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"Big Eye" TEC/APM 10" F6.6 Achromatic Refractor



Background:

I was hooked on fast refractors when I read a 1968 article in Exploring Space with Astronomy. The article described Clyde Tombaugh’s reaction to using a Jaegers cemented 5 inch F/5 achromat at Lowell observatory. His personal description on how this instrument performed on deep sky was absolutely mesmerizing. He also let visiting astronomers at Lowell Observatory look through this particular scope and they parroted his observations. I knew then I wanted this particular telescope. Another article in the same magazine gave directions on how to build the instrument using parts supplied from the now defunctJaegers Corporation. Still a couple years short of getting a drivers license, there was no requirement back then to impress your high school sweetheart by spending all your hard earned lawn mowing money on upgrades to your father’s hand me down Rambler, I set my goal on saving enough to buy this scope before the end of the summer. Jaegers was selling the lens for $79 and the 2.7 inch military Erfle eyepiece Clyde Tombaugh also used for $32. This was a bargain back then when similar sized achromatic refractors (Unitron comes to mind) were selling in the thousands of dollars.

I thoroughly enjoyed the 5 incher, using it considerably while growing up under very dark sky’s in Southeast Asia but as you would have it, aperture fever kicked in and I “upgraded” to another fast achromat, this time an air-spaced 6 inch F/5 Jaegers.

In 1995, while carousing through the latest edition of The Starry Messenger there was an ad from APM Telescopes in Germany with several high-end refractors for sale. One was an 9 inch F/11 Astro-Physics (AP) apochromat and the other, a fast 10 inch F/6.6 achromat. A quick call to Markus Ludes of APM Telescopes confirmed that both scopes were still available. Relief was instantaneous. For those of you who still remember The Starry Messenger, depending on when you received your copy, many of the good deals were already consummated if you were at the tail end of the US Postal distribution system. Markus described the 10 inch lens as a product of Telescope Engineering Corporation (TEC) and that its thermocompensated push-pull lens cell, copied from the Celestron Flourite series, was also a product from TEC. At that time TEC was a little known startup operation out of Coshocton, Ohio.

After deliberating for 24 hours I decided to buy the achromat because it was a little lighter and somewhat more manageable than the longer and heavier AP Triplet. Being a deep sky buff I wanted a relatively portable scope with the widest possible FOV. I have seen too many scopes wasting away in someone’s garage because they were too difficult and time consuming to set them up. The TEC was also less expensive than the AP asking price of $11,900, though when you consider how much large used AP refractors are selling today, this 9 inch APO back then was a bargain.

Optical Tube:

The optical tube was custom made by APM. The tube appears to be a fiberglass composite with internal baffling and is quite light for its size. I was a little concerned at first that there could be some tube flexure but there is none. The focuser is a very smooth 2.7 inch Takahashi and internal to the tube is a circular baffled 9 lb counterweight attached the base of the scope. With the counterweight, the scope is nicely balanced on the tripod with the center of gravity just forward of midline which also helps to elevate the eyepiece for observing at zenith. At that time, the scope weighed 68lbs. I have since lightened the optical tube by removing the internal counterweight and attaching a removable brass counterweight to the rear of the scope, and utilizing a Borg 65mm finder with heavy 2 inch brass focuser knobs to help offset the weight of the objective. The weight of the bare scope now is 59lbs which for me is not a problem in hefting on to a Millennium mount, though the pucker factor increases dramatically if the ground is wet and slippery. More than once I have nicked the tube while trying to slide the tube through the rings.
Other personal modifications include installing centering screws to adjust the flint element, lining the interior of the tube with felt and insulating the interior of the dew shield with aluminum sided bubble wrap lined with felt. This particular modification has really helped postponing use of a dew heater and conserving electrical power at remote locations.

Optics:

Markus Ludes commissioned Yuri Petrunin, the owner of TEC to design the lens and build the aluminum lens cell. Yuri has since told me that the lens was designed by his current chief optical designer Eduard Trygubov, who at the time, was still in living in the Ukraine. Eduard obtained the materials and had the optics polished at the Moscow Institute. The lens is an air-spaced Fraunhofer using Russian F1 as the crown and K8 as the flint elements. Visually, the lens appears to be optically clear with no obvious defects except for a single, very small (¼ mm) bubble in the crown element. At the time the lens was produced, Russian coating technology for such a large lens was a hit or miss affair and the coatings on the crown element did show some sleaks. I also remember D&G optical, a producer of superb long focus achromats advertising at the time that if you wanted them to coat an objective 8 inches or larger it was at done at customer risk. My observing passions at the time lay in deep sky and a few sleeks in a ¼ wave coating would not affect performance at lower powers.

I do not have an interferometer report on the lens but several observers have looked through the lens and estimated overall correction to be about 1/6 wave with slight undercorrection and with a very small amount of roughness which is pretty good for such a fast lens which are more difficult to accurately figure than slower systems. An air spaced doublet cools down very quickly and I have found equilibrium be less than an hour with the well designed lens cell. Surprisingly, I have found the time for this lens to cool down to be only slightly longer than my AP 130EDFS triplet. I have also found tube currents not to be an issue with the 11.5 inch diameter fiberglass tube.

Performance:

I was living in Florida when I received the telescope and the first deep sky object I trained it on was M42. Using a 30mm Leitz 88 degree AFOV eyepiece revealed the nebula like I had never seen it before. The filamentation was beautiful with E&F very apparent and a rose colored hue coming from the center of the nebula. After looking at several other deep sky objects and marveling at their appearance I was completely enthralled with the deep sky views through this machine.

Valery Deryuzhin of Aries Optical was also able to observe through this scope in the Austrian Alps and he posted this on SAA several years back: “I was the very first person (after Markus, of course) who tested this scope under real sky. This were back in 1994 September in Austrian Alps at 1800m level. The sky was velvet black and Orion nebulae, M31 and, and, and was simply amazing. Of course I saw these objects a lot of times through much larger scopes and under same if not blacker skies, but I never before and after saw such fantastic wide field view of these objects through amateur scope. I can add also, that the optics at that moment was not coated and, probably this scope now works even better. As for color correction, it was excellent for deep sky, but with a lot of colors on bright objects like Vega and Jupiter and on Mars too. But, using deep orange filter, I was able to see a lot of details on a gobbon Mars disk. Mars was very small at that moment (can’t remember its size, but no more than 10”). Details, I repeat were amazed. Saturn through green-yellow filter was also amazed with sharp details and was only a little inferior vs view through nearby 10” F/9 planetary Newtonian made by Markus’ friend Mathias. Without filter, of course, 10” Newton was clear winner”.

About two years ago, I offered to beta test Valery's spherochromatism corrector device called the Chromacor on my refractor. The Chromacor is specially designed for slower F/8 - F/10 achromats and has already been reviewed several times by Arpad Kovacsy, Tom Davis and others in Cloudynights. The latest version, the Chromacor II has removed much of the false color from the planets and brighter stars. Sirius is now white with the chromacor as opposed to a bright yellow without though a dim purple halo remains. The haze around Jupiter has been mostly removed with the planet now a brilliant white orb and the belts much more clearly defined. Saturn is breathtaking with no discernable color aberration and I’m really looking forward to seeing how this scope performs on Mars this summer. Though the improvement in color correction is not nearly as good as my AP or as aesthetically appealing, I am able to see considerably more detail in the planetary disks with this refractor.

When doing the star test a green filter is needed to clearly see the fresnel rings inside and outside of focus without the chromacor. With the chromacor the pattern is clearly defined. I use the Chromacor as plug and play devise without an alignment adapter because the scope is already well aligned and there is very little slop in the Takahashi focuser. When observing nebula and faint fuzzies or when using nebula filters the chromacor is removed. When observing the planets or globular clusters I insert the chromacor. Since the chromacor is better able to concentrate unfocused light into the Airy disc, very faint stars in a globular cluster become more apparent with than without. Lateral color at moderate to high powers is significant on the planets but if you have a good tracking system and am able to keep the object centered in the FOV this is not much of an issue for me. My eyes are less sensitive to chromatic aberration than many having lived with achromats for most of my earlier amateur years. Also, my high power eyepieces are Pentax XP and SMC/Tak orthoscopics with narrow fields and the Millennium mount has a PEC of about 4 seconds and tracking is dead center.

Now that I had a scope that could be used for planetary observing I investigated the possibility of getting the lenses recoated to eek out the best possible performance. I contacted Barry Grenier of D&G Optical and Yuri. Both told me it could be done but it would be prohibitively expensive having to most likely repolish and refigure the lens due to the possibility of the coatings bonding to the glass at the molecular level, and being very busy it could be a while before getting to the project. I then contacted Al Misiuk from Sirius Optics who said he could take on the project almost immediately and at a very low cost and more importantly, very low risk. I shipped the lens off to Al who was able to chemically remove the old coatings without damaging the glass. Interestingly, he believed the old coatings could have been Cryolite, a good optical coating but designed to be used only in protected locations because it is water soluble. Looks like some of those damp Florida nights may have further damaged the optical coatings. Al did a wonderful job recoating the lens and its now beautiful to look at and through.

The obvious benefit of the chromacor was that I now have a scope that provides good planetary images at full aperture and very good planetary viewing when the scope is stopped to 8 inches which increases the effective FL to F/8, the FL optimized for the chromacor. I currently keep the scope in the garage on JMI wheely bars and when I get the urge to observe the planets which is often now weather permitting, I can quickly wheel the scope out to the driveway and be operational in just a couple of minutes with the scope already cooled down.

The scope, mount, accessories, camping gear etc.. all fit in my compact Subaru Forester. I get a kick when I pull up to the registration desk at a Star Party and when asked what scope did I bring I usually get a “yeah right” look when I respond with “10 inch refractor” as the individual looks at the compact Subaru. Again, I have found portability to be one of the greatest assets to be able to enjoy this hobby to its fullest.

Conclusion:

This scope has given me endless hours of pleasure. Obviously, fast achromats have their limitations but for low power wide field use at an affordable cost they are unbeatable. It is my understanding that APM had planned to produce some large TMB designed Chinese made achromats of varying focal lengths but I believe this project has now been discontinued because of manufacturing difficulties. RR Optical out of the Netherlands is now beta testing a 210mm F/5 Chinese achromat designated Fat Lady. It should be interesting to see how this project develops and whether there is a viable market for these scopes with a chromacor option. My only advice is to keep it light and keep it portable.


Special thanks to Ralph Gruen for providing the photogrphs.



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