Clive Gibbons
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Reged: 05/26/05
Posts: 13421
Loc: Oort Cloud
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Last night was clear and cool, but most importantly, seeing had improved since the last observing session.
Circumstances allowed another brief test of the Megrez 110ED with a Celestron 1.25" prism diagonal.
Since Epsilon Bootes (Izar) was ideally placed, it was target #1.
At 76x, the first thing noticed was how crisp the star looked. The second thing noticed was Izar's companion snuggled up close to the primary. Woah! That was a surprise... I didn't expect it to pop into view at so low a power. Upping the mag. to 147x made the double even more obvious and began revealing the diffraction image.
It was easy to see that, a) the amount of spurious red had been largely eliminated, b) the image was cleaner and contrastier than I'd ever seen before and c) the bluish color of the fainter companion was now easy to spot.
Seeing was good enough to boost the power some more, but would the optics hold up?
187x looked as sharp. Just more image scale and better separation. The color contrast of the stars was enhanced further. 229x and Izar was still glorious. Finally, 306x was all the power I could throw at the star... and it didn't suffer at all. The limit this night was the seeing.
In my 33 years of fumbling around under the night sky, I can't recall an accessory (the prism diagonal) making such a profound change in a telescope's optical performance.
The Megrez was very good when used with it's mirror diagonal. But switching to a prism has lifted it to a whole new level. Spurious color has been cut in half. Overall image correction is markedly improved. I previously suggested that the prism seems to have upgraded the FPL51 objective to the level of an FPL52 lens. Well, how about an FPL53 doublet?? It's gotta be almost there.
Now the Megrez 110ED is running much closer to the Sky-Watcher ED120.
The brief session was capped off with a peek at Epsilon Lyrae (the Double-Double, Canada's favorite star ).
At 229x, just a perfect image. Four hard, sharp Airy disks, each circled by a faintly visible diffraction ring.
The stars looked "whiter" than before. None of the slight greenish tint previously observed when using the mirror.
Again, I suspect this is due to the improved color correction when using the prism.
In conclusion, I can't say that a prism diagonal will improve the performance of every fast, inexpensive ED doublet on the market, let alone other Megrez 110ED's.
But, it wouldn't surprise me if it'll help more than a few.
For the price of a good quality prism diagonal (even cheaper if ya find a nice used one), you could add $500 worth of performance to your scope.
That could be the deal of a lifetime!
P.s.,
I also hope the manufacturers and vendors of today's inexpensive ED doublets will explore the possible performance advantages of a prism diagonal when used in their scopes. If it can routinely boost image quality so dramatically, perhaps prism diagonals should come as standard equipment?
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A few telescopes of dubious value.
Understanding wife, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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Mr Onions
Mr. Happy
  
Reged: 04/14/07
Posts: 4741
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Clive,that last point is very well made. Perhaps you could make a post of it on the WO site/forum. You seem very happy with this.
Have you tried and daytime terrestrial viewing,Clive?
I only mention this I have a ZS66 and it gets used mainly for low power daytime viewing.
If you havent used it for daytime viewing will you do so now,immediately, thankyou 
Barty Pickles.
-------------------- Barchester Onions.
http://www.flickr.com/photos/httpwwwflickrcomphotos_caladanbrood/
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Clive Gibbons
Mostly Harmless
   
Reged: 05/26/05
Posts: 13421
Loc: Oort Cloud
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Sorry Barty, but I'm burrowed away in a concrete bunker right now (aka, the workplace), so immediate daytime testing is a non-starter.
The info has also been posted to the WO group... but activity there has been pretty quiet lately.
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A few telescopes of dubious value.
Understanding wife, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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rick rian
The Lockster
   
Reged: 08/03/04
Posts: 21167
Loc: Upper Midwest
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Another excellent report Clive, Please keep 'em coming! 
Now I need a prism diagonal ... just to try to keep up!
-------------------- Rickster
NexStar 8i
TV85
Canon 15x50is Bins
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Clive Gibbons
Mostly Harmless
   
Reged: 05/26/05
Posts: 13421
Loc: Oort Cloud
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No problem, Rick.  If a nice minty Baader prism comes my way, I'll let ya know.
--------------------
A few telescopes of dubious value.
Understanding wife, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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Mr Onions
Mr. Happy
  
Reged: 04/14/07
Posts: 4741
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You need more than a prism diagonal,Rian!
-------------------- Barchester Onions.
http://www.flickr.com/photos/httpwwwflickrcomphotos_caladanbrood/
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rick rian
The Lockster
   
Reged: 08/03/04
Posts: 21167
Loc: Upper Midwest
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Post deleted by rick rian
-------------------- Rickster
NexStar 8i
TV85
Canon 15x50is Bins
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Clive Gibbons
Mostly Harmless
   
Reged: 05/26/05
Posts: 13421
Loc: Oort Cloud
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Getting back to prism diagonals for the moment... There's 1.25" units by Meade and Celestron which sell for a bit over $40. Vixen has one for about $70. The Takahashi 1.25" prism is $115. My experience with the Tak model was very favorable.
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A few telescopes of dubious value.
Understanding wife, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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diggy
sage
Reged: 05/09/04
Posts: 305
Loc: Vancouver Island, Canada
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The 1.25" prisms eliminate use of 2" oculars, right? That's a big price to pay!
-------------------- WO 110mm Megrez doublet on an HEQ-5 mount
Nikon 10x50 binocs
"Beyond here be there"
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scopedude
scholastic sledgehammer
Reged: 06/26/06
Posts: 831
Loc: 6 deg. South
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How about 45 degree correct image prism?
-------------------- Megrez 80 FD, Megrez 88, FLT-98, ZS110 APO
Vixen R150S, Vixen NA140SSf
Celestron C8 CF, C5 Spotter
WO 7x50
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roadi
professor emeritus
   
Reged: 08/18/07
Posts: 531
Loc: Denmark
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Quote:
The 1.25" prisms eliminate use of 2" oculars, right? That's a big price to pay!
Thats right, if you just want to use a 1 1/4" prism or you can get a 2" prism from baader!!
Baader 2" Zenit prism: http://www.baader-planetarium.de/zubehoer/okularseitiges_zubeh/prisspiegel.htm#zweizoll
-------------------- Regards Rodi
60mm Polarex, Sky90, µ210, SKW150 Binoscope
Vixen GpDx, Astro5, LX200
Modified Baader/Celestron Binoviewer.
Panoptics, Tak LE's, Baader Ortho's, Antares Erfle's
Couple of diagonals..
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roadi
professor emeritus
   
Reged: 08/18/07
Posts: 531
Loc: Denmark
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I use my prism diagonal with all my scopes, the only one the prism seems to reduce CA on, is the 6" f5 Achro. Nothing gained on my FS 102 other than contrast. Ofcause thats nice too!!
By the way, binowiewers do the same thing, mine does!
-------------------- Regards Rodi
60mm Polarex, Sky90, µ210, SKW150 Binoscope
Vixen GpDx, Astro5, LX200
Modified Baader/Celestron Binoviewer.
Panoptics, Tak LE's, Baader Ortho's, Antares Erfle's
Couple of diagonals..
Edited by roadi (07/04/08 10:28 PM)
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Danno2006
professor emeritus
Reged: 01/25/06
Posts: 661
Loc: Ontario, Canada
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Clive - I noticed the seeing has improved lately in the GWN (Great White North). CSC is on our side the past week or so. Could your perceived improvement be due to steadier, clearer skies?
Danny
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ValeryD
Vendor (Aries)
   
Reged: 11/26/05
Posts: 341
Loc: Kherson, Ukraine.
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The effect of the prism diagonal. It just shifts the red focus closer to the objective. Most simple ED doublets are designed in such way: an optimal color correction shifted towards blue-violet in trade-off of the correction in red - according to a night time vision sensitivity shift. With such correction violet is much less pronounced. This gives us a feeling of better color correction. But this is only feeling. Prism diagonal just return color correction to a better optimum. Nothing magical.
Clive, I think you never saw how a Chromacor and SAFIX do their job in image improvement.
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Thomas A Davis
professor emeritus
Reged: 03/25/05
Posts: 738
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Quote:
The effect of the prism diagonal. It just shifts the red focus closer to the objective. Most simple ED doublets are designed in such way: an optimal color correction shifted towards blue-violet in trade-off of the correction in red - according to a night time vision sensitivity shift. With such correction violet is much less pronounced. This gives us a feeling of better color correction. But this is only feeling. Prism diagonal just return color correction to a better optimum. Nothing magical.
Clive, I think you never saw how a Chromacor and SAFIX do their job in image improvement.
Valery,
Explains exactly what I have seen with some of the ED doublets out there. They tend to not handle the red well so that they can be well corrected in the blue/violet. My Orion 80ED fell out in the red, and worked better with a prism diagonal. My 80mm triplet and 127mm triplet are well corrected in the red, but fall out a bit in the blue/violet. I tried a prism diagonal in my 80mm triplet, and the color correction got noticeably worse on a white fencepost with the prism. Just a bare trace of violet at 160X with a mirror diagonal, that went to clearly evident blue/violet with the prism and same eyepiece. No magic, just a shift back to a standard correction on the ED scopes that seem so well corrected in the blue/violet. Too many fail to understand the effect of poorer red correction on the view of certain double stars (such as Izar) and the planets. Shifting the color correction does seem to sell scopes though.
As to the Chromacor, yes, definite, real improvement. I still remember the experience I had testing you first prototype on my 120mm and 150mm achromats. The improvement in planetary detail was immediately evident. The Chromacor provided a real improvement in color correction, I can vouch for that from experience.
Tom
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ValeryD
Vendor (Aries)
   
Reged: 11/26/05
Posts: 341
Loc: Kherson, Ukraine.
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Tom,
It is necessary to add, that for planets red correction is more important, than violet correction. Especially for Mars.
For deepsky astrophoto this is important too. Deep violet can be attenuated by MV filter and then color balance can be restored in Photoshop. If the red is poorly controlled it is much more difficult to fight against this disadvantage.
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Houdini
professor emeritus
Reged: 07/13/07
Posts: 523
Loc: Europe
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As Rodi mentioned, bino viewers do the same thing. I had already noticed an obvious improvement with a binoviewer on a Megrez 110, now I understand why.
Robert
-------------------- 16" f/4.9 motorized alt-az, 25" f/5 Dobson, 43" f/4 alt-az under construction
Mirror Edge Support Calculator
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Clive Gibbons
Mostly Harmless
   
Reged: 05/26/05
Posts: 13421
Loc: Oort Cloud
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As Valery and Tom Davis discuss above, there's certainly nothing magical about the improvement seen using a prism diagonal.
Since there's so many ED doublets being produced these days, which employ color correction that more benefits imaging rather than visual usage, I'd recommend that owners of such instruments try a prism diagonal and see for themselves.
Those scopes likely include (but aren't limited to) any 66mm ED doublet, 80mm ED doublet, 102mm (f/6.95 or f/7) ED doublet, 110mm f/5.95 ED doublet and the new 110mm f/7 ED doublet. The new doublets from William Optics (72mm and 90mm) would be candidates, too. Recent Vixen ED doublets.
Sky-Watcher ED doublets. So, as you can see, there's many many refractors which could see improvement.
A few more nights of testing with my Megrez 110ED confirms the initial impressions. The prism diagonal has reduced c.a. and s.a. at the focal plane. It also seems to have shifted the type of color correction from photographic to a more visual-friendly kind.
Seeing has varied over the past week. From poor to fairly good. I'm certain the improvement I've noticed has nothing to do with seeing conditions.
WRT the disadvantage of using only 1.25" oculars in the prism diagonal, I don't find it an issue, since I rarely (less than 1% of the time) use any 2" accessories. However, at times when the lowest power, widest field is wanted, I'd have a 2" mirror diagonal handy for those moments.
A 45degree amici-type prism diagonal has other optical issues which prevent it from being a good choice for high magnification observing. I'd keep that type for daytime and low power astro use.
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Clive Gibbons
Mostly Harmless
   
Reged: 05/26/05
Posts: 13421
Loc: Oort Cloud
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Here's a fast 'n' nasty digital pic of the crescent Moon taken last night.
Megrez 110ED, prism diagonal, 2.5x TV Barlow, 24mm Ortho ocular, Samsung D760 point 'n' shoot camera. Afocal method.
You can see some color in the image, but it's mainly lateral color caused by the eyepiece/Barlow/camera lens combo and some slight camera-telescope misalignment (the camera wasn't directly coupled to the scope).
Seeing was rather poor (Moon quite low to the horizon).
What this image shows more than anything is the very natural color balance the scope now shows.
--------------------
A few telescopes of dubious value.
Understanding wife, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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rick rian
The Lockster
   
Reged: 08/03/04
Posts: 21167
Loc: Upper Midwest
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Excellent shot there Mr. Gibbons ... shows you have a steady hand and a very good camera! 
Thanks for posting.
-------------------- Rickster
NexStar 8i
TV85
Canon 15x50is Bins
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