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 Post subject: Star Diagonals
PostPosted: Tue Aug 14, 2012 10:15 pm 
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I have been reading several threads on Cloudy Nights regarding the differences between mirror and prism star diagonals. There were several observers who felt that prism diagonals offer improved images over mirror diagonals in long focus (f/10 and greater) achromatic refractors.

I finally found the prism diagonal I have had for several years and decided to compare the views with that of an Astro Tech dielectric diagonal using a 4" f/10 achromat.

I did this a few days ago when the skies were clear and steady but murky with humidity. Double stars were the primary targets.

The prism diagonal offered the better views. Airy disks and diffraction rings were sharp and there was no light scatter. Splitting close doubles was much easier.

The double - double in Lyra was nicely split at 111x (9mm Ortho). A very interesting double is Zeta Sagittae (8 Sagitta). The primary is a magnitude 5.5 yellow and the 8.7 magnitude secondary is a pale blue dot 8.6 seconds away. Because of the magnitude difference it takes a moment to locate the secondary.

At the next opportunity I hope to do a diagonal comparison with a 3" f/15 achromatic refractor.

Vic Palmieri


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 Post subject: Re: Star Diagonals
PostPosted: Wed Aug 15, 2012 3:34 pm 
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Vic, I would like to see this comparison for myself. Maybe you could bring both to a future star party.
Bill

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 Post subject: Re: Star Diagonals
PostPosted: Wed Aug 15, 2012 6:55 pm 
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Bill:

Sure, I'll have both diagonals at the next star party.

From what I read the prism is better for long focus achromats. F/10 or longer. Scopes with f ratios shorter than f/10 have light cones that are too steep and a prism would induce abberrations. For these scopes mirrors are the better choice. If anyone has a short focus refractor we can try the experiment with that instrument.

The double-double in Lyra would be a good candidate for the test.

Vic Palmieri


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 Post subject: Re: Star Diagonals
PostPosted: Wed Aug 15, 2012 10:01 pm 
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My TV-140 (apo, 5.5" aperture) has a focal ratio of 5. Would that be suitable for your comparisons?

Did you know that 1.25" prism (Amici, direct-image) star diagonals give better resolution than 2" prism star diagonals? Al Nagler pointed this out to me. Is your prism star diagonal 1.25" or 2"?

It seems that Al knew what he was talking about. I once compared a 2" William Optics 45-degree prism star diagonal (borrowed from a friend) with my Celestron 90-degree 1.25" prism star diagonal. This was a terrestial test on the serial number of a dollar bill about a hundred yards away. My $40 Taiwan-import Celestron 1.25" prism star diagonal gave a sharper image using the same telesope (TV-140) and eyepiece (16 mm Brandon).

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 Post subject: Re: Star Diagonals
PostPosted: Thu Aug 16, 2012 10:48 pm 
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Just got in from some pleasant observing. All double stars. The sky was very steady but the transparency was not so good. Perhaps humidity carries skyglow?

I continued my prism vs mirror diagonal tests. The scope was an old Edmund 3" f/15 achromat. All eyepieces were Orthos.

Vega: The scope controls secondary color very well. 12.5mm eyepiece (91mmx): the prism had a tighter image with less scatter.

Double-Double in Lyra (AB sep 2.6"; CD sep 2.3"): Mirror: 12.5mm (91x) AB split, CD elongated; 9mm (127x) AB split, CD split at times; 7mm (163x) AB and CD split.
Prism: 12.5mm (91x) AB and CD split; 9mm (127x) AB and CD split (best image); 7mm (163x) AB and CD split.

8 Sagittae (Zeta) Mag 5.5 and 8.7, sep 8.6": Mirror: 9mm (127x) difficult to see the blue secondary. Prism: 9mm (127x) blue secondary easily seen. Perhaps the secondary was lost in the mirror's scatter but I have read that prisms when used with long focus achromatic refractors shifts the blue part of the spectrum and brightens the image.

The prism diagonal (not an Amici correct image diagonal) had less scatter and sharper images. Maybe its how I see things in my scopes but there was a significant visual improvement.

Looking forward to observing Jupiter next!

Vic Palmieri


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 Post subject: Re: Star Diagonals
PostPosted: Thu Aug 16, 2012 11:01 pm 
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Matthew:

My prism diagonal is one for astronomical use (not a correct image Amici) and is a 1.25" model.

Vic Palmieri


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