About 18 months ago I purchased an 80mm F5 optical tube assembly for AUD$99 to mount on my 150mm refractor. It has mounting rings, but no finder, no diagonal – just the bare scope. (I have a whole load of the extra accessories so I didn’t need them.)  I got a second 80mm to mount as an extra finder under the C14 tube – same price. I was very impressed with these little scopes – took star photos etc., and I thought I would mount another one as a spotter on an old ‘Unitron’ table mount.  Then I asked how much a 102mm F500 would cost instead.  (Getting greedy!)      It was only AUD$160, so just $61 extra and with a 6X30 finder AUD$25, I was set up.  With a 500mm focal length, a good Barlow lens is a useful accessory.

In mounting it I discovered it was a bit large for the table mount but I had an old modified mount that I had previously put on a wooden 60mm refractor tripod from the 1960’s and this works like a charm. So I have a real ‘grab & go’. The tube weighs just 2.5kg, and yes a reasonable, but manageable amount of chromatic aberration comes for free. Optical performance is surprising good. The size is quite appropriate for the mount, but had I got even greedier, the equivalent price for a 120mm F5 jumps from AUD $160 to AUD$346, the weight increases from 2.53kg to 3.97kg, and the figures(and reviews) tell me that chromatic aberration is much worse. Besides mounting would need to be heavier so apart from cost, the 120mm would be getting beyond being readily ‘grab &go’ stage.  Skywatcher had an even larger 150mm F5 and the Chromatic aberration figures are even worse, but (surprise surprise), apart from old stock in the UK, it doesn’t seem to be currently marketed.  I am keeping this new scope in my study, ready to use quickly.