rustynailz911
Pooh-Bah
Reged: 01/30/08
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Re: Prism Diagonal
[Re: Clive Gibbons]
#2626362 - 09/05/08 09:33 PM
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i have budget equipment and have a 45deg Meade diagonal with a prism. i had some serious ghosting as well i was thinking it was the MA eyepieces i removed the diagonal and went just eyepiece perfect focus and no ghosting so it was the diagonal.i just ordered a 90 deg mirror diagonal.im not saying that prisms are bad they just didnt work for me it might work awsome in another scope .plus i really disliked the 45 deg angle it is uncomfortable on my scope for viewing.
Rusty
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Loc: Oort Cloud
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Re: Prism Diagonal
[Re: rustynailz911]
#2630375 - 09/08/08 08:49 AM
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Just to reiterate...
a 45-degree erecting prism diagonal is an entirely different animal, compared to a 90-degree star diagonal prism. If you had a poor experience with an erecting prism (either 45 degree or 90 degree type), I'd strongly recommend that you try the simpler, traditional star diagonal prism. Much less chance of introducing stray reflections and other optical errors with that type.
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Bradley B
sage
Reged: 05/14/07
Loc: Sacramento, California
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Re: Prism Diagonal
[Re: Clive Gibbons]
#2630547 - 09/08/08 10:57 AM
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That old Burgess 2" Diagonal was of the "correct image" variety, so no wonder there may be issues of collimation . . . however 8mm is extreme.
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Loc: Oort Cloud
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Re: Prism Diagonal
[Re: asaint]
#2669604 - 09/29/08 09:34 AM
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Further testing with ED doublets has shown that star diagonal prisms using BK7 glass typically have a greater benefit with regards to improving color correction in many ED doublet refractors.
It's usually the less expensive star diagonal prisms which use BK7 glass.
More expensive prisms, such as the Baader T-2 model (Zeiss) and the Takahashi 1.25" star diagonal, use a higher index glass (BAK-4, or analogous). This results in more complete "total internal reflection" when used in a very fast optical system (f/5 or faster). However, since most refractors aren't of such a short f/ratio, it's not necessary to use a high-index prism. BK7 glass will work just fine.
But, MOST IMPORTANTLY, the high index glass in more expensive prisms doesn't correct the prevalent color error (red defocus) which so many ED doublets display.
In conclusion, it would appear that the optimal type of prism for use in many of today's ED doublet refractors, is one using BK7 glass.
Inexpensive 1.25" star diagonal prisms by Celestron, Meade and Orion all use BK7 glass.
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Loc: Oort Cloud
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Re: Prism Diagonal
[Re: Clive Gibbons]
#3990179 - 08/18/10 09:09 AM
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Addendum--
Further research and testing has shown that prism diagonals can also improve the image correction of some ED triplet lenses.
ED triplets typically suffer from significantly less c.a. and spherochromatism than ED doublets. Many ED triplets are optimally corrected to use a mirror-type star diagonal, but I've seen cases of triplets benefiting from a prism diagonal.
Most recent observations with a Stellarvue 115T (latest version) triplet have revealed that for high magnification views of bright objects, a Takahashi 1.25" diagonal prism results in perfectly apochromatic performance. A dielectric mirror diagonal used in the scope revealed very slight red defocus.
So, whether you have an ED doublet or a triplet, it's useful to experiment to see what diagonal performs best for high power work.
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EastAnglian
professor emeritus
Reged: 11/22/10
Loc: Cambridgeshire Fens UK
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Re: Prism Diagonal
[Re: Clive Gibbons]
#5040895 - 01/27/12 12:49 PM
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As I understand it the very best image is obtained from not using any form of diagonal, or zenith prism? They are supplied with a lot of (all?) lower end beginners scopes, for no reason other than to make more money when this 'beginner' decides to upgrade his equipment, and having gotten used to unecessarily using one, he buys another. I AM one of these poor unfortunate people who've been conned into using one, and am making a big effort to rid myself of the habit.
Cheers
Max
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Mark9473
Postmaster
Reged: 07/21/05
Loc: 51°N 4°E
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Re: Prism Diagonal
[Re: EastAnglian]
#5041124 - 01/27/12 03:06 PM
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I can think of three scenarios why you would write that: (1) you observe only things relatively close to the horizon, (2) you're rather young and an accomplished acrobat, or (3) you have a fantastic home-built mount contraption that positions your telescope just right over a reclining chair.
I hope it's 3 and you'll post a picture of the mount!
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astrogeezer41
super member
Reged: 08/27/10
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Re: Prism Diagonal
[Re: Clive Gibbons]
#5041793 - 01/27/12 10:29 PM
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I put some of this on the Refractor Forum a few days ago as I was referring skyjim to this thread.
I was first told about this thread on prism diagonals on another forum when someone had asked about a red halo on an ED scope. I went to this thread and read about many of the EDs being corrected towards the red end of the spectrum.
Since I believed that my 80mm,f/7 achromat was also corrected a bit to the red side I replaced my mirror diagonal with a Tak prism diagonal, and tested it on Jupiter at 150x.
The difference was amazing. Instead of doing the tweaking that I had to do with the mirror diagonal I simply had to start outside of focus with the Tak until the image was sharp. When the seeing was steady the image of Jupiter was virtually white in the zones. Since then I have always used the prism diagonal with this scope.
I also have a less expensive Orion prism diagonal which also
works well with that scope.
I am grateful for the work that Mr. Gibbons has done on this issue and to the people on the other forum who provided the link.
So thanks everyone,
Robert
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EastAnglian
professor emeritus
Reged: 11/22/10
Loc: Cambridgeshire Fens UK
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Re: Prism Diagonal
[Re: Mark9473]
#5043797 - 01/29/12 08:40 AM
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Quote:
I can think of three scenarios why you would write that: (1) you observe only things relatively close to the horizon, (2) you're rather young and an accomplished acrobat, or (3) you have a fantastic home-built mount contraption that positions your telescope just right over a reclining chair.
I hope it's 3 and you'll post a picture of the mount!
It's 3; almost! I intend to start work this week on a mount similar to that made by Grendel, here:
http://www.cloudynights.com/photopost/showphoto.php?photo=25003&size=big&password=&sort=7&thecat=3257
Then, by sitting on an astronomer's stool with the eyepiece under my eye, all that's required is to raise the stool to observe objects at a lower altitude. I'll post a pic when I'm finished.
Also see this from The Amateur Astronomer's Handbook by James Muirden:
http://f1.grp.yahoofs.com/v1/0EIlT8x_ujv05WRAvn6kzpgTPg4Rj27stXiyomWzpxSXc4SCjVUurZZkI9tpeyazzuj-GYmCiX1kFTLlJipyeC2GH2j5t4pNvcA/cut.jpg
Cheers
Max
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grendel
Pooh-Bah
Reged: 04/12/09
Loc: Canterbury, Kent, UK
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Re: Prism Diagonal
[Re: EastAnglian]
#5044353 - 01/29/12 02:47 PM
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Max I have to say that with that pier and a 1250mm f/l scope near the horizon I am standing my 6'4" on the step next to the pier, and when looking at the zenith 'straight through' I am sitting on the step with the eyepiece just a few inches from the pier and quite low really. but the mount is really super steady on that 1/2" thick 6" diameter cardboard pier - that weighs in at about 20lbs.
Grendel
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grendel
Pooh-Bah
Reged: 04/12/09
Loc: Canterbury, Kent, UK
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Re: Prism Diagonal
[Re: grendel]
#5044361 - 01/29/12 02:50 PM
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oh and when I use a diagonal its always a prism one - I even have a .965" / 1.25" hybrid prism diagonal.
Grendel
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EastAnglian
professor emeritus
Reged: 11/22/10
Loc: Cambridgeshire Fens UK
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Re: Prism Diagonal
[Re: grendel]
#5044856 - 01/29/12 07:43 PM
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Quote:
Max I have to say that with that pier and a 1250mm f/l scope near the horizon I am standing my 6'4" on the step next to the pier, and when looking at the zenith 'straight through' I am sitting on the step with the eyepiece just a few inches from the pier and quite low really. but the mount is really super steady on that 1/2" thick 6" diameter cardboard pier - that weighs in at about 20lbs.
Grendel
Hi Grendel!
From my home position I can't observe anything near horizontal, and due to atmospheric conditions, don't do anything serious below thirty. Using that info as a starting position, actually diminishes the up/down eye movement considerably. I like to sit when I observe, preferably almost recline, so a comfortable astonomers stool with notched risers in fairly small increments, within safety limits should suit me fine. I hear what you're saying about closeness to pier, and will either make a simple alt/az balanced mount with large altitude projection, or just simply not observe at zenith. On that note, re your other post, although I'm going to give it a go in not using a diagonal, I have to say that I've never seen a hybrid .965/ 1 1/4" PRISM diagonal; where did you get it?
Cheers
Max
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grendel
Pooh-Bah
Reged: 04/12/09
Loc: Canterbury, Kent, UK
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Re: Prism Diagonal
[Re: EastAnglian]
#5045386 - 01/30/12 04:43 AM
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I picked both mine up from telescope house - in their box of odds and ends, one is a 45 degree hybrid diagonal, the other a standard diagonal. not even sure what brand they are. maybe I just got lucky or they took a standard .965 diagonal and happened to have a 1.25 eyepiece holder that fit the same thread.or a standard 1.25 and had a .965 nose that fit the thread - never really thought about it much, it was just available when I went in and asked - just £10 each from the bits box too.
Grendel
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EastAnglian
professor emeritus
Reged: 11/22/10
Loc: Cambridgeshire Fens UK
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Re: Prism Diagonal
[Re: grendel]
#5045515 - 01/30/12 08:57 AM
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Very nice too! I have a couple of standard plastic bodied mirror type, but haven't seen a prism in this configuration.
Cheers
Max
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Sarkikos
Postmaster
Reged: 12/18/07
Loc: Suburban Maryland, USA
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Re: Prism Diagonal
[Re: asaint]
#5045631 - 01/30/12 10:36 AM
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FWIW, a couple years ago I experimented with five different prism and mirror diagonals on my ST80 (80mm f/5 achromat). The diagonals included 1.25" and 2" mirror diagonals, dielectric and otherwise. I used several 1.25" prism diagonals, correct image and reversed image, BAK4 and BK7. I did not see any improvement in CA correction using any of the diagonals.
Recently I acquired a 70mm f/12.9 achromat. I might try the experiment again using all the 1.25" diagonals. Maybe a prism diagonal will improve the slight CA in this scope?
Mike
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EastAnglian
professor emeritus
Reged: 11/22/10
Loc: Cambridgeshire Fens UK
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Re: Prism Diagonal
[Re: Sarkikos]
#5045677 - 01/30/12 11:01 AM
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Hi Mike,
That's a very interesting experiment you did there. I for one would be very interested to hear the result, if you do something similar with the new scope. It's not often that anybody has such a comprehensive set of diagonals to do such a test.
Cheers
Max
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grendel
Pooh-Bah
Reged: 04/12/09
Loc: Canterbury, Kent, UK
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Re: Prism Diagonal
[Re: EastAnglian]
#5046290 - 01/30/12 04:33 PM Attachment (37 downloads)
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I just dug my two hybrid prism diagonals out and took some pics, the 45 degree one is a meade and has what is obviously an adaptor thread for the 1.25" eyepiece holder.
the other looks like it might have the same, but I couldnt unscrew it.
Grendel
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grendel
Pooh-Bah
Reged: 04/12/09
Loc: Canterbury, Kent, UK
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Re: Prism Diagonal
[Re: grendel]
#5046291 - 01/30/12 04:34 PM Attachment (31 downloads)
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and the two.
Grendel
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grendel
Pooh-Bah
Reged: 04/12/09
Loc: Canterbury, Kent, UK
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Re: Prism Diagonal
[Re: grendel]
#5046312 - 01/30/12 04:47 PM
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a bit of research shows a 'Meade Diagonal Prism #917 Hybrid' with the 90 degree diagonal and
'Meade Hybrid Erecting Prism #931' with a 45 degree prism
so obviously they were made - both are on clearance at telescope house in the uk for £37
Grendel
Edited by grendel (01/30/12 04:48 PM)
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EastAnglian
professor emeritus
Reged: 11/22/10
Loc: Cambridgeshire Fens UK
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Re: Prism Diagonal
[Re: grendel]
#5046457 - 01/30/12 06:05 PM
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Hi Grendel!
Many thanks for taking the pics, and doing the research! The numbers are particularly useful.
Cheers
Max
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