CLEMENT FOCUSER ON TRAVEL STAR 8"
I live in Toronto and my “Dark Site” is an island in Georgian Bay (on Northern Lake Huron). In recent years it has been mostly “Grey Site”, since our skies often have humidity and particulate, making them less transparent and less dark. It is not always so, I recall a long stretch of wonderful observing weather in the Summer of 2000 when I bought my first scope, the Nexstar 5”. The go-to was my teacher, which I did not need for too long. My next scope was TV 102 APO, followed by Tak FS60-C, Tak Sky 90, and Portaball 12.5” with Zambuto, which proved fantastic on planets as well as on DSO. I also became interested in wide views of the galaxy, in rich field observing; I was amazed how much more detail a 6” achromatic refractor f/5 showed, compared to a 4” APO. The inevitable Chromatic Aberration, false color, was there, of course, but it did not detract much from the beauty of the star fields
and all the DSOs. That’s why I produced my 8” achromat f/6; I wanted to be able to travel with it to the truly dark skies of this continent, for example South East Arizona. I soon learned that I preferred using both eyes, and started with TeleVue and Denk binoviewers. When Harry Siebert perfected his 2” Binoviewer, I bought the excellent Elite 45. It makes the most logical marriage with Travel Star scopes, allowing the widest views in the 2” eyepieces. I use the 20mm Naglers type 5, and Burgess SW 26mm and 32mm. I find all of them very comfortable. For high power I use the Siebert 2” bino with 5mm and 11mm Naglers in his adapters (step-downs).
I have no ties of any kind to the designer and manufacturer, Don Clement, whom I have not met in person. All my focusers were purchased at regular prices from Don. I feel admiration for any innovator who dares to push the envelope of current thinking and conventions. Here, the "flexure" is employed instead of friction hinges. This allows much greater precision of movement.
(Flexure could be researched further by those mechanically inclined, I found it quite fascinating.)
Total focus travel: 3 inches
Focuser thickness: 1 inch
Fully opened focuser is 4 inches long
Available in 2 inch and 3 inch versions, (special sizes can be ordered).
Mounted by three #10 Cap head screws, the diameter of the mounting circle is 4.041 inches.
Made of black anodized aluminum, bellows are made of a special substance.
The feel is that of precision and beauty, neither particularly heavy nor light.
*very low profile of just 1 inch
* no need for dual rate focusing. This single knob works well for both coarse and fine focus.
* immune to temperature variations, no grease lubrication to stiffen in cold or attract grit
*SS sealed precision instrument bearings that support the only sliding or rolling parts are factory permanently oil lubricated. Delrin AF leadscrews are self-lubricated.
* Robo-Focus can be easily added at any time by the user to get autofocus and temperature compensation over 3" of travel (future plan is for an inexpensive add-on PWM controlled 2 button motofocus)
* 3 Delrin tipped 1/4-28 nylon screws hold with 3/4" diameter knurled knobs securely hold 2" diagonals, adapters, eyepieces and accessories
*Backlash and wear compensated leadscrew, focusing is linear over 3" travel
*built in elliptically tipped adjustment screws in base for squaring the focuser
*all ferrous metal is SS or black oxide coated SS
* the bellows also act as very efficient baffling, and there is no tube to flock
* no tube to project into the light cone
* no image shift when focusing
* The only minor disadvantage may be the projection of the three rectangular arms. The diameter of the arms in a closed position is 9.5 inches (fully opened is 8.80 inches). This had no effect on my operations, I never bumped into them.
* Slightly increased effort in turning the focusing knob is required when the extreme focusing positions are approached. The last parts of focus travel (when fully opened or fully closed) are somewhat harder to turn than the central positions. But this, also, is inherent in the design, and cannot be helped. It never bothered me; in fact I learned to enjoy it as a slight reminder that the end of focus travel is being approached. (I have to stress that this was noticed only during terrestrial use on very close objects or when playing with the focuser.)
OBSERVING EXPERIENCE IN THE FIELD
I have been using the excellent Takahashi and TeleVue focusers, which have been described as "buttery smooth". Clement focuser belongs to the same category. It is buttery smooth, but it has the added advantage of no-lock-no-adjustments. Don Clement "slowed down" the speed of travel and the result on his newest versions is a very desirable one: no need for dual speed. The Clement design allows perfect certainty where the best focus is. Even scopes with much shorter focal lengths would be excellently served by this focuser.
Star Test. I am not experienced enough to judge all aspects of the star test; most of the time our turbulence prevents definite conclusions. So far, I had the best opportunity in August 2003, during the Big Black-out: I found no aberrations; there was a complete symmetry and regularity to all the rings on both sides of focus. Tested both, at 5 and 10 defocus rings, at high power (240x) on Vega. I saw very little color on Vega in focus at my downtown Toronto location. My area does not really have dark skies, and I suspect the gray background does not show the widely dispersed violet as much as black skies do, or the low power images do. (In black skies the CA is, of course, noticeable.) I saw a brilliant red-magenta center dot outside of the focus, but not much else on the other extreme, inside focus. (First tests earlier in April 2003 were like that also, but I just could not believe them; I needed the August confirmation of my initial findings. But APO owners, relax, we all know that Travel Star cannot have a color-free performance. )
Mars showed a bright polar cap easily and sharply. The gray-green large shapes on the salmon colored planet were well defined and surprisingly contrasty in moments of good seeing. Best views were with my 8 mm RKE Edmunds (150x) and with my 5 mm Naglers, Type 6 (240x); both in Denkmeier Binoviewer, straight, no Barlow. (The wide FOV of the Naglers makes following the celestial object easier even at this high power.) The focuser worked perfectly.
Moon was superb: High contrast images, razor sharp at 240x, very easy focusing experience. I did not see false color on the limb nor did I see it at the Terminator side at this power, since the brightness of the Moon may have simply obliterated CA. (Grayness of our skies may also contribute to CA being less noticeable.) But I saw lime green in low power views, where the false color is concentrated into smaller area. Focusing throughout these tests was a pleasure, including focusing on very close double stars.
Clement Focuser is a high-end instrument available at a reasonable price. Don Clement is a good person to deal with. He answered my e-mails instantly and followed up with a phone call when necessary. I am happy that I can highly recommend him and his Focuser. I understand a $ 50.- update is available for the earlier versions of the focuser. Users of binoviewers may be particularly happy with his design; the three nylon bolts holding the 2” Star Diagonals have such a perfectly secure grip that never in my observing did the binoviewer set up rotated downward.
(This happened to me with other focusers.) I understand that SCT version and Robo-focus are also available.
Joseph Drapell 2003 (updated October 2005)
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