Third Light of the Millennium Telescope
Optician's evaluation of mirror performance
[ As expected, the seeing was the best so far with the 30", 3-2 if not 2-3 on the 1 to 5 scale even down at Jupiter - vastly better than on the previous night - in fact, the image of planet and moons initially razor-sharp, very impressive. Later in the evening, mirror going pronouncedly over-corrected, as often happens to thick mirrors in these conditions; even the 12.5" does this from time to time. - Christopher ]
The view last night (Monday 12th) was quite amazing and instructive. Amazing, because of the numbers of faint stars the telescope seems to bring in to view and instructive because I now have confirmation of the optical figure.
The general figure is pretty exactly as I had already seen under test, but very instructive to have confirmation of this on the sky.
The first view I had was to concentrate on one of Jupiters outer moons, I don't know if it was three or four. However, the very bright disc was clear and defined. Focus was very precise. There was no 'wander' in and out of focus. It was either 'in' or 'out'. This confirms in itself, that overall, the figure is close to the calculated curve, with no strong under or over correction.
Going in to focus, there was a softening of the image compared to going out of focus. This was caused by the two slightly 'high' zones on the mirror, breaking ranks first from the the bulk of the light forming the core of the image. This first view contrasts strongly with the final views we obtained.
I do not know if it was just subjective, but the air seemed to get suddenly very much colder in the last 30mins of observing, as it became hazier in fact. However, the view of the double was very poor compared to the first views.
After examining Jupiter and moons, I had examined a probable 6th mag. star that came in to view. This gave the same exactly defined focus as obtained on the Jovian moon, with just a slight 'softness' going in to focus. However the image of the double was really poor, with strong overcorrected haze as you went in to focus. This actually was the kind of view I had suspected a 30" to give, on a night when the mirror had been uncovered and was cooling.
You can see that a mirror with a clear glass edge and plenty of air circulating round it, if the air temperature drops, will rapidly radiate heat there. Result, contraction and instant overcorrection/overcorrected edge. If the mirror is going to be continually confronting those kind of conditions, rather than those for the first hour, there is a case for taking the figure well back to undercorrection. This would give the edge a chance to fall away as it cooled, without tipping over in to an out of focus figure.
Terry