Thanks Darryl and David,
Wal Best’s Observatory at Bardon, Brisbane: I supply some background. Wal Best was a retired gentleman who built his observatory in the early to mid 1970’s. His property was quite vegetated and his wife wasn’t too keen on cutting a number of trees down, so he elected to use an old water tank on his property that in past times had been used to supply gravity fed water to the house. He had recently retired so it was a suitable work intensive project. So he trimmed off the tank at around 4″ in height from the base and filled it with concrete upon which he lovingly built a single skin circular brick wall and crafted a very fine aluminium dome with marine plywood frame. The quality of the work was outstanding. The dome was about 10ft diameter and a 12.5″ Dall Kirkham Cassegrain was installed within. He used this for perhaps two decades.
When Patrick Moore arrived for the 1988 ‘Brisbane Expo’, he was keen to climb up, but was prevented by a wartime injury. (He was in the RAF and said he had fallen out of a bomber onto the tarmac during the war and the knee injury continued to be troublesome.) I know all this because I took the photo. The other person is Susan Niven, a local club stalwart.
150mm F8 Achromat compared with ED glass version: This is a typical example of me being too stingy. (‘Tight-arsed’ as my wife puts it.) I saw the Celestron 150mm F8 optic tube half price for $600 AUD and I always wanted a big refractor so I bought it and mounted it in a cheap superseded heavy duty DX mount. Less than 18 months later the Skywatcher 150mm F8 ED was reduced from $3,000 to $2,000 as a special and so I swapped over selling the Celestron for a loss. In the meantime I had done quite a bit of testing and it was easy to conduct the same tests again for a direct comparison with the same mount, and same camera etc.
I support David’s statement about the performance comparison, but I do feel that the standard achromat does have its points. Call me perverse, and maybe it was the spurious colour it introduced, but I felt that the standard achromat produced a bit more contrast on (say) the planets. From the lunar images – base of page 25 in the E&T News you will see that the spurious colour effects become troublesome towards the outer areas of the field. I have a series of these comparison lunar images if anyone is interested.