Telescope Reviews: D&G 6" f/12 Refractor

Ah, the high virtues of a simple (not simple to make!) D&G doublet!

Because those with large $$ "normally" go for big APOs these days, it doesn't mean everyone does - or even wants to. Small APOs, sure - big APOs, not so sure..........

On an absolute basis, the very, very finest APOs can "potentially" give the very finest visual images in the best of conditions (no temp drift), with the most light focused in the airy disc.

That said, D&G doublets are still preferred by some, price no object. Why? Well, as a highly regarded former director of ALPO once told me: "A D&G doublet is worth the wait no matter how long - - one from anyone else is likely not worth the wait no matter how short". (a couple of other quality american makers have joined the scene since that comment was made to me 12 years ago).

Here's what keeps 'em coming back:

When it comes to practical observing with LARGE refractors, an air-spaced doublet has distinct advantages over an oil-spaced triplet, perhaps less so over an air spaced triplet. When you go out to view, if you live in an area like me (Kansas) where temps drop and keep dropping 'till dawn, an air-spaced Achro like the D&G equalises very quickly and the design is optimized to make temp changes less noticeable at the eyepiece than triplet designs. This means that while the APO-man is often waiting for the slight softness in the image to "go away", the achro-man is getting maximum performance from his lens.

And with maximum performance in a long focal length doublet, carefully optimized for most color-free possible with spherical correction to 1/10 wavefront (not just surface ptv) or better, what you have is a perfectly corrected lens which has the capability to perform, big time!

A 5" doublet of this quality will often have planetary images preferred by most over the highest quality 7" Maks, for instance (and that's assuming the Mak has temp stabilized). Contrast does not always beat greater aperture for what you want to see, but it does do so often enough on planets, the moon, doubles and open clusters.

With only two elements up front and as few as three in the eyepiece, you have the potential of superb contrast which more glass or reflections can hardly match and blackest contrast is what some of us CRAVE.

Another virtue of this design is the very long focal length which makes for a scope that an owner takes such pride in as it "looks" like a real scope. I don't think too many of us realize the pride of showing off one of these long boys -- a point "visitors" certainly DO appreciate - it creates a sense of anticipation in them that makes their (often long awaited) time at the eyepiece doubly rewarding.

Long focal length, as we all know, but most of us seem to forget, allows higher powers without normally using barlows. This scope type simply puts the least amount of glass between you and the universe - think about it.

Great designs for relatively inexpensive roll-roof observatories have never been better or cheaper to build.

Put one of these in your backyard with a scope like a D&G in it and many could call that good enough - for life.

Want a little better even - get a chromacor corrector instead of that next Nagler or two and you will reduce what tiny chromatic aberration is remaining by as much as 60%.

Everything is a compromise of performance objectives when it comes to scopes. I don't think you'll find many who would trade a large, perfectly figured doublet scope for anything else.

It performs when sometimes the great names are "too hot" to handle and most of us don't have time to wait. Do you?

Wishing all the best of scopes under perfect skies!

p.s. I hear the nay-sayers - the ones who need to justify buying that huge APO instead of putting their kids through college saying things like - :"well, put that scope on Venus or Sirius and..." - Hey, get Serious! Fact is, a touch of color "may" be seen by some, but not by all. And if your idea of astronomy is looking at Sirius all night -- I'm sure we'd find it "interesting" to read an article about your observations.

--------------------
Charles

Stars are wonderful! And there are so many that when you see a distant star through your fine optic, you are likely the only man on earth beholden to this singular delight of the angels.