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Equipment Discussions >> Refractors

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Peter Natscher
professor emeritus


Reged: 03/28/06

Loc: Central Coast California
Re: What an ED refractor can and cannot do new [Re: Napersky]
      #5598736 - 12/31/12 08:55 PM Attachment (24 downloads)

Astro-Physics shows three color crossings in the 160EDF specs. -- Red, Green, Blue.


Quote:

I emphasise Yellow because Zeiss AS lenses correct in 3 colors, all which come to a common focus: Green and Yellow as all achromats do plus red. However they are achromats and not APOs because they miss the Blue.

So here you have a doublet which doesn't use ED glass at all and has 3 color crossings but is certainly not an APO.

ED objectives I think do bring 4 colors into focus: Blue, Green, Yellow and Red. The focus of those colors may be very good and put them solidly into the APO catagory or they might correct poorly yet much better than a simple achromat which only has a common focus of the yellow and green but loses the blue and red entirely.

So I would believe ED objectives might range somewhere in between a semi-achromat to an full APO depending on the quality of build but they certainly do not fall into the achromat catagory.

Mark




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Julio
sage
*****

Reged: 01/18/07

Loc: Pembroke Pines ,FL
Re: What an ED refractor can and cannot do new [Re: Peter Natscher]
      #5599017 - 01/01/13 01:17 AM

Well i like my Orion 100ED achromat just fine.

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orion61

*****

Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: What an ED refractor can and cannot do new [Re: Julio]
      #5599586 - 01/01/13 01:05 PM

Amen My. Meade 102 ED gives Planetary detail invisable to my 8" SCT and for the live of me cant get color from it.

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kevint1
professor emeritus
*****

Reged: 04/19/11

Loc: West Michigan
Re: What an ED refractor can and cannot do new [Re: orion61]
      #5600114 - 01/01/13 06:38 PM

Quote:

Amen My. Meade 102 ED gives Planetary detail invisable to my 8" SCT and for the live of me cant get color from it.




+1 on both counts for my ES 102 ED.


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: What an ED refractor can and cannot do new [Re: orion61]
      #5600330 - 01/01/13 09:02 PM

Quote:

Amen My. Meade 102 ED gives Planetary detail invisable to my 8" SCT and for the live of me cant get color from it.




How well collimated is your 8 inch? Are you "critically collimating" it? How about thermal equilibrium?

Jon


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mgwhittle
Pooh-Bah
*****

Reged: 08/24/11

Loc: Chattanooga, TN
Re: What an ED refractor can and cannot do new [Re: orion61]
      #5600337 - 01/01/13 09:08 PM

Quote:

Amen My. Meade 102 ED gives Planetary detail invisable to my 8" SCT and for the live of me cant get color from it.




Gosh, that just doesn't sound right!


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Napersky
Carpal Tunnel
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Reged: 01/27/10

Loc: Chicagoland
Re: What an ED refractor can and cannot do new [Re: mgwhittle]
      #5600825 - 01/02/13 08:02 AM

Wolfgang Rohr on ED correction

http://translate.googleusercontent.com/translate_c?depth=1&rurl=translate...


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Napersky
Carpal Tunnel
*****

Reged: 01/27/10

Loc: Chicagoland
Re: What an ED refractor can and cannot do new [Re: Napersky]
      #5615614 - 01/10/13 02:11 PM

The MASTER says it best:

Fast 6"F8 Cde achromat: 550 - 650 nm
Long 6"F15 CeF achomat: 480 - 650 nm
Fast 6"F9 ED doublet: 450 - 650 nm
Fast 6" fluorite doublet: 420 - 1000 nm
Fast 6" FPL52/53 triplet: 380 - 1000 nm
Fast 6" fluorite triplet: 360 - 1000nm

It would be interesting then to divide the cost of each lens by its
useful wavelength range. For instance, a 6"F8 Cde achromat selling
for around $800 today would come in at $10 per nanometer. (our 6"
EDFS at $4900 comes in at $7.90 per nanometer). Interestingly, an 8"
SCT selling for around $900 comes in at $3.81 per nanometer. No fair
asking how a Newtonian would fare!

Seriously, why would you need correction well into the blue-violet
past 480nm? With black and white emulsions, this was necessary
because they have considerable sensitivity down to 380nm. Today's new
blue sensitive CCD cameras also need good correction in the violet.
Also, CCD cameras pick up lots of IR light below 650nm, so correction
to 1000nm is a distinct advantage. For pure visual use, it would be
quite sufficient if the useable range extended only from 440 to 650
nm. So, check the above table for your particular needs and make your
choice.


http://geogdata.csun.edu/~voltaire/roland/color.html


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Gem1021
newbie


Reged: 01/06/13

Loc: Parrish Florida USA
Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5616168 - 01/10/13 07:53 PM

Guys I was lead to believe ED glass in a refractor is to improve the light passing through the glass. Makes the image brighter

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Jared
Postmaster
*****

Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: What an ED refractor can and cannot do new [Re: Gem1021]
      #5616177 - 01/10/13 07:58 PM

Welcome to Cloudy Nights!

Unfortunately, whoever gave you that information was incorrect. The amount of absorption through any quality optical glass in a small refractor is trivial--on the order of 1% or less. That's true whether you are talking about ED glass or normal crown/flint achromats. Fluorite crystal (used on some really high-end refractors) is even lower.

The purpose of ED glass in a refractor (or a telephoto lens, for that matter) is to reduce chromatic aberration--to improve color correction. It gets rid of or at least reduces the "purple halos" around the moon, planets, and bright stars. It also improves telescope sharpness by properly focusing a larger range of frequencies of light than an achromat of the same aperture and focal length (though the differences are smaller than you might expect since the eye's sensitivity drops off as you move away from the green portion of the spectrum). It does all this at the expense of, well, expense. ED glass and the appropriate mating glass cost more than their traditional counterparts.


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