The entire serial number is SF58285:
SF = Starfire
5 = 5 inch diameter
8 = f/8 (nominally f/8.35)
285 = 285th 5 inch telescope made by Roland (NOT the 285th in this series, nor the 285th in this particular run, but the 285th 5 inch telescope of all types ever made by Roland up to this point). During a run, the serial numbers are only added as each set is tested and complete - and that is why the tube is not labelled with a serial number, only the edge of the lens set is, written there by Roland. Other marks which can be seen, especially on the 'older' optics are alignment marks for the rotation of the elements.
At some point Roland switched serial numbering to its current format. DDDFFSSS:
DDD= diameter in millimeters
FF = focal ratio
SSS = serial number
From everything I have gleaned over the years, there were 2 production runs of five inch Starfire EDT telescopes and both were 100% oil spaced. There were several prior makes and smaller runs of 5 inch telescopes which were NOT EDT, some of which were air spaced. The first runs of 5" EDT were possibly made from some surplus 155mm glass which did not make the cut for edge defects or inclusions or whatever reason. These telescopes are labeled f8 on the lens cell but are nominally f8.35. (155mm f/7.1 cut down to 130mm results in f/8.35). As well, there is absolutely no point in remaking tooling to build slightly different focal ratios. These telescopes are effectively identical to the later run of Starfire f/8.35 EDT scopes.
There was one run of 130 EDT f/8.35 scopes labelled as such on the cell which were made over a two year split run period. These were possibly made directly from new glass but used the same tooling as was used with the 155 f7 scopes. These were 130EDT were produced only for this period and then dropped from production as almost the entire demand was for the faster f/6 130EDF (nominally f/6.3!) scopes. ...
At any rate, as a visual observer, Thomas Back absolutely loved the Starfire EDT. I have a friend with the f8 and another with the f8.35. At some point I hope we can get these scopes together for a night of side by side viewing pleasure.
The information on serial-number conventions of AP scopes provided in this post is helpful and welcome. But it appears that the serial numbers of the much discussed AP 130 f8.35 FPL-53 versions departed somewhat from these conventions. There was a long CN thread in 2014 discussing prices of used AP scopes and focused on one particular offering of a used f8.35 that included several reports of f8.35 serial numbers, for both those in the first run, marked 'f8' on their rings, and those in the second run, marked 'f8.35.'
The serial-number reports were summarized in that thread, also by M13 Observer, as follows:
"Awesome, so now we 'know'. For AP130EDT telescopes, 131xxxEDF or 131EDFxx are the serial numbers of the AP130EDT f8.35 using FPL-53. We can make an assumption that the first run, likely ALL engraved with F8 on the retaining ring, goes from 131000EDF to least 131026EDF, and if Roland's comments can be relied upon, likely to around 131032EDF. The second and final run marked 131EDF33 (?) to at least 131EDF60, and likely to 131EDF64 is engraved with F8.35 on the retaining ring. Not definitive but probably quite close."
The f8.35 serial numbers thus had '131' in the 'aperture-diameter (mm)' field, included the design designation 'EDF' either after the actual serial number in the first run, or before it in the second run, and did not include a focal-length designation. (While the f8.35's design is in the 'EDF' group, it was named an 'EDT' to avoid confusion with the shorter focal length AP 130 f6 model offered at the same time; <http://astro-physics...edt/130f8edt>.)
M13 Observer's post in the present thread is absolutely right about Thomas Back's admiration for the AP 130 f8.35. In his 'Brief History of Astro-Physics Lenses,' he singled it out with the phrase "what a sweet telescope," and elsewhere cited it as ""truly Roland's finest moment" among AP scopes he had inspected, and as the inspiration for design of his own legendary TMB 130 f9.23 (aka 'f9.25') to replace it. See TMB's comments in <https://groups.yahoo...s/topics/26535>
Thomas Back's own AP 130 f8.35 was from the first run; the TMB 130 f9.25 was designed and produced after that run and before the second run. That sequence is described in a fascinating account of the history of TMB's 130 f9.25 by 'MrGrytt' (Harvey), relating his own involvement in it. He reported that, disappointed in the apparent discontinuation of the AP 130 f8.35 after the first run and before he had become eligible to buy one, he pressed Thomas Back to design a substitute. The result was the TMB 130 f9.25, and Harvey obtained essentially the first available production model of that scope, then was informed of the second run of AP f8.35's and the rise of his application for one to the top of the AP list. So, in the end, he got one of those, too! The story was outlined by Harvey in CN thread <https://www.cloudyni...to-tmb-optical/> and is well worth reading.