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Equipment Discussions >> Refractors

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saemark30
Pooh-Bah


Reged: 02/21/12

Re: What an ED refractor can and cannot do new [Re: wh48gs]
      #5311745 - 07/10/12 06:01 PM

NO I was wondering how well the Skywatcher 120ED f/7.5 doublet performs, like its smaller 80mm ED f/7.5 sibling.

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cjc
sage


Reged: 10/15/10

Loc: Derbyshire, England
Re: What an ED refractor can and cannot do new [Re: saemark30]
      #5311871 - 07/10/12 07:33 PM

Quote:

NO I was wondering how well the Skywatcher 120ED f/7.5 doublet performs, like its smaller 80mm ED f/7.5 sibling.



They perform well acording to owners and look at the tests here:

http://translate.googleusercontent.com/translate_c?hl=en&ie=UTF8&prev=_t&rurl=translate.google.com&sl=de&tl=en&u=http://www.astro-foren.de/showthread.php%3F6084-Verzeichnis-optischer-Berichte%26p%3D32999&usg=ALkJrhjdhOKfrg5kCdT0uxAm2znuTP2uQg#post32999

In passing one would also need threads on the quality of chinese manufactured optics and importance of ED glass...


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Jeff B
Carpal Tunnel
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Reged: 12/30/06

Re: What an ED refractor can and cannot do new [Re: wh48gs]
      #5311872 - 07/10/12 07:35 PM

Sorry, but Lovey gets my vote.

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wh48gs
Carpal Tunnel
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Reged: 03/02/07

Re: What an ED refractor can and cannot do new [Re: saemark30]
      #5311944 - 07/10/12 08:32 PM

I read it has fluorite element, but it doesn't really need it, so I'm not sure. The same site (vendor) states that FPL53 is Schott glass, and that this SW doublet has Schott crown as the positive front element (since this element has higher dispersion, it has to have lower optical power in order for the two elements to cancel out each other's longitudinal chromatism; and if it would be the positive element, such doublet could only produce diverging rays).

So those sources don't seem to be awfully reliable. But there is no real difference, FPL53 or fluorite, in the correction level. The latter merely has more matching glasses available for near-zero secondary spectrum and, since has somewhat higher relative partial dispersion, it could be stretched out to somewhat faster objectives than FPL53. It will also, in general, has less of secondary spectrum than FPL53 with less-than-perfect matching glasses suitable for fast objectives, but secondary spectrum is nearly negligible here compared to spherochromatism.

For example, if a common inexpensive crown like K5 is combined with FPL53 in a 120mm f/7.5 doublet, it will produce 0.933 limiting visual polychromatic Strehl (0.966 at 80mm). If combined with fluorite, it will have almost twice smaller combined error in F and C, but will also have about 50% larger error in the optimized wavelength, due to more higher order spherical. According to OSLO, its limiting design Strehl is only negligibly higher: 0.936 and 0.969, respectively.

As for the 3%, or so, difference between 80mm and 120mm aperture Strehl, it is not really significant either. It is comparable to the difference between a perfect aperture and one with 1/10 wave p-v of primary spherical. As long as the actual Strehl is over 0.90, there will be little difference in the performance level. A single most important factor for agood paper design to come close to its design level in the real objective is keeping the optimized line correction as close to the design minimum as possible.

Vla


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bratislav
sage


Reged: 09/07/06

What an ED refractor can and cannot do new [Re: wh48gs]
      #5311999 - 07/10/12 09:06 PM Attachment (22 downloads)

Quote:



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).


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bratislav
sage


Reged: 09/07/06

Re: What an ED refractor can and cannot do new [Re: wh48gs]
      #5312041 - 07/10/12 09:28 PM

Quote:

I read it has fluorite element, but it doesn't really need it, so I'm not sure. The same site (vendor) states that FPL53 is Schott glass, and that this SW doublet has Schott crown as the positive front element




I've read in some German forums it uses combination of ZKN7 and FPL53. Not really that much different from what you suggested (K5+FPL53).


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wh48gs
Carpal Tunnel
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Reged: 03/02/07

Re: What an ED refractor can and cannot do new [Re: bratislav]
      #5312206 - 07/10/12 11:02 PM

Quote:

Lets sort out some simple truths firts. 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.




Of course it can. And just a look at the relative partial dispersion diagram tells which ones will bring F, C and e lines together. Some doublet combination actually bring four widely separated wavelengths to a comomn focus (e.g. FPL53/KZFSN2-ouch!). On the other hand, if a triplet or quadruplet is made of two glasses that can't do it in a doublet, they won't do it as well. Combining more than two glasses can do it, but there's not too many viable combinations. For a visual instrument it doesn't even matter.

As the RPD diagram indicates, FPL52/ZKN7 secondary spectrum is practically non existent. The focal shift plot, showing paraxial focus deviation with wavelength, now indicates the magnitude of spherochromatism, which bends the LA (longitudinal aberration) lines enforcing the separation of paraxial foci in order to have best foci of these wavelengths come as close one to another as possible. Only in the absence of spherochromatism, the paraxial foci would have come together.

Crossings do indicate that the wavelengths are bunched up together, but what really matters is the amount of spherochromatism they have in them and, even more, how close the crossings are to their respective best focus locations.

Vla


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wh48gs
Carpal Tunnel
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Reged: 03/02/07

Re: What an ED refractor can and cannot do new [Re: bratislav]
      #5312227 - 07/10/12 11:18 PM

Quote:

I've read in some German forums it uses combination of ZKN7 and FPL53. Not really that much different from what you suggested (K5+FPL53).




It would up the Strehl to 0.935 design limit (vs. 0.933) (but ZKN7 is about 2.5 times the K5 price).

Vla


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jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: What an ED refractor can and cannot do new [Re: ckwastro]
      #5312720 - 07/11/12 11:40 AM

"1. Dob *and* refractor

2. MaryAnn....definitely!"

Pshah!

1. Dobs *and* refractors *and* catadioptrics

2. MaryAnn *and* Ginger *and* Lovey Howell *and* ...*and*...*and*...definitely!



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ckwastro
Pooh-Bah


Reged: 11/23/05

Loc: Tempe, AZ
Re: What an ED refractor can and cannot do new [Re: jrbarnett]
      #5312789 - 07/11/12 12:49 PM



Jim, I would expect nothing less from.......The Most Interesting Astronomer in the Universe.

(Although I'm not sure about Lovey)


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saemark30
Pooh-Bah


Reged: 02/21/12

Re: What an ED refractor can and cannot do new [Re: ckwastro]
      #5312883 - 07/11/12 04:01 PM

Since we like doublets so much
+2. MaryAnn *and* Ginger


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elwaine
Pooh-Bah


Reged: 06/18/06

Loc: Jupiter
Re: What an ED refractor can and cannot do new [Re: saemark30]
      #5313573 - 07/11/12 11:23 PM

Quote:

Even the 130mm AP is listed as having color correction that is somewhat inferior to the bigger models. AP list the color correction for the 130 as less than + - 0.006% focus variation from 706nm to 430nm (r to g wavelengths), while the larger scopes are listed as less than + - 0.004% focus variation from 656nm to 430nm (c to g wavelengths).




Anyone who can see the difference between + - 0.006% focus variation from 706nm to 430nm and less than + - 0.004% focus variation from 656nm to 430nm has got some seriously exquisite rods in their eyeballs. I cannot see any CA in my A-P 130mm, in spite of its poor correction to only + - 0.006% focus variation from 706nm to 430nm (r to g wavelengths).

And, BTW, I've done a lot of AP with an A-P 130mm and never encountered focus problems due to the incredibly small amount of residual CA.

As for what one should call an ED refractor with very good, if not perfect color correction: How about Anicescope?

Regards,


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saemark30
Pooh-Bah


Reged: 02/21/12

Re: What an ED refractor can and cannot do new [Re: elwaine]
      #5315981 - 07/13/12 12:47 PM

FPL-52 is no longer available.
So with a new 6" f/8 doublet being produced by APM, should we expect to see lower prices for used TEC160, FS152 even?


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Stacy
Star Partyer
*****

Reged: 09/15/02

Loc: Seattle, WA
Re: What an ED refractor can and cannot do new [Re: saemark30]
      #5316589 - 07/13/12 08:49 PM

I tried to read the whole thread, but I got a little over half-way before my brain started bleeding.

Please help me understand! I understand an Achro has two lenses and an APO has three. How can an ED doublet bring three wavelengths (typically red, green, blue) to focus? It seems impossible.

Also what is an ED Triplet? Is there any argument that ED Triplets are not "true apo's" ?

http://www.reference.com/browse/Apochromat?s=t


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gdd
Pooh-Bah


Reged: 11/23/05

Loc: N Seattle suburb, WA
Re: What an ED refractor can and cannot do new [Re: Stacy]
      #5316618 - 07/13/12 09:18 PM

Quote:

I tried to read the whole thread, but I got a little over half-way before my brain started bleeding.

Please help me understand! I understand an Achro has two lenses and an APO has three. How can an ED doublet bring three wavelengths (typically red, green, blue) to focus? It seems impossible.

Also what is an ED Triplet? Is there any argument that ED Triplets are not "true apo's" ?

http://www.reference.com/browse/Apochromat?s=t




Basically there are two ways to correct for 3 colors. One is to use 3 lenses to bring 3 wavelengths to a common focus. The other is to use low dispersion glass that does not separate the colors so much. So the ED doublet brings 2 colors to a common focus and relies on the low dispersion ED element to minimimze the out of focusness of the remaining colors.

Gale


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Astropin
super member


Reged: 08/30/07

Loc: Michigan
Re: What an ED refractor can and cannot do new [Re: gdd]
      #5317699 - 07/14/12 03:46 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.


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Napersky
Carpal Tunnel
*****

Reged: 01/27/10

Loc: Chicagoland
Re: What an ED refractor can and cannot do new [Re: bratislav]
      #5598287 - 12/31/12 04:14 PM

Quote:

Quote:



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?


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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]
      #5598289 - 12/31/12 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.


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Pete-LH
sage
*****

Reged: 03/25/09

Loc: Wilmington, DE
Re: What an ED refractor can and cannot do new [Re: Napersky]
      #5598365 - 12/31/12 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.


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Napersky
Carpal Tunnel
*****

Reged: 01/27/10

Loc: Chicagoland
Re: What an ED refractor can and cannot do new [Re: Pete-LH]
      #5598448 - 12/31/12 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


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