Telescope Engineering Company APO 140
By Riccardo Renzi
Since their appearance in the market, refracting apochromatic telescopes
have earned a reputation of being the "perfect telescope".
Even though the belief may be a little over stated (Simply because
there is no such thing as a perfect telescope), it is also true that in
equal aperture, there is no optical configuration able to rival these instruments
in visual observations (in high resolution as well as in low power-wide field), and their common
"fast" focal ratios (usually around f/6 - f/7, although options range from f/12 to f/4) makes them well-suited
for astrophotography.
Obviously, everything has a price, and the optics of an apochromat are definitely the most expensive
among other types of telescopes, up to 10 times that of an equal diameter reflector or catadioptric instrument.
The obliged question would be: Is it worth it? Can the pretended optical perfection of these instruments
cancel out the great difference in terms of diameter in an equal price reflector?
Thanks to the importer Unitron Italia we have had the opportunity to evaluate for another two months the
performance of an apochromat of highest quality, the TEC 140, a product of the US based Telescope
Engineering Company, and without the pretension to put a final word on this issue, this has helped us make
an opinion based on merit.
Exterior Appearance
The telescope's aesthetics appeal a lot to the writer: essential, it has a focuser which is a true work of art,
and the retractable Dew Shield makes the instrument compact for transport. The paint job, white, is slightly
textured which favors the mounting of the telescope. The lightness of the instrument is surprising: complete
with rings it weighs about 9 kilograms, which turns it into an instrument handled by mounts which are not
too pretentious.
The focuser (see picture page 68) is produced by Starlight Instruments (not related to the CCD company
by the same name) which has introduced the "Feather Touch" concept, a mechanical reducer which succeeds
in bringing about the most sensitive and precise motion, in practice its like having a two speed focuser,
normal and ultra fine. I personally find this solution bette than the electronic focusers, providing me
with sensibility in my touch, in contrast to the cold precision of a step motor, special for CCD/Webcam. The
focuser is also equipped with a graded scale - Specially useful when trying to reach focus with different
configurations- a compression ring for blocking the 2” accessories at the focal point, and on top of this, a
ring which, in the unlocked position, lets the whole focuser rotate holding focus (characteristic verified by
the author at the field on several occasions).
The 140mm.objective viewed at some angles literally appears to disappear, showing the three baffles inside
the tube, this is due to the fact that the internal lens, super ED glass, is oiled spaced (like Astro-
Physics triplets, or previously on Zeiss APQ) and forms together with the two other elements a "single
block" which does not let any reflection to be seen in between the lenses, while the other two glass-air surfaces
are fully multicoated with a blue colored classical treatment. The impression is that of having a single
llens, which is also what is achieved given the efficiency in terms of light transmission at the focal point.
Field Test
In contrast to what is commonly believed, even an instrument of "just" 140mm of aperture is compromised
in its performance by the "seeing". For this reason, my whole judgment on the TEC 140 is based upon
many observations on several occasions (even during star parties in which there was a chance of directly
comparing with several instruments of similar size and in identical environmental conditions).
Given the kindness of the importer, who has given me on loan the double plate 10-Micron, I have been
able to set up side by side the TEC 140 and my C11; an apparently unproposable confrontation, but interesting
in more than one respect. We therefore start from here.
During the testing period (preferred objects: Jupiter, Saturn, double stars) visually, the apochromat has always
and consistently provided better views, apart from the seeing conditions. Only the perception of colors
was better in the C11 (this is normal given the amount of light at play); small details were better perceived
through the refractor.
Stellar images were always like the TEC manual (airy disk with first diffraction ring), while in the catadioptric,
during the test period, I was never able to observe at the same time the diffraction pattern. Therefore,
the refractor turned out to be splendid for double star observing in contrast to the C11 with which it was
quite difficult to separate the double star from the scattered light. Webcam images of Jupiter an Saturn
have in turn showed, different from what is observed visually, that the C11 (given probably the amount of
light gathered at the focal plane) achieves superior results; at least in this observation field, therefore, optical
perfection has not paid (if you consider the fact that the C11 is a serial produced commercial instrument
at half the price and has double the aperture of the refractor).
Deep sky observing with the TEC was really exciting, with very sharp stars and exceptional contrast
which let me enjoy the best views of nebulae and galaxies, even with such limited aperture. Under good
skies and the right eyepiece it is possible for example to observe a great deal of the Veil Nebula, without
any filter. The instrument's limiting magnitude is about 13.7, characteristic observed in the Pleiades Star
Gluster at 140X. Most impressive is the contrast power: Even at extra low power - 25X achieved with a
40mm eyepiece the sky's background remains dark, where other instruments (even with lenses) fail.
Setup against another good "rich-field" refractor 120/600 and even with the smaller aperture of the last,
the TEC has showed a darker sky background at lower magnification, together with completely better images
(superior field correction, sharper stars, absence of chromatic aberrations).
I am of the idea that in deep-sky performance this telescope keeps up with the best Newtonian reflectors
of 20 cm f/6, low obstruction or catadioptric telescopes of at least 23 cm diameter. When compared to other
apochromatic telescopes (also great brands) at several star parties, the TEC has always come out among
the best, the sharpness in its optics, specially at great powers, is superlative even at 400X, where other
APO refractors - optimized probably for photography more than visual performance - evidently show limitations.
This instrument does almost not take time from observing in order to cool down. In conditions where the
temperature difference is of about 10 degrees, it takes less than 15 min to achieve the best performance.
Optimal result, due in part to the lightness and the materials which the tube is made of.
Optical Quality and Star Testing
The instrument is evidently optimized for high resolution visual observing; that is in order to get the most
from the optical path. Still, the corrected field is wide enough as to let the use of wide and ultra wide field
eyepieces, with stars reasonably sharp to the edge of field.
For 35mm photography or other, a field flattener would be desirable, as well as a focal reducer. A focal ratio
of f/7 is an intermediate focal length, neither too fast nor slow, optimal setup for visual applications (being
able to switch from diverse magnifications) but not the optimal for conventional photography or CCD, for
which a focal ratio of f/5 would be more appropriate.
The star test (see picture on the right) has brought about excellent results. In what respects
color correction, I have not found any trace of secondary spectrum, only a very
light shade of red in intrafocal position, which is completely absent in the in-focus images.
Spherical aberration is corrected to levels I had never before experienced for any telescope above 75mm
(inside and outside focus are virtually identical);
The polishing is also very good, in my inspection there was a tiny zone in the light path which defied interpretation
(an elevated zone? or a characteristic of the design?)
Anyway, it is the classical search for the "hair on the egg"; this telescope is closer to optical perfection
more than any instrument I have ever used before. In order to evaluate the quality of star images and its
surroundings, I have taken a picture with a webcam of a 6-th mag star for 60 seconds; the results being
that approximately 75% of the starlight was concentrated inside a single 5,6 micron pixel, while the other
25% on the adjacent 8 pixels. I figure it is an outstanding result, even though the field was obviously oversized
(the ideal field for 5.6 micron/pixel is that of about half of that used) the telescope demonstrated to
have an astounding efficiency in concentrating stellar light.
Photography Use, CCD, Webcam
As has been stated before, the need for a specific reducer/corrector is evident; no doubt soon the
Telescope Engineering Company designers will provide for this need.
If there is no extra sensitive CCD or hipersensitized film available, performing photography at f/7 may require
long exposure times; on the other hand, this optical configuration as is, is in condition of producing
stellar images of about 10 microns, an optimal result even for astrophotographers, for whom it may be
worth while taking long exposures. I am perplexed though when using a webcam; I could not ever reproduce
(obviously on planets) those details I easily observed visually.
Conclusion
It is really difficult to find any defects on the TEC 140, capable - among other things- of combining optical
power and portability. There is no other instrument (at least to the writer) capable of offering such characteristics
in less than 10 kilograms of weight. It is practically perfect for visual observation, especially in high
resolution, and therefore highly recommended for "purists" of visual observing, that is for those on the look
for "perfect, high contrast images, free of secondary color or scattered light. Given the extremely small size
of stellar images it produces, this telescope has enormous potential also for those who perform conventional
photography or CCD imaging, with some conditions as has been stated before.
All this quality sure comes with a price, in this case of around Euro 7200 for the optical tube with rings and
carrying case, a telescope at reach, then, of very few amateurs. Finally, is it worth it? Let’s put it this way: it
is if you can afford it, given the fact that the price is still competitive with other instruments of similar category.
Italian-English translation
Ricardo Valverde Volio
valverdevolio@hotmail.com |
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