, Chris Hopper, Tony Questa and Tony Marsh went to Holmbury to collimate the mirror. This is not as easy as it sounds since the mirror was originally designed as a Cassegrain telescope and has a 4 inch hole in the middle. The telescope was modified some time ago to be a Newtonian. This means that the usual laser collimation technique for Newtonians will not work.

However, we did put a laser in the focuser to help align the mirrors. We were all convinced that the secondary mirror had not been touched during the refurbishment. When we put the laser on, the beam was almost at the edge of the Perspex plug in the Cassegrain hole, i.e. 2 inches out of alignment. After some discussion, fiddling with all discernable adjustments and checking the mirror was free to move, we concluded that there was no way to move the main mirror that distance. So, reluctantly, we decided that the secondary mirror must have been jarred out of position. (The laser was centred perfectly on the secondary). Once that painful decision was made it was relatively easy to achieve alignment of the optical axes albeit with the laser beam missing the main mirror completely at one point. The Holmbury secondary has 6 adjustment screws rather than the usual 3.

Since there was no means of using the laser to adjust the collimation bolts of the main mirror we decided to star test. (Even if you stick a mirror on the plug in the main mirror to replace the missing main mirror and mark the centre point, there is no way to ensure that the added mirror surface is flat with respect to what would be the centre of the main mirror. If it was not flat the laser technique would mis-collimate. A Cheshire collimator would work since it uses the whole of the mirror surface - but we didn’t have one and it was dark!).

The honour of first refurbished light had to be Chris’. “I can see stars” he said. A good start. All three of us tried in and out of focus trying desperately to remember Martin Lewis’ excellent talk from March. We concluded ‘ not too bad’. We were sure that the mirror is almost certainly not as good as it could be – we will need, perhaps, to use a web-cam as Martin Lewis described to improve things further.

At this point we decided to ‘have a look around’. Since we had no finder and the Telrad batteries were dead, we had to guess where to point the scope. We were using the Meade 2-inch 35mm eyepiece giving us a magnification of just over x70 and a field of view of just over 1 degree. Those of you who have tried to find objects, even the Moon, with such a small field of view will appreciate the problem. It was only on the drive home that the penny dropped – the torch batteries would have fitted the Telrad. We were, of course, also learning how sensitive the drive slewing system was. Nonetheless, we managed to find Altair after discovering that the stepladder was not long enough to see Vega at the zenith. By this time the sky was very good for the summer. So we were drawn to the teapot of Sagittarius with the steam of the Milky Way rising from the spout. Time to take tea at the restaurant at the centre of the galaxy. We found the globular cluster NGC 6293 in Ophiuchus by chance. It was well resolved by the >20-inch Holmbury mirror showing a nice chevron of stars across the cluster. It looked like a bejewelled cobweb. If only we had the CCD set up!

But it was Monday night and work the next day, so we reluctantly had to give up. The sky went from black to blue as Chris discovered that putting the cover on the secondary moved it out of alignment.

Now we only have to get the guide scope and APO mounted, the Telrad aligned and set up the drive using 2 or 3 stars and check polar alignment, then check for periodic error, then the.………… needs doing.

There is plenty of work for those in the society who want to get involved in this exciting project – just contact Chris.

Last updated July 2006 [Back]