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12mm Radian and 12mm Pentax XF


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12mm Tele Vue Radian & 12mm SMC Pentax XF



Welcome, Reader!
Here you see some smaller participants of this comparison, surrounded by two Swiss Mountain Crystals. From left to right, in the background: Variable Zeiss Abbe Barlow, 1.8x TMB ED Barlow, 2.5x TV Powermate, 13 & 12mm Nagler, 14mm Pentax XW, 12mm Pentax SMC ortho, 12mm TMB Supermono & 10mm Zeiss Abbe ortho. The 12.5mm Circle-T ortho is missing for unknown reasons. Finally in the foreground the 12mm candidates: Monsignore Radian Tele Vue & Signorina S.M.C. Pentax XF.

1.1 Why These Two Eyepieces?

More than a year ago, when I asked my vendor for a SMC Pentax XW 10mm, he gave me also a 10mm Tele Vue Radian to try out. I was astonished by the Radian's performance, when I looked at daylight through a Dob at an "artificial star" consisting of an isolator for an electric wire in about 50m distance. The choice was not easy, but at the end I preferred the XW's superior ergonomic and comfort. The view offered by the Radian was not forgotten, and so I ordered some months later a 12mm Radian. I own the Radian since a year now, but I must admit that this eyepiece was very seldom used.

From the moment I heard about the new SMC Pentax XFs I was very curious about their performance. Unfortunately I was not able to find any longer review about them. I thought I could get one of the XFs myself to be able to inform my fellow astronomers. Out of this curiosity I ordered both XFs, the 8.5mm and the 12mm. Only the 12mm was on stock, the 8.5mm took longer to arrive here.

So when the 12mm XF arrived at the end of december 2005, my first thought was to compare all of my seven 12mm eyepieces. But very soon I had to understand that this would become a too difficult thing to do for me. This is why I chose the most similar eyepiece to the XF, and this is the Radian, because these two show a 60° apparent field of view.


Picture 2: Both eyepieces with fully extended eyecups.

1.2 My Former Experiences

I observe the stars since 11 years. I have owned six telescopes and several binoculars. By now I call 3 telescopes and more than 50 eyepieces my own. My main observation interests are deep sky objects, especially galaxies, but in nearly all observation sessions I also watch other objects of all kinds, planets included. While I like to observe the Moon I can call myself only an occasional Moon observer.

1.3 About This Test

Please understand that I do not have any prejudices against any of the eyepieces I mention - since they are all mine. Of course I am not related to any of the mentioned companies. I consider myself as a beginner in comparing eyepieces. I have performed an internet research about testing methods, have tried out some ideas by myself, and I tried to perform this comparison the way I would have liked to read it - but all the same I am very sure I still have a lot to learn about this.

I want to thank my CloudyNights teachers Mike Hosea, Daniel Mounsey, Tom Trusock, Don Pensack, Sven Wienstein, Alexis Cousein and many more for all the help they offered to the astronomic community. I also want to thank Thomas Back, Walter Koprolin and others for publishing their testing methods in the internet.

Please do not expect a comparison with a winner. There will be none to be found here, and the work of the involved companies will be respected as it is - even if I will feel free to express my personal feelings. As I am fascinated by eyepieces and as I own many of them, my main goal is to find out how to use a specific eyepiece. So my actual question is:

*On which kind of celestial objects does this eyepiece fit best?*

1.4 Used Material

I used my three telescopes:

  • A 30cm GSO Dobsonian, f-5, with Pyrex mirror, flocked opposite the focuser, equipped with an JMI NGF-DX3 focuser, collimated by using the Catseye collimation tools.
  • An 8cm William Optics Zenith Star fluorite apochromat, f-6.9, used with a dielectric diagonal (and sometimes without diagonal) on a Vixen Porta mount.
  • In the last sessions I also used my 11cm TS achromat, f-6, together with the Williams apo, both together on an Swiss AOK AYO mount on a Berlebach tripod.
Besides the two tested eyepieces I used my following oculars as a reference (in alphabetical order):

  • "Circle-T" ortho 12.5mm (in the USA known as UO "volcano top ortho")
  • Pentax SMC ortho 12mm (discontinued)
  • Pentax SMC XW 14mm (only for the section about ergonomics)
  • Tele Vue Nagler t4 12mm
  • Tele Vue Nagler t6 13mm
  • TMB Supermono 12mm
  • Zeiss Abbe ortho 10mm (discontinued)
All eyepieces got checked and if necessary cleaned before the sessions, using a camera cleaning liquid and the procedure described on the Tele Vue site.

Same with my three Barlows:

  • Tele Vue Powermate 2.5x
  • TMB ED Barlow 1.8x
  • Zeiss Abbe variable Barlow 2x (or more than 3x with the extension rings)

Picture 3: The end of a daylight resolution testing on a specially cold optical bench...
On the left is the 11 cm TS achromat, right the 8 cm WO apo, both on a Swiss AOK AYO alt-az mount on a Berlebach tripod, aiming at the right test pattern sheet, 90 steps away. The left sheet is from a family looking after their dog. The dog is called "John Russel", which is a rather unusual Swiss dog name. If ever you meet John Russel, tell me, ok?

1.5 Resulting Magnifications

Dobsonian:1500 / 12 = 125x
Apochromat:555 / 12 = 46x
Achromat:660 / 12 = 55x


1.6 Testing Conditions

Most of the comparisons were performed in the Swiss Prealps, on a snow covered hill at 1100 meters altitude, during end of december 2005, january and beginning of february 2006. While this site is not as dark (measured 20.55 - 21.11 mag/arcsec^2 without Moon) as my usual alpine observation sites, I found the seeing conditions to be more stable. This site was high enough to be above the high fog. I encountered near-ground temperatures of -5°C down to -12°C. Note that the ground temperatures vary from the actual temperature on eyepiece-height, being some 6° to 10°C warmer because of the influence of the cold snow. Please note also that while cold can affect an eyepiece's performance, I was told by an expert that an eyepiece should perform normally until -5°C. So the temperature should have caused no issues.

As for the influence of the cold on the observer, I can say that temperatures down to -15°C feel good to me because of the used high quality clothing. All my sessions were lived without serious cold problems - not at all because I enjoyed them.


Picture 4: Dobson in the Night

The very first and the third-last night sessions were try-out sessions without writing anything. I think it was a good idea just to play in between of the testing. I don't really believe in first impression-reviews. All my results have been tested and retested at least two times, mostly more, as I always wanted to be very sure.

The last night session was a 7 hours observation at 1600 meters over sea level, with a measured darkness of an astonishing 21.62 mag/arcsec^2 (estimated near 7mag - despite the snow's reflection!), with bad seeing and ground temperature down to -15°C.

The last sessions were at several afternoons and one evening in the second half of march and in april, near my home.

The total number of observation sessions for this comparison was: 8 night observation sessions of 3 to 7 hours, plus four daylight/early-evening sessions of 2 to 3 hours (plus some shorter sessions for trying out my optical test patterns).

1.7 Short History And Description Of The Eyepieces

The Tele Vue Radian was introduced around 1999 and got many enthusiastic reviews. Tele Vue writes: "(...) with fully multicoated exotic glasses, we reached our new goal of full field sharpness with true orthoscopic linearity, highest contrast for critical planetary viewing, and compact size." In 2006, I can still read in a most recent German catalog: "Regarding sharpness and contrast the Radian reaches absolute peak votes and can be compared to the very best orthoscopic eyepieces". Similar can be read on many reviews from all over the world. The Radians were intended as astronomical eyepieces. The American optical designer Al Nagler intended them especially for the astronomers using eyeglasses. He writes in his sympathetic open way: "As the hobbyist population ages (along with me), more amateurs need eyeglasses". The Radians are also aimed to users of small instruments and bino-viewers.

The Radian line is impressive:

3 / 4 / 5 / 6 / 8 / 10 / 12 / 14 / 18mm

The 3 to 6mm are 7 elements, 5 groups designs, the 8 to 18mm have 6 elements in 4 groups. Apparent field of view is 60°, eye relief is 20mm. Barrel diameter is 1.25". The weight of the 12mm is indicated as 240g. I controlled this with my Tanita KP-400M electronic scale, and found out that my Radian weights 249.4 g without caps (± 0.2 g). Unfortunately, as always with newer Tele Vue eyepieces, there are no additional eyepiece design or coating informations available. Televue writes not to disassemble the Radian, as reassembling requires a special tool.

The Radian comes in the usual black cardboard box together with a plastic ring, the Pupil Guide (intended as a help especially for novices), and two lens caps.

My Radian has engraved "Taiwan ROC".


Picture 5: Left is the SMC XF, right the Radian.
You can see that the filter thread of the XF is blackened - so if one uses filters the usual way, some colour might go away. Pentax warns about this in the instruction sheet.


Picture 6
It is better to put this eyepiece in a threadened extension tube when filters are in use. I recommend this method for all eyepieces as changing eyepieces also goes faster this way. The only question about this method is if you'll have enough in-focus-travel with your scope (it worked with all of my scopes).


The SMC Pentax XFs must have been introduced around 2004. I have found only one internet review about them, on a birdwatcher site. There the XFs got compared to the XWs, and the opinion about the XFs is not really overwhelming. While the German importer writes about "special ED lenses with the use of lanthanum glass", the birdwatcher site says: "[the XFs] do not have the extra low dispersion lanthanum glass optics". They are advertised in Germany and Canada as "compact, cost-efficient eyepieces {designed for spotting scopes}". The XFs were not really intended as astronomical eyepieces. In the instruction sheet Pentax declares both XFs as daytime eyepieces, but also usable for astronomy, using the following words:

"This eyepiece is developed for use with the PF-65ED / 65 ED and other Pentax Spotting Scopes. The 31.7mm diameter screw enables use with astronomical telescopes. However, note the above upon use."

I find the last sentence to be very diplomatic... ;-)

The XF serie consists of only an 8.5mm, a 12mm and a 6.5 - 19mm zoom. The XFs are also a 6 elements, 4 groups design. Apparent field of view is 60°, eye relief is 18mm. Barrel diameter is 1.25". The weight of the 12mm is indicated as 155g. My 12mm XF weighs 146.4g without caps.

Both XFs use a scaled design. The Pentax Multicoating is used, as indicated by the name "SMC Pentax XF".


Picture 7: SMC XF eyepiece design

The XF's eyecup can be easily unscrewed to clean the eyelens or to attach an adapter, probably to attach a camera.

Unfortunately these eyepieces come with caps only, the bolt box used with the XWs is missing. Each XF seems to have a serial number, on my 12mm it is a 6-digit number.

They are made in Japan, like all my XWs refering to the engraved origin. I write this because I read a rumor about Pentax eyepieces made in China, which seems not to be true.

1.8 Test Ratings

For a better understanding I have rated my impressions after most sections. The ratings of the differences are:
  • big
  • noticeable
  • small
  • very small
  • extremely small
  • none
I have also written at the end of the sections in how many stars the difference results:
  • 0 * (zero stars difference)
  • 1 * (one star difference) etc...
In the final section all results will be collected in a chart - there, 1 star is the minimum and 5 stars is the maximum.

2. Test Results in 21 Sections

2.1 Coatings and Reflections

I took away the eyecups (the Radian's rubber, and the complete upper part of the SMC XF) to get a clear picture of the lenses. I left the bottom caps on the eyepieces to get dark optics. I used two white lamps, one from left and one from right, to take these two pictures:


Picture 8: The smaller eyepiece, here up, is the SMC XF.
Picture 9: The bigger eyepiece, up in the picture, is the Radian.

Note that I have changed the places of the eyepieces, and still, both pictures show two additional reflections on the Radian's front lens. I have marked them with two red arrows on the second picture. On the next pictures the same two additional reflections will be found again.

Here there is also a difference in reflections, but at this angle you can recognize this is due to the different heights of the eyepieces. These pictures have only been made for the purpose of showing the different colors:


Picture 10: SMC XF in front, Radian behind.
Picture 11: Radian in front here.

... Difference in reflections: I am afraid I can not rate this, as I lack experience doing so. It seems that the SMC XF is slightly better constructed.

2.2 Handling (always with two pairs of gloves)

Radian: Mostly normal handling, but when I observed the whole night through, getting tired I found myself in the late night hours taking the Radian out of the focuser holding it at the eyecup, or taking the eyepiece out of the focuser too fast, which caused the eye cup to rise every time. So it had to get readjusted: "1, 2, 3" clicks, which I found annoying. I think there is a way to change the tension of the eyecup mechanism, but I am not quite the technician to change anything on my eyepieces.

SMC XF: Normal, easy and easy-weight handling.

... Difference in handling: small = 1 *

2.3 Ergonomics

Radian: The 12mm Radian uses the Instadjust system: a push-pull, click-stop adjustment with 8 positions. I always used the position 5 (or 3 from the top), as observing with the fully extended eyeguard resulted in a reduced field of view. I did not use the Pupil Guide plastic ring, as it did not seem necessary to me. The soft, fold-down rubber eyeguard shielded my observing eye against wind and brightness caused by the snow. The Radian was very easy to use. The soft rubber felt good on my face, warm, comfortable and not sticky. I found it to be very ergonomic. This is a very comfortable eyepiece, nearly as comfortable as the Pentax XWs. Due to the adjustable eye cup I could lean my face on the eyepiece (the eyecup never moved down when I did so), which resulted in a more relaxed observation. Only the Pentax XW is a tad more comfortable than the Radian. The XW also "closes better", which means the eyecup shields a little bit better from sidelight. But this may vary by the form of your face. SMC XF: The 12mm SMC Pentax XF uses a 4-position, turn, click-stop adjustment. I only used the most extended position. Even this proved to be slightly too short for the needed eye relief. With the eyepiece being new to me, I had some initial difficulties to find the right viewing position, resulting in kidney beaning. I found my ideal viewing position by leaning my eyebrow against the eyepiece. From then on, no kidney beaning occurred anymore - already at the very first session. The rubber felt warm and not sticky either. The diameter of the eyecup is a little bit too tight for my eyes. With the XF's slightly too short eyecup, the shielding effect was not as good as the Radian's. The XF's ergonomic does not reach the degree of perfection of the Radian or the Pentax XW. This is an eyepiece one must learn to observe with. But once found out how to use it, the eyepiece was easy to observe with.

... Difference in ergonomics: noticeable = 2 *


Pictures 12 & 13
The eyecup of the XF moves up by turning it. The eyecup has 4 positions which are quickly reached (not like the XWs, which need much longer time). First I thought the inside lying thread would be made of plastic, but no, it is aluminum (under the rubber).

2.4 A Special Word About Blackouts And Kidney Beaning

As mentioned before, these eyepieces both work well. Both can show blackouts or kidney beaning if the eyecup is not adjusted correctly. Once the eyecup is adjusted I did not experience any disturbances of this kind. Even on the bright Half-Moon, while I noticed there was a tendency to blackout in both eyepieces, I could avoid this by adjusting the eyecup correctly.

... Difference regarding blackouts and kidney beanings: none = 0 *

2.5 Dewing

In all these observation sessions the XF dewed up exactly once more than the Radian. I suppose this is because of the smaller air volume between the eye and the eyelens of the XF. In a night specially prone to dew, I simply pushed down the eyecups of both eyepieces, by this allowing the air to circulate better, which partly resolved the dew problem. I was able to wave the dew away by using my hands.

... Difference in dewing: extremely small = 0 *

2.6 Observing with Eyeglasses

While I don't wear spectacles, I tried to simulate this situation by observing Jupiter with my sunglasses. I encountered no specific problems with the eye relief. Of course this remains a very personal and not perfect impression. Because of the rubber eyeguards both eyepieces did not scratch the glasses.

2.7 Two First Viewing Impressions

#1 At the very first moment of the comparison the Radian seemed to show a wider field.

#2 The Radian was easier to focus at the beginning.

#1 I wondered if the Radian really has a wider field. So I took the Moon's edge at one edge of the field of view and a star at the opposite edge as a measurement. Both eyepieces showed to have the same field of view. Probably the Radian seems to have a wider field by being the bigger eyepiece. Maybe the easier ergonomic was involved into this feeling, too. More about this in the next section.

#2 In the very first session I found the Radian easier to focus. Once I had found out how to slow down the motion to focus the XF, I felt the XF easier and more precise to focus.

2.8 Apparent Field Of View

During one of the last sessions I checked the apparent fields of view. Watching a wood pile at daylight I noticed the slightly wider field of the XF. Some hours later I observed several times the Orion's star Mintaka and measured the time this star needed to cross the fields, after that I calculated the average time. The difference of the averaged times was 10 seconds. According to my measurements the Radian shows in my apo a 1.28° field, while the XF shows 1.32°. If the magnification is really 46x this results in apparent fields of view of quite exactly 59° for the Radian and nearly 61° for the Pentax XF.

... Difference in field of view: small

2.9 Ghosting

While observing bright objects as the Moon, Jupiter, Saturn, Sirius, Vega, etc. I experienced ghosting only once with the XF. I had my eye about 4cm away from the eyelens and the rapid movement of the ghosting indicated a reflection of the light on my own eye. When I moved my eye to the normal observing position this ghosting disappeared.

... Difference in ghosting: none = 0 *

2.10 Scatter

I tried to find out about scatter observing Saturn's moons. This was not easy. On most sessions I found the XF to have a tad less scatter, as Saturn's moons were slightly easier to be seen. But in two of three times, when I verified, the moons were also in the Radian to be seen.

So I tried to observe bright stars with fainter neighbours, such as double stars Polaris, Castor, or faint stars near Rigel etc. The difference was minimal. A big surprise was the Zeiss Abbe ortho which showed nearly no scatter at all.

Later I tried to observe the scatter of a bright object, watching the scatter with averted vision. The XFs "scatter cloud" was a tad smaller.

... Difference in scatter: small = 1 *

2.11 Glare

Positioning a bright object outside the field of view and using the XF in the apo I detected a short ray of light coming in, of maybe 1/20 of the field of view. I tried this in the Dob also, but there was no ray. So this must be a problem of the telescope and not of the eyepiece.

... Difference in glare: none = 0 *

2.12 Contrast On Deep Sky Objects

This is my greatest concern about eyepieces.

I often read about contrast being measured on objects like globular clusters, nebulae like M42 (the Great Orion Nebula), or other targets. By now I have learned that colour fidelity and scatter could interfere with the contrast observations.

So my contrast testing was done by only observing galaxies. I wondered if it was possible to see differences with a sky darkness of less than 6mag. So I have tried this with all of my 12mm eyepieces, and, well, this is not easy. Slight differences are not to be detected under a relatively bright sky. But under these conditions, while comparing the Radian to the XF, the differences were very clear.

I only tested on galaxies with clear structures:

#1 Leo edge-on spiral galaxy NGC 3628 (member of the Leo Triplet or M66 Group), observed with the Dobson:

First view with the Radian showed a black dust lane. The XF showed the black dust lane and the fainter part of the galaxy beyond the dark lane. As I know that one learns to see an object by observing it longer, I went back with the Radian, then XF again, etc. until the final check with the SMC ortho.

Results:

The SMC ortho showed both parts to be seen using direct vision. Also some mottling in the bigger part of the galaxy.

The XF did not show this mottling, or only extremely faint. Both parts to be seen using direct vision. The galaxy is longer in the XF as in the Radian.

The Radian showed the smaller part of the galaxy only faintly, even using averted vision. No structures. The right side of the galaxy seemed like a cloud, very unclear.

(This has been rechecked, using the apo and observing M65 and M66. The XF showed again a clearly better view.)

#2 Canes Venatici galaxies M51 (Whirlpool Galaxy and companion), seen with the apo: Radian: I need averted vision to recognize the two cores and faint halos XF: I see both cores with direct vision. Averted vision reveals even faint mottling in the bigger halo.


Picture 14
Drawings of the views of the Sombrero Galaxy M104 offered by the Radian, the SMC XF and the SMC ortho.

#3 Virgo edge-on galaxy M104 (Sombrero Galaxy)

Using only direct vision the Radian shows only the brighter part of the galaxy and the dark dust band. The XF, too, but also the fainter part of the galaxy is to be seen. The SMC ortho showed the galaxy clearer (also slightly brighter).

Of course I have retested this several times, as this difference was surprisingly big. Last testing happened with the Dobson under the best sky I have ever measured with my SQM (21.62 mag/arcsec^2), galaxy M51 again:

Radian: Clearly less spiral arms to be seen.

SMC XF: Best view of M51 of my life!!! Many spiral arms obvious with direct vision.

I found the SMC XF's performance similar to the "Circle-T" ortho and close to the SMC ortho.

Of course I have also tested on M31, M33 etc. Frankly, I was not prepared to such an obvious contrast difference between these two eyepieces. Of all my seven 12, 12.5 and 13mm eyepieces, the Radian showed by far the faintest contrast.

... Difference in contrast on deep sky objects: big = 3 *

2.13 Contrast on the Moon

While I tried to find out about the eyepieces' sharpness using the Moon, I found to see all details of the SMC XF also in the Radian, but a tad less pronounced. After some switches I understood the XF showed a clearly more contrasty view of the Moon. All details seen at these low magnifications were to be seen in both eyepieces, but the black-grey contrast was more pronounced in the XF, making details easier to be observed - even for me, an occasional Moon observer.

... Difference in contrast on the Moon: noticeable = 2 *

2.14 Colour Fidelity

I have read several times that "Tele Vue eyepieces tend to show a yellowish tint". While I don't know about Tele Vue eyepieces in general, as I have not tested them all, I have a different opinion: I do see that the Radian shows a more reddish colour. Actually I have already seen a blue star seeming slightly violet in the Radian. Of course I know that violet stars do not really exist - but one can read about violet stars in many places, like in the "Night Sky Observer's Guide", or on the beautiful site about double stars: http://www.belmontnc.4dw.net/dblstrs.htm

So this was intriguing me. I had to try this and find out the truth. While I know that violet stars can be the result of an optical illusion or of a colour contrast phenomenon, I have seen a transmission measurement of a Panoptic eyepiece, having its peak at 610 - 650nm, so tending to the red (= 650nm), so I thought there was the possibility that this could be seen, since: Blue plus Red = Violet.

End of january I tried to measure the light throughput of the eyepieces using a hand held spectroscope. I did not detect any change in the spectrum.

So I used the following techniques:

#1 I defocused until I saw some few Airy discs. Now I concentrated on the border of the moving area: Here is the colour!

#2 I used a Zeiss Abbe ortho as a reference eyepiece, since it is known as showing "white" or "neutral" colour.

#3 Finally, since I know that my eyes do not have the same colour perception (one shows me the world slightly "more blue", the other slightly "more red") I therefore performed these experiments with both eyes.

Here are my results:


Chart 1:Checking star colours

Panning, somwhere in the sky: The Radian shows all yellow and orange stars stronger, kind of more fun. But, watching attentively, the XF showed more subtle differences. Kind of another, less obvious and more detailed fun.

So, in both my eyes, there is no doubt: The Radian shows the stars slightly reddish. While I found this to be disturbing on some blue stars, especially after having used another eyepiece, as these stars seemed kind of greyish for example, this does not mean that it is rejectable everywhere. For instance the view of Betelgeuze was wonderful! I really think it depends on knowing on which object to use a certain eyepiece. The SMC XF's colour fidelity came rather close to the Zeiss.

... Difference in colour fidelity: noticeable to big = 3 *

2.15 Sharpness on-axis

This was the most difficult test.

While both eyepieces showed the same parts of the Cassini Division even at the low magnification of the apo, I was able to see more details on Saturn's globe using the Radian - but on the other side I saw more details on Jupiter using the XF. So I understood this was not about sharpness, but about colour fidelity. I think using Saturn or Jupiter to measure the sharpness of an eyepiece does not allow an universal conclusion because of the colour-factor. And, as I have found out by observing the Moon, Luna is not really the best test-object for sharpness either - this could be confounded with contrast. So this was my biggest problem of this test. I thought to have found two solutions during the last night:

In the apo I found the stars slightly smaller dots using the XF. But I was not really sure about this impression. During the last session, using the Dobson and its higher magnifications under bad seeing, I found the difference to be clear: The XF shows smaller stars.

In the morning, before the sun raised, I aimed the Dob at a distant mountain top. I saw pine trees, amazingly at this high altitude... While I initially thought both eyepieces showed the same details of this tree maybe 5 - 10km away, I found two small, thin branches of the tree near its top. Here the difference became clear: The XF showed a clear and sharp picture, while the Radian only showed a softer view of these very faint details.

I have sent these results to Mike Hosea and he encouraged me to continue the sharpness comparison. So I searched after a better comparison possibility. After a discussion with a Swiss astronomer I tried to use a downloaded photographic camera test, but here the problems started already after printing it with my bubble jet, resulting in Moiré-patterns. I tried a laser printer, which did not really help much. Now I started an intense and long research about other photographic camera tests, then about applications able to draw lines of 1/20 points difference, about laser and ink-jet printers, about dpi and ppm.

While all these researches I developed a test myself, and after several unhappy trials I invented a working one, which is consisting of parallel lines of different thickness.

I tried an earlier version of this test on a sunny afternoon in the snow. I pinned my test-sheet on the wall of a hay hut, drove about 300 meters away and set up the apo and the achro together on the AYO mount. But this was much too far away, so I carried the equipment closer to the hut, this several times, until I found the right distance where the eyepieces started to resolve the patterns in the less-than-1.00-line range. Later I counted my steps until the hut: 90 steps in some low snow.

I set up the scopes on the snowy field, to avoid air turbulence of the warmer road.


Picture 15: This is one of the early Windows-versions of my test patterns, before I started redrawing them with my Mac. This is approx. version 6.

So here is what I saw:

With the apo, at 46x, both eyepieces were able to resolve the pattern of 0.75 lines. Sometimes it seemed that the XF had an ever so slight advantage, but after some time I understood that I saw an extremely slight contrast difference even under the sunlight, as the Radian showed a yellowish view. But no eyepiece, not even my orthos or the Supermono showed more resolution. I really tried hard and very long (always again during 2 hours), but there was no way to resolve the 0.70 lines-pattern.

After this I had to refine my test, which took me several weeks, until I found out how to correctly use the clever technical software, then how to correct the limits of my printer, then found out about the differences in printing papers. And redrew it for the 27th time.


Picture 16
This version 27 is, so I hope, the very last version. This picture shows only a corner of the test sheet (which has all patterns from 0.60 points up to 2.25 points in steps of 0.05 points). The original print has nearly no Moiré patterns anymore - finally.

Now my test worked well.

Using both refractors in another try-out showed no difference in sharpness between the Radian and the XF, even when using two Barlows together and by this magnifying far over 230x. Only one of my specialized planetary eyepieces showed a tad more sharpness.

But in the last sunny afternoon session, using a longer distance, the Dobson showed a very subtle difference: I was able to resolve the 0.60-pattern more often and more easily with the XF.

I also watched a building on the top of a high mountain 30 kilometers away, and the details of the window were slightly clearer defined in the XF.

So, with these four impressions (the smaller stars, the far-away branches , the last result with the test-patterns and the 30 kilometer distant window) I am quite sure that the XF shows a better resolution.

... Difference in resolution: very small - small = 1 *

2.16 On-axis and Off-axis Resolution

f-5 Dobson:

Radian: field curvature starting at around 80% from center to border of field of view, getting slowly stronger.

XF: field curvature starting at around 60% from center to border of field of view, getting faster stronger. The stars were getting "bothering" around 75%.

Later, using my test patterns, I focused only once, in the center of the field, moved to 50% of the field, and to the border of the field without refocusing, and read:


Chart 2: On-axis and off-axis resolution

I present you this in a diagram: (m = meters = approx. distance scope to test sheet)



Nice to be seen how the field curvature of the Radian progresses uniformly. By this is also said that even the Radian shows a certain, smaller, amount of field curvature.

On the other hand, in every measurement, the field curvature of the SMC XF follows more or less the Radian's curve from the center to the second measurement and then rises faster - in every scope. Here I suspect to see the influence of the SMC XF's design which is aimed at spotting scopes.

I find remarkable how the XF's curves of the Dobsonian and the achro are parallel, while after the second measurement the apo's curve rises far more dramatically. So I think there is more to find out about field curvature (as has already been discussed in CN's forae) - because this very example shows clearly that field curvature is not automatically created by a short focal ratio. There must be a completely different interaction between the SMC XF and the apo's optics, which leads to this faster rising curve. And, finally, I do know that these my measurements do not show only the field curvature, but also include the various aberrations of the eyepieces and telescopes.

... Difference in field curvature: noticeable to big, depending on the telescope = 2 * - 3 *

2.17 Lateral Colour Aberrations

Using the apo, and after that also the Dobsonian, I moved Jupiter through the field of view: The Radian showed much more colour, starting at 50% between center and border, getting stronger fast. The XF started a little bit later, at around 60%, getting very slowly stronger.

The "final image" of Jupiter at the border of the field of view was the following:

Radian: Both main cloud bands of Jupiter to be seen, but Jupiter drowned in many colours.

SMC XF: Both main cloud bands of Jupiter to be seen, a little colour, but Jupiter awfully unsharp because of the field's curvature.

I have tested the lateral colour aberration on the Moon also, and the results were the same.

... Difference in lateral colour aberrations: big = 3 *

2.18 Pincushion Distortion

I used Jupiter to control this, but also the Airy discs of bright stars. I found no pincushion distortion in both eyepieces.

... Difference in pincushion distortion: none = 0 *

2.19 Impressions Of Various Observed Objects

  • M42: Playing with my eyepieces, after having observed M42 with all my other eyepieces, the Radian came into the focuser: I got the sudden impression of a slightly overall darker image
  • M13 (globular cluster in Hercules): XF shows some more stars, I see also more depth in the core.
  • M3 (globular cluster in Canes Venatici) Here I chose a line of three stars "right of" the globular cluster. Radian: with averted vision seen diffusely as "a line" XF: 2 of 3 stars seen with direct vision (same as "Circle-T" ortho) XF shows more resolved stars in the core.
  • M57 (Ring Nebula in Lyra): XF: slightly green colour Radian: colour unclear, no colour
  • M42: In a night with Moon, I found the Orion Nebula to be slightly green-bluish in the XF. But I had difficulties to name the colour seen when using the Radian: Was there any colour at all?
  • M37 (open cluster in Auriga): More effectful with the Radian, especially the orange star in the middle of the open cluster.

2.20 Accuracy of Manufacturer's Descriptions

This is probably a typical Swiss need, so please excuse me. - Here I consider the precision of the descriptions given by the manufacturers in their advertising. We customers have very few possibilities to verify all these descriptions about coatings and exotic glasses etc. This is why the few differences I have found are very important to me, as they tell me about the confidence I can draw.

I have found negative differences in:

Weight: Tele Vue Radian 249.4 g instead of the advertised 240 g. (data found on the Tele Vue site: 0.53 lb or 8.5 oz = 240 g or 241 g)

Contrast: Instead of the advertised "highest contrast" the Tele Vue Radian shows the worst contrast of all tested eyepieces. (data found on the Tele Vue site)

I have found positive differences in:

Weight: SMC Pentax XF 146.4 g instead of the advertised 155 g. (data found on the instruction sheet and on the Pentax Canada site)

... Difference in Accuracy of Descriptions: big = 3 * (mainly because of the promise about the contrast)

2.21 Prices

Radian: $250 / ?279

SMC XF: $150 / ?149

Only the Circle-T ortho comes at a lower price than the XF. In Germany one Radian costs nearly twice an XF.

... Difference in price: big = 3 *

3. Summarized Results and Final Conclusions

3.1 Results in a Spreadsheet (in a rating including all trials with all nine involved eyepieces)


° means:depending on the used telescope

Chart 3: Rating

3.2 Final Conclusions

Both eyepieces should come with better cases.

The 12mm Tele Vue Radian is an ergonomically pleasing eyepiece. It is suited for eyeglass wearers. It makes observations very easy and can be used easily by beginners. With its flat field the Radian seems to be specially suited for observing bright open star clusters and for panning the sky. By its reddish coloured glasses especially yellow, orange and red stars will show their colours in all their magnificence. As far as I can say this due to the low magnification, the 12mm Radian will also allow a pleasant general view of Saturn's globe.

This eyepiece is not recommended at all for galaxy observers.

I wish Tele Vue would be more accurate with its declarations about weight and contrast, as I am used to get precise informations from high-price suppliers.

The Tele Vue Radian's price is quite high, because for a little bit more money one can get eyepieces giving far more possibilities.

The 12mm SMC Pentax XF is an eyepiece which, ergonomically seen, after a short time of getting used to it, will work without problems, also for beginners who are able to learn. It is suited for eyeglass wearers. With its high contrast it will show galaxies as good as a common ortho. The rather neutral glass colour of the XF will cause many objects to show even subtle colours, as planetary nebulae, star asterisms, open star clusters and double stars, to show their true colours and reveal interesting colour differences. Also globular clusters' views will profit. As far as I can say this due to the low magnification, the neutral glass colour of the 12mm XF will also allow pleasant general views of Jupiter and the Moon. On the Moon the high contrast will be noticed.

This eyepiece is not recommended for observers requiring a really flat field.

I found the informations provided by Pentax to be precise or to be even rather conservative. I like this, as they don't promise too much.

The Pentax XF comes at an interesting price.

****************

A Big Thank You to Mike Hosea, Don Pensack and Sven Wienstein for all your advices and for answering far more than my questions during the last phase of this work! Thank you for letting me do what I wanted and giving me so much background knowledge!

So this is what I have seen, and I hope this helps. Thank you for reading!

Amalia von Allmen, Switzerland

P.S. This afternoon I have googled the name of the dog, you surely remember John Russel. It turned out to be an... astronomer!!! John Russel Hind, a British astronomer, discovered 10 asteroids in the 19th century. I hurried to shoot a picture, but unfortunately the sheet has been taken away. An astronomical dog near my eyepiece testing - What a funny coincidence!

http://en.wikipedia.org/wiki/John_Russel_Hind

And hopefully John Russel, the dog, has found his home safe again!




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