Erecting eyepiece diagonal: opinions?

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Discussion Overview

The discussion revolves around the use of Amici (roof) prism type erecting diagonals for terrestrial viewing with telescopes, specifically focusing on the ED80 OTA. Participants explore the optical characteristics, limitations, and availability of these diagonals compared to traditional star diagonals.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants express interest in the Amici prism type diagonals for their convenience in terrestrial viewing, particularly the 45° versions.
  • Concerns are raised about the optical quality of cheaper Amici prisms, with questions about their performance at higher magnifications and potential issues like streaks in star images.
  • There is a discussion about the availability of erecting diagonals compared to non-erecting versions, with some participants suggesting that the latter may be more common due to perceived quality issues.
  • Some participants note that traditional star diagonals produce a mirror image, which can be inconvenient for terrestrial viewing, while erecting diagonals are expected to provide a more natural view.
  • Technical challenges associated with the manufacturing of Amici prisms are mentioned, including the potential for light loss and diffraction spikes due to their design.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the suitability of Amici prisms for astronomical viewing, with some expressing skepticism about their performance while others question the rationale behind their market availability. The discussion remains unresolved regarding the quality and effectiveness of these diagonals.

Contextual Notes

Participants mention limitations in the availability of diagrams and technical information about Amici prisms, which may affect their understanding of the optics involved. There is also uncertainty regarding the performance characteristics of different diagonal types.

sophiecentaur
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I have this nice little ED80 OTA and it gives smashing images. I want to use it for terrestrial viewing. Are there any opinions about the Amici (roof) prism type erecting diagonals? I can't find much in the way of diagrams of the optics; the only one I found shows a 90° version, which is not as convenient as a 45° version
I only have nice 2" eyepieces, which limits what's available. Seems like there is no cheap solution. There is apparently a limit to magnification with these prisms. Are the cheaper ones worse in this respect?
 
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sophiecentaur said:
I have this nice little ED80 OTA and it gives smashing images. I want to use it for terrestrial viewing. Are there any opinions about the Amici (roof) prism type erecting diagonals?

hey sophi

not sure if this is the sort of thing you are after ?
Am not sure if it will erect the image ?

http://starizona.com/acb/Enhanced-Aluminum-Mirror-Diagonal---2---Satin-Finish-P2506C105.aspxDave
 
Thanks for getting back but that's a normal star diagonal, as far as I can make out. It is a posh (high reflectivity ) mirror and I already have one (reasonable quality, I think). It produces a mirror image and, for terrestrial viewing. the 90 degree angle is a pain - you have to look directly down into it or the image slopes drastically up/down hill. The erecting versions often has a 45 degree angle, for convenience and, like with binoculars, the roof prism gives a normal view.
This link has a picture of an amici prism system. There is a lot wrong with the system - hopeless of astronomy I think because of the streaks it gives stars and not good for high magnification because of the stripe down the middle but roof prism binos are absolutely fine with normal magnification.
 
sophiecentaur said:
This link has a picture of an amici prism system. There is a lot wrong with the system - hopeless of astronomy I think because of the streaks it gives stars and not good for high magnification because of the stripe down the middle but roof prism binos are absolutely fine with normal magnification.

Are you sure it's not suited for astronomical viewing? I have a hard time believing they sell them for that exact purpose if there are large problems with the design.

sophiecentaur said:
Seems like there is no cheap solution. There is apparently a limit to magnification with these prisms. Are the cheaper ones worse in this respect?

They could be, but I have no personal experience with them. I can understand why 2-inch eyepieces have expensive diagonals. That's a lot of added glass compared to the 1.25-inch eyepieces. Probably almost double the amount of glass and possibly more.
 
Drakkith said:
I have a hard time believing they sell them for that exact purpose
That's just it. Afaik, the erecting variety is more used for spotting scopes or for low magnification 'finder' purposes. They don't seem to be as available as the non-erecting, 90° versions on the market and I am just wondering if that's because the quality is not so good.
What I was hoping for was that someone would actually tell me they have one and it's brilliant or terrible.
It was interesting to find out about the Roof Prism principle but there are very few diagrams that make much sense. It's a very clever idea though.
 
sophiecentaur said:
They don't seem to be as available as the non-erecting, 90° versions on the market and I am just wondering if that's because the quality is not so good.

What do you mean a "non-erecting" version? All versions should be erecting.

As for quality, the only thing I know about them seems to be that they are more technically demanding than other types of prisms, especially the roof edge. Which makes sense, as any light that hits the peak of that roof and isn't reflected will result in a loss of light and alteration to the final image (possibly a bright diffraction spike according to some sources). The difficulty of making high-quality Amici prisms appears to be the reason that they are often marketed for terrestrial viewing or for low magnifications.

sophiecentaur said:
What I was hoping for was that someone would actually tell me they have one and it's brilliant or terrible.

Sorry, I wish I could.
 
A 'normal' star diagonal gives a mirror image. That is not an erecting diagonal (afaik). To get a terrestrial view, you have to be looking straight down which gives a crick neck. The 45 degree versions give a normal ('erected') image and save your neck. Good for spotting and finding. I have read that they give streaks on stars (roof effect) and also a band across the middle of a terrestrial view. I just wonder how much I will need to spend.
 

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