Based on the serials, there was no distinction between Telephoto, Spotter and Astro versions in numbering, although Telephoto versions were styled ‘Mirror Lens’ on the gold band around the rear of the OTA on which the serial number is printed and the Spotter and Astro versions were styled ‘Maksutov Telescope’. From what I have read, other than this, the OTAs are the same between all three versions.
The optics have different numbers and, unlike the Schmidt Cassegrain scopes, the optics number on the corrector can be read without removing the corrector, although as it is written on the edge, when you look at it from the front through the corrector you see a reversed image and need to allow for this, as can be seen in the post below.
By the time the C90 was introduced, in 1978, Celestron already seemed to be thinking of moving away from the old style serial numbers used for the C5s and C8s which showed the quarter and year of production. Instead, the C90 serials started with a 9. Initially 4 digit serials with the pattern 9nnn were used with nnn being a sequential number (the earliest number is serial 9136, a Spotter reported by Dmala) and when they reached 9999 an extra digit was added giving a 9nnnn pattern. However it would seem that the sequence did not restart at 1, to give 90001, but continued from 1000, ie going from 9999 to 91000, as we have the Spotter reported by Apophis Dude with serial 9896 and the next one is a Telephoto with serial 91687 reported by Charles425. But it could be that there are C90s with 90nnn serials out there.
However, when 9999 was reached, although they moved to a six digit format of 9nnnnn, this time the sequence does appears to have been restarted from 1 as we go from the Spotter reported by LostInStool with serial 99429 to serial 900212 reported by Geo.
Some time between the Spotter reported by Antonio Spinoza with serial 915626, and the Astro reported by Cometeer with serial 921082, the C90 seems to have changed from an orange tube to a black one.
To confuse matters further, there are also black C90s with 8nnnnn serials as reported by Joe Cepleur (#887061) and SkyCruzr (#896298). Does anyone know how and when the 8 series numbers fit in?
Finally, the f5.6 Telephoto, also styled a ‘mirror lens’ on the gold band, had serial numbers starting with a 5. Only one has been reported (by Geo) with serial 501649 so it’s not clear yet whether the f5.6 version followed the f11 version with initially having 4 and 5 serials as well.
As always, any further contributions, clarifications additions and corrections gratefully received. If known, it would be good to have the corrector number and the original purchase date if known and, for astro versions, any date on the motor. Also, if there is a case, is it a leatherette covered wood case or a plastic case and for Spotters and Astros, what manufacturer’s symbol is on the finder scope?
A low resolution image of the Registry is shown below and a higher resolution image can be seen by clicking on C90 Registry.