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Correct-image diagonals?

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#1 steve-in-kville

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Posted 19 February 2013 - 05:05 PM

Not sure if this belongs here or in the EP forum, but here goes: I would like to use my scope as a spotting scope on occasion. Does anyone have any experience using a correct-image diagonal? Are there certain makes/models I should be looking at?

Thanks.

#2 Jon Isaacs

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Posted 19 February 2013 - 08:06 PM

Not sure if this belongs here or in the EP forum, but here goes: I would like to use my scope as a spotting scope on occasion. Does anyone have any experience using a correct-image diagonal? Are there certain makes/models I should be looking at?

Thanks.


There are, I have a couple but I use a star diagonal. Sharper image and the mind soon adjusts to the left-right reversal....

jon

#3 Mark9473

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Posted 20 February 2013 - 07:16 AM

The (only) one that's worthwhile to have, because it is good even if you use high magnifications, is the Baader T2 90° amici prism.

#4 Jon Isaacs

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Posted 20 February 2013 - 07:40 AM

The (only) one that's worthwhile to have, because it is good even if you use high magnifications, is the Baader T2 90° amici prism.


And it has a limited clear aperture...

Jon

#5 Darren Drake

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Posted 20 February 2013 - 08:35 AM

I use a 2 inch version on my ST120 to match the star maps and computer displays for finding some of the dimmer objects. Works nicely but when I want to use the scope for higher reso stuf I switch to a regular mirror diagonal.

#6 Jon Isaacs

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Posted 20 February 2013 - 10:29 AM

I use a 2 inch version on my ST120 to match the star maps and computer displays for finding some of the dimmer objects. Works nicely but when I want to use the scope for higher reso stuf I switch to a regular mirror diagonal.


Darren:

Which RACI, which scope?

Jon

#7 Darren Drake

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Posted 20 February 2013 - 10:37 AM

I believe its a baader 2 inch version and I use it on an Orion ST120. Makes an awesome finder....

#8 Jon Isaacs

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Posted 20 February 2013 - 10:41 AM

I believe its a baader 2 inch version and I use it on an Orion ST120. Makes an awesome finder....


I use a handheld device for my star charts and a star diagonal in a refractor.. If I want to match the starfield, I just reverse the chart left to right on the handheld.

Jon

#9 Darren Drake

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Posted 20 February 2013 - 11:43 AM

I used to use the Sky program from my pocket computer and it did not have a mirror reverse function which was a pain and the reason I had to seek out a correct image 2 inch digonal. Now I have skysafari pro which can do a mirror flip but I save time by never needing to go back and forth.

#10 core

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Posted 20 February 2013 - 12:36 PM

I would like to use my scope as a spotting scope on occasion. Does anyone have any experience using a correct-image diagonal?


Televue 1.25" 60° diagonal, dielectric mirror, so it's L-R reversed. Occasional use on either my AT66 or C90, and compared with a Televue 45° amici (which looks to be re-badged generic unit), there is some difference during daytime. Planetary/Lunar use, the 60° wins hands down.

#11 timwetherell

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Posted 20 February 2013 - 08:27 PM

I have William Optics 1.25" and 2" erecting prisms. Both work superbly at low magnifications and both go a bit soft at very high magnifications compared to a mirror. The 2" one is a bit pointless really since the clear aperture is only about 35mm (which they do state in the specs)so it vignettes with pretty much any 2" EP. The 1.25" is a great product for converting a refractor into a terrestrial telescope though

#12 Tamiji Homma

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Posted 20 February 2013 - 09:11 PM

Hi Tim,

I have this William Optics 2" Erecting Prism Diagonal.
http://www.williamop...n90deg_spec.php

The spec says clear aperture is 40mm. I haven't measured actual clear aperture myself but I remembered Nagler 31T5 wasn't vignetting that bad.

So it must be bigger than 35mm :)

Tammy

#13 TONGKW

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Posted 20 February 2013 - 09:52 PM

I have a few correct image diagonals for Lunar observations and bird watching as shown hereunder.
From left is a William Optics 90-degree 2” giving quite a good view, though falls a bit behind a mirror diagonal.
The second, third and the last one are 45-degree and good for bird watching at low power up to 100x.
The fourth is a Orion 1.25” for low power observations up to 100x.

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#14 timwetherell

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Posted 21 February 2013 - 06:45 AM

yeah you may be right Tammy, I'm guessing 35mm from memory - may indeed be 40 :)

#15 Lane

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Posted 21 February 2013 - 07:42 PM

Not sure if this belongs here or in the EP forum, but here goes: I would like to use my scope as a spotting scope on occasion. Does anyone have any experience using a correct-image diagonal? Are there certain makes/models I should be looking at?

Thanks.


Ok for daytime use, but I would not use one at night. I have one of the better 2" ones made but it was discontinued. Comparing it to a regular diagonal at night it is easy to see that it does not measure up.

Update - I was wrong, only the Orion rebranded version was dropped the Williams Optics version is still around.

#16 JerryOrr

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Posted 09 March 2013 - 10:06 PM

I have one from Orion. I use it mostly for Lunar and planetary observing, when the seeing is not at its best. I simply prefer the correct-image for aesthetic reasons. But when the seeing is excellent, and there is a good chance to see some fine detail, I switch to the standard reverse-image diagonal. Remember, everytime the light is reflected off a mirror, there is a loss of light and resolution.

#17 Jim Romanski

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Posted 11 March 2013 - 01:20 PM

Jon no matter how you adapt your finder charts/program to the equipment there's nothing like a fully corrected view. When you move the scope it moves the way you expect it to. When you look up from the eyepiece everything in the real sky is oriented the way you expect it. Very important for finders and great if you can have it in the main scope as well.

My Father-In-Law who was my astronomy mentor was a big believer in corrected views. He built a correct view low power setup for his C8 using one of those old surplus tank sight telescopes. They have massive prisms and large erfle eyepieces. He built a setup to attach to the C8 and used it all the time. It was so nice doing a Messier Marathon with his scope and this setup.

Sure there are ways around it but I for one would welcome and pay good money for a quality amici prism that was large enough to use on my scopes that use a diagonal. I think lots of people would if they had the opportunity.

#18 Jon Isaacs

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Posted 11 March 2013 - 06:42 PM

Jon no matter how you adapt your finder charts/program to the equipment there's nothing like a fully corrected view. When you move the scope it moves the way you expect it to. When you look up from the eyepiece everything in the real sky is oriented the way you expect it. Very important for finders and great if you can have it in the main scope as well.



Jim:

I find it somewhat disconcerting when someone seems to be telling me how I experience the world. The fact is that I have no difficulties with properly moving a refractor with a left-right reversed image or a Newtonian with a straight through finder and totally reversed images both in the scope and in the finder.

But put a correct image diagonal on the refractor or a RACI finder on a Newtonian and my instincts are just wrong and its confusing. The human mind is a power processor and quickly adapts. Here's a for bit for you...

The image from a Newtonian it correctly oriented at the retina. It appears reversed because you eye lens inverts the image. Normally an image is upside down and backwards but your brain knows this and so automatically reinverts it...

Jon

#19 Jim Romanski

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Posted 11 March 2013 - 10:04 PM

Sorry Jon. I didn't mean to imply that there's anything wrong with the way you look at the world through a telescope or otherwise.

My point is that when I show a newbie or someone that doesn't observe much the view through a corrected diagonal vs. an inverted image they prefer the corrected one. And it's much easier to show them how to navigate the sky as well. I use Newtonians and refractors both straight through and left/right reversed. I've become used to the image and can correct in my head. But I still find it much simpler to use a corrected image in both finder and scope.

#20 Ziggy943

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Posted 11 March 2013 - 11:58 PM

I also prefer a corrected image on the finder as well as the main scope. I have always used an amici prism on every refractor I have and have had. For me visualizing the actual mechanics of Jupiter and it's moons are much easier to see when the view matches reality. I have used other finders but find them not as intuitive as corrected vision. Jon's MMV

A corrected view in the main scope is more important to me than in the finder.


#21 Jon Isaacs

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Posted 12 March 2013 - 01:00 AM

A corrected view in the main scope is more important to me than in the finder.



One difficulty with correct image prisms is that they degrade the image at high magnifications and most do not have the clear aperture necessary to provide the sorts of widefield views that refractors excel at.

Correct images, they aren't going to happen with a Newtonian.... faint, faint fuzzies, they are not going to happen with a refractor..

Jon

#22 Jon Isaacs

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Posted 12 March 2013 - 01:18 AM

Sorry Jon. I didn't mean to imply that there's anything wrong with the way you look at the world through a telescope or otherwise.


Jim:

It's not the implication that I was doing something wrong. I didn't see it that way. Rather it's simply the statements these below imply something me and my expectations:

"Jon no matter how you adapt your finder charts/program to the equipment there's nothing like a fully corrected view. When you move the scope it moves the way you expect it to."

The simple fact is that when I move a scope with a star diagonal, it moves the way I expect it to, when I move a scope with a correct image diagonal, it does not move the way I expect it to.

The reason this is disconcerting is that implies that what you experience is what I experience and by extension what others experience....

In my experience some do, some don't. My own experience suggests the vast majority of observers quickly adapt to the left-right inverted views as well a left-right-top bottom inverted views.

I do think there is a place for correct image diagonals but I find the downsides are significant in terms of the overall quality of the view and adjusting to a star diagonal is for most, an relatively easy process.. I bird watch with star diagonals, track flying birds with star diagonals...

Jon

#23 Ziggy943

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Posted 12 March 2013 - 11:37 AM


A corrected view in the main scope is more important to me than in the finder.



One difficulty with correct image prisms is that they degrade the image at high magnifications and most do not have the clear aperture necessary to provide the sorts of widefield views that refractors excel at.

Correct images, they aren't going to happen with a Newtonian.... faint, faint fuzzies, they are not going to happen with a refractor..

Jon


I have not found the image to degrade even with magnification in excess of 800x with the 9" Clark. With the 160mm TEC I have compared the Baader to the AP 2" diagonal on double stars and Saturn and can't say that one is better optically than the other. If anything the tilt would be slightly in favor of the Baader Amici prism. It also has the advantage of showing the Moon the way it is in the sky. That is a big plus for public star parties.

#24 mark8888

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Posted 12 March 2013 - 01:17 PM

The (only) one that's worthwhile to have, because it is good even if you use high magnifications, is the Baader T2 90° amici prism.

And it has a limited clear aperture...

I have not found the image to degrade even with magnification in excess of 800x with the 9" Clark. With the 160mm TEC I have compared the Baader to the AP 2" diagonal on double stars and Saturn and can't say that one is better optically than the other. If anything the tilt would be slightly in favor of the Baader Amici prism.




OK, sooo........does the Baader Amici prism actually have less clear aperture than, say, the Baader T2 Prism Star Diagonal (the one that used to be supplied with the Mark V binoviewer). Looking at them, they look the same but the Amici is significantly bigger, FWIW. Here they are, with the Amici on the right.


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#25 BluewaterObserva

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Posted 12 March 2013 - 05:27 PM

I use reallhy inexpensive 1.25" models with good results. I really only use uit for finderscopes.. Mainly st120 OTA's thee days. OPT puts these on sale for rediculously low prices every now and then. I usually buy several when they do. I find lots of uses for them.






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