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Equipment Discussions >> Eyepieces

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ThomasM
sage


Reged: 04/19/09

Eypiece transmission
      #5502437 - 11/03/12 01:47 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.amateurastronomie.com/Astronomie/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)

Edited by ThomasM (11/05/12 01:42 PM)


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jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: Eypiece transmission new [Re: ThomasM]
      #5502589 - 11/03/12 03:49 PM

Did you clean the eyepieces before testing them?

Regards,

Jim


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ThomasM
sage


Reged: 04/19/09

Re: Eypiece transmission new [Re: jrbarnett]
      #5502595 - 11/03/12 03:55 PM

Quote:

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?


Edited by ThomasM (11/03/12 04:09 PM)


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Eypiece transmission new [Re: ThomasM]
      #5502607 - 11/03/12 04:07 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?

Edited by Starman1 (11/03/12 04:10 PM)


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ThomasM
sage


Reged: 04/19/09

Re: Eypiece transmission new [Re: Starman1]
      #5502624 - 11/03/12 04:27 PM

Quote:

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


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jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: Eypiece transmission new [Re: ThomasM]
      #5502642 - 11/03/12 04:44 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.

- Jim


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robboski2004
member


Reged: 01/14/08

Re: Eypiece transmission new [Re: jrbarnett]
      #5502696 - 11/03/12 05:38 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 !!

Regards,
Ian.


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denis0007dl
sage
***

Reged: 04/17/12

Loc: Croatia
Re: Eypiece transmission new [Re: robboski2004]
      #5502768 - 11/03/12 06:22 PM

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

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Paul G
Post Laureate
*****

Reged: 05/08/03

Loc: Freedonia
Re: Eypiece transmission new [Re: ThomasM]
      #5502905 - 11/03/12 08:07 PM

Quote:

Quote:

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


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jtaylor996
scholastic sledgehammer


Reged: 09/02/08

Loc: North Texas
Re: Eypiece transmission new [Re: Paul G]
      #5503127 - 11/03/12 10:34 PM

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

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robboski2004
member


Reged: 01/14/08

Re: Eypiece transmission new [Re: jtaylor996]
      #5503146 - 11/03/12 10:51 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.


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ThomasM
sage


Reged: 04/19/09

Re: Eypiece transmission new [Re: Paul G]
      #5503419 - 11/04/12 05:09 AM

Quote:

Quote:

Quote:

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


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ThomasM
sage


Reged: 04/19/09

Re: Eypiece transmission new [Re: robboski2004]
      #5503421 - 11/04/12 05:15 AM

Quote:


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 !!

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


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davidpitre
Post Laureate
*****

Reged: 05/10/05

Loc: Central Texas
Re: Eypiece transmission new [Re: ThomasM]
      #5503576 - 11/04/12 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%.


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astroducky
sage


Reged: 10/02/06

Loc: South East Asia
Re: Eypiece transmission new [Re: ThomasM]
      #5503645 - 11/04/12 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.

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ThomasM
sage


Reged: 04/19/09

Re: Eypiece transmission new [Re: davidpitre]
      #5503681 - 11/04/12 10:10 AM

Quote:

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


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ThomasM
sage


Reged: 04/19/09

Re: Eypiece transmission new [Re: astroducky]
      #5503687 - 11/04/12 10:17 AM

Quote:

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

Edited by ThomasM (11/04/12 03:46 PM)


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robboski2004
member


Reged: 01/14/08

Re: Eypiece transmission new [Re: ThomasM]
      #5504134 - 11/04/12 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.


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robboski2004
member


Reged: 01/14/08

Re: Eypiece transmission [Re: denis0007dl]
      #5504148 - 11/04/12 03:55 PM


Hello Denis,

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

Regards,
Ian.


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ThreeD
Pooh-Bah


Reged: 12/23/08

Loc: Sacramento suburbs
Re: Eypiece transmission [Re: robboski2004]
      #5504857 - 11/05/12 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?

Edited by ThreeD (11/05/12 01:46 AM)


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