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TAMING THE WILD 8” F6 ACHROMAT

Posted by Doug Peterson   07/17/2006 08:56AM

TAMING THE WILD 8” F6 ACHROMAT
Like you, I have always wanted a big refractor.

As prices dropped and means increased, I eventually found myself with an 8” F12 D&G tube assembly mounted on a Mountain Instruments MI-250, in turn standing on the Meade Giant Tripod. The color correction was in the solid “ED” class, which is to say very good, if not the last word as in modern color free triplets from AP and TMB. Detail on Jupiter was astounding: The last time I can remember seeing so much detail was with a 16” Newtonian. It is said the “spokes” in Saturns rings were seen with an 8” refractor before we actually sent hardware up there. I can now believe it.

The setup is massive, huge. The tube is so long that birds perch in the dewshield and peck at their reflections in the objective. Always looking for a way to achieve high performance in a compact package, I looked at the APM-manufactured TMB-designed 8” F6 tube assy. Marketed as a comet sweeper RFT, Markus assured me the objective would meet my requirements for a “very good” star test. The first unit was damaged in shipment, despite the heroic packaging job; the second unit, after collimation and tweaking of focuser tilt for the ultrasensitive Chromacor, I am pleased to report meets that criterion.

For the D&G, with the Chromacor threaded on the front of a two inch diagonal and a 4” extension tube after, I was able to achieve Valery’s calculated 300mm optimum distance from focus. At the different standard position recommended by Valery for 6” F8 achromats (no extension tube) the 30mm aperture chromatic aberration corrector will vignette the 8” F6 light cone down to 7”F7. What! you say?! Throw away an inch of aperture? I thought only cheesy far-eastern and unwitting near-eastern marketeers would do such a thing. But the fact is that we are talking about a 40” long 39lb retracted tube , i.e. the “bulk factor” (volume x weight) is about what one would expect out of a solid 7” F7 triplet.



Uncorked--without the Chromacor--I still have 8” of aperture for deep sky, but a bit colorful for the planets. All that violet defocused (read: smeared) energy belongs back in the planetary image from whence it came. With the Chromacorr in path, the color correction is actually quite good, again in the ED doublet class. Certainly the violet halo is gone, replaced with rudy red diffraction rings and a suprisingly white airy disk. The image reminds me of the TeleVue Genesis I once owned. Other scopes with similar color correction that come to mind are the Nikon Fieldscope ED and the Takahashi Sky90.

Note that I have not necessarily achieved the optimum position, even better correction may be around the corner. It may also be possible to set up the Chromacor for a shorter distance to focus, opening up the aperture but at reduced color correction. More experimentation is clearly in order.

A word on the Chromacor and need for alignment: To correct the massive amounts of color defocus in an achromat, the chromacor needs to work hard. It uses highly bizarre glasses known only to Valery and maybe the Russian military. To essentially pull together the violet focus located millimeters away from the green minimum focus requires a heroic effort. But it works. Now the job of doing this for a point on axis is hard enough. I doubt that it is even theoretically possible to do the correction over a wide field. But it is not really necessary: for planetary imaging you want to keep the object in the center of the field if for no other reason than minimizing ocular aberrations. Lateral color is a result of the fact that the correction geometry is upset and unbalanced color error creaps back in at the edge of field—color that was already there in the achromat. In addition, the Chromacor must be square to the optical axis. Few commercial scopes in my experience are tightly spec’d enough. You must loosen and shim the focuser. Alternative methods would be to place the Chromacorr in an extension tube with radial screw adjustments like a finder scope to accurately adjust tilt. The Moonlight refractor focuser is available with collimation ability. For the Feathertouch that came with the 8” I expect to adapt Feathertouch’s tiltable Newtonian baseplate, this will require a little machinework.



The Aries Chromacor is available from Astrobuffet.com
The TMB-designed achromat is available from APM Markus Ludes.
Both are Astromart supporters.