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Posts Made By: Andreas Braun

May 6, 2006 08:46 AM Forum: Eyepieces

Eyepiece field stop measurement

Posted By Andreas Braun

Hello everybody,

all of us know about the importance of the diameter of the field stop of a particular eypiece since it determines together with the focal length of the telescope the true field of view (TFOV). One proven method is the star-drift-method which is based on counting the time a star near the celestial equator takes to cross the field stop.

My method can be performed at daylight and even indoors.

The measurement setup is as follows:

You place a measuring rod (yardstick or alike) at a convenient distance of your telescope's objective and note the length of the part of the yardstick to be seen in the eyepiece. That's all you need. The evaluation is quite straightforward.

If you name

DT = distance of the target (the measuring rod) to the objective,
WT = width of that part of the measuring rod which is seen with the particular eypiece under test,
WE = field stop diameter to be determined,
FL = nominal focal length of the telescope,
DP = actual distance of the focal plane with the telescope focused at DT,

then the simple lens formula

1/FL – 1/DT = 1/DP

will give you the value of DP.

You now can derive your desired field stop diameter WE as


Example: Eyepiece Nagler T5 26 mm, Telescope TEC 140 with FL = 980 mm

DT = 9.35 m ; WT = 298 mm; FL = 980 mm; DP = 1/(1/FL – 1/DT) = 1.095 m
(the short distance DT = 9.35 m has been made possible through the use of a 2"-extension of 80 mm length)

yielding WE = 298 mm(1.095 m/9.35 m) = 34.9 mm

This result is pretty much in accordance with the value of 35 mm published by Tele Vue for the Nag26. As can be seen the ratio DP/DT is nothing else but a scaling factor between the observed width of the target and the field stop diameter.

The advantage of this method is that it doesn't matter whether the field stop is accessible to direct measurement with a caliper or hidden within the eypiece.


September 20, 2007 11:07 AM Forum: Eyepieces

Re: 35mm Panoptic, improved

Posted By Andreas Braun

Scott Mortimer said:

I couldn't take it anymore ...


same here.

My solution is a far from being as solid as yours but very simple. From the DIY market I have self adhesive aluminum tape 0.05mm (0.002") thick and 2" wide. From the depth of the groove I know the number of stripes needed to be cut from the tape. After applying them to the barrel it works pretty much like without groove.


March 23, 2009 02:18 AM Forum: Eyepieces

The new Leica ASPH zoom 17.6-8.8mm

Posted By Andreas Braun

Two weeks ago I took delivery of my long (VERY long) awaited new zoom: Leica 25x-50x ASPH. The designation "25x-50x" refers to the available magnifications together with the Leica spotting scope's focal length of 440mm. I will call it 17.6-8.8mm. The designation "ASPH" refers to the aspheric lens(es?) employed. It is used with my TEC140 (f/7).

- Body dimensions: 59mm/2.3" diameter, 89mm/3.5" body length (without barrel)
- weight approx. 400g/14oz
- twist-up eyecup (3 positions, 9mm difference)
- comfortable eye relief (18mm)
- smooth zoom-ring movement

Having been a fan of my Leica zoom 22-7.3mm for quite some years my expectations were rather high. My benchmark was this:,,,,,,All_Forums,,,,,,&Words=&Searchpage=3&Limit=25&Main=2742856&Search=true&where=&Name=3690&daterange=&newerval=&newertype=&olderval=&oldertype=&bodyprev=#Post2742856

Fortunately the skies were cooperative at the very first evening showing a 12 day's moon with fairly good seeing conditions. I did compare the new zoom against the ZAOII's and the old zoom at different focal lengths of 16mm, 10mm, 8mm, 6mm, and 5mm. The main target for the higher mags was crater Gassendi. As reported in the given link the old zoom and the ZAOII's again were so close that it wasn't possible to see differences. The new zoom instantly could be seen as being on par with both. But after the comparisons had been performed for more than one hour it even seemed to excede both for coming slightly better into focus, not by a large margin but repeatedly so. It kind of looked a tad "sharper" which I did interpret as being may be even more contrasty. At any rate: it can easily compete for contrast, definition, and absence of stray light. And of course it substantially surpasses the ZAOII's for its huge AFOV and its generous eye relief.

Last week the sun made for a very tough test target for stray light and ghosting. With a filtering of ND5 (100 000 times) the apparent brightness of the solar disk is still 1.6 magnitudes brighter than the full moon. Putting the sun just outside the field stop there could not be seen even the faintest glow. This is equal to the ZAOII's where the approaching solar disk can be seen already through the triangular cuttings without generating stray light within the field of view.

Saturn gave a very pleasing view with natural colors slowly drifting through the large field of view without the necessity to refocus. To conclude this little report: the double cluster looked magnificent at different magnifications displaying pinpoint stars up to the field stop. Finally, the E and F stars within M42 could equally easily be observed with both Leica ASPH zoom and the ZAOII's.

- wide AFOV: 58.5° at 17.6mm up to 79.5° at 8.8mm
- virtually flat field (within my measurement uncertainty of 0.05mm)
- no astigmatism up to the field stop
- extremely sharp
- extremely high contrast due to complete absence of straylight and ghosting
- no eyeball reflections
- neutral color rendition
- parfocal
- no kidney beaning
- replaces three high quality eyepieces (even more with barlowing)
- barlows very well
- with a quality barlow lens element and extension rings it covers all focal lengths

- expensive (800 Euros/1080 USD)
- 2"-adapter not yet available (I'm using a modified Baader adapter T2-#16)
- no click-stops (I don't need it: see below)

Is it the "ultimate" eyepiece? For me: YES. For others as always: YMMV (or better: YMWV). Together with a premium barlow lens (Baader VIP Modular) and a two-stage scheme of adding extension rings I have 3 barlow factors of 1.5x, 2x, and 2.5x. Thus a focal range of 17.6mm down to 3.5mm gives me everything I would need. For widest TFOV I'm keeping my WO UWAN28 which gives incredible views with my TEC140 for having the same field curvature (approx. –380mm).

My UWAN16 and the old Leica 22-7.3mm zoom have been sold already, and eventually I will even sell my ZAOII's since I don't think them to be used often any more after – of course – much more comparisons. I haven't noticed any differences between them and my old Leica 22-7.3mm zoom within a period of 5 months – see link above - and again no differences between the ZAOII's and the new Leica zoom under favourable seeing conditions. OTOH due to the ease of use with this new zoom with its 18mm eye relief, large AFOV's and – most noticeable advantage – the possibility to zoom in and out it is vastly superior to fixed focal length eyepieces of narrow AFOV's and fairly short eye reliefs.

Another thought for observing no differences came from thoroughly reading the famous "6 mm Lunar/Planetary Eyepiece Comparison" by Bill Paolini. Paragraph 3 of his conclusions goes like this:
"3. Larger aperture instruments or brighter celestial objects are required to show significant differences between eyepieces. Many observing sessions demonstrated this, as reported in results sections of this comparison. This leads to the following sub-conclusions:

a. Planetary observations can yield significantly improved details using the highest ranked eyepieces in larger aperture instruments.
b. Lunar observations can yield significantly improved results using the highest ranked eyepieces even in moderate aperture instruments."

My 140mm aperture is way too small to comply with letter "a". But with lunar observations my scope should be able to yield different results – if there are any. As a final thought: may be my northern lattitude of 52.3° will make the neccessary seeing conditions rather unlikely. Anyway, I'm strongly tending to see no future use for the ZAOII's.

The attached picture gives an impression. From left: Zoom with 2"-barrel, barlow lens for 1.5x, 30mm T2-extension for 2x, 32mm T2-extension for 2.5x. The two combos 1.5x and 2x fit within the 2" eyepiece holder of my Baader Maxbright diagonal, the second extension will be placed outside the eyepiece holder. The white markings on the eyepiece body are little pieces cut from a nylon cable binder and glued onto small strips of transparent selfadhesive tape. They enable "feeling" the chosen magnification and are placed such that they give with my TEC140 mags of 60x, 80x, and 100x unbarlowed and the corresponding magnifications multiplied with the barlow factors.


March 5, 2010 12:59 PM Forum: Eyepieces

Leica ASPH Zoom 17.8-8.9mm

Posted By Andreas Braun

One year ago I took delivery of this newly introduced wide field (60-80°) zoom eyepiece. My first impressions have been posted here:

After quite some observations throughout the months and after many comparisons with the ZAOIIs I've published a review about this zoom:

Those of you who haven't seen it already might be interested.


June 24, 2006 01:41 AM Forum: Eyepieces

Experience with Zoom Eyepieces

Posted By Andreas Braun

Well, I'm afraid being kind of an exotic since I almost exclusivly use with my TEC 140 apo my Leica zoom 22mm...7.3mm. My only other eyepiece is the 26 mm Nagler T5 (38x, 2.1 deg TFOV).

In addition I have a Zeiss-Baader VIP-barlow screwed directly into the 2" filter thread giving 1.75x barlow factor (to be varied between 1.6x and 2.3x by means of T2-extension rings).

Thus I cover the complete range from 3.7 mm exit pupil down to 0.6 mm exit pupil with just two eyepieces and a barlow.

The zoom gives extremely sharp and contrasty views with high light transmission. The low AFOV for the longer focal lengths (above 16 mm fl yields less than 50 degs) is not used (exit pupil above 2.1 mm), since this range is covered nicely by the 26Nag.

I've compared the 10-mm-setting against a Pentax XW 10mm and with 2x barlowing against a 5 mm Nagler T6, and the Leica could compete very well.


December 21, 2006 10:40 AM Forum: Eyepieces

Interesting Site w/ eyepiece tests

Posted By Andreas Braun

"Not sure yet how to navigate the site to reach all of the tests but here are a few to start: "

Just click on the "+" (plus-sign) left to the brand name to open the particular site.


March 22, 2007 03:56 AM Forum: Eyepieces

Too Many Eyepieces?

Posted By Andreas Braun

Alan French said:
"Amateur astronomers seem to develop real eyepiece fetishes."

Well, I think I'm the other side of the coin. I've just 3 eyepieces and one barlow for my only telescope TEC 140 (or should I've said "tango-echo-charly").

WO UWAN 28, WO UWAN 16, Leica zoom 22...7.3mm, and Baader VIP barlow.

For deep sky I use my WO UWAN 16 solo (61x, 1.3° TFOV) or barlowed to yield 9mm (108x, 0.73° TFOV) and 6.3mm (155x, 0.51° TFOV). Rather seldom the UWAN 28 (35x, 2.3° TFOV) comes into action.

For solar system the Leica will be used barlowed 1.7x to give 13.2mm/74x/31' TFOV down to 4.4mm/222x/19' TFOV. Additional T2 rings will give additional barlow factors of 2x or 2.3x.

This gives me any desired focal length.

BTW, I'm glad we are back to eyepieces.


May 4, 2009 12:59 AM Forum: Eyepieces

10mm ZAOII & AP SPL...

Posted By Andreas Braun

Evan Chan said:

I was out observing the moon tonight with my TEC6MC and was using the 10mm AP SPL yielding 180x - this was the max the seeing would allow tonight. Pulled out the brand new ZAOII 10mm to do some comparisons.

Here's the thing...try as I did, I could not discern any differences between the two. Having read the comparison review of numerous 6mm eyepieces where the ZAOII came out tops, I was expecting to see a visual difference. Swapping them out numerous times, I tried to see if there was any differences in the following areas:

1) Color of the moon
2) Contrast differential when observing the "shadows"
3) Resolution

Nope. Couldn't do it. No difference. Nada. Is this because of the relatively slow F/12 Mak-Cass? Is it because the 10mm ep? Perhaps is it a combination of both? Or maybe my visual acuity just isn't that great?

Really hoping you eyepiece experts can shed some light on this

Just one more craterlets in Plato were visible tonight so perhaps it is seeing related?



your last remark gives the answer: seeing.

I've done lots of comparisons (even with seeing conditions enabling mags of about 220x) between my ZAOII's and my barlowed Leica asph zoom without detecting differences. May well be there are none or that subtle to not be easily detected.

But I remember a recent posting of a very experienced fellow astronomer on "the other forum" saying: "Seeing conditions trump all eyepiece differences. If the seeing is poor, a Zeiss ortho won't see doodleysquat. If the seeing is truly superb, the lowliest (fill in eyepiece of your choice) will see a fantastic planetary image." (Don Pensack hopefully will not object being cited here!)

Two days ago I've had glimpses of Alpine valley rille with both eyepieces at 180x but again no differences.


September 21, 2009 01:07 AM Forum: Equipment Talk

What are the tradeoffs of a zoom eyepiece?

Posted By Andreas Braun

Brent Campbell said:

What are the trade off's of a zoom eyepiece? I am getting back to observing again, and I have a variety of orthoscopic eyepieces from University Optics, Celestron, and a no-name import.

One of the things that bugs me the most is changing eyepieces. When looking at something having to fiddle around with the eyepiece, take it out of the case, decap it, etc....can be very annoying. Of course you loose the caps in the dark!

A zoom such as the 3-6mm Nagler Zoom, Orion has a "premium Zoom", or maybe even a pentax zoom such as this:

Another possibility is to use a Denkmier Power swicth Diagnol- favorably reviewed in Rod Mollise's book and also on cloudy nights.

Anyone have any thoughts on this? I will be using the eyepieces with a Celestron 8 Ultima which I just purchased.

You should consider the Hyperion 24mm-8mm zoom. The 24mm setting isn't of much use since the TFOV is only slightly different from the 20mm setting. But all the other settings are really attractive. And so is the optical quality as reported by many, many astronomers.

I used to test it for a short period of time and did some measurements.

The data are (first figure: focal length, second figure: effective field stop, third figure: AFOV, calculated without distortion):

24mm/18.2mm/43.4° - 20mm/17.7mm/50.7° - 16mm/15.2mm/54.4° - 12mm/12.7 mm/60.7° - 8 mm/10.1 mm/72.8°

With your f/10 scope it would give you almost everything up to 250x. I don't know the current price but it should be somewhere around 200$.


July 2, 2010 02:34 AM Forum: Refractors

What is the BEST visual mount for a TEC140

Posted By Andreas Braun

David Standen said:

Guys and Ladies, I just got a TEC140 (used) but need a mount for it. ...

.... then third choice would be a plain old ALT. Looking for something steady for visual use that won't break the bank (scope already did that!) p.s. Can't believe the wife let me buy it.

For visual use I'd highly recommend the Swiss AOK mount. I'm using it for four years now without any problems up to any reasonable mag.

See the pic.