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Which is the best mid-aperture apochromatic refractor? Which is the best mid-aperture apochromatic refractor? Which is the best mid-aperture apochromatic refractor? Which is the best mid-aperture apochromatic refractor?



TEC140/980 - second run
TEC140/980 second run Turret offload index ruby ball eyepiece spring collar five port turret
My TEC140-980APO #022 first run

To effectively observe Mars throughout the 2003 apparition I needed a
portable telescope. Mars from the latitude of East Anglia (approx. 52º)
would be placed behind nearby trees most of the time as seen from my
dome. A high quality, high resolution portable telescope would enable me
to see Mars from elsewhere in my garden. I also wanted a telescope
sufficiently portable to take with me on holiday to Corfu during the
period of opposition, where I could take advantage of better observing
conditions. After due consideration a weight limit of about 20kg all up
seemed reasonable for both 'scope and equatorial mount. Charter flight
luggage limit was 30kg apiece (60kg for a couple), and hand luggage not
exceeding 8kg each.

On the face of it there is a wide choice of compact medium aperture apo's:

Astrophysics (EDFS 130 f/6 StarFire)
Borg (125ED 125 f/6.4)
Stellarvue (SV4 4-inch f/6.4)
Takahashi (TOA130 130 f/7.7)
Telescope Engineering Company (TEC140/980 140 f/7.0)
TeleVue (NP127 127 f/5.2)
TMB (130 f/6)
Vixen (ED114SS 114 f/5.3)
Williams Optics (Fluoro-Star FLT-110 f/7)

What I was after was the largest aperture, cost no object,
with a barrel assembly less than 10kg. that could be broken down
into not more than a 2ft. length. I also wanted an apo with a high quality
rackmount; one which can rotate, carries the finder with it, and is
capable of carrying a 645 format SLR. The best of the initial batch
I looked at was the TMB130, which weighed in at a little over10kg.
OG Colour Correction
At February's AstroFest 2003 I saw, on Broadhurst, Clarkson & Fuller's
stand, an even better mid-aperture apo. The TEC140/980; an
f/7 oil-spaced triplet, similar to the design of the Zeiss APQ. BC&F
were asking £4500. What impressed me was the specification.
The objective was figured to 1/55 wave RMS in the green Mercury e-line
corresponding to a Strehl ratio of 99%. The Feathertouch rackmount was
excellent, with a long rack, very smooth backlash free action and no
perceptible sag. It had large machined dural knobs, plus a milled fine
focus knob, easily handable in the cold with a gloved hand. The
rackmount could be rotated and locked by means of a large scalloped
collar, and the racktube was 90mm I.D. The eyepiece tube also had a
collet locking arrangement. The whole OTA weighed in at 8.2kg, and the
machining of it was superb. This I thought was the apo of my dreams.
This has to be the 'scope to see Mars in 2003.

So out came the flexible friend, in for a good hammering. But no! BC&F
had taken delivery of five and they only had one left and they weren't
willing to sell it. They were however willing to take my order.

"When could I expect delivery?"
TEC 140 focuser Delivery was anticipated sometime towards the end of July.
"July, I want it now, not in July!"
"Is that the soonest you can get me one?"
"Oh well, put me down for one, what sort of deposit do you need?"
They accepted 10%, i.e. £450, which was drawn from my account on
March 1st 2003.

Later that month I e-mailed Yuri Petrunin at TEC and explained my predicament.
Yuri obliged by advising me he had a couple of TEC140/980's from his first
batch in reserve. If #022 came up to specification I could have it
shortly after Easter. That sounded good to me. Easter or shortly
thereafter had to be better than sometime in July.

Easter came and went and Yuri told me that it was
with Sky & Telescope undergoing a comparison trial.
with another rival mid-aperture apo. (see Dec'03 issue)
Yuri asked, "Did I still want it seeing as how it had already been used?"
"Did I still want it?"
"Are you crazy, of course I do.!"
This was getting better all the time.

I had already ordered and taken delivery of a Vixen GP-DX with a short
tripod, half pillar, and their SkySensor2000PC controller, and a 75S
guide 'scope, and the University Optics 8x50 Amici finder. All I was
waiting for now was the TEC140/980. Things went quiet for about a month,
then Yuri e-mailed me the invoice. $4880 + $240 shipping. This was
equivalent to £3140, £1360 less than BC&F's asking price.
Yes I'd have to make the wire transfer and pay import duty and VAT,
but Yuri's price included tube rings, a TEC 9" dovetail plate,
and a Scopeguard case, plus a shipping crate that would come in
useful for air freighting it to either Corfu, or to CDEPA in the
Algarve June 2004 where I intended to observe the Transit of Venus.
Yuri was paid in full and I eagerly awaited the telescope.

BC&F of course had to be advised because I wanted my deposit back. To
say they were not happy about the situation would be putting it mildly.
However they did repay the £450, albeit with poor grace. Business though
is after all business. When I telephoned Yuri at 7:15pm (early morning
Colarado time) BC&F had already been onto him giving him grief,
threatening him with hellfire and damnation. He asked my advice and I
told him not to let them intimidate him, they weren't going to stop
taking delivery of his 'scopes, they'd be cutting off their nose to
spite their face. I explained they were trying to have their cake and
eat it too; after all the next batch due to be sent to them at the end
of July would be all snapped up; they'd also had the interest off my
deposit for almost four months. He did seem worried, and that concerned
me. I then advised him not to give them an exclusive dealership in
either the UK or Europe, which is clearly what they were after.
Edward Trygobov & Alex Richenko
So what is it about dealers than makes them expect folk like us to act
in their interest all the time? Sure if you buy Meade, Celestron or
Vixen kit, you are going to have to go through a dealer. These companies
are so large they cannot be bothered dealing with one off orders
themselves. There is no easy way round those franchise dealerships. But
when it comes to smaller firms like Telescope Engineering Company, run
essentially by one man, in this instance, the optician who designed the objective,
and has them made to his specification, and personally tests each
and every one, matters are a little different.

If Yuri was prepared to deal direct, then that's his prerogative. After all
he doesn't need dealers, its the dealers who need him. He could sell
each TEC140/980 ten times over, it is after all the best apo in its
class. Furthermore the only reason BC&F were ordering small batches
every six months or so is because they were testing the market. Their
bread and butter is Meade kit, which they seem to shift by the lorry
load every month. If, because they objected to Yuri taking orders from
indivduals in the UK, they decided not to order another batch, they'd be
other dealers quite willing to step in and take their place, and they
knew it. There's a limit to how many telescopes Yuri can produce, and
because of his reputation, demand exceeds supply. In other words its a
seller's market.
Yuri Petrunin
I've heard it said that the dealer provides after-sales service. For
after-sales service substitute warranty support. In other words if
something goes wrong whilst its still under warranty your earstwhile
dealer will have it shipped back to wherever it came from and allow you
to await their pleasure whilst it is repaired, or replaced, and
returned. So what's the difference between me and a dealer returning a
telescope directly to the manufacturer? A warranty is after all a
warranty. If it applies through a dealer, it can apply from the company
too. In this instance Yuri guarantees the quality of his workmanship and
stands by that guarantee. And what about when something goes wrong after
the warranty period has expired? Well the dealer will provide a
similarly protracted service and then charge you for it. You will have
to pay for the repair, shipment to and from the plant, and the dealer's
service. But you could do the same yourself if a dealer wasn't involved
like the proverbial middle-man, and save yourself his service charge.

So what precisely is the benefit of exclusive dealerships? Do exclusive
sales rights between a manufacturer and the dealer benefit the customer?
I suppose if you know little or nothing about telescopes it could be
argued they provide a useful service. After all you can visit their
showroom and see a selection of 'scopes and receive, hopefully, unbiased
advice; and to be charitable I have always received such advice from the
helpful staff at BC&F, but then they know me and I know them. I have
long suspected that what they tell me may not be couched in the same
terms when pitching a sale to the man in the street who comes into their
emporium on a whim.

Serg Kapinus
But what about when you do know your 'scopes, or can find out either by
word of mouth, or directly from the horse's mouth? I would say not,
exclusive dealerships penalize such a customer. You are paying a markup
for which in reality you receive no benefit, either directly in terms of
service or after-sales service, or in kind. Which brings me to one of my
pet gripes; the rise and rise of the dealership, replacing what used to
be manufacturers and retailers. But that is another story.

Word on the internet is that the TEC140 blows the opposition away.
Typical Yankee hyperbole no doubt, but there must be a grain of truth to
it because some of the commendations are being made by die hard apo
users who seem to have owned just about every possible mid-aperture apo
over the past 20 years. One guy even traded in his Takahashi OTA130 for
a TEC140, having made a side by side comparison.

Takahashi have only just brought the OTA130 onto the market (late 2002).
It is a super ED triplet as well, only f/7.7 rather than f/7. I can understand
why some apo's are not as good visually as the TEC140/980. For a start
it's the same type of OG as the earlier 6-inch f/9 Zeiss APQ which was
also an oil-spaced triplet. The Zeiss APQ was accepted to be the best
apo ever. However the Takahashi is an air-spaced triplet, but despite
the longer f/ratio, it might not be quite so well corrected, or more
likely the production correction tolerance might be wider. Many apo's
though are Petzval or Dialyt systems, typically two widely spaced
doublets, sometimes using Fluorite elements.

TEC Team @ AstroFest Chicago 2002
In small apertures Fluorite is OK, but the crystal has to be
synthesised, and by the time the single crystal grows to be of use in a
5-inch+ lens it starts to contain significant errors in its atomic
lattice that produce diffraction effects. Also some of the Petzval
systems are corrected for wide field 7cm x 7cm format photography, not
for high power visual work. Wide field correction is obtained at the
price of sphero-chromatism , whereas triplets corrected for visual use
minimize sphero-chromatism at the expense of field curvature. Some
observers overlook these niceties.

Dealers of whom I have pointedly asked the question, "How does the apo
you market compare to its rivals?" whilst happy to champion their
product
, become very indignant at the suggestion that there might be a
superior apo in its class. They say it is unfair, invidious and
unwarranted to make such comparisons because what matters is, "Is the
owner happy with the 'scope of their choice?"

So what if they are? What has that got to do with the question? These
same dealers are perfectly willing to trade on a brand reputation supported
by P - V & RMS wavelength corrections and Strehl ratios, not merely as
indicative of their product's superiority, but as a guarantee of
anticipated performance. I say, he who lives by the sword should be
prepared to die by it. In other words if you support your claims for the
excellence of your product with P - V & RMS wavelength corrections and
Strehl ratios, you should be prepared to back your claims against
rivals, and not try to obfuscate by moving the goal posts.

The top three mid-aperture visual apos I would say are the TEC140 f/7
(1/55 wave RMS e-line oil-spaced triplet Strehl 99%), followed closely
by the TMB 130 f/6 (1/30th wave RMS e-line air-spaced triplet Strehl
95.8%) and the Takahashi OTA130 f/7.7 (1/28th wave RMS e-line air-spaced
Ortho-Apochromat triplet Strehl 95.5%).

Apo users trade such concepts as colour balance, and colour correction,
and OG-eyepiece combinations, but at the end of the day, its down to the
accuracy of the OG. If three elements are involved then spherical
aberration can be corrected for two wavelengths. and in my view in makes
little difference whether its the usual C-F lines as is the case for the TEC140 OG
or the less common B-F lines. Sphero-chromatism is always going
to be slightly more noticeable in a fast f/ratio OG than in a slow f/ratio OG.
The only ultimate arbiter is the Strehl ratio. The closer it is to 100%,
the better the overall visual performance.

It is common practice amongst apo designers to quote colour correction
at the Mercury e-line 546.074nm. This should not be confused with the
Fraunhofer e-line due to Iron at 438.3547nm. The Fraunhofer C-line is
due to Hydrogen alpha in the Balmer series at 656.2816nm. The F-line
is due to Hydrogen beta in the same series at 486.1327nm. The B-line
is also a Fraunhofer line due to molecular oxygen in the earth's atmosphere,
part of the Telluric lines, at 686.7-688.4nm.


An air-spaced OG gives the designer four more degrees of freedom, than
an oil-spaced (and hence essentially surface contact) design, but there
is inevitably a lot more work involved in figuring all six independent
surfaces. Contact designs are easier to test and therefore less time
consuming to figure accurately. There will also obviously be no scatter
or possible internal reflection within a contact triplet whereas there
will be at least some within an air-spaced triplet. All these things in
themselves may be nit-picking niceties, but summed, they no doubt
account for the marginally enhanced performance of Yuri Petrunin's
TEC140/980.

This is what Yuri Petronin had to say in response to questions about the TEDC140APO's colour correction:

Q1). With you allowing a small amount of residual color for the sake
of higher polychromatic strehl ratio and presumably sharperimages,
does the APO 140 still meet the Abbe conditions for apochromatism?

- Yes it meets  - see our webpage

or file folder named "Color correction" in the File section.

For the APO140 we have chosen the design with minimum f/ratio, that
still holds the Abbe criterium for APOs, that stands for the
following : the wave aberrations do not exceed 1/4 wave for:
C-line (656nm - red)
e-line (546nm - green)
F-line (480nm - blue)
and 1/2 for g-line (436nm - violet) and corrected for coma (aplanat).

Q2). How does trading off color for sharpness work (how can
optimizing for whiter images create softer views)?

- The color balance was chosen according to the relative sensitivity
of the human eye for visual spectral range (450-650nm), with weight
coefficients for each color according to human eye relative
sensitivity for these colors. Since the color sensitivity of our eyes
corresponds with our nearest star radiation intensity (see Color
diagram in the same file) - such optimization gives the  highest
Strehl (best MTF) or in the other words sharpest images and best
contrast on planets and the Moon. Also no problem on deep sky,
but with slight violet halo around bright white-blue stars like Vega
(see Vega's intensity diagramm in the same file), Deneb, that radiate
most of their energy in 300-400nm range - for which our design is
less corrected (1/2 wave for 436nm).

With the given glasses and type of objective (oiled triplet) it is
possible to put more weight on g-line, and move the color correction
the way when the image of, particularly, these kind of stars will
look "whiter", but such "move" will cause larger spherochromatism,
lesser contrast and softer images since the other colors are affected.

Going farther, you may ask this question: is it possible to have
color correction better than we have in APO140?
- Yes, and there are a few ways to do so:

1. To make f-ratio longer, for example at f/8 color correction will be
slightly better (appr. 15-20% and 95% people will not notice this
improvement, but 100% will notice that scope is not so compact!

2. To use  more expensive glasses (LaL-s or KZF-s) that will improve
color correction for given type and f-ratio, but at the same time
price for scope will get up to between $6000-6500, comparing to $4500 -
that for sure will be noticed by 110% of customers (-: 
There are some other limitations for these glasses - they are 1.5 -
1.7X more dense compared to crowns, and also are not as good
internally!

3. To change design to air-spaced and use two ED glasses - that can
make even better correction (price goes up - no question), but we
would prefer to keep ED glass (that, BTW, is 3-4 times softer and not
as chemically stable as crowns... do you live near the sea? ) between
two crowns.
 
And with such approach we are back to the comparison - what is more
practical: to do 6 surfaces or only two? Remember, contrast and
sharpness are more affected by accumulated surface errors (6 in Air
design and only 2 in Oiled), than by not "ideal" color correction.

1/4 wave spherical aberration of third order error reduces Strehl
20%, (see fig. 18.7, "Telescope Optics, evaluation and design" or
Fig. 15-1, page 267 in "Star testing of ..." Suiter's book)
- compared to only 7% Strehl loss from optimized - 99.9% (546nm) to
93% - polychromatic, that counts errors of ALL (!) colors.


Q3). Does this design decision have more to do with the selection of
glasses used, or the curves that you generate on them?

- The main limitation arises from the limited number of glasses
that are currently available on the market; the next limitation comes
from dimensional limits and internal defects of the glasses, etc.

Finally, the given telescope design is always a compromise between
many  issues that must be taken in account by the manufacturer. With our
APO140  we feel this compromise is done in the best way:
you have a compact, light weiht telescope with precision mechanics and
optics for a reasonable price, taking into account quality of optics
(1/10 PV average) that could not be easily found on today's market.

Regards, Yuri


It's also touted that the oil seeps out over time and needs replacing.
This I can assure you is a modern myth. I know of no instance where this
has occured. However there are well known recorded instances of owners
of the 7-inch Meade Fluorite apo having had to return their OG's because
the Fluorite element has cracked in extreme cold. Fluorite has a very
different coefficient of expansion than ED and super ED glass and Meade
in particular experienced early manufacturing tolerance problems with
their OG cells. The instances I know about all occured in Canada, in
temperatures around -40°F, where the owners had taken the telescope out
of doors from a heated garage or house. No such instances have been
reported for ED & super ED O.G.'s.

How do I judge my TEC140/980? Let me tell you; I've used some excellent
'scopes in my time, but Yuri Petrunin's TEC140/980 is thee best
telescope I've ever looked through - it is nigh perfect. Don't just take it
from me, take it from none other than Dennis deCicco, read the review article in S&T.


Post Script

Had I stuck with Broadhurst, Clarkson & Fuller, would I have received
a second run TEC APO140-980 before 15th August, the date I was due
to depart for Corfu?

Well from what I read on the TEC Yahoo User Group, no:-

On Sunday, August 10, 2003, at 05:02 am, davidwhitworth27 wrote:
From : "davidwhitworth27"
Date : Sun, 10 Aug 2003 04:02:25 -0000
To : tec-scopes@yahoogroups.com
Subject :[tec-scopes]APO 140 update, was Okie-Tex Star Party thread
Reply-To :tec-scopes@yahoogroups.com

I received an email reply from Yuri today and wanted to share his
response with those of you that are expecting a 140 from the second
run. I had asked Yuri if I would have mine by August 22nd and he
replied that that was the approximate date they would ship the first
10 (of 20) in that run. He said that the lenses for all 20 were
complete and that they were now working on that OTA's. So, those
that have #'s 036 through 045, if Yuri's schedule is correct, should
see them in the next 3 weeks. Let's hope that schedule sticks.

David


--- In tec-scopes@yahoogroups.com, "davidwhitworth27" wrote:

Hi Mike,
That would make you run 3. I placed my order (run 2) in February
for a late May / early June delivery. There was a delay getting glass
for run 2 of about 5 weeks, so by my math they should be finished by
now. I ordered that early to have it in time for Mars but it isn't
looking good for that now. I sent an email to Yuri on Wednesday asking if
it would be here in the next two weeks but I haven't heard back from
him yet.

David


In tec-scopes@yahoogroups.com, "davidwhitworth27"
I would hope that Run #2 of the TEC 140 APO's would be
completed and shipped at least a month before Sept 21.

David




for a survey of refractive systems for astronomical telescopes go to Roger Ceragioli's monograph
and if you feel the urge to make your own refractor go to Roger Ceragioli's Refractor Construction Page.

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