EYEPIECES - DISTORTION
Distortion
Curves for Various Eyepieces.
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Copyright – P. J.
Smith
PURPOSE
I have placed this material
on my web site to enable more informed discussion on distortion characteristics
of various eyepieces.
I must point out that one
should be very wary about condemning an eyepiece on its distortion
characteristics. An eyepiece is a
compromise between complexity, sharpness, apparent field of view, eyerelief,
field curvature, and distortion.
Distortion is low on my
list except for a few special cases.
With small or even moderate apparent fields of view it is usually of no
consequence and almost ‘invisible’. The
advent of the ‘superwides’ has brought the subject into prominence with some
people quite vocal on preferences.
In most cases, information
has been taken from patent literature.
Since the US Patent material is most accessible, I have used this and
will give references. In a few cases,
some examples have been taken from Japanese Patents and the US MIL handbook of
Optical Design because this refers to them as good, typical, examples. Thus, all of this information should be traceable
for a serious researcher. It is of
interest that this handbook, which is now quite old, implied eyepiece design
had matured to such an extent that there was little to be gained spending much
time on more advanced eyepiece design.
Whoever authored that probably now regrets the statement!
In no cases should
this information be used to malign some eyepiece
without a full study of all other
aberrations.
This is only one
aspect of performance and in my opinion
other factors are far
more important.
COMPARISONS
To aid in making comparisons, I have arranged all plots to the
same scale. In some cases, this has
caused certain extreme wide field examples to produce plots off scale.
The range of the plots has
also been standardised as much as possible.
In the case of the moderate field examples, a 60 degree (30 degree
semi-field) has been used for all plots.
This is often more than what the eyepiece can stand in terms of
sharpness and astigmatism. Realistic
fields will be given and you will have to apply this to the graphs in some
cases.
The reason this has been
done is to allow a more valid comparison between the distortion figures of
different types of eyepiece constructions.
It should also be pointed
out that the choice of different glasses can have a major effect on distortion
so one should not assume all eyepieces which look to have a similar structure
have the same distortion characteristics.
It is tempting to classify all eyepieces by element/group
construction. While this is something I
use a lot for naming eyepiece files, it is more for the convenience of locating
specific files among the plethora of eyepieces I have on file rather than being
a good predictor of performance.
Extreme field eyepieces may
have different fields. These will be
given. Be careful when comparing
superwides with other eyepieces.
Although the extreme field distortion figure is of interest, any fair
comparison should only be made out to the same apparent field of view.
All output is from Zemax and I have arranged superimposed plots of
distortion based on two criteria.
Distortion is measured by considering the
departure from various ideal criteria.
F Tan(θ) curves are what is usually called ‘Standard’ distortion.
F (θ) curves are related to angular distortion.
In some cases, especially
where eyepieces have been originally designed for instruments with very short
focal length objectives, the eyerelief figures originally given are
inappropriate when used in longer telescopes.
For this reason, for assessment purposes, I have often modified material
in patent literature in this respect.
This reflects what would happen if a person took measurements of these
eyepieces with astronomical telescopes.
The actual eyepiece prescriptions have NOT been changed.
Distortion curves are not changed much by doing this.
ANALYSIS
– MODERATE FIELD
Albert Koenig. USPat.
2217281. Filed 1938 Afv = 50°
One of many designs for Carl Zeiss.
50° of the 60° plot is used.
Mil Hdbk Afv = 50°
40° of the 60° plot is used.
These are examples of what, today, is usually called the ‘Orthoscopic’
or Abbe configuration. It should be
noted that over the years, the word ‘Orthoscopic’ has been used in conjunction
with other types. Most assume that any
‘Orthoscopic’ type has almost zero Rectilinear distortion.
Albert Koenig. USPat.
2217281. Filed 1938 Afv = 50°
One of many designs for Carl Zeiss.
50° of the 60° plot is used.
This is an unsymmetric design and as such is somewhat different from the
common ‘Ploessl’ as sold today.
Mil Hdbk Afv = 50°
50° of the 60° plot is used.
Albert Nagler USPat. 4482217.
Filed 1983. Afv = 50°
50° of the 60° plot is used.
Because it is so common, yet made to high standards, this has been
included. Definition is better in this
example of a ‘Ploessl’ than many others.
It is also better than some other ‘Ploessl’s’ for ghosting because the refractive index of both glass types is
almost the same.
Satoshi Fukumoto USPat. 5546237.
Filed 1994. Afv = 56°
56° of the 60° plot is used.
While not all eyepieces using aspheric surfaces have distortion
characteristics similar to this one, it shows how the extra freedom of aspheric
surfaces may be used to tailor the distortion curves as one wishes.
It is also possible to produce this type of distortion characteristic
with all spherical surfaces although it is very uncommon. The two eyepieces below from Japanese
Patents are examples.
Japanese Patent 07-063996. Filed
1993. Afv = 56°
56° of the 60° plot is used.
Japanese Patent 07-281108. Filed
1994. Afv = 60°
60° of the 60° plot is used.
ANALYSIS
– WIDE FIELD
Heinrich Erfle. USPat.
1478704. Filed 1921 Afv = 65°
One of many designs for Carl Zeiss.
65° of the 80° plot is used. This
was the first really wide field eyepiece.
Christian von Hofe ( Goerz) USPat. 1759529. Filed 1924 Afv = 75°
75° of the 80° plot is used.
Ludwig Jacob Bertele. USPat.
1699682. Filed 1925 Afv = 70°
70° of the 80° plot is used.
Albert Koenig. USPat.
2206195. Filed 1938 Afv = 70°
One of many designs for Carl Zeiss
70° of the 80° plot is used.
Ludwig Jacob Bertele. (designed
for Wild, Herbrugg), USPat. 2549158.
Filed 1947 Afv = 80°
80° of the 80° plot is used.
Ludwig Jacob Bertele. (designed
for Wild, Herrbrugg), USPat. 2549158.
Filed 1947 Afv = 80°
80° of the 80° plot is used.
Albert Nagler USPat. 4525035.
Filed 1984. Afv = 70°
70° of the 80° plot is used.
ANALYSIS -
NAGLER TYPE
Albert Nagler USPat. 4286844.
Filed 1979. Afv = 80°
This is the original ‘Nagler’ eyepiece known to many. It was not the first eyepiece to cover such
huge fields, nor is it the first to use this general layout. It was, however, the first readily available
eyepiece of its type to be made available to ATM’s with excellent definition
extending far off axis. Many other
superwide eyepieces before it were for military use where definition near the
edge is considered less important than an extremely wide field.
Yasunori Ueno Nagler USPat. 5638213.
Filed 1995. Afv = 80°
One of many of his designs for Nikon.
Many other designs of this type exist although it would appear that many
have not been manufactured. Some are
less extreme.
These two have been chosen to illustrate that the distortion
characteristics can be tailored to suite demands to some extent. In both cases, the afov is designed for, and
plotted for, 80°.
ANALYSIS -
ASHPERIC
It is possible to exercise more control over distortion with the extra
freedom introduced by using aspheric surfaces.
Of course not all aspheric eyepieces are of this distortion type.
Another example using aspherics has been included under Moderate field
types.
Robert Richter USPat. 1968222.
Filed 1933. Afv = 65°
The eyepiece was only intended to cover about 65° of the 80° plot used
here.
65° was quite a wide field back
in 1933.
Jean Cojan. USPat. 2528468. Filed 1950 Afv = 80°
One of many of his designs, all with a parabolic surface. The plot covers 80°.
COMMENTS
The ready availability of software to quickly analyse these designs has
made it possible to scrutinise claims made as to properties of eyepieces. I believe it leads to a need for the
reappraisal of many of the commonly held ideas about distortion characteristics
of eyepiece ‘types’.
I will let people be their own judges.
I would like to repeat an earlier warning NOT to judge eyepieces simply
on these distortion figures. In some
cases, the nicest looking distortion curves match with poor performance in
other areas.
All of this begs the question of what is the best type of distortion
characteristic for an eyepiece. I think
it depends a little on purpose and a lot on personal preferences and have no
answer.
Others might like to comment here because one person’s preferences is
just as valid as another.
Finally, I would like to apologise in advance for any errors. If you find any, please let me know.
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