ASTRO-PHYSICS SUPER PLANETARY SERIES EYEPIECES

Overview: a telescope is the light bucket that gathers light and forms one virtual image, the eyepiece (or ocular) enlarges that image and focuses it to a point where it can be seen by the eye. It used to be common wisdom that a refracting telescope could be usefully operated at magnifications as high as something on the order of between twenty five to fifty magnifications per inch of aperture (1 to 1-1/2 per mm); and so a four inch aperture telescope (about 100mm aperture) could operate at between 100 to 200x. However, with dramatic improvements in refractive lens technology attained over the recent decades this limit is now on the order of better than three times the aperture - or about 300X or more for a very well made four inch apochromat. Just as the objective lens technology has advanced, so has that of the eyepieces that go onto a telescope. A number of names can come to mind whom we can credit for the advances in eyepiece design over recent years - and now that list includes Roland Christen.

Right: Astro-Physics Super Planetary Eyepieces showing (left to right): 8mm, 10mm, 5mm, 12mm, and 6mm.
The SPL 4mm eyepiece was not available to be included at the time this article was written (80,303 bytes)
Click on image for higher quality, enlarged view (172,912 bytes).

The Astro-Physics "Super Planetary" (or SPL) is an original three element eyepiece design by Roland Christen, the founder and lens designer of the world renowned Astro-Physics Company near Rockford, Illinois. Mr. Christen has attained an uncommonly good understanding of optical theory and practice, with experience he has gleaned from decades of of designing, manufacturing and testing many of the best refractive telescope optics in the world. Previously, Astro-Physics customers selected a telescope and then bought their eyepieces from third parties. By 1994 Astro-Physics and Company Seven became the only American source of a then new high resolution eyepiece benchmark: the Carl Zeiss Abbe Orthoscopic. The term "orthoscopic" denotes an eyepiece that introduces no barrel or pincushion distortion, so that an object will have the same size when observed anywhere in the field of view. The Abbe design employs a triplet field lens and a singlet eyelens. The Abbe Orthoscopic eyepiece were optimized for use on the planets and in their shorter focal lengths of from 4mm to 10mm are not recommended for use when wearing spectacles.

Any new product from Astro-Physics is greeted by much enthusiasm and speculation. Accordingly true to form, Mr. Christen has provided the astronomy community with another fine choice of product. However, this time it is his first foray into production eyepieces. The three element "Super Planetary" series eyepieces were first publicly hinted at in late 2003. The first examples of the SPL in 5mm focal length were shipped in December 2003. While the first several nearly complete sets were delivered in Spring and Summer of 2004 the introduction of the 4mm SPL was delayed at the time this article was first written; when the 4mm came available we added information about it to this article. The original factory plan was that these eyepieces would become available in some numbers by the Fall of 2004, and in stock by 2005. As it turns out the ramp up process and sporadic production have rendered the availability poor, we have been so confused about what is available and when that we have generally not been accepting new orders for these eyepieces - even while some other retailers have promised delivery dates that are incongruous. However, these are all on display at Company Seven's showroom.

The SPL's will not replace all other choices for use on Astro-Physics telescopes since they were developed only to meet the requirements of a small percentage of the observing population. And so most of our community will continue to rely on other highly perfected complimentary designs most noteworthy which are the innovative Nagler, Panoptic and Radian series devised by Al Nagler the founder of TeleVue Optics for example. Regardless, the preliminary and sensible comments made by Roland Christen never seem to keep people's imaginations from running away.

OUR IMPRESSIONS

We at Company Seven tire of what we refer to as the "eyepiece du jour" syndrome. This is when someone obtains the first of a new eyepiece (usually one that few others who are credible have evaluated) and then they rave to all that it is the best in the world - they remind us of an old elementary school acquaintance who used to boast "I have one and it's the best, and you don't". First of all they are probably wrong, and secondly there are too many design properties of an eyepiece to factor before can set forth such a blanket statement. Many of us are willing to give up some fractional amount of fine detail provided by one design of eyepiece for the increased comfort or field of view of another design. And there is a good reason why some eyepieces have six, seven or more elements - and it has nothing to do with bragging rights; each element is there for some reason. Until aspheric designs become practical to produce, we are likely to see several elements in the better wide angle designs, and a few in narrower field of view designs. Some of us seek out the finest telescopes regardless of cost, and the eyepiece that will show the most clear and clean image regardless of all other considerations. So just as they make chocolate for some and vanilla for others, there is not one clear best choice of eyepiece for everybody - the technology is not yet at that point.

We compared the SPL against a number of the better regarded production eyepieces. And there was also some comparison of the SPL's against similar focal length eyepieces out of production from our collection. Visual evaluations on very fine resolution targets were performed in daylight outdoors, and indoors. Indoors testing was also done with an artificial star, under reflected powerful and distant (to reduce heat effects) quartz halogen, and also with artificial daylight sources in a controlled environment. The advantage indoors is that we know the distances, and can control the seeing conditions. The comparative evaluation out doors were performed looking at fine details on distant leaves under overcast seeing conditions; a Leica rangefinding binocular was used to check distance to the closer (< 1,300m) targets. The telescopes we employed were:

1. a select 3-1/2" Questar f/14 Maksutov-Cassegrain
2. Astro-Physics 92mm "Stowaway" f/6.6 refractor,
3. an economical new f/9 refractor, the 100mm ED Apo, and
4. a TeleVue NP101 Apo refractor.

We employed these instruments at various f ratios by adding auxiliary negative optics in line made by a number of makers. The various measurements published below were taken by us although we did not measure Eye Relief at this time though we publish that information above as provided by Astro-Physics. We lacked access to a most capable testing device since this unit is now down and being replaced by a new GPI in mid September 2004; we were hoping to gather some MTF data just for fun. We took advantage of some other devices to help us to arrive at some of these conclusions:

1. The SPL series are an innovative accomplishment best suited for high resolution applications, their ability to reveal fine detail clearly is as good as we have observed. The contrast of the SPL is among the best two designs compared, and this is probably the best possible with recent lens technology.

Correction for astigmatism is very good, and these introduce no distortion. Distortion is a change of magnification across the field of view so that a square target might appear as a pincushion with edges bowed in, or barrel with edges bowed out.

The ability to transmit a high percentage of light and focus it so precisely means that one will not only see better detail on bright targets, but that one is likely to see fainter stars in the field of view than that provided by less capable designs.

2. There were no ghost images or undesired reflections from the field end of the eyepiece, and so you will not know when your telescope is near a bright star until the star actually enters into the field of view of the SPL. In an illuminated room we noted some modest reflection of light within the SPL caused by off axis room lighting or a camera flash, but this was modest or less than that found in most other eyepieces. Generally speaking if you set the eyepieces on a black cloth in daylight the lenses will most likely appear transparent, similar to the Zeiss Abbe.

An observer having to contend with stray lighting might find it helpful to wear a dense black cloth hood overhead (not white, especially in the Southern states) or use an eyecup. We prefer to use the design of eye guard such as those provided with the Orion Ultrascopic (Baader Eudiascopic) series which we will stock.

3. The glass used is very transparent, the coatings attenuate some wavelengths that would otherwise cause some reduction in perceived detail. This has to do with dealing with eye physiology and not simply pure light throughput.

4. The balance of design Field of View and Eye Relief provide a practical balance for the designed purposes. In these respects the SPL compare similarly to the other eyepieces which we considered to be the reference standard to date. We could comfortably observe with as short a Focal Length SPL as the 10 mm without making eyelash contact. Other eyepieces tested that were designed with better eye relief than the SPL's showed lower contrast to one degree or another.

5. We do not generally recommended the SPL eyepieces (or most other simple designs) in focal lengths of from 4mm to 10mm for use by spectacle wearers since the distance from the eye lens to the exit pupil is too short to permit seeing the entire field of view. However, some observers with astigmatism may find that when observing at such high magnifications then the exit pupil is so small in diameter that it may bypass the off center area of the Cornea that is introducing astigmatism. And so some spectacle wearers may find the SPL's useable.

Alternatively, it is a simple matter when observing a small object such as planet to back the eye away from the eyepiece. This will result in a narrowing the apparent field of view, in the case of the shorter focal length SPL's we found this to be around 30 degrees or somewhat less. This change of observing position can make several SPL's come within reach of many spectacle wearers.

6. These are not "wide angle" or long eye relief eyepieces, and considering their high magnification and comparably narrow field of view these would not be our first choice for applications involving:

• use on astronomical telescopes that have no tracking mechanism
  

• sharing views with a number of others, especially if they lack experience using eyepieces

7. The SPL can be an excellent choice for use in Eyepiece Projection imaging (film, CCD) techniques

8. Company Seven evaluated our first production SPL eyepieces for cleanliness and they present a very clear image free of internal lens contaminants. However, some field stops showed artifacts that would not be visible at night unless scanning across a well illuminated target such as the Moon, or on the Sun; we could remove these easily enough.

9. The classical Carl Zeiss Othoscopic design eyepiece of Ernst Abbe was designed for use with telescopes of f/10 or greater. The Zeiss Ortho design that was most recently in production were the Abbe Orthoscopic design, and these were optimized for telescopes of f/8. It is quite an accomplishment that the SPL series work well for their intended purposes at ratios as fast as f/4.

10. Color fidelity appeared excellent using a daylight source on targets across the visual spectrum. The SPL design correction peaks in the yellow green region of the visual spectrum as does the human eye. But even with the choice of coating employed, these remain useful beyond the visual and were measured with a sensor to transmit below 400nm in the UV, and above that notably into the IR.

11. When an observed target is off center in the field of view the SPL then the performance remains consistently good. And so these can be quite suitable for close in observing of star clusters which provide pinpoint stars across the field of view.

12. At the high magnifications these SPL's can provide, any speck of dust on the lenses of any similar focal length eyepiece will appear obvious and egregious. So it is important to keep these clean.

13. Considering SPL field of view and eye relief as compared against other first rate high resolution designs shows:

• The definition of fine detail was the close to the best probably a close second place, but the next closest eyepiece design of similar focal length was quite good
  

• The retail prices are similar among the top three choices (top in terms of resolving fine detail)
  

• The contrast was perceptibly best on the SPL and Zeiss Abbe Ortho than on other eyepieces of similar focal length
  

• The SPL provides between about 60 to 90 percent of the field of view (by diameter) of most of the better eyepieces of similar focal length which we compared.
  

• The SPL provides between about 30 to around 100 percent of the eye relief of most of the better eyepieces of similar focal length which we compared. Even the short SPL 5mm was considered useful for viewing by those who did not have to wear prescription spectacles.
  

• The SPL's are the most fussy among the eyepieces compared here regarding f ratio of telescope, and choice of auxiliary negative lenses. When using the SPL on the Questar for example some anomalies appear around the edge of the field of view that are not visible on the Astro-Physics 92mm f7, the Orion 100mm ED, or TeleVue NP101 telescopes. Those that did show up on artificially illuminated test targets however are not likely to be noticed when using the SPL eyepieces on targets at night or in daylight.

  During another procedure using the SPL on an illuminated target with a common Barlow, or on the Questar 3-1/2 with the internal Barlow engaged then a yellow chromatism is noticeable around the edge of the field, and a noticeable vignetting appears at the border. This is quite similar effect to that effect seen on a very highly regarded eyepiece design that is considered with no question to be the best in their respective class.

• Compatability with the Barlow lenses we tried was the one perceptible aspect where most other eyepieces tested were found to be completely insensitive.

14. An optional good quality negative lens (Barlow) can provide higher magnifications for those who demand an increase in magnification, or increase in the number of available magnification fine "steps", or a bit longer eye relief. The SPL is useable with all Barlow lenses, especially when using the SPL eyepiece on targets at night or when they are in the center of the field of view. However, the best performance across the entire field of view is attained when using a TeleVue Powermate lens; this is a negative lens incorporating a beam shaping component.

The least desirable visual results were obtained when using the comparatively simple and compact 1.25" Barlow lenses such as the Vixen Ultima (Orion "Shorty Plus") or the Barlow installed in the Questar 3-1/2" telescope. We also noted that even the better eyepieces (including the TeleVue Radian) performed better with more refined (and more costly) Barlow or TeleVue Powermate lenses.

15. The SPL is not very sensitive to eye placement. As long as you can line your eye up to the exit pupil then you will see the target even if your head is tilted a bit off axis or slightly off center. There is no black out effect when observing daytime or at night.

16. The ease of eye placement, and the apparent field of view which provides about the angular dimension across which the human eye can focus makes these a good choice for use with an optional Binocular Viewer, even more so if weight of the payload poses some concern.

17. The SPL eyepieces came were assembled from two production lots, these we evaluated are close but not parfocal.

18. The smooth Derlin housing is comfortable to handle even in extreme cold, and this is good since one will need to handle these with a firm grip to avoid having an SPL slip out of the hand. Some of us might find it better to have some ridge running the circumference of the Derlin. We all felt it is too easy to unscrew the stainless steel barrel from the upper assembly. These are included here as personal preference notes.

19. The engraved and painted writing on the SPL's are easy to read when they are held in low or red light. The focal length designation is only visible when the number is turned towards the observer. In practice, the owner may learn to judge focal length simply by noting the differences of height. However, it might be helpful to provide some kind of more obvious focal length coding (extra f.l. engraving, lines, etc.) around the SPL to more readily distinguish one focal length from another in the dark.

20. We did not test the SPL with an optional positive lens (telecompressor) in place. It seems pointless to buy high magnification eyepieces then place a lens into a telescope to reduce that magnification.

21. At such high powers, a telescope equipped with a smooth fine geared focuser (such as the Questar 3-1/2, Astro-Physics 92, etc.) can sure make the testing go a bit easier.

It makes little practical difference how the SPL series compare against obsolete, or out of production designs. The fact is these are about as good a performance, practical, high resolution "super planetary" eyepiece as one can obtain in 2004. Assuming their eye relief and field of view are compatible with observers needs, then these will be popular within the community of those who seek the highest possible detail: double star observers, occultation chasers, and planet watchers. And of course there will be some who just want to know they own a set for the bragging rights.

Right: Astro-Physics Super Planetary Eyepieces with and without lens caps in place, and with set of covers to the side (51,932 bytes).
Click on image to see a higher quality, enlarged view (137,698).

SUPER PLANETARY EYEPIECE SPECIFICATIONS

Part Number	Focal Length	Field Stop	Weight	Height
SPL040	4 mm	2.60 mm	2.5 oz / 72 g	33.50
SPL050	5 mm	3.57 mm	2.5 oz / 72 g	36.1 mm
SPL060	6 mm	4.37 mm	2.6 oz / 75 g	37.25 mm
SPL080	8 mm	6.04 mm	2.8 oz / 79 g	40.12 mm
SPL010	10 mm	7.65 mm	3.0 oz / 85 g	42.85 mm
SPL012	12 mm	8.91 mm	3.2 oz / 92 g	45.26 mm


* Specifications are subject to change without notice.