Dielectric vs Prism diagonal in refractors

[DavidU]

I have been doing some comparisons with my 150mm x 1200mm refractor regarding light scatter and colour
correction of various diagonals.
What I have found in some older Zeiss documents mention that to get maximum performance from their
refractors they suggest a 90deg prism diagonal.
This got me thinking so a few trials were in order.
A 1.25 older Al coated diagonal, a cheaper 1.25 BK7 diag and a newer 2" dielectric diagonal.A wide angle 25mm
and a UWA 6.7mm eye piece was used.
On Jupiter the Al 1.25 appears clean and bright with quite an extended violet halo. Next the
2" dialectric showed a very slightly brighter image with a slightly extended violet halo however
showing some light scatter that reduced visual contrast.The BK7 prism diagonal provided quite a bit
more control of the violet light and gave a noticeable increase in contrast.Also noticeable was
a greater violet control on the moon giving a much sharper limb.Further
A noticeable increase in contrast and detail on several DSO's in particular various gob clusters.
A prism diagonal (which adds overcorrection, depending on the f/ratio of the objective lens--
the faster the lens the more overcorrection results) can help null the optical system.
Slightly under corrected objective lenses are fairly common in the marketplace.
While my Achromat is no APO it may(in some instances)try a BK7 Diagonal
and see what you find.I would suggest that a BK4 diagonal may not work as well for colour control due to it's denser properties and different refractive index.
My $49 spent on a BK7 diagonal was well worth it.:D

[anj026]

Thank you for your post David and sharing your findings. I've not tried a prism myself but have read of others having similar experience. One review on an 8" f12 achromat (Cloudy Nights) mentioned that a binoviewer helped reduce chromatic aberation and that was put down to the prisms within the binoviewer.

[GeoffW1]

Hi,

Thanks also for this post. I need some information on this, to slot into plans for the next telescope.

It is like being on ice skates, I cannot stop.

Cheers

[casstony]

I imagine Wolfgang has had a lot of people retrieving their glaswegs from the bottom drawer :) Seems like BK7 prisms are the best choice for anything less than top of the line refractors.

[DavidU]

I looked for a Wolfgang test on prism diagonals.... I will have to look harder.... link?

[casstony]

Sorry David, I thought you were aware of the site:

http://translate.google.com/translate?hl=en&sl=de&u=http://www.astro-foren.de/&sa=X&oi=translate&resnum=1&ct=result&prev=/search%3Fq%3Dhttp://www.astro-foren.de%26hl%3Den%26client%3Dsafar i%26rls%3Den%26sa%3DG
Go to: Forum - testing of optics - optical directory reports(top of page)

In the image below, the top star test is without a prism - note the greater red fringe inside focus. The bottom star test is with a prism and the colors are brought closer together:

http://rohr.aiax.de/CelED80JK-02.jpg

In the next image, note the change in strehl ratios for the different colors - this test is done without changing focus. When focus is changed to suit each color the strhl ratio is high for all colors for the ed80:

http://rohr.aiax.de/CelED80JK-04.jpg

Note: the above is my interpretation of the article - I'm no expert.

[Kal]

If a lens is undercorrected, then some prisms will shift the colour error slightly to bring red more into focus while at the same time making blue more out of focus.



In the days of film, some refractors were made to be better corrected at the wavelengths that film was most sensitive to, which was blue. Perhaps the reference to using a prism in the Zeiss documents was to balance out this undercorrection.

[Visionary]

Based on this advice and advice generally I purchased a Baader' 90 degree 2" prism! It's going to take a while to get here but the documentary evidence suggests the $$$ will be well rewarded. Cheers

[GreyWolf]

G'day David,
Have you received yours yet from Baader (via local retail)?
Andy

[Peter Ward]

I've been down this path many years ago....

Many diagonals were tested (prisms, silvered, dielectric)

To cut a long story short... the Astro-Physics 2" dielectric.... while many fell by the wayside......at about 1500x magnification (Sirius+ Barlows+ 4mm eyepiece.) could not be faulted....we simply were running out of light.

It's not cheap.

But it is effectively THE perfect diagonal.

[Wavytone]

Prism diagonals are like Plossl eyepieces - very good in their day but there are better products now. With modern refractors around the f/7 mark I'd stick with a dielectric mirror diagonal.

Please note the only reason prism diagonals were popular many decades ago was because the average first surface mirror lost anything from 10%-40% of the light, depending on the state of the coating, and deteriorated with time. In comparison, the transmission of a clean prism with MgF coating was above 95%, and prisms don't deteriorate with time.

These days durable dielectric mirrors are available at modest cost with reflectivity at 99% or more, making prisms pointless.

There's another reason to not use a prism, too. Inserting a prism diagonal effectively inserts a thick slab of glass into the beam converging towards the focal plane. Because the beam is converging and the glass surfaces are flat, the result is that a prism adds a small amount of spherical aberration and chromatic correction to that of the refractor. The effect also depends on the focal ratio of the refractor.

In a refractor at f/9 or longer you won't notice. But at f/5 you definitely will notice. With an f/7 refractor the effect will also be noticeable at high power.

FWIW A few objectives are designed to be used with a prism, the vast majority are not.

In particular binocular objectives (typically f/5) are designed to match the contributions from the erecting prisms; trying to use them as refractors (sans prisms) is works perfectly well at low power, but at high power you'll realise they are under corrected for spherical and chromatic corrections.

Happy experimenting.