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Have an achromat? Get Huygens eyepieces!

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#1 memento

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Posted 25 September 2018 - 04:03 AM

Since I have the Vixen 90M I am also dealing with "false color" more than before (my previous scopes were lengthy f/15 refractors).

 

Yesterday I was watching the craters on the almost full moon with the 90M and my volcano top 7mm Ortho. At some point I decided to try my .965" eyepieces as well. In that size I have an 18mm Ortho (Vixen, so in reality it's more a Plössl), also a cheap (still Vixen) 18mm H.M. as well as better-made 12.5mm and 8mm Japanese H.M. eyepieces. Here's a pic with my 12.5mm H.M. just for reference.

 

DSCF2503.jpg

 

I was really astounded to realise that all three H.M. eyepieces with their ancient optical formula showed MUCH LESS false color than the more modern Orthos. While contrast and detail sharpness were basically at least as good as in the Orthos. The limb of the moon had a distinctive violet fringe in both Orthos, and that was reduced a lot in the three H.M. eyepieces. I'd say the limb was maybe half as wide and maybe half as bright. Hard to describe accurately but, again, false color was reduced a lot.

 

Actually I found myself preferring the views that the H.M. gave. Only my Vixen 18mm H.M. is let down a lot because its a cheapish plastic eyepiece, albeit with very nice optics, yet with only 30 deg apparent field of view and a rough plastic field stop. My better-quality 12.5mm and 8mm H.M. eyepieces show the same 40ish deg apparent field of view of an Ortho and a nice crisp field edge.

 

I now really wonder why there are no modern high quality versions of H.M. eyepieces, or maybe a modified modern eyepiece design, that also would reduce the color fringe and maybe give more eye relief than the old H.M. does. I am sure it could be done these days?

 

There are so many big more modern achromats out there, such eyepieces should actually appeal to a lot of buyers! Thomas


Edited by memento, 25 September 2018 - 04:04 AM.

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#2 happylimpet

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Posted 25 September 2018 - 05:17 AM

I wonder if they create false colour in an equal and opposite manner, cancelling it out? Win!


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#3 HarryRik9

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Posted 25 September 2018 - 07:07 AM

What is the focal length of a Vixen 90M?



#4 rogeriomagellan

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Posted 25 September 2018 - 07:14 AM

Hi, Thomas.

 

That sounds like a great idea. 

 

However, I wonder about the implications of such a measure to the industry. Wouldn't it be the end of anti-fringing filters? 



#5 Stan Lopata

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Posted 25 September 2018 - 07:15 AM

Thomas - have you tried microscope eyepieces?  If not, give them a try.  Most give very good results with telescopes.  The older microscope eyepieces are inexpensive and readily available.

Stan



#6 mitsos68

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Posted 25 September 2018 - 07:47 AM

Ramsdens show the color cancelling effect more apparent.



#7 dan_h

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Posted 25 September 2018 - 11:33 AM

I wonder if they create false colour in an equal and opposite manner, cancelling it out? Win!

If that were true then these eyepieces would also change the color correction of other scopes such as SCTs. APOs and reflectors of all sorts.  No wonder many observers find these eyepieces then less than acceptable.

 

The Huygens eyepiece design is corrected for lateral color so I wonder if the secondary color reduction seen is less in the center of the field?

 

Great that the HMs decrease the color on the 90M. That's a match made in heaven.  

 

dan



#8 memento

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Posted 25 September 2018 - 02:28 PM

The Vixen 90M has a 90mm lens, it's f/11.1 with 1000mm focal length.

 

I will try to verify if color correction with the Huygens eyepieces is similar at the borders and in the center of the image. Yesterday I kind of kept the Moon's limb around maybe 1/3 out of the image center in all my eyepieces.

 

I didn't try the H.M.s yet in my Celestron C5 SCT as I don't have an adapter at hand to use that scope with .965" eyepieces.

 

Still wonder if some new variation of Huygens eyepiece design could maybe overcome the disadvantages of the old ones, basically the tight eye relief and tiny eye lenses. Today we have aspherics and all kinds of special glass and such.

 

Huygens is a two-element design, so even when adding like aspheric shapes or something similar, and also ensuring high quality production standards, it should still be quite affordable to make such an eyepiece? Even if it only would work well in achromats, there would be a market? I mean people also buy all those fringe killer filters right now ...

 

@ Stan
Thanks for the tip with microscope eyepieces. Maybe I'll get around to try some of those as well with the scope!


Edited by memento, 25 September 2018 - 02:29 PM.


#9 Esso2112

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Posted 25 September 2018 - 05:02 PM

I have noticed this as well with certain HM eyepieces. I was observing with my Astro Optical (76 x 910mm) and using the Astro Optical HM eyepieces, the color was pretty bad. I through in some 1950’s Goto Kobaku HM eyepieces and the color essentially disappeared. As with all eyepiece types, there are good ones and not so good ones. 


Edited by Esso2112, 25 September 2018 - 05:03 PM.


#10 lylver

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Posted 25 September 2018 - 06:42 PM

You can use Zeiss Pk (k/c compensated) eyepiece, I got the same conclusions, as they were created to compensate achromatic long f/D microscope objective, they work well for astronomic long f/D objective.

... The old way to do before Nikon and Olympus create the CF microscope eyepieces around year 1980



#11 MartinPond

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Posted 26 September 2018 - 12:08 AM

Hi, Thomas.

 

That sounds like a great idea. 

 

However, I wonder about the implications of such a measure to the industry. Wouldn't it be the end of anti-fringing filters? 

Nope....those are for objective fringing, even center court.

 Most Huygens have good color

  because they are in  long barrels and are cropped to 30-35deg view.

In microscopes, the field curvature from the objective is equivalent to an

  extremely long telescope barrel.

 

I can get get extremely good color and 50 degrees with 2 pcx, (symm Ramsden) but

 they are both glass aspheriical field lenses.   (pillaged from x-wide bins).

Asphere pcx from a catalog are usually far too strong and super-costly.  Just pillage.


Edited by MartinPond, 26 September 2018 - 12:16 AM.


#12 Deep13

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Posted 26 September 2018 - 12:55 AM

Yeah, I have a 5" f/12 and a pair of 25 CZJ Huygens show a very clean image.



#13 jjack's

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Posted 26 September 2018 - 02:54 AM

Hi all

old HM eyepieces have poor coating (if any) and if the blue fringe is at the threshold of the visibility, it can decrease or disapear.

It is working like a light neutral filter. Some binowievers report the same effect because light is divided by two when reaching their eyes.

Old plossls have better coatings than HM and generaly better coatings drive you to a more evident blue or violet fringe.

So i am not shure it's due to the formula.

it is very easy to create CA but it is very very difficult to remove it.


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#14 memento

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Posted 26 September 2018 - 07:06 AM

If the missing coating works like a light NEUTRAL filter this would mean overall image brightness is a bit lower, right?

 

Sure that might be the case. But any 12.5mm or 8mm eyepiece pointed at the (nearly) full Moon in an f/11 scope still gives an über bright image. So in this case, that can't be the reason that the fringe is less visible.



#15 db2005

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Posted 26 September 2018 - 01:25 PM

On several occasions I've found that the full Moon is not the ideal target for seeing and assessing the amount of CA. The Moon is so bright that the eye's iris contracts below the exit pupil size, effectively reducing the effective aperture of the scope, and making CA harder to detect. I find CA is much easier to see on a bright star like Vega.

 

And I agree with jjack's that filtration of the light might be in play. One of my older Plössl eyepieces has a slightly yellowish colour tone, which may hide deep violet color fringing.

 

Nevertheless, your encouraging experiences with these simple eyepieces is healthy food for thought: Amateur scopes many years ago were often marketed as complete systems with OTA and proprietary eyepieces, and it's easy to see the advantages: It's easier for an optics designer to design an optical system for excellent optical performance if he is in control of the entire optical train (objective lens, choice of diagonal, eyepiece). On the other hand it's much more difficult to design a telescope that will perform optimally with all eyepieces that people might try to use with it, and vice versa. Hence all the frequent CN discussions we see going roughly along the lines of "just how good is X eyepiece in Y scope design of Z focal ratio/focal length". And for the same reasons we occasionally see the exact same eyepieces being given glowing reviews by some observers and "meh" (or worse) reviews by other observers using different telescope designs, focal ratios or focal lengths.



#16 Astrojensen

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Posted 26 September 2018 - 02:12 PM

To remove as many variables as possible, I've compared a 9mm Gordon huygenian with a 9mm Zeiss ortho, both uncoated. Both display images of similar brightness, but in an achromat where the ortho shows chromatic aberration, the huygenian shows little or none, depending on how pronounced it is. In an achromat, I actually prefer the huygenian, especially for planetary observing. 

 

It naturally still has the short eye relief the huygenian is infamous for. The AFOV is the same as the ortho, however.

 

My 25mm and 16mm (fully coated) Zeiss huygenians also show improved color correction in achromats, compared to a 25mm KK ortho and a 16mm Zeiss ortho (both also fully coated). 

 

In apochromats and reflectors, the orthos are better. 

 

 

Clear skies!
Thomas, Denmark 


Edited by Astrojensen, 26 September 2018 - 02:13 PM.

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#17 MartinPond

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Posted 27 September 2018 - 07:09 AM

Interesting to note the spot diagrams for the Hutgens:

 

https://www.telescop...berration_2.htm

 

At F/10, the cloud is fairly tight, but there is a large flare-up in blue.

At F/15, the blue is far more tightly contained.

 

You could say this is a very-long-barrel eyepiece, but another possibility is

   that some sort of blue/violet reject filter would cause a great tightening of spot size.

 

In the last few star parties, I've been gravitating more and more 

towards the fine details in the all-glass aspherics from mid-50s Japanese binoculars,

  even at F/5. I cannot find ray spot diagrams for the type.

That would be the type I think the minimalist should try next.

The plastic field lens aspheric doesn't give a fine spot, though.

 

A window-glass comes-with Huygens has astoundingly smooth and sharp lines in F8.

(blown up central image)

I suspect a glass defect property (greenish antimony glass) strips off some blue/violet

 and gives that great center-field view.  The total field is 33 degrees.


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#18 memento

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Posted 27 September 2018 - 12:59 PM

I wonder if that blue flare-up in those spot diagrams is the explanation why it cancels out (some of) the false color of achromatic objectives?




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