Necessary Facts: Testing the Stellarvue Correcting Diagonal Prism

Saturday, June 19, 2021

Testing the Stellarvue Correcting Diagonal Prism

Chromatic Aberration Test -- I mentioned that I bought this correcting diagonal prism along with some other astronomical instrumentation. On The Sky Searchers discussion board, I was warned that this device can introduce chromatic aberration in instruments with shorter focal lengths. This morning, I tested it. Here is my report to The Sky Searchers.

This morning from 0415 to 0550 local central US daylight time (UT -5), I viewed Jupiter with my Explore Scientific "First Light" 102-mm refractor (F=660 mm; f/6.47) and Stellarvue correcting prism diagonal, testing all (Ploessl) oculars 32 17 13 8 and 6 mm with and without 2X Barlow. My final judgment is that the prism does not introduce noticeable chromatic aberration.

However, some details provide a complete report. The easiest and perhaps most embarrassing is that I never noticed chromatic aberration before because my attention was always on the disk itself, the bands of the planet. Can I see them north and south? Are they brown or gray? My brain filtered out any distractions. I accepted the slight rainbowing as normal.

Understand that I am viewing from city skies that have degraded over the past two years. I can tell by reviewing my notebook and remembering the observations and sightings. I have more particulates in the air and more light and therefore more haze. So, this is not a clear, dark sky. Therefore, seeing includes subconscious interpreting. We understand sensations as percepts because of identifications within the brain and mind. If I were not intent on a qualitative measurement of chromatic aberration, I would not have perceived it.

That being so, it remains that chromatic aberration (CA) was evident in every view of Jupiter. CA appeared as a slight colored ring at the edge of the visible disk of the planet. The colors shifted within various sectors and also around the disk being mostly soft yellow and soft green but also some soft blue. This was true at all magnifications. It was less noticeable at the highest powers but only because the overall resolution was grainy and dark with floating clear disks overall.

I next viewed Jupiter with my 70 mm National Geographic refractor (F=700 mm; f/10) and the combination ocular 17mm with 2X Barlow and saw the same artifact. However, that NG 70 has its original mirror diagonal, not a prism. I attribute this to the design of the achromatic doublet objective. It is inherent in the design of a "Dollond" type telescope. (See my comments [below] Thu Jun 17, 2021 5:13 pm about viewing the Moon.) The combination of crown and flint glass removes problems, hence the term "achromatic." But it is not apochromatic, not the complete correction delivered by triplet objectives of extra-low diffraction rare-earth-additive glass.

I also must report that in order to improve my viewing, I went back into the house and cleaned my eyes with Bausch & Lomb "Alaway" brand ophthamalic antihistamine solution.

I consider the Stellaevue prism to be an excellent addition to my instrument. I installed it to replace the manufacturer's mirror diagonal and I will continue to use it because it delivers the primary benefit of a helical focusser for fine adjustment.

Addenda: (1) The best test would be with comparable Newtonian reflector because a properly formed reflector delivers an image without chromatic aberration.

~~(2) One bonus to this morning's session was my first view of Jupiter's Great Red Spot.~~ [Data suggested the identification, but what I saw did not match the event.]

Red Spot Transits

Jovian Moon Phenomena

What I saw looked at first like the shadow of a moon. But there is no datat to support that.

23 March 2021. Viewing the gibbous moon about 1952 hours local time (UTC-6)

40 mm Svbony Plössl – Green mostly green some green-yellow to the right and occasional blue along the lower left.
32 mm Celestron Full Multi-Coated – Green all around
25 mm Unbranded (from Explore Scientific) Super Plössl FMC – Yellow-green all around.