Difference between Amici Prism and Star Diagonal, my experience

[Aquavit]

As a newbie one of the first things that struck me was the reversed image in the eyepiece and the difficulties this presented me, particularly when trying to find my way around the Moon.

Deciding that it was easier to buy a "right way round" diagonal in the form of an Amici Prism rather than training my brain to unlearn a lifetime of knowing right from left I ordered this https://www.firstlightoptics.com/diagonals/william-optics-125-90-degree-erecting-prism.html from FLO.

 

Fitted to my little Mak and tested with some Lunar surfing all was good, except it wasn't. It became apparent that I was struggling to get a clear view through the eyepiece , any eyepiece in fact although some were worse than others. My observing was hampered by a "tunnelling" effect, known as blackout as I now understand, and detailed in my earlier thread here where the conclusion was that the problem was caused by too much eye relief:

Since then I've done some experimentation and switched between the OEM Skywatcher diagonal and the new WO prism. To my surprise, viewing through the star diagonal eliminated the blackout issue and made all of my eyepieces viable. I've repeated the exercise both at night and in daylight and the difference is quite stark. On examining the two diagonals what is immediately obvious is that the aperture on the WO prism is significantly less both at the scope and the eyepiece end.

Putting my callipers over them reveals:

Scope end - SW Star Diagonal 28.2mm ID with no obstruction before the mirror, WO Prism 25.3mm ID with a further obstruction reducing the ID to around 19.2mm.

Eyepiece end - SW Star Diagonal 32mm ID with no obstruction before the mirror, WO Prism 32mm ID with a stepped obstruction reducing the ID to around 18mm.

Obviously these obstructions must reduce the light travelling through the diagonal but, more pertinent to me, they are I think what is creating the blackout I was experiencing  when using it.

Is this design common to all Prism diagonals or a feature of the WO product? I'm sure someone more technically astute can explain these differences and answer the "why".

Anyway, stumbling across what seems to be the cause of my problem has restored my faith in the eyepieces I have and determined me to retrain the brain to understand that right is in fact left.

[johninderby]

That’s why I prefer the Baader prism diagonals. Their T2 amici prism has a clear aperture of 31mm but not cheap. 

https://www.rothervalleyoptics.co.uk/baader-t-2-90-amici-prism-bbhs-coated-diagonal.html

Edited March 16, 2021 by johninderby

[vlaiv]

What you were seeing was vignetting rather than usual blackouts from eyepieces.

If prism is not sufficiently large - it will be stopped down and you can only use it with eyepieces that have field stop of similar diameter.

When looking at particular prism / diagonal - it's worth noting what the clear aperture of the item. You want it to be at least 28mm for 1.25" versions

[Aquavit]

16 minutes ago, vlaiv said:

What you were seeing was vignetting rather than usual blackouts from eyepieces.

If prism is not sufficiently large - it will be stopped down and you can only use it with eyepieces that have field stop of similar diameter.

When looking at particular prism / diagonal - it's worth noting what the clear aperture of the item. You want it to be at least 28mm for 1.25" versions

Thanks Vlaiv, there is so much to learn in this hobby and I suspect that all the information required to make a correct choice is not always obvious.

Why would WO reduce the aperture on their prism if it excludes the use of a lot of eyepieces?

Edited March 16, 2021 by Aquavit
Added question.

[Aquavit]

17 minutes ago, johninderby said:

That’s why I prefer the Baader prism diagonals. Their T2 amici prism has a clear aperture of 31mm but not cheap. 

https://www.baader-planetarium.com/en/baader-t-2--90°-baader-roof-prism-with-bbhs-r-coating-(t-2-part-02).html

As you say, not cheap!

Do you observe exclusively with a prism diagonal or do you switch between the two types and suffer the mental gymnastics??

 

[johninderby]

Actually I don’t use an amici prism very often. Usually use a regular mirror diagonal or a regular Baader prism. It’s just something I don’t think about as I automatically adjust to it. 

[vlaiv]

5 minutes ago, Aquavit said:

Thanks Vlaiv, there is so much to learn in this hobby and I suspect that all the information required to make a correct choice is not always obvious.

Why would WO reduce the aperture on their prism if it excludes the use of a lot of eyepieces?

Price / performance thing.

In order to get large clear aperture - you need to use oversized prism because of the way light bends thru it.

I tend not to use amici prisms for astronomy as astronomical quality amici prisms are very expensive. These cheap models are not as good as regular diagonal mirrors.

In space there is no really "up/down" - it depends on orientation of observer. Here on earth - we think there is down because of gravity - but different observers looking at the Moon will see it oriented differently depending where they are on Earth's surface. If you are on northern hemisphere and observing the moon and on southern hemisphere observing the moon at the same time - image will be flipped by 180° between the two.

I think it is better to get brain accustomed to that sort of thing right from beginning.

[johninderby]

Amici prisms are handy for nature watching though. 

[RT65CB-SWL]

Hi @Aquavit and a belated welcome to SGL. 

I used to get a 'blackout' or a 'black-spot' with my ETX105 with some e/p's unless the eye was centred directly over it, especially at higher magnifications. 

Out of curiosity, what e/p's are/were you using?

[Louis D]

I just double checked my cheap 45 and 90 degree Amici prism diagonals, and the OP is correct.  They are vastly stopped down.  I only use the 45 degree ones for terrestrial observing and the 90 degree one in a 60mm RACI finder, so the vignetting has never bothered me.

I've never experienced blackout with either, even with widest field eyepieces; but that was always with refractors.  Perhaps there is some interaction with the Mak's central obstruction going on as well here?

[Aquavit]

3 minutes ago, Philip R said:

Hi @Aquavit and a belated welcome to SGL. 

I used to get a 'blackout' or a 'black-spot' with my ETX105 with some e/p's unless the eye was centred directly over it, especially at higher magnifications. 

Out of curiosity, what e/p's are/were you using?

Thanks for the welcome Philip!

I was using an Astro Essentials 32mm Plossl, Explore Scientific 26mm/62º and my OVL Zoom 7.2mm - 21.5mm. The Plossl and the ES were the worst affected, the Zoom seemed to cope better with the prism, but all are much better with the mirror diagonal.