Telescopes I have known, part II

In the first part of this post, I talked about three telescopes I owned in the years following the reignition of my passion for the night sky in 2001. I won’t follow a strict chronological order in this installment, but rather will group my telescopes into two categories: small refractors and big refractors. While I have owned (and still own) reflectors in the last few years, it’s been refractors that have left the most indelible impression on me.

There’s something about an unobstructed view of the cosmos that pleases me. Remember the OA5.5 I owned, an exotic sub-species of the two-mirror reflecting telescope invented by (and named after) Sir Isaac Newton, put up high-contrast images without any diffraction artifacts like the thin crosses caused by the vanes that hold the secondary mirror. In that way, the OA5.5 was “refractor-like”. Optical theory says that an obstruction 10% or less should not have any discernible effect on image sharpness, and even the Meade 628’s secondary wasn’t much bigger than that theoretical limit. Still, seeing a field of tiny of cross-shaped stars is very different than seeing a field of pinpoints.

The primary limitation of refractors is aperture…telescopes based on lenses get heavy, long and expensive as the size of the lens gets larger. Mounting also become a problem as the optical tubes grow in length and weight, and the viewing position, at the bottom of the tube, means a mount much higher off the ground than a Newtonian, which was the viewing position at the top of the tube; Newtonians often have their mounts right on the ground!

The question is whether refractors reasonably small enough to mount easily are worth it compared to the much-larger reflectors that take up the same space and often cost less. Can a smaller aperture provide compelling enough views of the cosmos?

Vixen FL90s/FL102s

I tried these scopes a few years apart, but both were similar in aperture and performance. Which was stunning. The Vixen scopes have doublet objectives with one element made from a special kind of glass, well, in this case, actually crystal: calcium fluorite. The refractive properties of this glass allows the lenses to focus more wavelengths of light in the same spot than a doublet made of traditional crown and flint like my Tasco. (This inability to focus all wavelengths of light leads to false color or chromatic abberation…usually a purplish, sometimes a greenish, tinge around bright objects.) This kind of refractor is generally referred to as an apocromat, as opposed to crown and flint telescopes that are generally called achromats.

Here’s my Vixen FL90s:

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The Vixen flourites were some of the finest doublet apochromat telescopes ever made, with the flourite lenses supplied by Optron, the company that supplies Canon with special lenses as well as the famous Japanese telescope brand Takahashi. The FL102s was sold in the US under the Celestron brand, as a C102F.

I had, bar none, the best planetary views in my life from these scopes! Saturn and Jupiter were razor sharp, with no false color and no discernible scatter. Jupiter’s cloud belts were full of festoons and other details, amazing to see “weather” on a planet so many millions of miles away! I watched Jupiter’s moons cast transiting shadows on the gas giant’s surface, tiny, perfectly round dots. I think, if I had to pick a “desert island” telesope, it might just be a Vixen FL102s. It wont’ show as much as a larger refector, but what it does show is simply stunning.

The Vixen scopes tend to be a little undervalued compared to their Takahashi kin, and I think unfairly so. Vixen scopes are much lighter, and yet sturdy. The FL102s, with a 4″ aperture, weighs about as much as many modern 3″ refractors. The focusers, while not as hefty as the sand-cast ones Takahashi uses, are solid and when well-tuned, quite nice to use.

For deep sky use, galaxies, nebulae, and the like, no three or four inch refractor is going to be that great. It’s just not a lot of light-gathering ability. The Vixens were fine for this, for what they were. But turn the scope on a planet or the moon and wow, step back, whistle through your teeth, then step forward for another look. This is why people love refractors!

Vixen Custom 80M and ed81s

Given how good their apochromats were, I wondered about Vixen’s achromats. I had heard good things, and so when I found an 80mm f11 Vixen “Custom” for sale for a good price, I jumped on it. It turned out to be a great scope, very reminiscent of my Tasco in the views it provided but much shorter and lighter.

Here’s the 80m doing outreach duty, followed by a close-up of the focuser end.

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I have to admit that, as a design kind of guy (part of my day job involved poster, document and periodical design), I love Vixen’s green and white color scheme, and their lovely red-and-white ID plates.

For a variety of reasons, Vixen stopped producing calcium fluorite apochromat refractors. Their follow up scopes used special kinds of glass generally know as “ED” or extra-low dispersion. These later scopes are even more undervalued and unappreciated than the fluorites. My experience with the FL81s, which is Vixen’s current three-inch Japanese-made model, was very positive. It puts up very sharp, tight views, and appeared to me to be sharper and more contrasty than several other ~80mm ED doublets originating in China (which were fine scopes too!) And again, all that in a thinner and lighter-weight package. Here’s the ED81s:

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Lastly (for this post, I could write about telescopes all night, as long as it’s cloudy that is), I wanted to see what a large refractor was all about. I again turned to Vixen, in part because of their svelte designs, and in part because of a unique optical formula that until recently only two companies bothered with: it’s called a Petzval, and it used four elements, two up front like a normal refractor, and two in the back. The rear elements shortens the focal length and often flattens the field of view (doublets and even triplets distort the field of view, making it appear curved and making it hard to get everything into focus at the same time.)

The Petzval design was poplularized in modern times by Al Nagler of Tele-Vue, who introduced his first Petzval telescope, the four-inch Genesis, in 1989. They improved it with the Genesis-SDF, Televue 101, NP101, and so on, each iteration getting better optically and the recent scopes are generally known as being color free, and are highly regarded, especially as flat-field photographic instruments. I actually used a Genesis in 2003, the year my daughter was born, and I enjoyed it, though looking back I think that scope was not very well collimated.

Vixen’s take on the Petzval four-element refractor does not use low dispersion glass like the Tele-Vue. They are flat-field achromats that Vixen calls “Neo-Achromats”. I had the 140mm model:

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It was the largest refractor I ever owned. I loved it. Whereas the smaller refractors were primarily planetary and lunar scopes, the NA140 was all about deep sky, yielding a completely flat field of view about 3 degrees across. This is a scope for cruising the star belts of the Milky Way, and exploring the ghostly disks of planetary nebulae, and even hunting down the brighter of the dim galaxies. Looking through the NA140 was a pleasure.

Mounting it was not. It’s still a pretty long and heavy scope, and the low eyepiece position made for a tall and unwieldy tripod. Would I rather have the NA140 or the FL102s on that castaway island? It’s actually a hard call. Planetary views were not bad with the NA140 at all, and deep sky observing was far more rewarding.

People pan the NA140 for being an achromat and doing what achromats do, which is, in part, showing false color on bright objects. But they don’t get enough credit for their high-contrast deep sky views.

Not that I live in 250 square feet, the NA140 is too much scope for me. But I loved it when I had it, and recommend it to anyone who can give it a sturdy mount and appreciate what it does do very well.

I hope you have enjoyed this idiosyncratic, and incomplete, list of telescopes that I have been lucky enough to watch over and use for a short time. All of them have tons of potential. They can last a long time and inspire thousands with the stirring views of our universe they can provide. I wish them well!

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