Alan French
Night Owl
Reged: 01/28/05
Loc: Upstate NY
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Re: What an ED refractor can and cannot do
[Re: gdd]
#5305243 - 07/06/12 11:39 AM
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An ED triplet can be, and often is, a fluor-crown and two crowns. The two crowns can be the same glass.
Clear skies, Alan
Edited by Alan French (07/06/12 11:40 AM)
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saemark30
scholastic sledgehammer
Reged: 02/21/12
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Re: What an ED refractor can and cannot do
[Re: hfjacinto]
#5309896 - 07/09/12 02:13 PM
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What ED scope was that image taken with?
The 80ED f/7.5 is definitely in the APO category.
Is the 120ED f/7.5 in the same league?
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dscarpa
Carpal Tunnel
Reged: 03/15/08
Loc: San Diego Ca.
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Re: What an ED refractor can and cannot do
[Re: gnowellsct]
#5310019 - 07/09/12 03:38 PM
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I wish there was a very good not too expensive 5" doublet available. I would like to have a bigger refractor in addition to my WO ZS-110 but without the expense and weight of 5" triplets. The 120 Eon and 120 SW Pro are too close size wise to my ZS. I'm visual only. David
Edited by dscarpa (07/09/12 03:40 PM)
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kevint1
sage
Reged: 04/19/11
Loc: W. Michigan, USA
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Re: What an ED refractor can and cannot do
[Re: dscarpa]
#5310138 - 07/09/12 05:12 PM
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I wish there was a very good not too expensive 5" doublet available.
Have you looked at the ES AR127?
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dscarpa
Carpal Tunnel
Reged: 03/15/08
Loc: San Diego Ca.
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Re: What an ED refractor can and cannot do
[Re: gnowellsct]
#5310212 - 07/09/12 05:56 PM
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I would want a ED doublet. I do a lot of lunar-planetary so good color correction is a must. David
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hfjacinto
I think he's got it!
Reged: 01/12/09
Loc: Land of clouds and LP
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Re: What an ED refractor can and cannot do
[Re: saemark30]
#5310500 - 07/09/12 09:12 PM Attachment (25 downloads)
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Quote:
What ED scope was that image taken with?
The 80ED f/7.5 is definitely in the APO category.
Is the 120ED f/7.5 in the same league?
This was taken with the F6.25 Orion 80MM
Edited by hfjacinto (07/09/12 09:13 PM)
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hfjacinto
I think he's got it!
Reged: 01/12/09
Loc: Land of clouds and LP
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Re: What an ED refractor can and cannot do
[Re: hfjacinto]
#5310504 - 07/09/12 09:14 PM Attachment (25 downloads)
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This was also taken with the 80MM
Edited by hfjacinto (07/09/12 09:14 PM)
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hfjacinto
I think he's got it!
Reged: 01/12/09
Loc: Land of clouds and LP
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Re: What an ED refractor can and cannot do
[Re: hfjacinto]
#5310506 - 07/09/12 09:15 PM Attachment (29 downloads)
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This was with a 120MM EON
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wh48gs
Pooh-Bah
Reged: 03/02/07
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Re: What an ED refractor can and cannot do
[Re: saemark30]
#5310650 - 07/09/12 11:09 PM Attachment (41 downloads)
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What would you think of the old Meade 152 ED?
It's halfway between a FS152 and normal 6" f/15 achromat, or similar to what AP was before ED triplets.
It uses Hikari's FK01 ED glass with KF3 in front. Visual polychromatic Strehl is 0.915 (design limit), which compares to 0.78 for the 6-inch f/15 achromat. This glass combination has some residual secondary spectrum (less than 1/5 of the standard achromat), but it matters less than spherochromatism.
Similar glass combination was probably used for the Vixen f/6.6 ED102SS. There is, actually, no better choice for such a fast doublet, since matches with near-zero secondary spectrum (including FPL53/ZKN7)don't have sufficient Abbe number differential to supress spherochromatism below 0.04 wave RMS in the optimized wavelength. A 102mm f/6.6 KF3/FK01 objective has 0.021 wave RMS in the optimized wavelength and 0.87 visual polychromatic Strehl design limit, which is quite good considering its speed (OSLO file attached).
Vla
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Jeff B
Carpal Tunnel
Reged: 12/30/06
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Re: What an ED refractor can and cannot do
[Re: wh48gs]
#5311135 - 07/10/12 10:01 AM
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Ok, now that we've completely settled the glass and doublet versus triplets issues and are all in agreement, I think we need take up the air spaced versus oil spaced issue.
Jeff
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jmiele
Patron Saint?
Reged: 12/04/10
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Re: What an ED refractor can and cannot do
[Re: Jeff B]
#5311260 - 07/10/12 11:35 AM
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Nooooooo! Not again.. 
Joe
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jmiele
Patron Saint?
Reged: 12/04/10
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Re: What an ED refractor can and cannot do
[Re: jmiele]
#5311262 - 07/10/12 11:37 AM
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If I concede that all ED Doublets are Apo's can we skip the Air vs Oil discussion.
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ken svp120
Carpal Tunnel
Reged: 12/19/04
Loc: Ohio
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Re: What an ED refractor can and cannot do
[Re: jmiele]
#5311289 - 07/10/12 11:58 AM
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As an Aside: After years on this forum, there most certainly are recurring threads such as...
Air spaced v. Oil spaced
Aluminum v. Carbon Fiber
Doublet v. Triplet
What do Glass types mean
Is ED or Semi Apo meaningful/valid
It would almost be nice if the moderators would start an individual thread for each of these recurring items, let it go on indefinately, and redirect any such future threads/posts to the appropriate thread. That way, we could debate on and on, all the info would be in one place, and these items wouldn't keep cluttering up other threads.
Just a thought...
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Jeff B
Carpal Tunnel
Reged: 12/30/06
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Re: What an ED refractor can and cannot do
[Re: ken svp120]
#5311450 - 07/10/12 02:23 PM
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Quote:
As an Aside: After years on this forum, there most certainly are recurring threads such as...
Air spaced v. Oil spaced
Aluminum v. Carbon Fiber
Doublet v. Triplet
What do Glass types mean
Is ED or Semi Apo meaningful/valid
It would almost be nice if the moderators would start an individual thread for each of these recurring items, let it go on indefinately, and redirect any such future threads/posts to the appropriate thread. That way, we could debate on and on, all the info would be in one place, and these items wouldn't keep cluttering up other threads.
Just a thought...
Don't for get Achro vs APO...or whatever you term an APO to be..DOH,  it's going to start all over again..my bad. 
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csrlice12
Post Laureate
Reged: 05/22/12
Loc: Denver, CO
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Re: What an ED refractor can and cannot do
[Re: Jeff B]
#5311514 - 07/10/12 03:09 PM
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Quote:
Quote:
As an Aside: After years on this forum, there most certainly are recurring threads such as...
Air spaced v. Oil spaced
Aluminum v. Carbon Fiber
Doublet v. Triplet
What do Glass types mean
Is ED or Semi Apo meaningful/valid
It would almost be nice if the moderators would start an individual thread for each of these recurring items, let it go on indefinately, and redirect any such future threads/posts to the appropriate thread. That way, we could debate on and on, all the info would be in one place, and these items wouldn't keep cluttering up other threads.
Just a thought...
Don't for get Achro vs APO...or whatever you term an APO to be..DOH, it's going to start all over again..my bad.
You forgot the favorites: Dob vs Refractor, and Ginger or MaryAnn?
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ckwastro
Pooh-Bah
Reged: 11/23/05
Loc: Tempe, AZ
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Re: What an ED refractor can and cannot do
[Re: csrlice12]
#5311530 - 07/10/12 03:17 PM
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You forgot the favorites: Dob vs Refractor, and Ginger or MaryAnn?
1. Dob *and* refractor
2. MaryAnn....definitely! 
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ken svp120
Carpal Tunnel
Reged: 12/19/04
Loc: Ohio
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Re: What an ED refractor can and cannot do
[Re: ckwastro]
#5311550 - 07/10/12 03:29 PM
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+1 for MaryAnn - no argument there. 
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saemark30
scholastic sledgehammer
Reged: 02/21/12
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Re: What an ED refractor can and cannot do
[Re: wh48gs]
#5311568 - 07/10/12 03:40 PM
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Can you workout the Synta 120ED refractor?
We know it uses S-FPL-53 at least.
How good is it?
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Eddgie
Postmaster
Reged: 02/01/06
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Re: What an ED refractor can and cannot do
[Re: csrlice12]
#5311641 - 07/10/12 04:30 PM
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Ginger... Tina Louise was a bombshell compared to Dawn Wells. Gilligan's Island over-did her and ruined her career. She has been compared to Jane Mansfield (who was originally offered the part I think).
This to me seems more interesting than the original premise of the thread that took a look at one of the fastest ED refractors ever sold and attempted to make a generic statement regarding ED scopes in general by using an extreme sample of the class as being represntitive of all ED scopes.
I wonder why Astro Physics makes their 140 triplets f/7.5? Gee, couldn't they have made it f/6.5? That would be more better, eh?
And what about the 175??? couldn't they have at least made it f/7??? I mean its a triplit! Wouldn't it be just perfect if it were f/7? And yet they made if f/8.. It's a triplet, but a slow one. Why oh why didn't they make it f/6.5?
I still think using such a fast doublet and inferring that the result applies to all ED scopes is a little bit of a stretch.
It is very difficult to make even triplets color free if the system gets to fast. Bending light is easy. Getting all the wavelenths bent to the exact same focus is almost impossible. And the faster the system, the more difficult it becomes.
Tina Louise in a hearbeat though. She should have been a big screen bombshell, but Gilligan's Island robbered her of that chance.
Just like the OPs post is attempting to do to ED telescopes in general by choosing an extreme sample and making a blanket statement that implies that all ED scopes are inferior to all triplets.
Quote:
A true top end triplet apo would do a better job with this test.
"D'uh" of the Decade award for this statement....
Edited by Eddgie (07/10/12 04:33 PM)
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wh48gs
Pooh-Bah
Reged: 03/02/07
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Re: What an ED refractor can and cannot do
[Re: saemark30]
#5311664 - 07/10/12 04:46 PM
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The triplet? It is almost certainly FPL53, because it is probably the cheapest full-fledged ED glass available these days. More or less, it would be similar to downscaled TEC 140, which has 0.95+ polychromatic Strehl when visually optimized, and 0.92 claimed officially (which should be due to some compromises toward photo-visual correction and/or factoring in greater actual error in the optimized wavelength than the design minimum). With only little more expensive matching glass (ZKN7/FPL53/ZKN7 instead of K10/FPL53/BK7) it could reach 0.97 visual Strehl at 140mm aperture, which gets closer to 0.98 at 120mm. In any case, its design limit is probably 0.96-0.97 Strehl, which would boil down to about 0.95 with quality fabrication.
Vla
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