TheSkySearchers

We are in a rainy season. I have not had two nights out in six weeks. Last night I did get out for about an hour. Nominally "clear", the skies had a layer of high humidity and not much was visible. I waited about an hour for a break and got to try out the diagonal on delta Corvii and alpha Librae (Zubenelgenubi).

They say:
This 1 1/4", 90 degree erecting prism is the same as used on our top rated Stellarvue finder scopes. This is a correct image erecting prism. The helical focuser has a rubber grip and provides about .6" of travel. Many purchase these to be able to do fine focusing adjustments on telescopes that suffer from image shift when also needing an image that is right side up and correctly oriented.

The erecting prism is fully multi-coated providing extremely high light transmission (greater than 99%). Compared to prisms using Mgfl2 coatings, the contrast is much higher and the stars appear brighter.

Side plates are carbon fiber. The insert tube is threaded for filters. Plastic dust caps are provided.
https://www.stellarvue.com/90-degree-er ... h-focuser/

When I got it, I found the screw adjustment to be tight in one place with some light friction and resistance, but running it up and down about 20 times cleared that.

I bought this to use with my Explore First Light 102-mm refractor which could use a fine focus. Granted that last night was not easy, the diagonal worked as expected and I was able to get somewhat sharper views of the binaries. I would like to try it on planets and the Moon, of course, but I will have to wait.

One thing is that as a correcting prism, my Right-Left were corrected opposite to how my hands have learned to direct the telescope, but it is not a big deal.

Finder scopes typically use Amici prisms to deliver correct image. They work fine at low power, but at higher powers typically show diffraction spike on bright targets. Let us know how it goes with Moon and planets.

Thanks, Bigzmey. This is the first diagonal I saw with a helical focus. I read more about "star" diagonals versus prisms and I understand the potential problem. I will report here for sure... soon as it clears up...

As I understand it, "star" diagonal is a misnomer because it does not refer only to mirrors but includes prisms, but we use it to mean "mirror" when we want to differentiate it from a prism.

Star means 90 degree angle vs terrestrial which is 45 deg. Star could be prism or mirror. Star mirror is erecting where up is up, but right and left flipped. Prism could be erecting or correct image depending on prism design.

The prisms used in my SV finders are out of collimation, and have a very narrow aperture. I called SV about the mis collimation, and was told it was normal. I took out the objective and placed it in a shorter tube, where I could use a regular diagonal. I won't recommend their finders. The $50 ones from GSO are just as good in my opinion.

John Fitzgerald wrote: ↑Sat Jun 05, 2021 3:27 pm The prisms used in my SV finders are out of collimation, and have a very narrow aperture. I called SV about the mis collimation, and was told it was normal. I took out the objective and placed it in a shorter tube, where I could use a regular diagonal. I won't recommend their finders. The $50 ones from GSO are just as good in my opinion.

That's the "luck of the draw". Mine was nearly perfect and had incredibly good star images for a Kellner eyepiece used in a 200mm focal length doublet refractor.
I suspect they vary a lot, whether SV or GSO.
There are some decent 50mm finders out there--Vixen, Explore Scientific, etc.
Sometimes you just have to try more than one.
I miss the Lumicon finder I had in the '80s. It was as perfect a finder as I've seen (though you had to add a dewshield).

Don,
The objectives are fine. The diagonals are so much out of collimation, that the image shifts as much as two degrees after rotating the diagonal. It is made to easily rotate. The only way to fix it would be to rotate the entire finder, or do what I described in my post above; put the objective in a tube that allowed a removable diagonal, and put a collimated diagonal in there. Very frustrating on an eq mount.

mikemarotta wrote: ↑Mon May 31, 2021 1:22 pm As I understand it, "star" diagonal is a misnomer because it does not refer only to mirrors but includes prisms, but we use it to mean "mirror" when we want to differentiate it from a prism.

I'm not so sure I would refer to it as a misnomer. A misnomer would mean that "star diagonal" is a wrong or inaccurate name or designation. IMO "star diagonal" is a correct name for diagonals used for astronomical or nighttime use hence the "star" in its name, whereas "terrestrial" or "erecting" diagonals are for primarily daytime use (there are exceptions). I think the correct terminology would be to say a mirror star diagonal, or prism star diagonal. And actually, given the different technologies for mirrors need a little more words to differentiate those, e.g., dielectric mirror diagonal, aluminized mirror diagonal, silver mirror diagonal, gold mirror diagonal. I think it is dangerous to alter the common definitions of things in public spaces as it just causes confusion and misunderstanding for many readers. Of course, marketing messes it up too as one can find Orion Erecting Star Diagonals so over time it has gotten all messed up. Adding mirror or non-mirror differentiation to "star" diagonals will just further conflate the already confusing terminology.

But on a slight tangent, I evaluated the very expensive Baader Premium AMICI diagonal and while yes it showed the horizontal spike on brighter stars due to the split in the prism, it was quite a thin fine line due to the extreme precision of the prism and it could also handle 300x and more on planetary observing with virtually no difference from a top of the line dielectric or prism diagonal. I was also amazed when using it how differently DSO present themselves when they are viewed in proper orientation. Ans of course star hopping was so much easier with it. But it is a heavy unit and the $700+ price is a little over the top for me. Would love to have one in the stall though!

Bigzmey wrote: ↑Sun May 30, 2021 7:46 pm ...but at higher powers typically show diffraction spike on bright targets. Let us know how it goes with Moon and planets.

I went out this morning from 0330 to 0500 just to view Jupiter and Saturn. The correcting prism with focusser worked very well. I was able to get sharp views of the planets at 41.25X and 77.6X. (ES 102 refractor; 32mm ocular with 2X Barlow and then 17mm with Barlow.) When I tried the 8mm and Barlow, the resolution was dark with eye floaters. But at 41 to 77, it was what I always wanted: a sharp focus, easy to obtain. Even at this nominally low power and given my Bortle 7-8 skies, this time I was able to view the turbulent scallops in the northen band of Jupiter. And, again, I was using the ES-102 which is f/6.47.

Viewing stars has been a different matter. I was warned here about chromatic aberration with the prism. it is not so much the colors but focussing stars into nice Airy disks has been elusive. Also, it so happens that I had a prism from Edmund Scientific when I was about 12 and mostly, I just played with it. And here I recognized some of the rainbowing, but it goes away with eye relief: move my head in or out, left or right, realign my eye, like we do when we get blackouts with large low-point eyepiece. That was while viewing alpha Libra Zubenelgenubi.

However, last nght, I installed the correcting diagnonal on my National Geographic 70mm f/10, viewed Mizar, and it was fine.

Sounds like it works nicely for you. The fuzzy airy disks could be from bad seeing.

Bigzmey wrote: ↑Sun May 30, 2021 7:46 pm Finder scopes typically use Amici prisms to deliver correct image. They work fine at low power, but at higher powers typically show diffraction spike on bright targets. Let us know how it goes with Moon and planets.

Having the helical focusser makes a difference.

It looks like clear weather forecast for the weekend coming up through Sunday at 0600 AM. I can run the magnification up the ladder and see how all of the oculars perform with and without the Barlow. I have 40 32 25 17 13 8 and. 6. Those are just Ploessls. I have a Televue 82 degree and a Meade 82 degree still in boxes to use with my APO 115, also still in its carrying case. But I will do the experiments with both my 70 and 102 and let you know.

In the mean time, I did view Jupiter and Saturn last weekend, and the Moon last night. I viewed Jupiter with the ES 102 and 17 mm w 2X Barlow and it was sharp, the best view I have had that I can remember, surely in an smaller instrument. I could make out the scallops in the northern equatorial band.
I viewed Saturn with the 32mm and 2x Barlow and it was also sharp. With the 17mm and Barlow, I could make out the northern band on Saturn under the ring, or else it was the shadow of the rings if that orientation is possible now. Again, having the helical focusser for fine tuning makes a difference.

The Moon was fine, of course.I used both the 70mm and 102mm and each with 17mm and no Barlow. That was about sundown at 2000 PM. However, when it was dark, I viewed with the 102. Still sharp. However, I did notice a chroma halo around the bright circumference. However, I have seen this before with the mirror diagonal and I attribute it to the refractor. I tried to write it up for CN a couple of months back, long before I got the prism, but they moved the discussion and it went nowhere.

I feel that the amount of CA added by prisms is exaggerated on forums (maybe by mirror diagonals vendors, ). It is below detectable level in my F6.3 80mm ED scope. On the other hand, reduction in light scatter compared to the dielectric diagonals is quite evident.

As I have stated before, if you want to check if a prism will be a benefit, insert a eyepiece in the focuser and do a star test. If you can see a slight reddish tinge on the inward focus disk then a prism may be beneficial.
If you do not then a mirror will work for you.

For all of the discussion about mirrors and prisms, I bought the diagonal for its helix focusser to provide fine focus for my 102 mm refractor. If I replace my aged 70 mm National Geographic with another or similar, the focusser can serve well there, also.

Chromatic Aberration Test

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 views 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 the Stellarvue star diagonal. I attribute this to the design of the achromatic doublet objective. It is inherent in the design of a "Dolland" type telescope. (See my comments above 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 a 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.

Congrats on catching GRS! I do prefer prism diagonals over mirrors on planets.

For planets the Baader prism is always my choice.
The AP mirror is very close but the prism has a little bit more contrast for me.

Lady Fraktor wrote: ↑Sat Jun 19, 2021 5:44 pm For planets the Baader prism is always my choice.
The AP mirror is very close but the prism has a little bit more contrast for me.

Same for me.