Prism Diagonal = Better Color Correction?
Posted 02 July 2008 - 07:45 AM
To achieve the very best correction, those scopes require the use of a prism-type star diagonal.
Some apochromatic refractors work best with a mirror diagonal. Others benefit from the refractive properties of a prism. Rarely does the vendor indicate which works best with what.
I've often used a prism with my apo doublet refractors. The dearly departed Tak FS78 was always coupled to a Tak prism 1.25" diagonal and displayed flawless images.
More recently, I tried my Sky-Watcher ED120 with both mirror and prism diagonals and observed that the mirror-type showed slightly more chromatic aberration in the image. Thus, a '70s vintage Celestron 1.25" glass diagonal (original equipment with C8's of the day) has been used with that scope since the testing was done.
This past weekend, I was trying out a used TeleVue 2.5x Barlow with my Megrez 110ED, to see how it compared with a 2.5x Powermate. The Powermate wasn't showing it's best with the Megrez's f/5.95 focal ratio and FPL-51 doublet optics. Sure enough, the old TV 2.5x Barlow (actually 2.8x as measured) worked better than the Powermate. No stopping down to 101mm (f/6.5) was required to sharpen up
the diffraction image.
Just before deciding to bring the scope in for the night, a thought popped into my head ... why not try the
old Celestron 1.25" diagonal with the Megrez, in place of the William Optics 2" dielectric unit? It would be much lighter than the WO diagonal, but how would using a prism work in such a fast ED refractor??
I suspected it wouldn't be pretty.
The Megrez was aimed at Delta Cygni. Using a 12.5mm Ortho and the TV 2.5x Barlow (147x) showed the companion fairly plainly even in the so-so seeing. Not bad. But, SURPRISE-- switching to the Celestron diagonal revealed Delta's faint companion with slightly less difficulty and it looked like there was reduced red defocus in the primary's image. Markedly less!
The view was so promising, it tempted me to up the power to 183x and swing over to Epsilon Bootes (Izar) to compare the two diagonals once again.
With the dielectric, the main star's yellowy color showed it's typical ruddy excess at best visual focus. Swapping
to the Celestron diagonal reduced the unfocused red quite a bit. There was a real improvement.
It was like the FPL51 doublet had been upgraded to FPL52. How best to quantify this?
I'd say the 110ED at f/5.95 using the prism diagonal, showed better correction than the same scope stopped
down to 101mm (f/6.5) using the WO dielectric diagonal. In fact, the Megrez stopped down further to 94mm f/7
displayed about as much c.a. as at full 110mm aperture with the prism. Very surprising.
The upshot of this improvement is the WO 2" dielectric is now packed away in it's box.
Itâ€™s stylish shape and carbon fiber trim sure is impressive looking on the back of the Megrez and delivers a wonderfully bright image. A great piece of kit.
BUT, the smaller, lighter Celestron unit results in significantly reduced spurious color at best focus.
All I can suggest from this observation, is that if you currently own a relatively fast ED doublet refractor, it's
high magnification performance (ie. above 150x) could benefit from switching to a prism-type star diagonal.
But, you won't find out until you try.
If you have the chance to pick up a good quality right-angle prism diagonal for a reasonable price, it's fun
to experiment and possibly discover for yourself how to optimize your scope's image quality.
Posted 02 July 2008 - 08:09 AM
I would find that I got similar results on my Orion 80ED.
That scope tended to have the red fallout a bit, and to have
some undercorrection. What I seemed to notice was that the
prism added a bit of overcorrection that improved the star
test on the scope. Like you, I found the image to be
sharper with the prism diagonal versus the mirror diagonal.
Since most scopes tend to have a bit of undercorrection,
this does indeed help. Where a refractor is figured with
a bit of overcorrection, and I had one that was this way,
it actually can make things worse. My 90mm fluorite was
better with the mirror diagonal than the prism. What was
a small amount of overcorrection became enough to be
noticed in the final image. While on the 80ED, it helped
both the spherical correction, and improved the red
Posted 02 July 2008 - 10:41 AM
With the growing popularity of relatively fast, inexpensive ED doublets, perhaps trying out a prism diagonal will result in a cheap way to improve optical performance in some of those scopes.
Long live the prism diagonal!
Posted 02 July 2008 - 11:23 AM
Thanks for posting.
Posted 02 July 2008 - 11:40 AM
When I first received my Antares 105mm f/14.4 achromat, I used a 2" Meade Series 5000 dielectric mirror diagonal to test it out. All well and good, but I then swapped in an old 1.25" Vixen prism diagonal to try out the focuser extension tube that shipped with the Antares. On that particular scope I found that the prism diagonal was actually better for higher magnification resolution. On double stars especially companions were more easily spotted using the prism rather than the mirror. I didn't think to check for differences in chromatism though.
Since that swap, the prism diagonal hasn't left the Antares' focuser. Very nice since I hadn't messed with that prism diagonal in a couple of years and it was collecting dust.
Posted 02 July 2008 - 12:37 PM
I wonder how this would compare to a top of the range diagonal?
Im going to have to see if they"re for sale here in UK.
I could use it on the ZS66.
Thankyou, young Clive Gibbons.
Posted 02 July 2008 - 12:49 PM
The most disappointing part of the experiment was felt by Pip (see bottom of picture in original posting).
She thought it was feeding time, but was badly mistaken.
Posted 02 July 2008 - 12:54 PM
I was going to ask what your helper (pip) thought of all that .
Posted 02 July 2008 - 12:55 PM
Based on your report, I'll give it another try. I think the extenders or maybe the laser are the problem. If it's the laser, there's really no collimation issue.
Posted 02 July 2008 - 01:00 PM
Posted 02 July 2008 - 01:05 PM
I strongly suspect that the moderately long f/ratio (f/8.1) of the FS78 combined with it's already excellent correction, would result in very little performance difference between a prism or mirror diagonal. Perhaps nothing noticeable.
If you've got the mirror and prism diagonals handy, tho... it would be fun to see if there's any difference.
Posted 02 July 2008 - 01:10 PM
Posted 02 July 2008 - 06:33 PM
Posted 02 July 2008 - 07:25 PM
They were usually recommended for f/10 and above, the reason being that the refraction COULD cause problems with a steep light cone. For faster refractors, mirrors were usually recommended.
Now of course, mirrors with Dielectric coatings can match the light transmission.
I would personally think that it would be more likely that the mirror would contribute less to spectrum disbursal than a prism diagonal.
I still prefer prism diagonals for small scopes because of the efficency. I see no benefit to a dielectric mirror in a small medium speed telescope. I think that Diagonal manufacturing has been pretty much perfected, but Dielectric coating technology is a bit more difficult. I don't know if cheap Dielectric diagonals are any better than even cheap prisms except maybe in build quality.
In 2", the problem with prism is that it is big and heavy, and harder to get three sides perfectly flat than it is to get one surface on a 2" mirror.. But again, I think that the dielectric coatings are quite tricky to get applied evenly. I trust AP and Televue to get it right, but I don't know if I trust some of the mass produced products. If the surface isn't applied correctly, apparently it can build up around the outside and give a result similar to a turned edge on a mirror...
Haven't tried a prims in a fast refractor.. Maybe I will try on in my TV 101. Nah... TV 101 looks 100% color free to me usint Everbright.. I'm happy...
Posted 03 July 2008 - 12:01 AM
Posted 03 July 2008 - 08:03 AM
Don't want to argue, but I notice that my low cost Vixen 1.25" prism will ADD color to the image in my F7 SV85S APO, but not to my F11 Vixen-spec achro. This is only barely noticable on bright daytime observations. Haven't really looked if the prism improves for spherical undecorrection though. I'm still quite inexperienced, but for some reason I just trust my mirror diagonals more. Maybe I need to try a high end prism?
No problem, Sir.
Yup, a prism will change the overall correction of a system, depending on a few factors. The f/ratio of the objective lens is one. The level and type of color correction the objective lens has, is another. As Tom Davis reports, if the objective is slightly undercorrected for spherical aberration, the natural overcorrection a prism adds can help null it out.
In the case of your SV85S, the lens is as visually well corrected as as ED doublet of it's speed can be, so the refractive properties of the prism won't help. In fact, as you saw, it adds some color.
That's why folks have to test for themselves and assess the results on an individual basis.
Posted 03 July 2008 - 08:51 AM
This posting probably sums it up best.
But the way some ED scopes are corrected these days, the prism can actually reduce color (or shift the type of correction), rather than add c.a..
It seems to be one of those factoids that gets overlooked and then "rediscovered".
There's so many more inexpensive ED doublets on the market these days (which are somewhat undercorrected) perhaps the timing is right for those users to reconsider the prism option.
Posted 05 July 2008 - 09:01 AM
Posted 05 July 2008 - 10:27 AM
Posted 05 July 2008 - 10:49 AM
Posted 05 July 2008 - 03:17 PM
star tests show a significant improvement in color correction.
obviously it was designed for the use of a prism.
header is: Â´with zenit prism almost an apoÂ´