11 replies to this topic

[ericb760]

One of my acquisitions over the last two years yielded me a Celestron Erect Image 90 degree diagonal. I used it last night because it was the first one I grabbed from my astronomy junk drawer. It gave almost freaky sharp views of the moon in my Bushnell SkyChief Jr. I'm just wondering why these diagonals aren't that popular. I couldn't see any degradation when compared to a regular diagonal. Is there a reason these aren't more popular?

[TOMDEY]

"Does an Erect Image Diagonal degrade views when compared to a regular diagonal?"

 

Not if it's of premium quality = built well.     Tom

[Guydive]

Quality aside, they usually come with small clear aperture

[Jon Isaacs]

"Does an Erect Image Diagonal degrade views when compared to a regular diagonal?"

 

Not if it's of premium quality = built well.     Tom

 

Baader says that even with their best correct image diagonal, the roof prism line can be seen.  With Celestron correct image diagonals, I see a definite degradation at higher powers, particularly when viewing double stars.

 

The prism splits the light into two paths and then joins them back together..  The way around this is an oversized prism and only use half of it.  I believe that some binocular telescopes are done this way.

 

 

Jon

[Guydive]

Baader says that even with their best correct image diagonal, the roof prism line can be seen. With Celestron correct image diagonals, I see a definite degradation at higher powers, particularly when viewing double stars.

The prism splits the light into two paths and then joins them back together.. The way around this is an oversized prism and only use half of it. I believe that some binocular telescopes are done this way.



Jon

Here's an example for an oversize prism half used:
http://www.kasai-tra....jp/ezp-en.html

[Jon Isaacs]

Here's an example for an oversize prism half used:
http://www.kasai-tra....jp/ezp-en.html

 

 

They are 33,000 yen which seems to be about $230.  Now how about a 2 inch?

 

Jon

Edited by Jon Isaacs, 08 May 2025 - 03:16 AM.

[Guydive]

They are 33,000 yen which seems to be about $230. Now how about a 2 inch?

Jon

Just imagine the weight

[deSitter]

One of my acquisitions over the last two years yielded me a Celestron Erect Image 90 degree diagonal. I used it last night because it was the first one I grabbed from my astronomy junk drawer. It gave almost freaky sharp views of the moon in my Bushnell SkyChief Jr. I'm just wondering why these diagonals aren't that popular. I couldn't see any degradation when compared to a regular diagonal. Is there a reason these aren't more popular?

 

There will always be a diffraction spike artifact from the apex of the "roof" where the image flipping happens, no matter how well made the prism. For a finder or terrestrial use, this is unimportant.

 

I often use a RACI diagonal on the Moon. Never on planets. It can be useful to check the position angle of a double star, without having to do the mental gymnastics to flip the image in your head, or subtract from 360 etc. 

 

Making roof prisms was once a difficult job, and WW-2 created a great demand for them, for gunsights. The two sides of the roof must be perpendicular to an extreme exactness, otherwise you get doubled or missing views along the roof line. There was a pack of amateurs known as "The Roof Prism Gang" who worked to fill the demand under the leadership of IIRC Albert Ingalls. It is an interesting story.

 

-drl

Edited by deSitter, 08 May 2025 - 03:57 AM.

[deSitter]

Just imagine the weight

 

I have a 2-inch regular prism diagonal, and it is surprisingly light. Either that, or my mirror 2" is surprisingly heavy  

 

-drl

[Guydive]

I have a 2-inch regular prism diagonal, and it is surprisingly light. Either that, or my mirror 2" is surprisingly heavy

-drl

So the type of prism in my link, is double the size to avoid the roof - so a 2" would be at least X4 the weight

[apfever]

Some semantics are in order for beginners and the confused. Outside of some extreme oddballs, All regular 90 degree diagonals will give an upright image, and most 45 degree diagonals now give an upright image also. The issue is having the image corrected from left to right. 

 

"RACI" is still common for Right Angle Corrected Image. "Right Angle" means upright not upside down. "Corrected Image" refers to 'left and right' being correct and not reversed like a mirror image.  

 

NOW this use of "Erect Image" seems to be applied to the 'left/right' issue which is somewhat confusing.  "Erect Image" used to be the upside right or upside down issue in my book. 

 

Consider looking at the ground, terrestrial viewing is easiest to understand.  All regular mirror or prism diagonals will give an upright image so the sky is on top and ground on the bottom. However, the image will be reversed left to right so a bird looking to the right will appear to be looking to the left in the scope. 

 

The special Amici Prism corrects the mirror image in a single regular sized prism. It will have a diffraction line created from an edge in the prism. These prisms do have extra tolerance needs in production. I use them and have no issues with a quality one. The inherent drawbacks are minimal.  If you are doing high magnification or critical viewing (a small percentage of use) then you should have experience with the issue, deal with it, or not use an Amici. 

 

The large single sided Amici diagonals mentioned above seem awkward and expensive to me. Perhaps something for the critical viewer using the least complicated glass. 

 

"Erect Image" is a misnomer that seems to be getting accepted as a reference to mirror images. ....things change. 

Edited by apfever, 08 May 2025 - 07:51 AM.

[deSitter]

Some semantics are in order for beginners and the confused. Outside of some extreme oddballs, All regular 90 degree diagonals will give an upright image, and most 45 degree diagonals now give an upright image also. The issue is having the image corrected from left to right. 

 

"RACI" is still common for Right Angle Corrected Image. "Right Angle" means upright not upside down. "Corrected Image" refers to 'left and right' being correct and not reversed like a mirror image.  

 

NOW this use of "Erect Image" seems to be applied to the 'left/right' issue which is somewhat confusing.  "Erect Image" used to be the upside right or upside down issue in my book. 

 

Consider looking at the ground, terrestrial viewing is easiest to understand.  All regular mirror or prism diagonals will give an upright image so the sky is on top and ground on the bottom. However, the image will be reversed left to right so a bird looking to the right will appear to be looking to the left in the scope. 

 

The special Amici Prism corrects the mirror image in a single regular sized prism. It will have a diffraction line created from an edge in the prism. These prisms do have extra tolerance needs in production. I use them and have no issues with a quality one. The inherent drawbacks are minimal.  If you are doing high magnification or critical viewing (a small percentage of use) then you should have experience with the issue, deal with it, or not use an Amici. 

 

The large single sided Amici diagonals mentioned above seem awkward and expensive to me. Perhaps something for the critical viewer using the least complicated glass. 

 

"Erect Image" is a misnomer that seems to be getting accepted as a reference to mirror images. ....things change. 

 

Yes I think the words RACI, erect, correct, and erector get mixed up. RACI to me seems least ambiguous.

 

45 degree diagoanls use a different type of roof prism but operate on the same principle.

 

-drl