Yepper. 518x (well, 517.5 to be precise) from my 90mm scope.
The Math: 621 f/l / (3mm / 2.5x) = 621 / 1.2 = 517.5 then the "mag per inch rule" yields 517.5 / 3.5 (aperture) = 147.8x per inch.
The Players: the Moon, a Megrez 90FD APO, some DeLite EP's and a 2.5x PowerMate.
The Story: Generally, I'm not a planetary guy but I'd been working Cygnus a lot and Cassiopeia was still too low on the horizon for good cluster hunting. The Moon was up and the air was particularly clear and settled albeit with some light moonglow. So, swapping DeLites around I worked my way down to the 3mm, or 207x which, using the 60x per inch rule, should theoretically be close to the Meg's mag limit of 210x. Just for the heck of it, I put the 2.5x PowerMate in the chain and boy, was I amazed! I had a sharp image at 518x. The clarity blew me away. I literally shook my head a few times thinking "this can't be" as I've never hit that level of magnification with any refractor I've owned before. I had been poking around the terminator and the relief and contrast was incredible. I did the magnification calculation several times manually and used these links as well: https://astronomynow.com/scope-calc/ and https://skyandtelesc...ope-calculator/ (The only difference I can see between them is one uses focal length and the other focal ratio)
Effective Focal Length: 1555mm (You are using a 2.5 x Barlow lens) Effective Focal Ratio: f/17.3 Magnification: 518x True Field of View: 0.1° (Less than half of the full Moon's disc would fit into the field of view) Eyepiece Exit Pupil: 0.2 mm Theoretical Resolving Power: 1.29 arcseconds Approx. Limiting Magnitude of Telescope (under dark, moonless skies): +12.6
The 0.2 exit pupil should have been a problem but it wasn't. I had plenty of light and while eye placement was a bit more picky than I'm used to with the DeLites, it wasn't bad at all. There is something about the optics in the DeLite series that I can't explain. They are almost magical in their performance.
I have a new appreciation of both William Optics and Tele Vue. If I can get it to work, I intend to try getting a projection image using my cell phone in an upcoming night and splitting some tough binaries.
Edited by BlueMoon, 21 June 2021 - 12:34 PM.