Here, I wish continue the work I started on another forum in July through August 2014. It concerns the field performance of an Orion 180mm f/15 Maksutov cassegrain ( known affectionately as 'Cornelia'). For reference see here:-
As an experienced tester of telescope optics, I am still of the opinion that a large maksutov such as this makes an excellent alternative to a top-rated medium aperture apochromatic refractor, because of its optical quality, ultra-portability and lack of maintenance. It also makes much better economic sense. Some folk have expressed concerns about whether an instrument like this might exhibit abnormal thermal issues. One purpose of this work is to establish whether that is true or not.
My methodology remains the same; I begin by testing the maksutov against a fully acclimated 5" f/12 classical achromat.
Here are some of my most recent observations.
Ambient; 9C, 1016mB, rising. Nice clear sky. Seeing fair to good. Very autumnal; cold.
Instrument: 180mm Orion Mak.
Enjoyed my first glimpse of the vast lunar regolith this evening but at an altitude that was far from ideal. The 32mm Plossl, which offers the widest true field so far as I know for this telescope, couldn’t quite fit the entire Moon into the field of view. Still the image was intensely bright, the ray craters prominent and the eastern lunar limb crisply laden with impact craters.
Drank up some beautiful sights after visiting a suite of colourful double stars from the comfort of my back garden: Albireo, Mizar, Iota Cassiopeiae, Almach, 61 Cygni, Iota Trianguli, Eta Persei, Zeta Lyrae, Gamma Delphini. Beautiful stable images throughout the evening at 113x. Packed up at 00:30h (7th), final temperature 6.0C. No obvious thermal problems encountered.
Ambient: 10C, 1018mB, rising, fully clear sky, almost a carbon copy of last night, save for a twist in the seeing conditions later in the vigil.
Once taken out of the shed, the Maksutov took a wee while to deliver its best images. Temperatures fell a bit more rapidly this evening, reaching 8C by 22:06h. Still the images of Delta Cygni were excellent from about 21:40h, so no big deal. Seeing deteriorated approaching midnight and it became much more difficult to see its companion. Fainter pairs like Iota Cass showed the same behaviour; still recognisable as a triple but noticeably degraded. That said, the instrument continued to be eminently useable at lower powers right from the get go, and I enjoyed visiting some more bright wide doubles with a full moon (or very nearly so) low in the south.
Final temperature at local midnight: 6C.
Edited by astroneil, 08 September 2014 - 12:02 PM.