Amici Prism Diagonal

[Roger Corbett]

Hi all,
I'm becoming a bit of a Moon and Planets visual observer and in consequence, I'm considering an Amici Diagonal. I have read that at high magnification your view can be obscured. That said, I have read that it is of little consequence when using a Mak and a decent diagonal like this one - https://tinyurl.com/y75z4n5v from Baader Planetarium. I'm really keen to make my East East and my West West. Any advice or experience would be welcome

 

[johninderby]

Don’t like amici prisims for night time observing. Good for terestrial daytime use. Just never seen the need for one for night time observing.

Prefer a dielectric mirror diagonal and better still a prisim diagonal with a mak.

This is probably the most popular

https://www.baader-planetarium.com/en/accessories/optical-accessories/prism-and-mirror-star-diagonals/baader-zenith-prism-diagonal-t-290-°-with-32mm-prism--(t-2-part-14).html

This a slightly better version

https://www.baader-planetarium.com/en/accessories/optical-accessories/prism-and-mirror-star-diagonals/baader-t-2-stardiagonal-(zeiss)-prism-with-bbhs-r-coating-(t-2-part-01b).html?___SID=U

 

I use one with my mak.with a helical focuser on top for fine focusing.

Edited April 16, 2020 by johninderby

[Doc]

While I do agree that AMICI are great for daytime use I do think they have a place for night time viewing as well. I have the APM AMICI diagonal and it's pretty awesome at night. it does have the light defect on very bright objects but it's not off putting. The best thing is the ease of star hopping as it's the correct way up, I like them a lot and think it's a good purchase.

[Floater]

For Moon viewing the prism is always my go to. The Baader Amici should not be confused with cheaper prisms ... It is a good piece of kit and if you are to concentrate on Moon/planets then I would say go for it.

Edited April 16, 2020 by Floater
Clarifying which prism

[Ruud]

For terrestrial observations amici prisms are OK. This is because these observations are limited mostly by looking horizontally through thick atmosphere, when magnifications over 80x are hardly usable anyway.

On a good night however, allowing high magnifications, the amici prism becomes the weak link and a noticeable source of image degradation.

[Don Pensack]

2 hours ago, Roger Corbett said:

Hi all,
I'm becoming a bit of a Moon and Planets visual observer and in consequence, I'm considering an Amici Diagonal. I have read that at high magnification your view can be obscured. That said, I have read that it is of little consequence when using a Mak and a decent diagonal like this one - https://tinyurl.com/y75z4n5v from Baader Planetarium. I'm really keen to make my East East and my West West. Any advice or experience would be welcome

 

Some issues for nighttime observing:

--more internal light loss than a conventional prism or mirror

--due to the split in the field, there is a phase loss in the field.  Critical observers say it results in a loss of contrast and sharpness

--there is a vertical line in the field that divides left from right.  Because this is not infinitely sharp, there is a flare of light when anything crosses the center line.  It's most noticeable with stars, but can also be an issue when the object fills the field, as in the Moon, or looking at the daylight sky, where that line is usually visible.  It would be especially apparent when a planet crosses the line.

--well made Amici prisms can be very expensive--as much as the high end silver or dielectric ones or more.

--long focal ratios have smaller true fields that occupy less of the prism, but that means the center reversal line is even more apparent.

--there are some good 2" Amici prism diagonals out there, but I only recommend them for low power viewing during daylight.

Edited April 16, 2020 by Don Pensack

[Peter Drew]

As Don rightly says, unless a very high end example is used there are some noticeable fall offs compared to regular high end diagonals.  However, it is a balance.  I too find it more convenient to observe the Moon in correct orientation and use a home made Amici unit incorporating a high quality ex WD prism.      🙂

[Space Hopper]

7 hours ago, Ruud said:

For terrestrial observations amici prisms are OK. This is because these observations are limited mostly by looking horizontally through thick atmosphere, when magnifications over 80x are hardly usable anyway.

On a good night however, allowing high magnifications, the amici prism becomes the weak link and a noticeable source of image degradation.

Something i'm going to put to the test.

The Baader Amici (T2 version #2456130 , not the mega expensive one) is a very good performer.

It will take magnification well, and i mainly use mine for daytime terrestrial / birding and Lunar viewing.

As an example, i can read a car number plate with it, with a binoviewer, at a distance of 1.9km. This was at 88x mag.

 

I've only so far noticed the horizontal line aberration with Venus, and you can't really get any brighter than that.

Its my intention to purchase a good quality Baader mirror diagonal as well at some point soon, and do some comparisons, with the Amici and my other Baader prism diagonals.

I'm not expecting to see a 'night and day' difference, but who knows, i may be surprised ?

I think the biggest factor here, are the UK skys, rather than our diagonals that we use in our scopes.

 

[John]

9 hours ago, Space Hopper said:

....I'm not expecting to see a 'night and day' difference, but who knows, i may be surprised ?

I think the biggest factor here, are the UK skys, rather than our diagonals that we use in our scopes.

 

I used to think that but I feel that diagonal choice does matter a bit more now:

https://www.cloudynights.com/articles/cat/articles/mirror-vs-dielectric-vs-prism-diagonal-comparison-r2877

 

[Don Pensack]

8 hours ago, John said:

I used to think that but I feel that diagonal choice does matter a bit more now:

https://www.cloudynights.com/articles/cat/articles/mirror-vs-dielectric-vs-prism-diagonal-comparison-r2877

 

The observer who wrote that comparison is almost exclusively an observer of planets and double stars and uses exclusively small scopes.

That doesn't invalidate his comparison completely, but his results might have been different had he used larger scopes or evaluated the diagonals on ultra-faint objects.

Not to mention that some of his comments might have been eyepiece or scope-dependent.  He has also reviewed the Baader BBHS silvered star diagonal and never even mentioned

the weakness of silver coatings in the violet end of the spectrum.  An H-Beta test on the California Nebula at a dark site showed me it was no better than a dielectric diagonal at 1/5 the price.

On planets, it's superb, as you would expect from its high % flat transmission from yellow-red.  So the evaluations are also target-specific.

Most of us cannot afford specific star diagonals for specific objects, or to replace them every couple of years, so take such comparisons with a grain of salt.

I put them in the same category as those who claim that multi-coatings scatter more light than single-layer coatings or that gold wires sound better than copper wires in an amplifier.

Myself, I'd rather have a star diagonal that can handle a heavy 2" eyepiece and can be canted to the left without unscrewing the barrel.

Star diagonals vary in quality a lot, but it's more about the surface accuracy and smoothness than the type of coating or the material the diagonal is made from.

They are usually mirrors.  Our primary mirrors vary in quality a lot from scope to scope.  It's not hard to understand Star diagonals do too.  It makes it almost impossible to generalize

about a specific brand and model with a sample of one.

 

[John]

1 hour ago, Don Pensack said:

The observer who wrote that comparison is almost exclusively an observer of planets and double stars and uses exclusively small scopes.

That doesn't invalidate his comparison completely, but his results might have been different had he used larger scopes or evaluated the diagonals on ultra-faint objects.

Not to mention that some of his comments might have been eyepiece or scope-dependent.  He has also reviewed the Baader BBHS silvered star diagonal and never even mentioned......

...... so take such comparisons with a grain of salt.....

 

Will you tell him on CN Don, or shall I ?

I have the Baader T2-Zeiss prism, an Astro Physics Maxbright and a couple of TV Everbrites and I honestly can't tell the difference between them.

Some folks apparently can though

 

[Peter Drew]

If you get OCD over diagonals try not using one.  Perfection.    🙂

[Louis D]

5 hours ago, Peter Drew said:

If you get OCD over diagonals try not using one.  Perfection.    🙂

I've read that diehard Japanese observers regularly eschew diagonals in search of the best possible view regardless of ergonomics.

[mikeDnight]

I observed for years using a Takahashi FS128 mounted high on a GM8 and without ever using a diagonal. The backache it caused created more of a distraction than any of the almost non-existent artefacts potentially introduced by using a good diagonal. I now use a diagonal prism every time, and there is no discernable difference in image quality. With the prism diagonal I can easily exceed 400X in my FC100D without any deterioration of the image caused by any optical component, diagonal included.  Having said that, I've always steered clear of amici prisms because of their very nature. No matter how good they are, there will always be some detremental effect caused by the diffraction spike. This may appear to only be a problem on bright objects like Venus, but never the less it is still present on all targets, and will subtract from the scopes definition and contrast. In a hobby where everyone seems to obsess about the finest optical advantage of one scope, eyepiece or diagonal over another, even outlaying thousands of pounds just to obtain a fractional edge in performance; why add a known destructive influence into the equation? 

On a personal level, I've used mirror and prism diagonals for so long now that I've no trouble interpreting charts or coordinates in the night sky or on planetary surfaces. I know it drives some people nuts, but you can always use a mirror reversed lunar map for the Moon. Or you can draw your own map of Mars mirror reversed, or even mak a globe that can be used as a visual aid alongside a refractor with a diagonal in it.

S&T mirror reversed lunar map:

Mirror reversed map of Mars made from observations from the 2016 apperition:

A mirror reversed Mars globe based on the apperition of 2016:

[Space Hopper]

15 hours ago, Peter Drew said:

If you get OCD over diagonals try not using one.  Perfection.    🙂

But i'd be stuck without the extra out focus they give !! High power and 2" glass would be a no-go

[Peter Drew]

2" extension tube ?    🙂 

[Paz]

I use a correct image prism diagonal if I am doing difficult star hopping and sometimes on the moon when it can get confusing with a mirror image. The frequency with which I use a correct image diagonal has gradually reduced over time but I think they will always have a place in the arsenal for me.

[Roger Corbett]

There are plenty of people here with a great deal more experience than me, as I have seen from this thread. But I'm going to give it a go. I have a pretty good regular diagonal to use at high magnification if I find it necessary. It only takes a few seconds to plug in a diagonal, and a diagonal is an absolute necessity with any kind of Cass. Spend an hour or too looking through one without and you wont be concerned about anything other than finding a chiropractor.