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45 or 90 degree diagonal? Which is better and why?

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#1 LyraCygnus

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Posted 12 June 2020 - 06:27 PM

Hi,

I'm a complete beginner and do not know what difference it makes having a 45 degree or 90 degree diagonal on a telescope.  I'm trying to decide between buying a Celestron Powerseeker AZS 80mm scope with a 90 degree diagonal or a Skywatcher 102mm scope with a 45 degree diagonal.  The focal length on the Skywatcher is 510, the Celestron is 400.  Focal ratio on both scopes is f5. 

 

I have read that a 90 degree diagonal is better but am not clear why this is.  The cost difference is around $100 extra with the Skywatcher.  Would it be better to spend the extra and have a bigger aperture or is a 45 degree diagonal a big disadvantage?  Any advice and opinions greatly appreciated. 



#2 SeattleScott

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Posted 12 June 2020 - 06:34 PM

45 is better for terrestrial. For stargazing it will vignette.
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#3 photoracer18

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Posted 12 June 2020 - 06:49 PM

In the 45 degree diagonal the light bounces off surfaces 5 times due to the type of prism used to get a completely correct image. 90 degree is an astro diagonal and no one cares about the orientation of the image (or shouldn't) and the light bounces off only one surface. Thus the light path is shorter and there is less light loss before it gets to your eye. The longer light path can sometimes keep eyepieces from reaching focus.


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#4 ShaulaB

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Posted 12 June 2020 - 07:01 PM

If you use a 45 degree diagonal and view an object that is straight up, or even well above the horizon, it would be awkward on the neck, uncomfortable.

Look at more telescope pictures showing telescopes used only for astro. None of them have 45 degree diagonals.
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#5 Shorty Barlow

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Posted 12 June 2020 - 08:07 PM

90 degree is an astro diagonal and no one cares about the orientation of the image (or shouldn't) ...

I care. I use 90 degree Amici prisms (Correct Image) for lunar viewing, and some deep sky. Of course, good quality Amicis can be expensive, but they all aren't.

 

med_gallery_249298_5348_465638.jpg

 

These Omegon Amici and William Optics Amicis are quite reasonably priced. You may see a diffraction spike on bright objects although I've never seen it on the Moon. Amici prisms differ in focal plane lengths, although occasionally there can be in-focusing issues.

 

sml_gallery_249298_5348_46298.jpg

 

45 degree are best suited to terrestrial viewing for a variety of reasons. Vignetting and often uncomfortable angle of observing being a couple. 


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#6 Sketcher

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Posted 12 June 2020 - 08:54 PM

A 90° diagonal will generally be considerably more comfortable for astronomical use.  Also, all other things being equal (and they never are) a 90° diagonal will be more likely to provide a higher quality (sharpness, contrast, freedom from "artifacts") view; but that difference may or may not be quite subtle -- if noticeable at all.  Consequently, astronomical observers tend to favor 90° diagonals.

 

That being said, I would suggest you purchase the telescope you want, based on other reasons, without regard to the diagonal it comes with.  Either telescope should be perfectly usable with either diagonal.  If your preferred telescope doesn't come with your preferred diagonal, you can always purchase the other diagonal separately -- and enjoy the option of using either diagonal depending on how you're using the telescope (for example, astronomically or terrestrially).

 

Note:  Some diagonals, of either type, if cheaply made, may vignette the views with low-power, wide-angle eyepieces due to an under-size mirror or prism (to save cost).  This is more common with the 45° diagonals; but is sometimes found with the cheaper 90° diagonals as well.

 

P.S.  I have both, 45° and 90° diagonals of various sizes and image orientations, It's very rare for me to use a 45° diagonal for anything.  For myself, the preference is primarily a comfort issue.

 

P.P.S.  If I want a "correct-image" (as opposed to a mirror-reversed image) I'm more likely to use a more specialized, 90° correct-image diagonal than a (more commonly available) 45° correct-image diagonal.  This time, my preference is due to both, comfort and quality.


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#7 Avgvstvs

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Posted 13 June 2020 - 12:50 AM

The 45 can be useful for a correct image finder. I sometimes use one on my Orion 70mm finder.

The 90 comes in mirror and prism varieties. Mirror is generally preferred as there is less light loss.


Edited by Avgvstvs, 13 June 2020 - 12:51 AM.

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#8 Jon Isaacs

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Posted 13 June 2020 - 05:32 AM

The 45 can be useful for a correct image finder. I sometimes use one on my Orion 70mm finder.

The 90 comes in mirror and prism varieties. Mirror is generally preferred as there is less light loss.

 

There are 90 degree correct image diagonals. Correct image diagonal general vignette the widest field eyepieces and bounce the light around several times.

 

Star diagonals reverse the image left to right and are much simpler. In optics, simple is good.

 

Jon


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#9 LyraCygnus

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Posted 13 June 2020 - 01:51 PM

A 90° diagonal will generally be considerably more comfortable for astronomical use.  Also, all other things being equal (and they never are) a 90° diagonal will be more likely to provide a higher quality (sharpness, contrast, freedom from "artifacts") view; but that difference may or may not be quite subtle -- if noticeable at all.  Consequently, astronomical observers tend to favor 90° diagonals.

 

That being said, I would suggest you purchase the telescope you want, based on other reasons, without regard to the diagonal it comes with.  Either telescope should be perfectly usable with either diagonal.  If your preferred telescope doesn't come with your preferred diagonal, you can always purchase the other diagonal separately -- and enjoy the option of using either diagonal depending on how you're using the telescope (for example, astronomically or terrestrially).

 

That sounds like very good advice.

 

 

 



#10 LyraCygnus

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Posted 13 June 2020 - 01:55 PM

Thanks everyone for the info and advice, it's made it a lot easier to decide what to buy.   ;) I'm inclined to go for the 102mm and buy a 90° diagonal at some point just to make the viewing more comfortable.  In the long run it's probably the better option since more DSO can be seen with the larger aperture. 


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#11 Starman1

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Posted 17 June 2020 - 07:00 PM

Hi,

I'm a complete beginner and do not know what difference it makes having a 45 degree or 90 degree diagonal on a telescope.  I'm trying to decide between buying a Celestron Powerseeker AZS 80mm scope with a 90 degree diagonal or a Skywatcher 102mm scope with a 45 degree diagonal.  The focal length on the Skywatcher is 510, the Celestron is 400.  Focal ratio on both scopes is f5. 

 

I have read that a 90 degree diagonal is better but am not clear why this is.  The cost difference is around $100 extra with the Skywatcher.  Would it be better to spend the extra and have a bigger aperture or is a 45 degree diagonal a big disadvantage?  Any advice and opinions greatly appreciated. 

45° diagonals:

--are harder to use at the zenith, where you will do a lot of observing.

--are smaller in clear aperture so the light on the edge of the field is cut off or heavily reduced (vignetted) in a low power eyepiece

--have more internal light loss than a 90° diagonal

--are rarely made to the quality standards of 90° diagonals (there are exceptions)

--are designed to make terrestrial observing easier, when the telescope is mainly horizontal.

--are typically not designed strong enough to hold heavy eyepieces

--are erecting prisms and have many optical issues for nighttime use (I won't go into those here)

 

So, buy the scope you want, but figure if it doesn't come with a 90° star diagonal, you will be getting one.


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#12 LyraCygnus

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Posted 19 June 2020 - 02:10 AM

45° diagonals:


 

--are designed to make terrestrial observing easier, when the telescope is mainly horizontal.

--are erecting prisms and have many optical issues for nighttime use (I won't go into those here)

 

So, buy the scope you want, but figure if it doesn't come with a 90° star diagonal, you will be getting one.

Thanks for the info.  The 2 points you make there raise a few questions for me.

 

1  If using it for terrestrial use when the scope isn't horizontal  does it mean it will be uncomfortable to use?

2  Is there a difference between a diagonal and a star diagonal or are they the same thing?

3  If you have the time (or anyone else) could you mention a couple of the major optical issues that come with a 45°  diagonal?

 

Thanks a lot.
 



#13 Paul Sweeney

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Posted 19 June 2020 - 04:16 AM

A correct image diagonal can be a real blessing to a new astronomer. The image you see has the same orientation as that of the naked eye and of your star chart. That simplifies things a lot. It takes time to learn to convert the "normal" image of the 90° diagonal to correspond with the star chart.

The biggest negative with cheap 45° diagonals is the 5 surfaces. Each surface is a potential error, and they add up quickly. My cheapo-cheapo 45° diagonal makes single stars into close doubles. For bird watching it is ok, for astronomy it is junk.

I would look around and find the scope you want with the diagonal you want. Call the dealer and see if they will swap the diagonals for you.

#14 jaraxx

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Posted 19 June 2020 - 06:32 AM

I have and have used both, using scopes for looking at the sky as well as the dirt. As has been pointed out, the 90° is better for looking at the sky. I would add that in general, I find a 90° diagonal to be more useful looking at dirt than the 45° is for looking at the sky. If I had to choose one it would be the 90°  - it functions well unless you're looking downhill, where it becomes awkward.

I would also point out that you might be replacing the one that comes with the scope pretty quickly unless you're buying a more expensive scope.


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#15 Ulmer Spatz

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Posted 19 June 2020 - 06:33 AM

1  If using it for terrestrial use when the scope isn't horizontal  does it mean it will be uncomfortable to use?

With ergonomics questions, simulations often work well. Roll up some paper, crush one end of the roll, fold it to 45 degrees as shown and simulate viewing from the horizon to straight overhead. Next, fold the crushed end to 90 degrees and simulate viewing again. 

 

I predict that your neck will say NO when doing the 45-degree straight overhead simulation. When looking at something on or near the horizon, the 45-degree angle allows you to switch from scope view to naked eye view by just rolling your eyeballs up a bit. There's no need to move your head. It's the only advantage of the 45-degree angle I can think of right now. 

 

diagonal simulation.jpg


Edited by Ulmer Spatz, 19 June 2020 - 07:53 AM.

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#16 Jon Isaacs

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Posted 19 June 2020 - 08:03 AM

Thanks for the info.  The 2 points you make there raise a few questions for me.

 

1  If using it for terrestrial use when the scope isn't horizontal  does it mean it will be uncomfortable to use?

2  Is there a difference between a diagonal and a star diagonal or are they the same thing?

3  If you have the time (or anyone else) could you mention a couple of the major optical issues that come with a 45°  diagonal?

 

Thanks a lot.
 

 

-  I use a 90 degree "star diagonal" both for astronomy and for terrestrial viewing, primarily birding.  I have no issues with the comfort when viewing terrestrially. 

 

-  2.  A star diagonal is either a mirror diagonal or a simple prism diagonal that inverts the image so it is correct top to bottom but leaves the left-right reversed, assuming the diagonal is oriented vertically.

 

-  The two optical issues that come with a correct image diagonal, 45 degrees or 90 degrees are:

 

- Limited clear aperture.  Most 1.25 inch Correct image diagonals have about 20mm of clear aperture.  With wider field eyepieces, this will cause significant vignetting, the edge of the field is dim and may be black.  

 

- The complicated light path.  The diagonal splits the light beam in two and they travel separate paths.  Getting them back together is not so easily done.  

 

kisspng-light-amici-roof-prism-amici-pri

 

Binoculars that use roof/amici prisms are "phase coated" because the two light paths end up with some phase shift.  If they are not phase coated, there is a loss of contrast.  I have not seen this done with correct image diagonals but it seems like it ought to be.

 

Jon


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#17 LyraCygnus

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Posted 20 June 2020 - 09:24 AM

 Roll up some paper, crush one end of the roll, fold it to 45 degrees as shown and simulate viewing from the horizon to straight overhead. Next, fold the crushed end to 90 degrees and simulate viewing again. 

 

I predict that your neck will say NO when doing the 45-degree straight overhead simulation.

 

attachicon.gifdiagonal simulation.jpg

Very useful thank you.

 

 



#18 LyraCygnus

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Posted 20 June 2020 - 09:56 AM

 

2  Is there a difference between a diagonal and a star diagonal or are they the same thing?

3  If you have the time (or anyone else) could you mention a couple of the major optical issues that come with a 45°  diagonal?

 

Thanks a lot.
 

 

 

 

-  I use a 90 degree "star diagonal" both for astronomy and for terrestrial viewing, primarily birding.  I have no issues with the comfort when viewing terrestrially. 

 

-  2.  A star diagonal is either a mirror diagonal or a simple prism diagonal that inverts the image so it is correct top to bottom but leaves the left-right reversed, assuming the diagonal is oriented vertically.

 

-  The two optical issues that come with a correct image diagonal, 45 degrees or 90 degrees are:

 

- Limited clear aperture.  Most 1.25 inch Correct image diagonals have about 20mm of clear aperture.  With wider field eyepieces, this will cause significant vignetting, the edge of the field is dim and may be black.  

 

- The complicated light path.  The diagonal splits the light beam in two and they travel separate paths.  Getting them back together is not so easily done.  

 

kisspng-light-amici-roof-prism-amici-pri

 

Binoculars that use roof/amici prisms are "phase coated" because the two light paths end up with some phase shift.  If they are not phase coated, there is a loss of contrast.  I have not seen this done with correct image diagonals but it seems like it ought to be.

 

Jon

Thank you for the detailed info on the optical issues, very useful.



#19 LyraCygnus

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Posted 20 June 2020 - 10:06 AM

I just watched this short and basic video which talked about the difference between a diagonal and a star diagonal. From it I understand that a 45° diagonal shows the image correct side up and correct way round (which like others here have said is good for daytime use) , while a 90° diagonal is a star diagonal which shows the image correct side up but not the correct way round, i.e. the left hand side image will look as though it's on the right hand side and vice versa. 

 

Thanks again to everyone for the replies.

 

https://youtu.be/iXCxnWSMOjc


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#20 Starman1

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Posted 20 June 2020 - 10:26 AM

I just watched this short and basic video which talked about the difference between a diagonal and a star diagonal. From it I understand that a 45° diagonal shows the image correct side up and correct way round (which like others here have said is good for daytime use) , while a 90° diagonal is a star diagonal which shows the image correct side up but not the correct way round, i.e. the left hand side image will look as though it's on the right hand side and vice versa. 

 

Thanks again to everyone for the replies.

 

https://youtu.be/iXCxnWSMOjc

Simple:

 

Star diagonal--any device behind the scope that bends the light sideways to make it easier to view, as on a refractor or catadioptric scope.

It can be a mirror, a prism, a correct image prism, and of 45°, 60°, or 90°.  All are star diagonals if used for viewing stars.

 

Diagonal--a secondary mirror in a newtonian telescope that diverts the light out the side of the scope.


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#21 Jon Isaacs

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Posted 20 June 2020 - 10:26 AM

Just to add to the confusion:

 

There are correct image 90 degree diagonals.  

 

Jon


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#22 LyraCygnus

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Posted 20 June 2020 - 11:25 AM

Simple:

 

Star diagonal--any device behind the scope that bends the light sideways to make it easier to view, as on a refractor or catadioptric scope.

It can be a mirror, a prism, a correct image prism, and of 45°, 60°, or 90°.  All are star diagonals if used for viewing stars.

 

Diagonal--a secondary mirror in a newtonian telescope that diverts the light out the side of the scope.

Yes  it can be a mirror or prism.    I was just initially wondering if there was a difference between the two and if so what it was.  Now I know.



#23 LyraCygnus

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Posted 20 June 2020 - 11:36 AM

Just to add to the confusion:

 

There are correct image 90 degree diagonals.  

 

Jon

Yes right, that's what my original question was about - the difference between the 45 and 90°.  I was going to ask at some point about the difference between a diagonal and star diagonal when someone mentioned a star diagonal in their reply so I thought I'd ask the question here. 

 

I've decided to get the bigger aperture telescope and get a 90° later on.  I just think it'll be more cost effective in the long run if I really get into the hobby instead of having to buy a bigger scope later on and having to sell the smaller one or keep it mostly unused.



#24 Starman1

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Posted 20 June 2020 - 01:30 PM

Yes  it can be a mirror or prism.    I was just initially wondering if there was a difference between the two and if so what it was.  Now I know.

And prisms can be correct image type (45-60-90°) or conventional orientation like mirrors (90° only)


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#25 LyraCygnus

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Posted 20 June 2020 - 05:57 PM

Simple:

 

Star diagonal--any device behind the scope that bends the light sideways to make it easier to view, as on a refractor or catadioptric scope.

It can be a mirror, a prism, a correct image prism, and of 45°, 60°, or 90°.  All are star diagonals if used for viewing stars.

 

 

If you were using it to view planets would it still be called a star diagonal or just a diagonal? 

 

 




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