76 mm f/9.2 Newtonian (Polaroid)

Department-store scope, Polaroid branded. Checked on 13/09/2026 for the failure mode these usually have and it came back clean.

It is not a Bird-Jones. A lot of cheap 76/700 tubes hide a small corrector lens in the drawtube in front of a spherical mirror — optically mediocre and horrible to collimate. Looked down the focuser with no eyepiece in: nothing there. It is an honest Newtonian.

The f-ratio is doing a lot of quiet work here. A spherical mirror gets easier to make well as the focal ratio grows, and a 76 mm sphere needs to be f/6.5 or slower to reach the quarter-wave standard:

	f-ratio	wavefront error if spherical
this scope	f/9.2	0.09 waves, diffraction limited
[[newt-76]]	f/4.6	0.69 waves, nearly 3x the limit

So even a cheap spherical mirror performs properly in this tube, while the 350 needs a genuine parabola to work at all and may not have one at its price. This may well be the sharper of the two small scopes, which is the opposite of the assumption that the newer, shorter, more convenient one is better.

Star test still to do. Defocus a bright star equally inside and outside focus and compare the ring patterns. Near-identical means a good figure; obviously different means spherical aberration. Five minutes, and it decides which of the two is worth keeping.

Where it sits

Same 76 mm aperture as [[newt-76]], so the same 2.84 km on the Moon — double the focal length buys scale, not resolution. And 76 mm will never out-resolve the 102 mm [[mak-102]] at 2.12 km.

	field	panels for a 31' Moon
[[newt-76]]	44.9' x 29.7'	2
this scope	22.4' x 14.9'	6
[[mak-102]]	12.1' x 8.0'	12

Wider than the Mak, narrower than the 350, less aperture than the Mak. No job of its own unless the star test shows the 350 is the compromised one.

Under consideration for sale to part-fund the [[asi662mc]]. Realistically worth tens of dollars rather than hundreds, so there is no hurry and no cost to keeping it until the star test is done.