Posted 28 August 2008 - 08:51 AM
Not really being a 'fractor guy, this is new-ish territory for me. Coupla questions:
1. So Clive, you're saying that a lot of the more affordable ED refractors have intentionally shifted the CA toward the red, which in your opinion is not such a good thing....and that a prism diagonal can help shift the CA back to the "normal" range. Is that more or less correct?
2. "Another optical characteristic of a diagonal prism is that it's naturally overcorrected for spherical aberration." I don't understand this. Please explain in a bit more detail. Thanks!
Posted 28 August 2008 - 09:14 AM
Re. #1, yes that's my take.
I've also heard from another CN member with a TV-76, who reports it has similar color correction, which has been improved by using a diagonal prism. So, it's worth checking the other TV doublets for that, too.
Re. #2, It's my understanding that when converging light passes through a diagonal prism, a small amount of spherical aberration is introduced. I've read about this in other fora. It's a fairly small effect, especially if the objective lens is of long f/ratio.
I'm not an optician, so please don't ask me for the technical details...
Thanks to Allister for posting the article!
Posted 28 August 2008 - 10:07 AM
I have an Astro-Tech 66ED doublet that I am quite pleased with overall. The color correction is very good for doublet, but it's not perfect. I currently use a 2" SCT-style di-electric mirror diagonal, because I enjoy low-power wide-field views of the Milky Way, clusters, occultations, etc. (the strength of this scope, IMO). To this end, I have a 2" widefield eyepiece that gives me 5+ degrees of sky at 15x. I also have some RKE and UO orthoscopic eyepieces that I use for general purpose observing. Since the 2" diagonal threads-on, it's a minor pain to install and remove it just for the sake of using a smaller diagonal. Ideally, I'd like to have a 2" 90-degree prism diagonal - so I can enjoy the benefits of the increased correction for higher power planetary/lunar viewing, and still be able to use my 2" eyepiece for low power views without swapping out diagonals.
So, where can I find a 2" 90-degree prism diagonal? (preferably one that doesn't cost several hundred dollars)
Regards and clear skies,
Posted 28 August 2008 - 11:02 AM
Posted 28 August 2008 - 12:52 PM
Most of us think of the compensating element being part of the objective lens, like the flint glass in an achromatic lens, but there are many other ways to reduce chromatic aberration along the optical train. Schupmann and petzval designs are examples that use medial elements to improve "color correction" in telescopes.
Posted 28 August 2008 - 01:16 PM
Posted 28 August 2008 - 01:53 PM
Posted 28 August 2008 - 02:07 PM
It's really only been in the past few years that ED refractors with this optical formula have risen to great popularity. Just a very nice coincidence that such an inexpensive and readily available accessory can help them achieve even better performance.
If the data points keep accumulating to confirm that diagonal prisms improve image quality in a significant number of ED scopes (and perhaps other designs), this could be an opportunity for vendors to offer more star diagonals of the prismatic type-- especially better quality units.
More like the Baader T-2 model, but perhaps a bit less expensive. If there's competition at that end of the market, pricing should come down.
Finally, it seems to me that the term "right-angle star diagonal prism" is a bit of a mouthful.
Doesn't exactly roll off the tongue...
In the spirit of simplified nomenclature, I'd like to humbly suggest that from now on, such an accessory be called "the Clivagonal" (pronounced Clive-agonal).
Uhhhhhh, no good, huh?
Sorry.... never mind.
Posted 28 August 2008 - 02:37 PM
Probably a good thing I'm outta wedgie range.
Posted 28 August 2008 - 03:50 PM
Early in the review you mention the star diagonal prisms started waning in popularity because they do not work well with fast optical systems. But then you said nothing more about the performance of these diagonals with respect to different focal ratios. What is the fastest focal ratio that prisms can be effectively used with?
Would a star diagonal prism help with CA correction in an achromat, slower optical system?
Posted 28 August 2008 - 04:01 PM
Posted 28 August 2008 - 10:17 PM
how would the prism help one of these?
Posted 28 August 2008 - 11:09 PM
I'm not sure about the prism diagonal, but a violet-reduction filter works wonders with the Orion 100mm f/6. I used to own the Orion 100mm f/6, and I also had a William Optics VR-1 filter - which was a must for planetary and lunar viewing. The benefit was negligible on other targets, like deep sky, but it certainly helped tame the color on brighter targets and at higher powers. FWIW, I tried 2 other violet reduction filters (Orion and Baader FK), and the WO was the best.
Regards and clear skies,
Posted 29 August 2008 - 07:53 AM
An achromat might benefit a bit from a prism diagonal.
I believe Jim Burnett tried one in his long focus 4" achro and liked the result.
An achro (even an f/15 one) has quite a bit of c.a., compared to a good ED doublet (even a rather fast one). So, I doubt a prism will make a huge difference in the view... but for the price of trying one out, I'd say it's worth experimenting.
For a 100mm f/6 achro, well... it has much more c.a. and the subtle assistance a prism provides would get largely overwhelmed by the objective's spurious color.
I know how well a prism works in my F/5.95 110mm scope, so other ED's of smaller aperture in the F/6 range shouldn't present a problem using one.
IIRC, prisms really started to fall out of favor when folks with SCT's began using f/5 telecompressors to make them into "RFT" systems. Then, along came TeleVue with their F/5 (or thereabouts) Petzval refractors in the mid to late 1980's.
Using a prism with an F/5 probably isn't a good thing. Too much spurious color and spherical aberration gets added.
Posted 29 August 2008 - 09:56 AM
Even with a dielectric I think I'm losing something. When playing with a makeshift "artificial star" I noticed that the diffraction rings were cleaner when viewed without the diagonal in place.
Posted 29 August 2008 - 10:26 AM
I think you'll see the best image with the Mak, when you don't use any diagonal. A friend of mine tried 1.25" dielectric and prism diagonals with his 6" SCT recently. He then looked through the scope without a diagonal. Target was Jupiter (thankfully quite low in the sky-- no neck straining required). The view "straight through" was contrastier. Of course, taking the diagonal out of the optical system will result in a somewhat shorter effective focal length if the scope uses a moving primary mirror to focus. The shorter e.f.l will lower the magnification a bit, which naturally results in a brighter, contrastier appearing image.
His opinion about the prism vs. mirror in the SCT:
The mirror (dielectric 99% reflectivity) looked brighter but the prism appeared a little bit contrastier. Best view was "straight thru".
FWIW, Questar Mak-Cassegrain telescopes have always used a built-in star diagonal prism, so that company seems to think they're worthwhile.
Posted 29 August 2008 - 11:06 AM
Posted 29 August 2008 - 11:29 AM
Posted 29 August 2008 - 01:39 PM
I recall back in the '90s when the APQ's were being offered, that a Zeiss prism diagonal was recommended for best image correction. It was that factoid which first got me thinking about prism vs. mirror diagonals.
Posted 29 August 2008 - 02:27 PM
I haven't seen anyone else comment on this, so perhaps it's unique to my prism or high-power eyepiece choice (TV Radians).
Posted 29 August 2008 - 03:06 PM
Yes I remember you mentioning it.
Worth noting for owners of Radian (or similar) oculars.
Posted 29 August 2008 - 04:17 PM
Maybe a simpler design, like an orthoscopic, would solve those ghosting issues......or maybe it was the prism. Do you have a simpler eyepiece with less elements in it to try?
Regards and clear skies,