Real-world gearing

With 3:1 gearing, a 100 degree movement of the telescope translates into only 300 degrees of Pot movement. However a typical CITEK Pot. (from CPC) has a rotation limit of 267 degrees, whilst the cheaper 'audio' types are limited to only 225-240 degrees !

Taking 225 as the limit (and 100 degrees as the telescope movement), we will need a gearing ratio of 1:2.25

Sticking with the Meccano pinion (19 teeth) this means a spur gear of 42 (or less) teeth.

However our choices are limited:
The 'part 27' starts at 50 teeth, '27a'=57, 27b'=133, '27c'=95, '27d'=60, '27e'=55, '27g'=45, '27h'=66, '27j'=56 '27k'=65, '27t'=70.
3 different part '31' designs exist, all with 38 teeth, as is part '31b' (also with 38 teeth).

Using '27g'=45 gives a ratio of 45/19 = 2.37:1, so 100 degrees = 237, which is well within the range of a CITEK Pot.

However using Meccano parts is (very) expensive - so instead I ordered a couple of hundred plastic gear wheels from China for about £5 (less than the cost of one Meccano spur wheel). When these arrive, I hope to find a better 'fit' :-)

If the 'correct' ratio can't be found, 'end stops' can be fitted to the telescope mount to limit the inclination to 0-90 (rather then 0-100) degrees ...