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2" diagonal with 1.25" EP question.

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#1 BlueMoon

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Posted 09 August 2020 - 09:50 PM

Howdy folks. Just my curious nature again. I'm considering the purchase of a Correct Image amici prism diagonal. The correct image part is desirable but I want to understand a bit more before investing some serious $$$ in a quality piece.

 

I understand how using a 2" diagonal allows for a wider field of view and better illumination of the field. I'm assuming it's due to the mirror's increased area for reflecting light and this is beneficial for 2" EPs which can utilize it.

 

But how does it benefit 1.25" pieces? I'm trying to grok how a 1.25" mounted in an adapter in a 2" diagonal benefits from a larger mirror. My widest aFoV is 68 degrees with my working set at 62. Again, my assumption is that a 1.25" is only going to allow so much light due to it's decreased barrel and field stop(?) diameters.

 

In other words, if one isn't using 2" EPs, if there any reason to use a 2" prism with an adapter when a 1.25" will do?

 

Cheers and thanks.

 

 



#2 barbarosa

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Posted 09 August 2020 - 10:12 PM

The Astronomy Tools Field of View Calculator does not ask for the specs of the diagonal, only the scope, eyepiece and Barlow or reducer if used.

 

The diagonal would not  be a factor unless it a had stop that caused severe  vignetting. An Amici prism ca introduce artifacts you may notice such as a line down the center. I have a 1.2 Amici prism diagonal from a Celestron C-5 spotter. I never found it useful for astronomy and my recollection is that I could see a line if I consciously looked for it. When I list the C-5 (grudgingly) the Amici and a standard prim diagonal go with it.

 

People disagree but I found no gain from using a 2" mirror diagonal with a 1.25" EP at least out to 68 degrees APOV which was the widest field EP I owned.


Edited by barbarosa, 09 August 2020 - 10:15 PM.

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#3 rkelley8493

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Posted 09 August 2020 - 10:14 PM

You can always use 1.25" eyepieces with 2" accessories as long as you have an adapter, but you can't the other way around without cutting off part of the light cone. 


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#4 BlueMoon

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Posted 09 August 2020 - 10:23 PM

 

People disagree but I found no gain from using a 2" mirror diagonal with a 1.25" EP at least out to 68 degrees APOV which was the widest field EP I owned.

This is my thinking currently. I'm not planning on anything larger than 68 degrees due to vision problems I have using larger aFoV eye pieces. If I pull the trigger on a CI amici I can't see spending the extra $$$ for a 2" if there is no benefit to my 1.25" eye pieces. Thanks for the link as well. :)

 

 

You can always use 1.25" eyepieces with 2" accessories as long as you have an adapter, but you can't the other way around without cutting off part of the light cone.

True that. I don't think I'll ever go back to 2" so this problem shouldn't crop up. :)

 

Cheers.


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#5 jrbarnett

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Posted 09 August 2020 - 11:13 PM

Howdy folks. Just my curious nature again. I'm considering the purchase of a Correct Image amici prism diagonal. The correct image part is desirable but I want to understand a bit more before investing some serious $$$ in a quality piece.

 

I understand how using a 2" diagonal allows for a wider field of view and better illumination of the field. I'm assuming it's due to the mirror's increased area for reflecting light and this is beneficial for 2" EPs which can utilize it.

 

But how does it benefit 1.25" pieces? I'm trying to grok how a 1.25" mounted in an adapter in a 2" diagonal benefits from a larger mirror. My widest aFoV is 68 degrees with my working set at 62. Again, my assumption is that a 1.25" is only going to allow so much light due to it's decreased barrel and field stop(?) diameters.

 

In other words, if one isn't using 2" EPs, if there any reason to use a 2" prism with an adapter when a 1.25" will do?

 

Cheers and thanks.

I honestly don't think there are any quality Amici prism diagonals.  Other than allowing intuitive sweeping at low power on an alt-az mount, I think they're inferior in performance to a standard diagonal.  For one thing with anything bright in the FOV they tend to put up an artifact in the form of vertical spike.  Also Amicis split the light into two paths and tend to be dimmer (lose/scatter some light) than conventional diagonals, and often are not "full aperture" so they vignette with very wide field/low power eyepieces.

 

:)

 

Still sure you want to plunk down your hard earned dough on an Amici?

 

Best,

 

Jim


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#6 Taosmath

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Posted 09 August 2020 - 11:30 PM

The measurements of TFOV  I made using a C6 & a  C8 with either a 1.25" diagonal or a 2" diagonal actually showed a 10% decrease in TFOV of the 2" diagonal when compared to a 1.25" diagonal.  I used a variety of EP's including several short focal length Naglers (82 degree AFOV).

 

I attributed this to a corresponding approx 10% increase in apparent focal length of the system when using a 2" diagonal compared to the 1.25".  This was consistent across all 1.25"  EP's  I used, regardless of manufacturer, model, EP focal length or AFOV.


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#7 BlueMoon

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Posted 10 August 2020 - 12:41 AM

 

Still sure you want to plunk down your hard earned dough on an Amici?

Ah, there's the rub Jim. Frankly, I'm pretty happy with my Stellarvue dielectric pieces but always curious about what else is available. I found Bill's article here very interesting: https://www.cloudyni...omparison-r2877 In particular this sentence from his summary intrigued me: "Quality prisms are prime candidates for the diagonal component in telescopes with focal ratios of f/7 or greater." My 100ED is an f/9. 

 

As with most everything it's all a trade-off. I read what I can find and there are a whole lot of folks here at CN with much more experience than me so I value all your opinions in forming mine. :)

 

Clear Skies!


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