erect image diagonal

[Luna-tic]

Why is it these aren't recommended for night viewing?. I just ordered a Williams Optics 45 degree correct image diagonal, been wanting one for terrestrial use. Ad said not recommended for night (astronomy?) use.

[Stu]

It's normally because the prisms needed to correct the image can lead to star shapes that are not so optimal, plus you really want the minimum number of reflections/surfaces possible for maximum light transmission.

A mirror diagonal or non erecting Prism should give better results or astro assuming they are good quality.

[Louis D]

The roof prism has a vertex in the center that leads to a spike on bright objects.  Here's an excerpt from Astronomics's image erecting prisms page:

Some people are tempted to use a 45-degree viewing image erecting system for astronomical observing, feeling that a correctly-oriented image that matches the orientation of a star chart would help them locate objects more easily. This use is not recommended, however, as the vertex in the prism puts a small spike of light on every bright point in the image. This is of no consequence in the daylight, as the spike is dim and the image is usually lit brightly enough to wash out the spike. At night, however, when the background is dim and dark, the spike of light is quite visible and reduces the resolution of binary stars, star clusters, and subtle lunar and planetary features. In addition, observing the sky with a porro prism or 45-degree prism leads to uncomfortable and contorted observing postures, as the eyepiece is pointed towards the ground when observing near the zenith.

There are 90 degree image erecting prisms, but it is my understanding that they have the same vertex issue as the 45 degree ones.  I have both and never use them at night, so I can't speak from experience.  I keep them around for spotting scope usage in the daytime.

[Ruud]

The lower parts of the atmosphere are dense and turbulent with heat currents. This deteriorates the image when you look in a horizontal direction, limiting the magnifications you an use. Magnifications higher than about 70x are rarely useful in terrestrial viewing. 

Looking up and out of the atmosphere you look through less air and better air too. Magnifications over 150x are regularly feasible. 

Even a decent rectifying prism deteriorates the image, reducing the maximum useful magnification by a factor two or so, which means little for terrestrial viewing where the atmosphere is the limiting factor. But for astronomy the rectifying prism becomes the limiting factor, greatly reducing the highest useful magnification. The effects are diffraction spikes, loss of contrast and blur.

[MarsG76]

Every time there is a reflective or refractive object in the light path it does interfere and degrade the amount of light reaching the eye, but in practice on a decent diagonal most of the time it's not noticeable.... if having the erect image diagonal is more comfortable for you observe with than I think you would see more in comfort with the diagonal and the very minute degradation rather than if you had to wrap yourself into a twisted pretzel to look at something, like for example at the zenith on a lowish tripod...

 

[Floater]

I use a Baader 2” erecting prism for lunar. I just prefer to see a correct image on the Moon. Quite happy with L/R switch on stars, etc., but I want the Moon to appear in my eyepiece the way it does when I look at it without a scope.

Any degradation is minimal and doesn’t impinge on my observing experience - in fact, the correct image enhances the experience. 

[iPeace]

It doesn't seem to be easy to consistently manufacture correct-image diagonals to the very highest standard.

I have happily oscillated between correct-image and reverse-image (at least to save some setup weight for travel) since then; I do love the two T2 amici prisms that Baader managed to supply in the end.

The preference for correct-image is a personal one, as is the level of tolerance for degradation of the view in comparison with "reverse view". How noticeable said degradation will be may vary depending on quality of the prism and type of observing - perhaps more at higher magnifications.

[Alien 13]

A very interesting topic, I do all my viewing using the DSLR liveview screen where everything is the right way round and the thought of using a standard diagonal gives me the shudders, its just wrong to my brain.

It would be interesting to know if there are any half decent reasonably priced 2 inch 90 degree versions out there? I am only interested in ultra low power observing with nice big exit pupils (double figures).

Alan

[Doc]

I have ordered the APM erect image prism should be here before Christmas fingers crossed.

http://www.apm-telescopes.de/en/optical-accessories/stardiagonal-mirror--prism/apm-2-inch-erect-image-prism-fast-lock-ultra-broadband-coating

Once I use it I'll write a report on it and post it here on SGL.

[iPeace]

2 minutes ago, Alien 13 said:

It would be interesting to know if there are any half decent reasonably priced 2 inch 90 degree versions out there? I am only interested in ultra low power observing with nice big exit pupils (double figures).

@Floater Gordon's 2" Baader is very decent - I say so with confidence, having passed it on to him.

[iPeace]

1 minute ago, Doc said:

Once I use it I'll write a report on it and post it here on SGL.

Looking forward to it.

[iPeace]

@Alien 13 - this is the one Gordon has:

http://www.baader-planetarium.com/en/accessories/optical-accessories/prism-and-mirror-star-diagonals/baader-2"-amici-erecting-prism-90°-with-2"-holder-and-2"-sleeve.html?___SID=U

****

As an aside: the 2" 'Astro Quality' version (not quite so attractively priced) has been out of stock since shortly after the Big Bang:

http://www.baader-planetarium.com/en/accessories/optical-accessories/prism-and-mirror-star-diagonals/baader-2"90°-astro-amici-prism-with-bbhs-r-coating.html

I gave up on that one.

[Doc]

The expensive Baader one must be very hard to manufacture, as there are hardly any around, and none in stock anywhere.

[LukeSkywatcher]

I personally prefer to use an erect image diagonal. I just like to see objects the right way orientated as my eyes would see them. 

[Luna-tic]

I kinda figured it had something to do with prismatic aberrations, and that they'd be more noticeable in dim/dark lighting. As I said, I wanted one for terrestrial viewing (specifically to use my C6 as a spotting scope at the rifle range). Just for kicks and giggles, I'll see how it works at night, too, but don't expect it to perform. I'm quite satisfied to get a high-quality reversed image with the astronomical-intended stuff I have; I chose the Williams erect image diagonal because of the good reviews I read about their telescopes. Down the road a bit (and not too far I hope), I plan to buy a widefield refractor, looking at Williams' GT81 Apo. Any comments?

[timwetherell]

I have the William optics 1.25" prism and it's lovely terrestrially but on bright stars there is a noticeable diffraction line from the joint in the prisms. I had their 2" version briefly but have to say I was very disappointed to find that the clear aperture was only about 35mm so it was essentially useless with 2" eyepieces which begs the question why even make it? I've never used the Baader 2" but they say it's designed for 2" EPs so presumably it has a full 46mm clear aperture

[25585]

I feel this is where on-screen star charts score. Right way up views can be swivelled in both axis. 

My Sarna Deep Sky Atlas shows DSOs with south on top for Newtonians, but I have never heard of any other printed publication giving through-the-eyepiece perspectives. 

[25585]

On 7 December 2017 at 00:06, Stu said:

It's normally because the prisms needed to correct the image can lead to star shapes that are not so optimal, plus you really want the minimum number of reflections/surfaces possible for maximum light transmission.

A mirror diagonal or non erecting Prism should give better results or astro assuming they are good quality.

Yet binoculars, even complicated roof prism types, manage to give reflection & refraction free views. So could an equally good prism not be fitted into a diagonal?

One good advantage for those starting out with hand held bins, & even going up to giants, is the "correct" orientation. 

[Luna-tic]

I received my Williams Optics erect-image diagonal today. Nicely made, but I'll have to wait for the snow to melt before I haul my kit out to test it.

[25585]

Tele Vue make an erect image prism, has anyone got or tried that?

[Louis D]

8 hours ago, 25585 said:

Yet binoculars, even complicated roof prism types, manage to give reflection & refraction free views. So could an equally good prism not be fitted into a diagonal?

One good advantage for those starting out with hand held bins, & even going up to giants, is the "correct" orientation. 

Certainly porro prism types wouldn't have any issues with spikes.  I would imagine roof prism binos would still spike stars.  Since I don't have any, I can't verify this.