Hi guys,sorry for taking so long to reply, i'm on holidays at the moment traveling with my wife and our fifth wheeler.
I'll try and address some of the questions you have raised and add some-more information.
Stew,both lenses are 60mm not 40mm. You say the ''comet'' was a 40mm scope, yet this 60mm scope fits perfectly into a case with a label that clearly says '' Comet NO 3''. Perhaps the 40mm comet was a N01 or N02.? You tell me.
Chuck is correct in his observation and reasoning as to to the cell size and the tube size.
The discrepancy in the cell sizes is that the f15 is made of thin steel and non collimateble and the f20 cell is made of a brownish Bakelite and collimateble . The f20 cell is much larger and material thicker due to the fragility of the material its made of and the mechanics of the collimation feature. Hence the need for a larger tube for the extention.
I'm not at home so can't provide pics of lenses at this time,a description will have to do.
The f15 is an air spaced doublet,three spacers,most likely a Fraunhaufer, not coated. For its age,63 years minimum,its in very good condition, no scratches i can detect and only the slightest evidence of cleaning. Still a brilliant performer.
The f20 is a totally different animal and has me rather mystified.
When i was first offered this package i almost rejected it because of the condition of this lens. There was a dense scummy opaqueness between the two lenses and i had real concerns about fungal etching etc.
The rather unique feature of the of the interchangeable focal lengths proved too intriguing so i took the plunge and bought it.
Believe me i didn't get it for nothing,i'm not a gambler but i'm not against an educated punt.
Once it was in my hands i dismantled the lens with great trepidation, fearing the very worst.
To my great relief the scum wiped straight off with a little lens cleaner and a very soft cloth leaving two beautifully polished surfaces. .
The two elements were surface to surface and appeared to be dead flat,no spacers,the first surface is convex as is the forth surface.
The first element,or crown?,is almost twice as thick as the second,the flint?.
The big surprise was that the first element or the''crown'' when viewed from the edge is a of a very green color,very close to window glass,my experience with any camera or telescope lenses is that the edge was always ''white'' or clear. Viewed front on the glass virtually disappears,
like looking into very deep clear pool of water. I really don't get this, i'm not at all familiar with this design of lens.
So,this lens had no spacers, should it have? Is this a known design or an oddball Goto offering.
What is this green glass?
Disclosure,neither of these lenses is marked as ''GOTO''. But i have seen other ''GOTO'' lenses on c.n.that were not marked.
Stew, you asked for any documentation with the scope, all that exists are the labels in the case. One states who made it and when and the other outlines what was in the box on delivery.
So, it might just be a Tasco.The only ''Goto'' logo on the scope itself is on the focuser.
The most interesting documentation,to me, is the penciled in name and address of one of the previous owners from the U.K.
''JAMES M LONG. F.R.A.S. (FELLOW OF THE ROYAL ASTRONOMICAL SOCIETY). I'm thinking that he knew his apples when it came to buying a telescope..
That's what convinced me to buy it.
Stew, the other bits and pieces you asked for pics of will have to wait until i get home.
Edited by fsphotography, 15 August 2017 - 09:21 AM.