TEC 140 APO

Motivations

Objective qualities of apochromatic refractors are:

- a very large flat field (2° to 3°)
- a very good "availability" (fast cool down, permanent alignment)

But an apochromatic telescope is also (almost always) a hand figured equipment, built in small series and of high quality; for this reason, rather a "fact" that an intrinsic property, these telescopes are also very praised for planetary observation.

In the years 2000, having studied various solutions around 100 mm - 115 mm, I quickly realized that price (in €/mm²) was really very high for the service particularly in the small diameters. Incidentally, I discovered the availability of TEC APO 140 at a reasonable price (everything is relative) regarding to its many qualities and its significant diameter. Ordered directly to the manufacturer from France, taking advantage of the nice $/€ exchange rate (in 2004), a TEC 140 costed - here - about the same price than a TAK FS-128 and was only a little more expensive than a 115 TMB APO.

Relationships and communications with TEC

Telescope Engineering Company (TEC) has been created in the USA in Golden near Denver (Colorado) in 1994. Yuri Petrunin is the President and the team comprises experienced opticians of the old Soviet Union. TEC is specialized in high quality Maksutov Telescope and, since 2002, manufactures apochromatic refractors of 140mm 160mm and 200mm aperture. Unlike it can be read sometime, all optical components are manufactured by TEC in Golden as well as most mechanical parts and OTA.

This telescope has been ordered in 2003. Ordering (and importing) directly such an item from 8000 km is in itself a little experience. But Yuri Petrunin makes the communication easy by concise but clear answers to the questions an amateur may have in these situations; confidence was immediately established and this was an important part of my decision. After the down payment has been wired, the only thing to do is "waiting". Current delay is about one year with little risks depending on the availability of glasses. Finally, an evening (not a morning here) you receive the so much waited message: The scope will be ready to ship to you shortly after we have received the final payment... Then all is going very fast since the large package travelled the 8000 km in two days only ! It should be noted that, at this crucial moment, quality of shipping is as important as quality of the instrument but all has been anticipated by TEC and everything went well.

The scope described here was the first TEC instrument delivered in France. It is a scope of the third run (RUN III).

Characteristics

TEC 140 OTA is made in the USA by TEC in Golden. Focuser is made by Starlight Instruments in collaboration with TEC; finder is a Japanese product.

The optic

The optic is an oil-spaced type triplet: in AP and Zeiss APQ objectives, spaces between lens are filled with a thin oil film whose refractive index is near that of the glass; in fact TEC140 does not use oil but a special two components mix sealed between lenses. The result is an impressive transmission factor and an excellent contrast due to the reduction of air-glasses reflections. The counterpart is that sometime oil-spaced objectives may have an higher F/D ratio than air-spaced objectives since the engineer has less degrees of freedom to correct the aberrations (spherochromatism and coma). The difficulties are not the same but of same level in both cases and generally each manufacturer masters only one technique. For the user, improved transmission factor and contrast against air-spaced objectives may largely compensate the - theoretical - lack of correction...

TEC Objective

For a given geometry, chromatic correction is achieved thanks to a strict control of the indexes and dispersions of the three elements of the objective; for the extra-low dispersion glass of the TEC140's central element, TEC uses FPL-53. For the 160 and 180mm OTA, TEC selected fluorite (CaF2) for the central lens but estimates it is useless for the 140 whose F/D=7 ratio stays compatible (regarding ~Abbe criteria) with an FPL53 ED glass that is far less expensive; a TEC 160 ED F/D=8 using FPL51 for the central lens as been available in the past and was giving an excellent visual correction too. The TEC110FL uses CaF2 for the central lens but this scope is extremely short (F/D=5.6).

Since 2018 TEC offers a new version of the TEC 140 with CaF2 for central element. The advantage is, of course, a better correction of spherochromatism in the blue and a reduction of the focus shift. A practical reason put forward by TEC is that it is difficult to obtain FPL53 glasses in sufficient quantity and quality but TEC may also have an interest in repositionning its entire line of refractors at very high end. In any case, it is interesting to know that a new field flattener-reducer would be available bringing the focal ratio to 6.2 and giving a field of 3° in FF (24X36) which corrects a criticism made until now on the lack of a dedicated reducer. The old (non-reducing) flattener is still usable because both objectives have the same field curvature (however to be tested with regard to the blue correction optimized for the ED version). TEC could also have reduced the focal ratio of the TEC140 (and the size of the tube...) but being given the production difficulties of the TEC110FL (at F5.6) which led to its abandonment, it was probably not a good option while a flattening reducer solves different problems at the same time... In Yuri's own opinion, the visual difference between the ED version and the FL version is almost zero. The difference is certainly more noticeable in imaging.

The TEC140 described here is a TEC140ED.

Some tests

As a user of reflectors with opened tubes, I have been surprised by the fast cool down of the refractor. In fact, despite an in/outdoor difference of 8° (approximately), diffraction pattern is altered but visible (while not perceptible in a reflector due to the tube currents) and you can almost immediately start the observing session; then, diffraction pattern is stabilized after 30 to 45 mn.

Once OTA is stabilized, startest at 500X immediately near focus plane does NOT show any geometrical flaw and theoretical diffraction patterns are obtained without any difficulty. Collimation is perfect and in/out patterns are absolutely identical with same apparent colors as far I can see. Since this startest is perfect I can't give any quotation of the residual aberrations...

Regarding chromatic correction, by examining Vega at high power, a slight purple halo can be expected but is not obvious... While I am not an expert in chromatic correction, I find the color balance perfectly optimized and the chromatic correction extremely well controlled. But the most impressive and important for me is that Vega's Airy disk is almost visible with only few rings (may be two) and sky is perfectly black around the star with almost no diffusion; this is very rare and promising for great views of the planets but also globular and open clusters.

I finally did a Roddier test with an EOS 350D by decomposing the image into the three colors; wavelengths are only approximations probably as well as results in red and especially in the blue (where the corrections fall quickly) but this gives an idea at least for a visual use. The results are excellent ~1/40 RMS in the green ; the results are consistent over several sequences with a good consistency of the shape of the wavefront and confirmed with Winroddier in multi-iterations mode (3 to 5 iterations); they fall a little during cool down as demonstrated below; the optical accuracy in the blue is lower as expected but sufficient and matches the specifications:

Red (~650nm) : >1/50 RMS Strehl 0.99 (warning: sensitivity of the DSLR is degraded in the red)
Green (550nm) : >1/40 RMS Strehl 0.98
Blue (~460nm) : ~1/15 RMS Strehl 0.80

I also confirmed this result in the Green using a Baader Solar Continuum filter BW=10nm @540nm : so, extracting RGB from the RAW file seems a valid method at least for the Green and a TEC140.

TEC140 - Roddier in Green (images 150-200 pixels)

For the record the specifications defined by Abbe for an APO objectives are 1/2PV in the blue and 1/4PV in the red or green. Here, the range of results are well above that and within the TEC specifications (TEC indicates 1/8 PV in Green and a less correction in Blue); moreover, the intra/extra images almost indistinguishable, speak for themselves. The quality of this optic is ... "above any suspicion" ... which corresponds to my expectations. The absence of diffusion around Vega also attests for an excellent polishing quality and effectiveness of the oil objective.

The cool-down is relatively quick and predictable, but requires at least one or two hours by cold weather. During the cool down, it may appear a very slight astigmatism (visually barely detectable) and/or a tetrafoil (only detectable with a Roddier test) as long as temperature is not uniform. The images below show effects of cool down on the TEC140 telescope: left, during the cool down (10°, one hour after being outside), the wave front exhibits a slight tetrafoil (~1/5PV) for an overall correction of 1/30RMS and on the right, about one hour later (i.e two hours after being outside), all mechanical stress have disappeared and the correction is back to 1/45 RMS. The difference is in fact so slight that it does not affect the image at focus (the Strehl ratio only varies from 0.96 to 0.98) and this explains why the scope appears almost immediately usable. You can read more about the conditions of these tests in the pages in French.

Effect of cool down TEC140

Shack Hartmann test

Last year (2014) I tested my TEC110 with Frederic at Airylab during my vacations in Provence. This year (2015) we tested the TEC APO 140. This was the first TEC 140 tested by Airylab and one of the rare professional lab-independant-test made with a TEC140... Airylab uses an Shack-Hartmann (HASO^tm) analyser. The results are impressive and match my own Roddier test with a nice 0.975 Strehl in the green (543nm). The sperochromatism appears very well controlled and balanced between red and blue. The total focus shift is 110µm (= 0.011%, +/- 0.006%) from 473 to 635nm.

TEC140ED alone Color	Strehl best focus (modal mode)	Min ray spot (strehl *) (zonal mode)	Focus Shift (HASO)	Strehl Best focus (Roddier)	Design (TEC data)
RED (635nm)	0.974	0.962	0	0.99	0.85
GREEN (543nm)	0.974	0.959	-70µm	0.98	0.99
BLUE (473nm)	0.910	0.762	+40µm	0.80	~0.8 (**)

We also tested the TEC140 with the Field Flattener (FF140) and appreciated its very clever design : with the FF, the color balance is slightly shifted toward the blue for a better match to the sensitivity of a sensor (with a greater shift between red and blue, still within the specifications provided by TEC). The field is very flat within 0+20µm (0-17mm) and 0-60µm(17-27mm). Spot size is about 12µm @27mm (TEC Spec=15µm !). So, whatever the format of your sensor, the FF is a must for the best possible color correction and large images up to 6X7 !

TEC140ED + FF Color	Strehl best focus (modal mode)	Focus Shift (HASO)
RED (635nm)	0.893	0
GREEN (543nm)	0.938	-130µm
BLUE (473nm)	0.960	+160µm

The full report is available on the AiryLab web site (TEC140+ Zeiss Diagonal) and a more comprehensive review of the TEC 140 ED has been published in a french astro-magazine.

(*) Modal mode: Like Roddier and many other software (Zygo, AtmosFringe...), HASO^tm reconstructs the wave front as the sum of a series of known modes (Zernike's equation). This algorithm allows a quantification of the various aberrations including a variable "defocus". This method optimizes the wavefront exactly like the user does it by focusing. In Zonal mode (denomination which seems specific to this software) HASO^tm optimizes the wavefront for the smallest ray spot. This allows a raw quantification of the spherical aberration and of the optical design (and comparing them) ; it is an interesting data for imaging (with or whitout FF) but is not really representative in visual use. In fact both are interesting but generally, only the Strehl for the best best focus is calculated and advertised. For more information, see telescope-optic.net.

(**) TEC indicates 0.29@438nm and 0.92@486nm.

What mount for the TEC140 ?

I measured the distribution of the weights as follow (+/- 0.2 kg):

(1)	OTA only (without front cover !)	8.6 kg	19.0 lb
(2)	(1) + two TEC rings	9.3 kg	20.5 lb
(3)	(2) + Vixen AM-AD-3809 dovetail	9.4 kg	20.7 lb
(4)	(3) + finder	9.8 kg	21.6 lb
(5)	(4) + 2" TV diagonal + PAN 19 mm	10.4 kg	22.9 lb
(6)	(3) + 2" TV diagonal + PAN 19 mm	10.0 kg	22.0 lb
(7)	(4) + TEC turret + 3 EP	11.0 kg	24.2 lb
(8)	(3) + Denk II + 2 X D21 EP	11.2 kg	24.7 lb
(9)	(8) + Finder	11.6 kg	25.6 lb

As you can see, TEC 140 equipped for observing weights about 10.4 kg (22.9 lb) and just reaches the limit of my GP-DX rated for 10 to 11 kg (22 - 24 lb). However, taking into account the moderate diameter of the OTA (150 mm) and its low eccentricity around the RA axis, it is possible to minimize the total weight applied to the bearings by using only two counterweights (3.7 kg + 2.8 kg = 8.2 lb + 3.2 lb); the most critical is the length of the OTA which moment of inertia is high and induces oscillations of the tripod. This is why a sturdy head and tripod are required (aluminium tripod HAL 110 delivered with the mount is not suitable and it is too short for a refractor); I chose the Berlebach UNI 28 (*) tripod to reinforce my setup.

The GP-DX mount reinforced by the Berlebach tripod behaves very well and dampening time is reduced to less than 2 seconds with only little or no oscillation when fine-focusing. OTA can be balanced for all positions and Skysensor's accuracy is not affected; however maximal speed has been reduced to 800X in order to take care of the motors.

Practically I don't see any difference with my 6 Kg (all equipment included) Clavé Cassegrain except, may be, a slight tendency to lose pointing in case of a continuous effort is applied to the end of the OTA: certainly, in such setup, length of the OTA is the most critical parameter...

Some additional information about the use of the Berlebach tripod with TEC 140 + GP-DX: the foots must be properly drilled into the ground; if the tripod lies on a flat surface (concrete, pavement...) dampening time increases and vibrations become a nuisance... Lastly, one must take care that, during pointing, focuser may hit the prominent levers of the tripod's head: the GP Half-Pillar may be useful and does not seem to have any influence on dampening time.


TEC140 without any heavy accessories is perfectly usable, at least for visual use, with a Vixen GP-DX or Sphinx mount reinforced by a rigid wooden tripod. With half pillar on the right.

Periodic Error measured with my GP-DX after a meticulous adjustment of the gear, mounted by 11kg of refractor + accessories, perfectly balanced is about +/-10 " (or less) what is very good while it increases up to +/-20" with my Newton (9Kg) whose perfect balancing is almost impossible. This demonstrates how balancing is critical for a modest mount.

However, it is probable that a more sturdy mount will be more comfortable than a GP-DX especially for imaging due to the length of the OTA and a possible increase of the periodic error in case of the mount is not properly balanced. In addition, equipped with a binoviewer, a turret or a CCD camera, weight may reach 11 to 12 kg or more, that is to say beyond the specifications of the manufacturer. This is why the TEC 140 is rather intended for mounts of the TAK EM 200, ATLUX and Losmandy G11 range. I think EM 200 suits this scope particularly well. Often, in the USA, TEC 140 is mounted on Losmandy G11; it is seen also on massive azimuthal mounts such as Giro II or DiscMount.

I tested a Vixen Sphinx SXD (see review in French here). While it is very portable, well done (mechanically) and stable enough for the TEC 140 and even the TEC MC200 Maksutov, I can not recommend this mount because of its very poor tracking ability: jittering with load (~8kg and above) and DEC/RA jumps (not taking into account the gaps of the Starbook).

I finally acquired a G11-Gemini upgraded with the Ovision Worm (reviewed here). Stability is very good ; Gemini is very precise and movements are very smooth. In addition the Ovision worm allows a very low periodic error (about 5" peak to peak). Overall, the Losmandy G11, despite the additional weight of the mount head, is more pleasant to use than the GP-DX ; however, I prefer the GP-DX as a travel mount.

TEC 140 now on G11-Gemini

(*) While UNI 18 and UNI 28 have the same head, UNI 28 may be preferable for a refractor since legs must be extended so that head is about 1.2m above the ground

Observing (to be updated)

For its first light under my suburban sky, and for Deep Sky, TEC 140 seems above my Clavé Cassegrain with a better contrast and just below my (super polished, low obstructed) Newton 210. I am waiting planets to do more interesting observations... and the next vacations for pertinent views of the deep sky.

Double stars

Double Double (Eps Lyr) : this double star is not a challenge for the TEC 140 but it is absolutely magnificent showing perfect and steady Airy disks surrounded by very thin rings in front of a black sky...

Delta Cyg (2.2" 2.9/6.9) : interesting and beautiful with a strong difference of brightness, obvious with TEC140

Struve 386 : (0"9 8.2/8.5) : Faint stars, faint but split

Struve 410 : (0"85 6.1 6.1) : Test pair for a 140mm refractor; same magnitude of both components. Split obvious.

16 Vul (0.8" 5.9/6.2) : test pair for 150mm, seen elongated with slight strangulation

Struve 2606 : (0."8 7.7/8.3) : Not split but elongation perceptible

Lambda Cyg (0"7 5 6.5) : not split but slight elongation of the airy disk easily perceptible.

Beta Del (Sep 0.5"" Mg 3.6/4.5). This double start cannot be split with any of my instruments (TEC 140, Clavius 160 and Newton 210). However, Airy disk is slightly altered... and something is visible:

- TEC 140: Airy disk shows a very little "tail" (not obvious but visible),
- Clavius 160: the "tail" is more prominent but reinforcement of the first ring makes observing more difficult and questionable,
- Newton 210: Airy disk is clearly elongated and double star is obvious while not totally split.

Mars (opposition 2005)

Mars requires high magnifications (250X to 300X) in order to reduce the strong planet's brightness ; binoviewing is also highly recommended. Observing Mars demonstrates why (if needed) ability to restitute the contrasts is so important for an astronomical telescope : most Mars's features can only be distinguished by subtil difference of Albedo.

By poor seeing, view is the same as the one given by a little 75mm refractor : main albedo features are visible (Syrtis Major, mare Acidalium) but it is difficult to perceive their limits. By better conditions (while not exceptional), many well known areas become easily visible : sinus sabaeus/meridiani, oxia palus, mare acidalium, solis lacus... However, delicate contrasts of the planet are only "supposed" by moment. By better conditions, shades of Albedo appear more obvious making possible to distinguish various features such as: mare Tyrrhenum, mare Serpentis, Hellas, Hellespontus, Hchronia, Aeria, Pandorae, Deucalionis... Other small but more contrasted features are visible along Syrtis Major (Deltoton S.) or Sinus Sabaeus (Sygeus Portus)... South polar cap is obvious while very small; in North large Utopia Planitia appear gray or brighter bordered by some darker features.

During moments of very steady seeing, using the DenkII, I have had absolutely astounding images showing Vallees Marineris... with the planet taking on the appearance of the best images taken by amateurs.

In comparaison with my 200mm Newton using 400X (both instruments in monocular viewing) and if conditions are favorable (i.e. 1 time out of 5 five against 1 time out of 2 with the refractor), this last scope does not show much more "topographic" features but gives a larger planet with slightly more colors and more subtil nuances of albedo (details easier to pick out with the 200mm). But if a high magnification is not practicable, it is not profitable to set up the 200mm since binocular viewing with the TEC140 is overall more pleasant and allows for much more profitable observations.

Generally on Mars and other planets, TEC140+DenkII allows viewing the same levels of details than my excellent 200mm in monocular viewing. See also below a comparison with a 8" TEC MAK.

See also :
- Realist simulations of Mars
- A good and classical map of Mars...

Jupiter

Using 200X, by correct conditions (periodic tremors, slight residual turbulence), but not exceptional, first impression is that an higher magnification is required to reach a full resolution… Thus, taking into account the planet’s brightness, the optimal magnification is rather 250X as for the moon (UO 4mm).

From start, the level of perception of the very subtle gas nebulosity’s is high. The main bands immediately appear with their contours and condensations and subtle contrasts appear easily between the holes of turbulences. The red spot is coloured pink, the festoons are visible and the poles appear striated by bands or granular... Ganymede appears with a significant diameter. There is really what to observe and draw!

By using Denk II binoviewer at 200X (that is to say a little less than by monocular viewing), details appear even more easily and more quickly even if the binocular observation does not reveal any more, in theory... The planet’s brightness and the excellent transmission of the TEC lenses make it possible to fully benefit from the binocular vision with an excellent comfort.

There is a similarity with my 200mm Newton: both instruments encourage using high magnifications in order to attenuate the brightness of the planet. For these average conditions, if my Newton allows using more magnification (about 285X) and makes detection of little contours or faint contrasts easier, there is no major difference between both scopes. Actually, it seems to me that, due to a "physiological" advantage (not optical), the use of a binoviewer renders the same quality of observation as that offered by my 200mm by monocular viewing…

By exceptional conditions (no turbulence), I remember my 200mm allows going further in magnification (400X) and detection of colours and contrasts... And the test of TEC 140 must be done. Unfortunately, I am not sure if the rather low altitude of Jupiter of these next years will make such conditions possible.

Saturne

Using 200X Saturn appears absolutely splendid: its five main satellites, the crepe ring all over the circumference of the rings and just in front of the globe are immediately visible; Titan takes a planetary appearance and its red colour is obvious. The main band on the globe appears contrasted as well as well as shading and darkening of the South Pole. In December 2004, the rings start to close but the Cassini division is still visible all over the circumference of the ring and becomes difficult just in front of the globe; a small end of the North Pole emerges behind the ring.

Views with Denk II binoviewer are still more impressive and absolutely splendid giving totally new impressions. I will not say Denk II shows more than a single eyepiece but it makes perception of fugitive details and subtle contrasts on the globe or the rings more easily perceptible.

Of course, by good conditions, my excellent N200 gathers more light and renders a little more coloured planet possibly increasing the 3D effect by the more obvious transparency of the crepe ring in front of the globe, but image quality of the TEC140 is just one step below. In addition, if the optimal magnification is about 200X (0.7mm exit pupil) the scope makes it possible to reach without notable loss of contrast 280X that is to say about the same practical magnification, that my N200 may offer by current conditions (often limited by atmospheric turbulence).

The comfort of observation to the TEC140 is moreover excellent, in particular compared to a Newton on equatorial mounting: while rotating the focuser to horizontal, comfortably sitting in a garden chair and using a long eye relief eyepiece (LVW 3.5) or a binoviewer, observing is an exceptional enjoyment.

Deep sky

NGC 7000 : Finally seen full size ! Perfectly identifiable and spectacular Mexico gulf. General shape is recognizable while NA continent is less prominent than on photography. 2,5° field seems necessary to have the full benefit of this spectacle (better seen with 30mm/74° eyepiece). Some traces of the Pelican nebula also identifiable beyond the field.

M 13 : Splendid and perfectly contrasted. Periphery resolved but core remains "milky" or granulous. Newton 200mm allows more resolution.

M 27 : Splendid object : "apple core" perfectly recognizable as well as some faint details (center star not visible) ; beautiful field with a Nagler 11mm. It should be noted that difference with my 200m Newton is less obvious with planetary nebulae than with globular clusters (which have a threshold magnitude of about 12 to 13).

M 8 & M 20 : Spectacular view of M 8 and M20 in the same field (modified Erfle 30mm/74°). Observed with a LVW22, M20 shows three distinct nebulous shapes and almost a photografic aspect : dark band between two lobes perfectly visible and M8 appears as an irregular clover (main nebulosity) using adverted vision. Splendid spectacle with a lot of details (much more to see than described here...).

M 31, M 32, M 110 : M 31 extends beyond 2°5 ...! Practically entirely seen using a 40mm eyepiece and beautiful spectacle with the three galaxies...

M 51 : This observation is interesting to compare my Newton 200 and TEC140 while I was not able to set up the both instruments together at the same place and at the same time. Like with other very large galaxies (M33 M101), observing M51 requires a very dark sky. Under a dark sky, the spiral structure is just suggested but not obvious by showing a dense core and an irregular distribution of light around the core. With my Newton 200mm, perceiving the spiral arms is more obvious and the suggestion becomes a slight perception (don't expect something spectacular but just a perception...). Of course the satellite is easily visible and its core shows an irregular shape but the bridge between the two galaxies is not visible. Despite this detailed description, view is a bit deceiving...

NGC 7662 : Very bright nebula in a beautiful field. Direct vision is possible.

NGC 7331 : Interesting : elongated galaxy with a more light in the core in a field of faint stars.

M 29 : Very large and spectacular cluster to observe with a large field eyepiece (2°).

Some conclusions about deep sky observed through the refractor: Difference with my Newton 200mm appears, in my opinion, more obvious with globular clusters than with other objects since they have a "magnitude threshold". Of course using more diameter to observe the planetary nebulae is always preferable and difference is perceptible between 200mm and 140mm but provided more magnification is used (1mm exit pupil), views are almost similar in both instruments taking advantage of the high contrast of the refractor. Galaxies remain deceiving in both instruments and certainly require more diameter (300mm to 400mm...) to be seen properly. However, views of large clusters and large nebulae are absolutely splendid and spectacular through the TEC140: observing NGC7000 full size or M8/M20 in the same field is much more spectacular than observing M51 in my 200mm Newton...

Useful accessories for the TEC140

Superb turret for five 31.75 eyepieces (weight 700g)

Zeiss-Baader 2" diagonal

It is said that on some APO scopes a large prism may improve the color correction (depending on the color curve). Zeiss APQ scopes are well known for being conceived to be used with a prism and not with a diagonal mirror and this "trick" has been found effective on some other APO scopes. This statement is not true for the TEC140 : a lab test done with Airylab shows that the prism shifts the correction slightly toward the red and adds a very small amount of spherochromatism (about -1/10 to -2/10 Strehl). To be honnest, these changes are absolutely not visible at the eyepiece with the TEC140 but render the prism unusable with my TEC110. Probably, the limit focal ratio for a large prism is around F7.

I purchased the 2" Zeiss Baader Universal Zenitprisma and below are my results against a 2" Televue Enhanced Diagonal.

This prism is very robust, multi-coated and perfectly collimated. Weight is about the same than my Televue Diagonal (500g). On the optical bench at Airylab the prism turns out to be perfect without astigmatism or other optical aberration ; the Strehl ratio only changes from 0.975 to 0.965 (average) due to the additionnal spherochomatism.

Baader 2" Prism diagonal tested at Airylab

Startest on Procyon : at focus, no false color is obvious and difference between Televue Enhanced diagonal and Baader prism is far from being easy to see... However, by quickly switching from one diagonal to another, a very faint purple fringe around the Airy disk can be suspected with the mirror while it is not present with the prism. Occasionally, this very subtle improvement may be useful on some very difficult and contrasted double stars. At a very short distance of focus, a purple spot can be seen for a very precise position of the focuser : it is obvious with the mirror and barely perceptible with the prism which gives very white out/in focus patterns even at short distance of focus. There is no geometrical difference in the In/Out patterns between the diagonals. This result may be subjective since TEC or Airylab test indicate that no improvement of color correction can be expected and more probably the opposite!...

On Saturn (planetary viewing) : in fact absolutely NO difference can be seen between the prism and the diagonal mirror as far color correction is concerned but the light scatter seems slightly reduced with the prism. Again, not enough to be conclusive.

Anyway, I found the prism an other interesting and unexpected advantage: it CLOSES the tube and thermal equilibrium is less affected when you put a new eyepiece in the diagonal; may be this was the better and the more obvious improvement! In addition, the prism does not require any re-coating and is easy to clean.

Anyway, this high quality prism matches the TEC 140 APO very well ; this is - overall - a good option and since 10 years I only use a pris with my TEC140. While an oversized 1.25" is available, I recommend a 2".

DDCAP

DDCAP is a nice, inventive and well made cap to protect your scope from moisture... (discontinued)

Cap + Dessicant + Indicator Pack + Vinyl plug

Denkmeier Binoviewer

I am using a Denkmeier II + Power Switch + a pair of D21 Eyepieces.

First, TEC140’ focuser handles the binoviewer very well. Its eyepiece collet is highly appreciated making this exceptional optical assembly very secure.

Denk II binoviewer & D21 Eyepieces - 2" Televue diagonal

With a pair of D21 eyepieces, Power Switch, OCS corrector, Televue 2" diagonal and 980 mm focal length, I have three magnifications estimated as follows: 200X, 120X and 70X, i.e slightly more than those calculated from the specifications of the manufacturer; actually, this range of magnifications suits a 140mm OTA perfectly.

Eye by eye, a loss of light is perceptible but you forget it (totally) during binoviewing : one of the two nights, on Saturn, six satellites were visible with a single eyepiece (without bino) or by remerging the split beam with two eyes through the bonoviewer. Startest on-axis is not affected with no sign of astigmatism or false colors.

On the Moon and planets (see tests above) : Views are absolutely fantastic and it is like you observe in a totally new type of instrument. In fact I was surprised to see details that I did not see - at first glance - with a single eyepiece: while a single eyepiece shows the same level of details, it is like if the binoviewer allows you to know “where to look at”.

I also quickly tested the binoviewer on M42 while the Moon was still above the horizon. View was very interesting and comfortable and promising for future nights dedicated to deep sky at least for the brightest objects; I don't think it is interesting on faint objects for which adverted vision is required unless using a bigger scope...

Comparison with a TEC Maksutov MC200 F15.5M

Since I wrote this review, I tested side by side during more than one year, in various conditions, my TEC 140 with my TEC Maksutov MC200 15.5M (reviewed here in French only). In some words, there is no doubt that this excellent 8" Maksutov conceived with a moderate obstruction (*) can give better results on planets and small deep-sky objects. Don’t think images in the Mak are totally different but improved brightness and contrast rendered by the Maksutov is enough to allow more magnification to be applied and to make observing of very faint planetary details easier during +/- short moments of good seeing. The difference is easily perceptible, even by a novice observer. But if seeing fails, I have to say that images are equally poor in both instruments… I rarely encountered real conditions (**) in which images of the APO are significantly "better" than images given by the Mak (by "better", I mean "more detailed" because they sometime appear more "aesthetic") thanks to its good thermal properties.

Optical quality is excellent has be determined being above 1/7PV (Strehl > 0.96) by Airylab using a Shack-Hartmann optical bench. I found similar results with several Roddier tests (1/30RMS average Strehl 0.96, EER ~0.82) however, I also found that optical correction may be variable depending on the thermal state of the tube. If it does not drop below a decent ~1/20RMS, this shows the sensitivity of the instrument to its thermal state that is common to all Maksutovs and probably explains their capricious reputation !... Running the fan at least during one hour before observing is mandatory in order to equalize the temperature between all the components. Fortunately, a solution has recently emerged in the use of bubble insulation to insulate the tube and avoid harmful exchanges between the tube and the outside. The results, regarding thermal stability, are spectacular.

In addition to its optical excellence, TEC MC200 has exceptional mechanical qualities with zero image shift, permanent collimation (checked at 1200X), smooth and precise focuser, good thermal properties, moderate weight… and despite the small illuminated field, vigneting is not sensible visually for a field of up to about 30' to 40'.

TEC MAK MC200 on ATS pier and GP-DX TECMC200+APO140

On Mars in February 2008 (diam 10") under 0°C and good seeing, Oxia Palus, Aurorae Sinus are still identifiable with the MAK but almost washed out with the TEC140... On the other hand, F companion of Teta Orionis was equally difficult in both instruments and even slightly more aesthetic and often easier with the refractor... Why? Certainly because image stability in the Mak, is - as you may suppose - more fluctuating and more sensible to instant variation of seeing: consequently, image quality is more “variable” and available information are more difficult "to process"... You have to be patient to get the benefit of the higher resolution! It is a well-known fact that patience is generally the "price to pay" to Aperture while other parameters such as Obstruction, Cassegrain and Thermal design may also play a role in the final result.

On Saturn in June 2011, under very good seeing, the crepe ring - in front of the globe - is totally evident with the 8" MAK @ 260X (RKE 12) while it requires attention with the refractor and equatorial bands also more contrasted. Cassini division is equally visible and evident in both instruments while the five faint visible satellites are more aesthetic with the refractor (like it is the case with all faint stars).

While it seems the 8" Mak competes for planetary observation with something like a "TEC 165 to 170" APO (which unfortunately does not exist), it is a very specialized instrument. Consequently, the choice between a 140APO and an 8" MAK is simple: even if you can have only one scope and can't afford a TEC160 or a TEC180, the TEC140 is still the way to go. The TEC140 APO has enough aperture for amazing observations and has more applications... Also, your choice is "simple" because, unfortunately, the 8" TEC Mak is out of production!

In conclusion, planetary resolution with the TEC140 is just one (but noticeable) step below a good 8" (but TEC!) Maksutov with a moderate obstruction. The refractor takes advantage for open clusters, deep-sky objects requiring large fields and certainly with no contest for its versatility, its aptitude to imaging and the ease of use thanks to its superb rotating focuser and its moderate length and weight…

(*) measured aperture 200mm, measured obstruction 54mm = 27%, Measured focal length ~ 3140mm (F/D~15.7)

(**) Unlike one can often read, the MAK (at least this one) can be perfectly usable under cold weather (0°C) provided temperature is stable (see text).

Conclusions

TEC is an exceptional company for its extreme care to satisfy its customers and the consequence is that all TEC scopes are exceptional instruments.

Laurent ()

TEC 140 Pros

- excellent service by TEC,
- premium optic,
- oil spaced design,
- premium focuser,
- excellent finishing,
- moderate weight.

TEC 140 Cons

- finder does not match the color of the OTA !...