Last night I observed the moon, Castor and Polaris with my two little 40mm Unitron, F17.5 refractors.
One has an air-spaced objective and the other, a 1950's model, utilizes a cemented doublet. Both displayed beautiful Airy disks and both provided a "3D" effect when the moon was viewed at low power. Lovely!
For comparison, I set up my Orion 120mm, F8.3 achromatic refractor on a Carton heavy-duty, counterweighted alt-az mount. Controls were butter smooth and tracking was effortless.
I was surprised at how well the Orion performed and how lovely were the Airy disks it displayed. It was like a different telescope!
In the past, the big refractor always gave good images but at focus, the brighter stars would usually display a sort of bulls-eye pattern with overly bright and numerous Fresnel rings. Classic spherical aberration to my understanding.
But last night...
The central disk was bright, hard and round and was surrounded by a single diffraction ring on most stars. What happened to the spherical aberration?
Only two things were different this time:
The seeing for once was essentially perfect, with no heat waves to distort the images and the telescope was perfectly acclimated (although it usually is when I observe). Can bad seeing mimic optical aberrations like SA?
The other difference is that I cleaned the lenses earlier in the day and removed the blackening I had applied to the edges. I had found the lenses difficult to remove because the thickness of the black paint was enough to cause the lenses to "hang-up" slightly in the cell. When reassembling, I made sure that each lens element was free and rattled slightly in the cell. Could the possible binding of the elements cause an unwanted increase in the lens spacing with attendant spherical aberration?
In addition to lovely views of Castor and Polaris, the 4.7 inch Orion achromat displayed compelling detail on the moon. Yes. there was a little color on the limb, but I seldom observe the limb anyway.
I am quite aware of the loss of information in the image due to chromatic aberration, but I was seeing too much detail to care. With a green filter (admittedly not the best aesthetically), the image sharpness was breathtaking!
Tonight, low back permitting, I plan to put my 3 inch Brandon, F/15 achromat through its paces. It is a great, all-around, small refractor.
Edited by Steve Allison, 28 April 2021 - 12:46 AM.