Jump to content

  •  

- - - - -

TEC 10" f/20 and Other Subjects



Eric with his TEC 10" F/20 and Meade 12" LX200

Editors Note

I encourage readers to pay particular attention to Eric's use of the star test in his excellent article. Prior to performing the star test, Eric understood from an optical design stand point how the scope should perform. I would encourage other authors to do the same as some telescopes do not perform as you would expect in the star test. If you lack such an understanding then take the extra step to contact the manufacturer. Otherwise, you run the risk of improperly applying a set of diagnostic standards to an instrument that has been both properly designed and executed. This is only fair to the companies that make them and to the amateurs that will read your review.

Several years ago, I was looking for a telescope with a reasonable aperture and a long focal length, that could outperform my 225mm. f/12 Takahashi SCT The Takahashi is a telescope that has been long advertised with an overall 1/8th wave front correction and I knew a telescope that could beat it would be hard to find. I've been using this instrument for high resolution Lunar and Planetary observing and imaging, but I wanted a bigger telescope with a longer focal length and a smaller secondary obstruction.

After a couple of visits to RTMC and the Texas Star Party, I had a chance to observe through some Aries, Intes and Intes-Micro Maksutov telescopes. I like the Russian optics very much, but not the mechanical craftsmanship. I found image shift, the focuser knob felt too hard or too loose at times and the Russian tank or submarine like finish was not very apealling. To be fair the ones I examined were some of the first types of Russian OTAs there were made available in the U.S. I found some of the units had exceptional optics, some good and the minority of them mediocre. Consequently, I assumed the optics were generally well made but with rather poor quality control.

I have read and seen, that these instrument have been thoroughly modified during the passing years, and that now they have a very good overall workmanship optically and mechanically.

Continuing with the story, I established communication with several distributors of Russian optics, including Telescope Engineering Company (TEC). Most of them promised me optics of 1/6th to 1/12th correction at the wave front. Contrary to all, Yuri Petrunin (TEC) just promised me at least an honest ¼ lambda correction at the focus front, or as he said, probably better.

A little background on myself. I've made and recorrected optics for my friends and myself for many years. At last count I've worked on more than 60 systems that include Newtonians, Cassegrains, Gregorians, Schmidt cameras, Schmidt correctors, a Wright camera, a 6" Maksutov, several optical flats, and many others. Consequently I know how to test optics and know how hard it is to get a compound system that is really diffraction limited and has smooth optics. I've recorrected many very well known new and old brands of commercial parabolic mirrors. Most of these were supposed to be corrected to 1/10th of a wave or more, some even advertised to 1/25th of a wave, but in reality, most of these were undercorrected or overcorrected parabolas, that did not even get to be really diffraction limited in the best scenarios. Some others had rough figures, were semi polished and had all kinds of zonal defects, on what resembled at the most, a poorly made sphere.

I had never seen a TEC Maksutov, but after talking to Yuri and looking at the beautifully made layout of the instrument that he sent me, I was hooked and immediately made the decision to buy the MC250/20 in July 1996. He told me that the optics, that is the primary Astrositall mirror, the Meniscus and the secondary were made in Ukraine, and that he made the tube assembly, including baffles, cells, focusing system, etc, in the U.S. He also retested the optics to insure the quality, rejecting the ones that he did not like.

Then came the hard decision, to sell or not to sell my beloved Takahashi that is serial number 001 (just around a100 were made). I needed the funds for the new and unknown telescope. At that time, I was afraid that I had made a really bad move, so I stoically waited for several months.

When I finally received the OTA, I was amazed by the superb craftsmanship of the tube assembly. It immediately reminded me of the pre-war Zeiss instruments. I said if it looks this good, it surely will perform incredibly well.

The OTA is 45" long (including the dew cap), 12" in diameter and weighs 42 lbs. After mounting it on my G-11 I discovered that I needed a bigger mount to keep the 5 meter focal length instrument steady. I then decided to install it on an HGM-200 that I had set aside for a future big refractor. Finally after almost a year and a half wait, I got an A-P 1200 Go-To German equatorial mount to install the TEC on. This combination works superbly and it's the one I still use today.

Eric and his former Takahashi 225

As for the optics of the 10" f/20 TEC, well I can say that it outperforms without any doubt my former Takahashi 225mm f/12 SCT. To my surprise I can also say that it outperforms all the telescopes that I had ever made, owned and observed through during my 52 years in the hobby. Well I must rectify this, because it probably equals in image quality a long focus 14" Newtonian that I looked through. I remember it had a beautiful riveted hexagonal aluminum tube with the most clever sliding diagonal/focuser I have ever seen. The owner said, that the primary mirror was made by Perkin-Elmer for NASA or something like that, and that he bought the mirror as surplus for a few dollars. I'll never forget how Saturn looked through that telescope at the Prude Ranch Texas Star Party many years ago. Sadly, I don't remember the name of that lucky fellow.

Going back to the 10" TEC, I can say that the optics are extremely clean, the Meniscus light scattering is almost null. Have you ever tried to illuminate your optics at night with a flashlight? If not, do so, probably you'll find a terrible scenario - thousands or millions of pits, scratches, bubbles etc that mar the corrector and the primary mirror. However, the TEC reflects the light almost without scatter. This speaks highly of the very well made and smooth polished surfaces and the new state of the art AR coatings. The spherical primary mirror is a smooth and perfectly figured sphere with no zones, no turned edge and no astigmatism. The overall system is superb! It gives clean pinpoint refractor like star images. Everytime I observe Orion's trapezium, it resolves the E and F Stars using a regular Meade 26mm Plössl that gives roughly 190X. This is not a special feat, but assesses the low scattering of the optics and the excellent contrast.

Double Stars are cleanly split to the aperture limit when the seeing permits and the Lunar and Planetary views are breathtaking. This telescope perplexes me every time I use it and I use it almost every weekend if possible.

When observing the Moon and Planets, you simply forget that you are using extremely high powers - sometimes even over 50X or 60X per inch. This is because the image does not break down. I usually observe with powers that range from 200X to 850X if seeing permits. I've even gone up to 1190X on special occasions just to convince myself again that the optics are superb. Most of my observing of the 1999 Mars opposition, was with powers that ranged from 625X to 714X. Some friends that are good observers could not believe the powers that we were using, until I showed them the eyepiece focal length and my "telescope-eyepiece power tables" that have the powers calculated for a given ocular and telescope (a handy alternative).

Contrast plays a big role in the TEC´s performance. Star fields look like diamonds on black velvet as there is no stray light in the field due to the very well designed baffles. Even when observing the Moon, you can see faint stars up to the limb embedded in a real velvety black background. These give you a funny and unreal feeling when observing with this instrument.

I don't have an interferometric test data on my system as this was one of the first TEC's built, (project # 4). However, I think it is well beyond an 1/8 lambda at the wave front. My "Ronchi eyepiece" gives perfectly straight and sharp bands in and out of focus when pointed to a Star. The null when using just one line of the 100 line grating or a knife edge at focus, is something to see.

The "Star Test" shows almost identical Fresnel diffraction patterns in the intra and extra focal images. You are probably thinking - "shouldn't a very well corrected system show identical diffraction patterns?". The answer is no, because the Meniscus corrector of an all spherical Maksutov system produces a balance of the 3rd and 5th order spherical aberration, that typically gives less than 1/8th wave correction. The reason for the difference is that the point of best focus does not coincide with the paraxial focus, and that shows as an asymmetry in the diffraction patterns.

To be fair, the symmetry of the Fresnel patterns obviously is a measure of optical quality, but that applied only to optical systems different than an all spherical Maksutovs. Now, if you introduce an aspheric surface to a variant of a Maksutov design, then you can get equal diffraction patterns in the in an out focus positions.

Image shift is negligible. To test for it you need high power and turn the micrometer focusing knob in and out very fast. A funny feeling when you are used to the normal terrible image shift of some SCT's (including to a certain degree, my former Takahashi SCT.).

When I purchased the TEC, I installed an NGFS Crayford focuser with a DRO system that was using on the Takahashi. Mostly I never use it as the micrometer focuser of the TEC is more than adequate for the precision and predictable manual focusing needed for imaging with a long focus instrument.

During the years, I have taken the optics out several times to clean and also to adjust and relubricate the focusing mechanism. A few months ago I sent the optics to Yuri to have them recoated. When I received them back, he told me that the Meniscus had been recoated with a new type of AR coat that gives better performance. To my surprise, it surely did. The contrast is better than before, a feat I could not believe possible

Concerning the process of taking out the optics (a practice that I do not recommend if you don't know what you are doing) it's fast and trouble free. The collimation is very easy to do precisely as the primary mirror cell is a push pull system that can be adjusted while the secondary mirror uses the normal 3 collimating screws.

I use this instrument only for Moon and Planetary high resolution imaging and observing, not for deep sky nor for double Star work. I just use double Stars to test optics.

For deep sky observing and imaging, I use an 12"LX200 that is highly modified as per MAPUG instructions. This telescope has been contrast enhanced by covering the inside of the OTA with flocked paper and repainting the baffles in an out with an Ultra flat black paint. Image shift in the LX200 is almost nil, due to the installation of 2 ball bearings in the focusing mechanism, plus having the slider and the sliding tube finely mated using a very fine grease emery. The optics are fairly good and are critically collimated.

I have both telescopes permanently mounted near each other, so it is easy to make comparisons using the same powers. This telescope (LX200) gives very good and contrasty images that can be augmented to around 500X (or a bit more when the seeing is excellent). After this the images start to break down rapidly. While the TEC is idling at 500X with a 10mm eyepiece, the LX200 is almost at its limit with an 6mm eyepiece.

You have to remember that a 12" is less prone to superb seeing as is a 10" or a smaller aperture telescope. This is due to the size of the "air cells" that hover in the atmosphere above your observing site. This limits most of the time the aperture of your high resolution instrument and in my case I have found that most of the time it is the 10" f/20.

Images look more luminous and contrasty with the TEC due to the good baffling low scatter and well corrected optics plus a small secondary obstruction (22%). My good old LX200 is no match for the Maksutov, except that it has a much wider field and can therefore use ultrawide angle eyepieces. It's also a bigger and faster system for deep sky imaging at "just" f/10 or f/6.3.

The Maksutov is less demanding on eyepieces, you can almost use any reasonable Kellner or Orthoscopic eyepiece and it will give you a very good image, though not a wide angle one. I especially like to use my small and weightless old Zeiss Jena oculars of the Abbe orthoscopic. However my newer U.O. Abbe orthoscopic set performs almost equally well, as also do the XL Pentax complete set, this last one performs incredibly good.

I can still make another fast comparison while writing this, because I also have next to the TEC and to the 12"LX200, an 6" f/12 Astro-Physics triplet apochromatic refractor, the one that was known as the "Superplanetary". This big telescope was given to me as a gift by my good friend Joseph H.C. Liu, a well known Chinese astrophotographer. It performs stupendously well and I use it now to image the Sun in white light and in H alpha. It gives razor sharp images of the Sun, Moon, Planets and Stars. It is a pleasure to watch in good seeing, all those perfectly pinpoint Stars and those sharp and hard contrasty craters of the Moon and sharp disks of Jupiter, Saturn and Mars, though much less luminous and with less detail than with the Maksutov. Nonetheless you can crank up the power of the refractor and the image does not break, just dims. It's hard to compare both telescopes due to the difference in apertures.

Probably some day, I will compare them by making an off axis mask for the TEC and an on axis one for the A-P to get similar apertures (around 4"). Even if there is a gross difference in f/ratios, I can correct this using a good Barlow.

So as you can see, I have found in my 3 telescopes the best I can expect for a given chore, but again I have to say, the TEC is something special.

The only drawbacks that I have encountered with the TEC is the price, (even that it is worth every penny) and the other the cooling time. However, with the installation of a fan that Yuri sent me to put behind the primary mirror, now it cools fairly fast. I just bought one of the "Lymax Coolers", but have not yet tried it due to the raining season down in Mexico (where I live) and the high humidity involved.

I have a sliding roof observatory that keeps the inside and outside temperatures almost even, so when I open the roof it cools rather quick.

My Maksutov telescope is now an old design, Yuri has been improving many aspects of the MC250/20 and the rest of his new telescopes. Improvements include using composite materials for the baffles, new optical designs, smoother optics with better coatings, a much better quality control, optical test facilities, a new temperature stabilizing system and overall, a much better instrument. Consequently, the new ones will perform better than mine, though surely I would not notice much difference.

If you are interested in some images taken with the TEC and with the 2 other telescopes, you are welcome to browse this website: http://voltaire.csun.edu/roel/roel.html



0 Comments



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics