PAGE 9

Copyright P. J. Smith

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PLANETARY EYEPIECES

Some very high magnification (efl = 6 mm) unusual eyepieces of exquisite definition are considered. They all have a very restricted apparent field. But for planetary use they will be very difficult to better no matter how complicated the design.


Initial thoughts

I have come to realise there is a place for a very specialised type of eyepiece that would never be stocked by normal suppliers.

Eyepieces for optimum performance on extended images have special requirements. Critically examing minute contrast differences in an extended image needs eyepieces with more than just 'resolving power'.

The surfaces must be polished to the best standards of accuracy and smoothness and mounted within tolerance. Mounting and spacing tolerances become awesome in very complex designs.

There must be a minimum of veiling glare. This is controlled by surface reflections, baffling, and blackening.

Narrow angle light scattering must be kept to a minimum. This requires the best smoothness on all surfaces.

Ghost images should not concentrate energy at the retina.

Surface quality is obviously very important. Errors in curvature may actually cancel each other to some extent, but small surface imperfections produce scattering and make matters worse with each surface.

Coatings themselves introduce more surfaces. Every time another coating is laid down on a multicoated surface, imperfections have the potential to increase in magnitude.

There is a small group of discerning observers willing to sacrifice anything to obtain the very best performance. This may well entail breaking with tradition and following a different drummer.


Philosophy

Is a bucket the optimum container for a drop of water?

Or a steamroller the best tool to crack nuts?

Why then use a 70 degree apparent field eyepiece to view a planet whose apparent image probably never exceeds a few degrees?

Why not you say ? After all, I already have one and it's a top of the line job. Super duper mega coated 20 element job. Got to be the best.

WRONG !

Like a desert flower, the special purpose but humble eyepiece described here should eclipse all but a very few when used for its one and only intended purpose.

Even if your super eyepiece had been optimised for a limited field of 10 degrees it would still be inferior. Certainly the spot sizes would be awesome. They would be mere pinpoints. But every superfluous surface in the overly complex design would do more harm than good. No amount of superpolishing and multicoating can ever achieve the same result as removing surfaces entirely.

So lets see how many surfaces we can remove and still have exquisite performance.


Three eyepieces will be compared. They are all of 6 mm effective focal length.

The first is a purpose designed achromat.

The second is an off the shelf achromat.

The third, used simply as a benchmark, is a typical good quality Abbe or Orthoscopic design.


Axial Performance

Since planets should always be centred in the field of vision, let's compare axial performance first. Off axis performance will be investigated later.

Since a good Orthoscopic eyepiece is often favoured by planetary observers, the following graph allows comparison with one.

Note how the custom designed doublet actually delivers better axial performance right down to F:5 systems.

Some readily available off the shelf small achromats were investigated and the performance of the most promising one included. Beyond F:8 it exceeds the performance of the Orthoscopic.

Even at F:5, when the Airy disk (semi diameter) is about 3 microns, performance will be reasonable

Because the designer has very few degrees of freedom with a doublet eyepiece it becomes important to pair the glass types with care.

Many commercial achromats are not good performers but some will pleasantly surprise you..


Custom designed doublet - off axis performance.

It is obvious the main limitation with a 6 mm eyepiece is the Airy disk system constraint. The dotted line represents the customary 1 minute of arc criterion usually applied on axis to eyepieces.

There is enough 'over magnification' to make the 1 minute criterion of little significance. Certainly at F:8 and F:12.

Although I believe the graphic above (in conjunction with a field curve plot) best summarises performance, for those who cannot live without spot diagrams the some information is presented in different form below.

Curvature of field.

Because apparent field has been limited to 6 degrees off axis, the amount of field curvature is about 0.05 mm. This corresponds roughly to 1 dioptre of defocus. If the image is kept away from the edge no problems should be apparent even to elderly eyes.

It is interesting that apparent field is limited to 12 degree total more on the grounds of field curvature than spot sizes.


Selected off the shelf achromat.

Performance has suffered but down to F:8 is very acceptable.

A very small amount of astigmatism may appear at the edges at the lower F:NO's.


Good quality Orthoscopic.

The following represents what can be expected from a good Orthoscopic (Abbe) eyepiece.

Certainly at small F:NO's performance is better and field curvature is less.

But the extra surfaces may do more harm than good when examining Planets near the axis.


Unobtainable small field optimised Orthoscopic

If the Orthoscopic eyepiece is deliberately restricted to 12 degrees total field and then optimised, the result is impressive.

This does NOT automatically imply better performance overall. For critical work on axis when trying to discern minute contrast differences in an extended image the extra surfaces may actually be a liability.

The difference between these two Orthoscopics makes one realise the price to be paid by pushing wide angle beyond sensible limits. Many mediocre performers would sparkle if field was restricted and the design optimised for narrower fields.


Eyerelief

Extremely small efl. eyepieces are an abortion when it comes to eyerelief. In fact, I cannot sensibly use any very small normal eyepiece with glasses because I only see about 15 degrees of field. Sometimes even less?

The simple doublet has in fact more eyerelief than multi component eyepieces of normal design.

Eyerelief is about 6 mm. Even spectacle users will be able to view the entire field reasonably when it is limited to 12 degree. In fact, this represents a very sensible eyepiece for spectacle users because if seen field can never be much more than 15 degrees, why screw up the performance by optimising for 60 degrees. (Yes I know - the sales department runs everything ........... That's life.)


Custom eyepiece

More specialised glass may be selected but I have tried to use a common range of glass in all designs wherever possible. The eyerelief is about 6 mm.


Off the shelf eyepiece

On p 36 of my old Edmonds Optics Catalogue (1995) the first achromat has an effective focal length of 6 mm.

Glass types are 728283 and 670472.

It is listed as Stock item M45089 at $35.70 and is only available coated.

The lens is best used with the flint element towards the eye in a manner similar to the above diagram.

It seems more suited than others for this application.

Eyerelief is 5 mm. Sounds dreadful but at 12 deg. total field this is OK.


Conclusions

 Serious Planetary observers, or anyone wishing to examine small portions of an extended image like the moon, might well consider this option.

Eyepieces of this type are by no means original.

I hope enough information is provided to aid rational choice.

Maybe, when the main body of observers has drifted away and only the intrepid remain, a few eyepieces of this type will be seen in use in future.


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