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#1 ThomasM

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Posted 03 November 2012 - 12:47 PM

There is a lot of discussion about eypiece transmission, but measurements are rare. This week I performed measurements of the transmission of several popular eyepieces using a laser diode with 532 nm wavelength and a 25 mm f=125 mm refractor and a thorlabs power meter.

Eyepiece/ Transmission in %

Baader 36 mm Asphric 94,9
Baader 18 mm Ortho I 96,3
Baader 18 mm Ortho II 97.9
Baader 9 mm Ortho 95,7
Baader 7 mm Ortho 95.8 (edited)
TV Panoptic 25 mm 95,2
TV Ethos 17 mm I 91.7
TV Ethos 17 mm II 92.4 (added)
TV Nagler 13 mm I 87,6
TV Nagler 13 mm II 87,3 (added)
Leica Aspheric Zoom with TMB Barlow 6-12 mm 93,4
Leica Aspheric Zoom 9-18 mm 94,4
Thorlabs Monocentric 10 mm fully multicaoted 93,7
No name Plössl 4 mm (MgF2) 86,5
TMB ED 1.8x Barlow > 99%


The statistical errors (standard diviation) are less than 0.5 %, systematic errors presumably less than 2 %, at least for the long focal length eyepieces.

A few things surprised me:

1. The transmission of the Leica Zoom (8 lenses) exceeds that of the Thorlabs monocentric, a three lens, single element eyepiece, although the Thorlabs is fully multicoated. Edited: After removing dust from the front lens I obtained 96.9%
2. The Transmission of the TV 13 mm Nagler is rather low compared to a previous measurement of the TV Nagler 9 mm, (http://www.amateuras.../tips/tips3.htm). I repated the measurement, to make sure that the transmission is indeed below 90%.
3. The two Baader 18mm ortho eyepieces differ by more than one percent, this is reproducable.

In practice these differences are probably academic, they are presumably almost invisible.
Best regards

Thomas

p.s. I repeated the measurement for the some eyepieces and added new values (Nov 5 2012)

#2 jrbarnett

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Posted 03 November 2012 - 02:49 PM

Did you clean the eyepieces before testing them?

Regards,

Jim

#3 ThomasM

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Posted 03 November 2012 - 02:55 PM

Did you clean the eyepieces before testing them?

Regards,

Jim


Actually, the eyepieces were very clean. Anyhow, I wiped the eyelense with a microfaser fabric and removed dust with a clean brush.

best regards


Thomas

p.s. would you suggest to do additional cleaning, e.g. with alcohol or with 'optical wonder' fluid from Baader?


#4 Starman1

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Posted 03 November 2012 - 03:07 PM

Thomas,
The 13 Nagler was an original Nagler, I presume.
That actually doesn't surprise me since my original 14mm Meade Series 4000 (smoothside, some uncoated surfaces internally) also transmitted about 85%, IIRC.
Coatings have substantially improved since then.
A fully multi-coated 4-element, 2-group, eyepiece could transmit as much as 98.0% You had one right there, at 97.9%
In theory, an Ethos could have a transmission of 94.1% Your 91.7% indicates it misses the theoretical by just a hair.

Since the highest transmission is 97.9%, you kind of want none of your eyepieces to fall below a loss of 0.1 magnitude, or 10%, which says it should be at least 87.9%

Only 2 of the eyepieces you tested missed that, and I bet the no-name Plossl was uncoated on its interior surfaces. Or the coatings were very much sub-par, as a good MgFl2 coating on all surfaces could yield as high as 94.1% transmission.

The fact that a ten inch reaches over 0.4 magnitudes deeper than an 8" shows how little effect transmission in eyepieces has. Probably of more importance are factors such as the spectrum of transmission and the polish quality on the glass.

Oh, one more thing: the transmission of 100% was equalized AFTER the refractor, I presume. What was the transmission of the refractor?

#5 ThomasM

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Posted 03 November 2012 - 03:27 PM

Thomas,
The 13 Nagler was an original Nagler, I presume.

...
Coatings have substantially improved since then.
A fully multi-coated 4-element, 2-group, eyepiece could transmit as much as 98.0% You had one right there, at 97.9%
In theory, an Ethos could have a transmission of 94.1% Your 91.7% indicates it misses the theoretical by just a hair.


...

The fact that a ten inch reaches over 0.4 magnitudes deeper than an 8" shows how little effect transmission in eyepieces has. Probably of more importance are factors such as the spectrum of transmission and the polish quality on the glass.

Oh, one more thing: the transmission of 100% was equalized AFTER the refractor, I presume. What was the transmission of the refractor?


Don,
thanks for your comments. Actually, the 13 mm Nagler is a five years old T6, that's why I am surprised that the measured transmission is 'only' 87.6%.

The transmission was measured by comparing the direct beam behind the focal spot of the fractor lens and the beam passing through the eyepiece. I have not measured the transmission of the fractor lens, but I principle I could have done that.

best regards

Thomas

#6 jrbarnett

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Posted 03 November 2012 - 03:44 PM

If you have a loupe or a magnifying glass, inspeact the eye lenses of each. If they look clean, you're fine. If you see any evidence of grime, haze, oil, etc., I would give 'em a cleaning with Pursol or similar cleaning agent.

It can't hurt to check. My eyepieces tend to get pretty dirty. I'm usually rewarded with a mess whenever I do a loupe inspection. :lol:

- Jim

#7 robboski2004

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Posted 03 November 2012 - 04:38 PM


Hello Thomas,

I also recently setup a 532nm laser to measure the following eyepieces.........

17.3mm Delos 96%
17mm Ethos 93.5%
32mm TV Plossl 95% ( what do you expect for $130 !!!)
41mm Panoptic 96%
32mm Meade SWA 88%
22mm Pan ( 1990's) 87%

setup : laser into optical fibre which expands the beam to 10mm entering the eyepiece, then through KG5 filter to isolate green from original diode wavelength into a newport intergrating sphere and power meter.

Have a 27mm Pan and 12mm Delos on order.
So my set will be 41mm / 27mm Pans 96% and 17.3mm / 12mm Delos all transmitting approx 96% half way between photopic and scotopic curves.

Tough time to be an amateur observer :jump:!!

Regards,
Ian.

#8 denis0007dl

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Posted 03 November 2012 - 05:22 PM

It would be interesting to test Explore Scientific 68" and 82" eyepieces :)

#9 Paul G

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Posted 03 November 2012 - 07:07 PM

Thomas,
The 13 Nagler was an original Nagler, I presume.

...
Coatings have substantially improved since then.
A fully multi-coated 4-element, 2-group, eyepiece could transmit as much as 98.0% You had one right there, at 97.9%
In theory, an Ethos could have a transmission of 94.1% Your 91.7% indicates it misses the theoretical by just a hair.


...

The fact that a ten inch reaches over 0.4 magnitudes deeper than an 8" shows how little effect transmission in eyepieces has. Probably of more importance are factors such as the spectrum of transmission and the polish quality on the glass.

Oh, one more thing: the transmission of 100% was equalized AFTER the refractor, I presume. What was the transmission of the refractor?


Don,
thanks for your comments. Actually, the 13 mm Nagler is a five years old T6, that's why I am surprised that the measured transmission is 'only' 87.6%.

The transmission was measured by comparing the direct beam behind the focal spot of the fractor lens and the beam passing through the eyepiece. I have not measured the transmission of the fractor lens, but I principle I could have done that.

best regards

Thomas


Interesting. I wonder how the methodology differs from this where the T6 Naglers did much better:

Eyepiece Transmission

#10 jtaylor996

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Posted 03 November 2012 - 09:34 PM

I wonder if this number changes across the FOV...

#11 robboski2004

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Posted 03 November 2012 - 09:51 PM


The 22mm Panoptic manufactured in the 90's has a distinct
change in coating colour on the steep eyelens and field lens curves.
When i measured the 41mm Pan, i obtained virtually the same transmission whether the 10mm circle was centered in the field lens or off to one side.

Ian.

#12 ThomasM

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Posted 04 November 2012 - 05:09 AM

Thomas,
The 13 Nagler was an original Nagler, I presume.

...
Coatings have substantially improved since then.
A fully multi-coated 4-element, 2-group, eyepiece could transmit as much as 98.0% You had one right there, at 97.9%
In theory, an Ethos could have a transmission of 94.1% Your 91.7% indicates it misses the theoretical by just a hair.


...

The fact that a ten inch reaches over 0.4 magnitudes deeper than an 8" shows how little effect transmission in eyepieces has. Probably of more importance are factors such as the spectrum of transmission and the polish quality on the glass.

Oh, one more thing: the transmission of 100% was equalized AFTER the refractor, I presume. What was the transmission of the refractor?


Don,
thanks for your comments. Actually, the 13 mm Nagler is a five years old T6, that's why I am surprised that the measured transmission is 'only' 87.6%.

The transmission was measured by comparing the direct beam behind the focal spot of the fractor lens and the beam passing through the eyepiece. I have not measured the transmission of the fractor lens, but I principle I could have done that.

best regards

Thomas


Interesting. I wonder how the methodology differs from this where the T6 Naglers did much better:

Eyepiece Transmission


I was surprised too that the transmission of the 13 mm Nagler T6 (I got 87.6%) is more than 4 % lower than that given in the link above. Actually, I have no idea how the measurements were performed. I own a scond Nagler 13mm T6 for binowing and could do an additional measurement.

best regards

Thomas

#13 ThomasM

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Posted 04 November 2012 - 05:15 AM

Hello Thomas,

I also recently setup a 532nm laser to measure the following eyepieces.........

17.3mm Delos 96%
17mm Ethos 93.5%
32mm TV Plossl 95% ( what do you expect for $130 !!!)
41mm Panoptic 96%
32mm Meade SWA 88%
22mm Pan ( 1990's) 87%

setup : laser into optical fibre which expands the beam to 10mm entering the eyepiece, then through KG5 filter to isolate green from original diode wavelength into a newport intergrating sphere and power meter.

Have a 27mm Pan and 12mm Delos on order.
So my set will be 41mm / 27mm Pans 96% and 17.3mm / 12mm Delos all transmitting approx 96% half way between photopic and scotopic curves.

Tough time to be an amateur observer :jump:!!

Regards,
Ian.


Ian,

these are very intersting results. The Delos seems to be very good, 3 % higher transmission than the Ethos. Your value for the 17 mm Ethos is slightly higher than mine this is probably within the error bars. Can you estimate your error bars? Do you intend to measure the transmission of shorter focal length eyepieces, e.g. 8 or 10 mm Delos?

best regards

Thomas

#14 davidpitre

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Posted 04 November 2012 - 08:55 AM

Check out the TMB barlow. I use one, and always wondered about the cumulative effect it added in terms of light loss when viewing extremely faint objects. Question answered.

It is notable that Ian and Thomas both measured the 17 Ethos, their measurements differing by almost 2%.

#15 astroducky

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Posted 04 November 2012 - 09:45 AM

Have you taken into account that the laser itself fluctuates a few percent over time? Unless you directly take the 'input' beam into the system and concurrently/ immediately measure the 'output' of the beam, it will not be that accurate. Also, the power meter itself has an accuracy typically of 3%. That could also answer why there are differences in results of similar eyepieces.

#16 ThomasM

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Posted 04 November 2012 - 10:10 AM

Check out the TMB barlow. I use one, and always wondered about the cumulative effect it added in terms of light loss when viewing extremely faint objects. Question answered.

It is notable that Ian and Thomas both measured the 17 Ethos, their measurements differing by almost 2%.


Yes, the TMB Barlow is excellent, the coatings are darker than that of any eyepiece I own, very low scatter and the transmission is better than 99%.

Regards

Thomas

p.s. I could also check a second 17 mm Ethos and see if the transmission is the same

#17 ThomasM

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Posted 04 November 2012 - 10:17 AM

Have you taken into account that the laser itself fluctuates a few percent over time? Unless you directly take the 'input' beam into the system and concurrently/ immediately measure the 'output' of the beam, it will not be that accurate. Also, the power meter itself has an accuracy typically of 3%. That could also answer why there are differences in results of similar eyepieces.


The numbers are obtained by first measuring the direct beam, then the beam with eyepiece, this cyle is repeated 10 times. After warm up the laser is very stable, short term fluctions less than 0,5%, this includes the error of the power meter. Standard diviation is 0.2%. Therefor, as stated above, I expect statiscal errors to be less than 0.5 %, systematic errors are much more difficult to judge but I would guess less than 1-2 %.

In other words, the differences in the transmisson of similar eyepieces are real.



best regards

Thomas

#18 robboski2004

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Posted 04 November 2012 - 03:45 PM


Hello Thomas,

Claiming absolute values is always fraught with danger !

However, the control in my setup is eyepiece / no eyepiece.

I would be confident to claim +/- 0.5% with the measurements i supplied.
These measements were obtained over extended periods,using slightly different setups which all became very consistent.

I have a 12mm Delos on order, so will be in a position to measure it's transmission in the next 2 weeks or so ?

Regards,
Ian.

#19 robboski2004

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Posted 04 November 2012 - 03:55 PM


Hello Denis,

Unfortunately, i do not have access to any ES eyepieces
at this time.

Regards,
Ian.

#20 ThreeD

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Posted 05 November 2012 - 01:41 AM

Very interesting stuff.

I know that Alvin Huey, who thrives on pushing the limits of observability, has switched to using Delos for his widefield EPs based on his own experience. These results seem to validate his observations.

Edit: Alvin also swears by his TMB ED barlow. Very, very interesting.

Are any of those Baader Orthos that were tested BGOs?

#21 ThomasM

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Posted 05 November 2012 - 01:49 PM

Are any of those Baader Orthos that were tested BGOs?


Yes, I tested the Baader 18 mm, 9 mm and 7 mm Ortho, see the beginging of the thread, the transmission ranges between ~ 95.7-97.8%

best regards

Thomas

#22 ThomasM

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Posted 05 November 2012 - 02:41 PM

Hello Thomas,

Claiming absolute values is always fraught with danger !

However, the control in my setup is eyepiece / no eyepiece.

I would be confident to claim +/- 0.5% with the measurements i supplied.
These measements were obtained over extended periods,using slightly different setups which all became very consistent.

I have a 12mm Delos on order, so will be in a position to measure it's transmission in the next 2 weeks or so ?

Regards,
Ian.


Jan,

I agree, claming absolute values can be dangerous. I got the impression that your error bars are a little bit smaller than mine, I have just started with eyepiece transmission measurements. I am pretty sure that my total error bar is less than 2 %, but presumably less than 1 %.

You got 93.5 % transmission for the 17 mm Ethos, I obtained 91.7 %. In order to check the discrapency I repeated the measurement and got 91.5 % and in addition I measured the transmission of a second Ethos 17 mm and obtained 92.4%.


Finally, the 'low' transmission of the Nagler 13 mm T6 was surprising, I got 87.6 % and now for a second Nagler 13 mm T6 I obtained 87.3 % ( compared to ~93% of the Nagler 9 mm T6 http://www.amateuras...ips/tips3.htm). I should add that the eyepiece was carefully cleaned, first with with a brush to removed dust, then with a microfaser fabric and 'optical wonder' cleaning agent from Baader.

So, please let us know when you have measured transmission of further eyepieces (Delos 12 mm and shorter focal length).


best regards

Thomas

#23 Alvin Huey

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Posted 05 November 2012 - 03:49 PM

Good stuff...

How about the ZAO-II? ;)

#24 5u4

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Posted 05 November 2012 - 04:07 PM

Wonder if the lowish transmission of the 2 13T6's are representative of all 13T6's or just ones from a certain period where the coatings may have been less than perfect or whatever. I have a brand new 13T6 which makes me wonder where mine falls in the transmission dept.

#25 robboski2004

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Posted 05 November 2012 - 09:43 PM

Good stuff...

How about the ZAO-II? ;)


Alvin,

At a guess , i would say 103% !!

Ian.






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