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What an ED refractor can and cannot do

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#176 Astropin

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Posted 14 July 2012 - 02:46 PM

Or three ways if you take into consideration the focal length. Focal length can also be used to correct (to a degree) CA.

Apo does not mean "triplet" Apochromatic is simply a description of high performance (CA correction). A very well made doublet using ED glass could certainly qualify as an APO.....at least visually.

#177 Napersky

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Posted 31 December 2012 - 04:14 PM



Wouldnit it be more correct to say these are the three types of refractors:

1. Chromatic (singlet, 1 crossing)
2. Achromatic (doublet, 2 crossings)
3. Apochromatic (triplet, 3 crossings)

The corrected colors are the number of crossings.


Lots of misconceptions in this thread...

Lets sort out some simple truths first. Two element objective (so called ED doublet) CAN bring three colors to focus. Here's one I've just cooked up in Zemax. FPL52+ZKN7. Three crossings. Triplets can be designed to bring four colors to focus (in some cases even FIVE).

BTW, I would not take the number of wavelengths that are simultaneously focused too seriously as a quality measure. A doublet with two color crossing and tight chromatic focal shift where it counts is vastly preferred than three color (but at wrong wavelengths and huge CFS).



Does Yellow Count as a Color? It's not a primary color such as Blue, Green, or Red?

#178 Napersky

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Posted 31 December 2012 - 04:16 PM

Obviously Yellow is not a color.

Only Primary colors count in Tom Back's definition, Although it's lacking in his definition of an APO.

#179 Pete-LH

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Posted 31 December 2012 - 05:01 PM

I responded to this and deleted it because my answer was somewhat incorrect. I am embarassed because in school I was heavily involved in spectroscopy but with time I have evloved/devolved into a paint chemist (ironic because I am red-green colour blind).
So as a paint chemist I responded that red, blue and yellow are the primary colours(reflective or subtractive).
However here we are talking about emission colours and here the primary colours are red, green and blue(or addtive primaries).
However, these are colour terminologies related to how we combine these for produced images: red, green, blue for Television or CRT's and red, blue, yellow for paint (or cyan, magenta, yellow for printing processes).
But in spectroscopic terms I believe yellow light emitted from a star is at the wavelenth of 570nm (or 589 nm from those damned Low-pressure sodium lamps used in parking lots).
This response is probably out of context since I came to this thread late and see it is many pages long(and I have just scanned it quickly).
Still I am grateful because it gets me back to thnking about the basics of what we visualize.
As for What an ED refractor can or can't do, if it is less than 5" it cannot resolve a globular star cluster to my satisfaction. But otherwise I find the visual experience to be just fine.

#180 Napersky

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Posted 31 December 2012 - 05:45 PM

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

#181 Peter Natscher

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Posted 31 December 2012 - 08:55 PM

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


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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#182 Julio

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Posted 01 January 2013 - 01:17 AM

Well i like my Orion 100ED achromat just fine.

#183 orion61

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Posted 01 January 2013 - 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.

#184 kevint1

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Posted 01 January 2013 - 06:38 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.


+1 on both counts for my ES 102 ED.

#185 Jon Isaacs

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Posted 01 January 2013 - 09:02 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.


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

Jon

#186 mgwhittle

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Posted 01 January 2013 - 09:08 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.


Gosh, that just doesn't sound right!

#187 Napersky

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Posted 02 January 2013 - 08:02 AM

Wolfgang Rohr on ED correction

http://translate.goo...rl=translate...

#188 Napersky

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Posted 10 January 2013 - 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...land/color.html

#189 Gem1021

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Posted 10 January 2013 - 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

#190 Jared

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Posted 10 January 2013 - 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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