An amusing review! I've tried to fix one of those National Geographic 50mm's too. It's an education. Those cheap scopes tend to be horrible, but they figure some kid will get it and destroy it in a couple of hours. So no big deal.
>What they can do with these is stop them way down with a baffle behind the objective with a much smaller opening so if it says f9 on the box it might actually be f18. That sharpens things up a bit and improves the color.<
(But) The best part is when they finally get some decent optics and it all takes off into a new dimension. These crappy starting scopes are a stage to pass through so you know the good stuff when the glorious images hit your eye and brain.
They are usually already stopped down because of the long drawtube. It clips the light cone, effectively reducing aperture to 40mm or perhaps even 35mm.
I found this out firsthand with the Svbony SV502 50mm kids telescope. Got one when they had them for $23 on their ebay store. Unlike most of these 50/360 refractors they come with a 1.25 inch visual back. I was surprised at how sharp it was at 20-60x in daytime viewing.
But the R&P drawtube was 9 inches long! Way, WAY! more than needed for focusing any eyepiece or eyepiece + barlow. So I took it apart and removed almost 4 inches of tube, easy to do with these plastic focusers.
Put it back together and……….Hey! What’s going on! The stars at the edge of the field all have tails. Is the objective cattywaumpus? Checked that, no, all squared up. Did I tilt the focuser when I put it back? No, that’s ok too.
Since stars in the center of the field didn’t have tails it finally dawned on me that I was now looking through the full 50mm of aperture. And even though the system operates at f7.2, there is still distortion off axis. This also sheds light on why my friends 50mm f12 appeared to be so sharp at 60x with a 10mm eyepiece. The slower objective has a flatter field due to the longer focal ratio.
Anyway, it makes me wonder just how many inexpensive refractors also have light cones clipped by their drawtubes.
Edited by John R., 30 March 2024 - 10:54 AM.
