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Astronomy for the Beginner or User of Small Telescopes

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The Vixen A70lf Long Focus Refractor

Vixen 70mm refractor

Above you see my Vixen 2602 A70Lf Telescope mounted on my workhorse Pipe Fitting Tripod. I use that tripod for all of my refractors, as well as for my ETX 90. The base of the tripod is not included in the image.

Most commonly, in the past the tripod held my Carton 60mm Refractor telescope, a very long focus, heavy duty 60mm telescope. When I ordered the Vixen, I wasn't sure if the larger (70mm) size of the Vixen would make my old standby tripod less adequate. Happily and surprisingly, the Vixen 70mm is very light, weighing even less than my long focus 60mm.

Vixen 70mm refractor focuser

One of the reason's the Vixen is so light, alas, is that there is a lot of plastic in its construction. The focuser, shown above, is plastic, except for the draw tube. So the focuser adds virtually nothing to the weight of the telescope. In its favor, however, it can be noted that the focuser is made of very sturdy plastic, thick enough where strength is required. It has a long draw tube, so it can accommodate almost any combination of eyepieces and Barlow lenses. It even has a locking screw to lock the the eyepiece in any position.

Vixen 70mm refractor lens cell

The lens cell, a bit to my disappointment, is also made of plastic. Again, it is strong enough to hold the lens securely. But I did read of one buyer who had one of the cell screw holes (shown above) crack at the narrowest point. I've not had that problem. The cell as shown above doesn't have the lens hood attached.

Vixen 70mm refractor objective

With the lens hood, the cell looks like the image above. The slender thing sticking up aways down the tube is the simple mounting screw I use to attach the telescope to the pipe-fitting tripod. It is held in place by a hose clamp with a hole to accomodate the screw, with some felt wrapped beneath the clamp to protect the telescope's finish.

Lest you think the telescope is a toy, you need to know that while there is a lot of plastic construction in the design, it is all adequate in strength. If the telescope is handled with some care, I don't see the plastic as being a problem. When I started viewing with my ETX 90, I remember being concerned about how long that telescope would last, given that nearly the entire base and drive mechanism is made of plastic. That was over a decade ago, and the telescope still works fine.

Most importantly, the Vixen objective is not made of plastic, but glass. Vixen literature says that the objective is the same as used in their more expensive models, so no expense was spared where it counts. Besides, the telescope carries an advertised 5 year warranty.

The telescope came with an adequate pair of Plossl eyepieces, a 20mm and a 6.3mm. Also included was a decent star diagonal, and a focuser extension that can allow direct viewing without the star diagonal. The only disappointing accessory was the barely adequate 24mm finder, which honestly is toy-like.

Views Though the Vixen

I've not had a lot of viewing time through the Vixen, but enough to make some comments. First, when I took it out the first night, I viewed the 3/4 moon. Crater images were very sharp and had high contrast. Since I observe a lot with reflectors of various designs, the steadiness of the images through the Vixen were pleasant to see.

However, I did notice what looked a bit like misalignment with out-of-focus star images. I used my Cheshire eyepiece to examine how well aligned the objective and focuser were. It appeared that there was in fact a slight misalignment. I was able to loosen the screws of the focuser and move it just enough to correct the alignment error.

altazimuth setting circles

I've not used the Vixen on planets as of yet, but in addition to some moon observing, I've done some star observing to see how it performs. I made use of two aids on a recent star outing to help locate several items in a single evening. First of all, I made use of the setting circles that are mounted on the pipe-fitting mount. Simple though the mount is, the setting circles provide me good star viewing capability. If I had only the Vixen finder to depend on, I doubt if I could have found many targets.

Star Pointer Utility

Second, I made use of the Star Pointer web utility, which tells me where to point my telescope to see any item in any of the Messier, Caldwell, or Herschel 400 star catalogs. In addition, I can choose to get pointing coordinates for a nice sampling of double stars. I view the utility from the browser on my smart phone so that I can have the computer aiding handy in my pocket. The utility displayed coordinates are updated about once every 30 seconds.

But before I even started with my Star Pointer session, I took a look at Epsilon Lyrae, the double double. I always take a glance at the double double if it's visible, with whatever telescope I'm using. It's a pretty good test of a telescope, as well as of the conditions.

Most of my telescopes can resolve all 4 components of the double double, but some not as cleanly as others. The Vixen gave one of the nicest views of the double double, with all 4 components split and with the 3 brightest of the double pair showing a classic diffraction pattern. The dimmest of the 4 wasn't bright enough to throw much of a diffraction pattern.

With the aid of setting circles and Star Pointer, in one session I was able to view M2, M73, M72, M15, M71, M27, M13, M92, M56, M57, M39, M31, and M34. I also viewed the double cluster in Perseus (Caldwell 14), and about 4 double stars, including Eta Cassiopeiae. All of these objects showed well in the Vixen, with M13 even showing a bit of granulation.

I was also quite impressed with the view of the double cluster in Perseus. It shows well in any telescope, but in my 60mm telescope is definitely not as spectacular as in my f/5 Newtonian. But the 70mm Vixen gave a very nice presentation. It framed well with my 25mm Plossl, just filling the field of view. There were plenty of stars to please the eye. I took a glance also at the Pleiades, and needed to drop down to a 40mm eyepiece to take it in. But it was brilliant.

When I get a chance to view the planets, I'll add some info on that as well, but given how well the double double was displayed, and how tack sharp the moon craters have shown, I expect good results.

What Makes the Vixen Perform so Well?

It's been known since the invention of the refractor telescope over 400 years ago that they had one flaw -- chromatic aberration. Look at some of the photographs of the oldest Refractor Telescopes and you can see how difficult it was in ages gone by to get rid of that color problem. Refractors made with a single lens had to be tens of feet long to give any decent performance.

Eventually the achromatic concept was discovered, which involved making the objective two lenses instead of one. By choosing the proper types of glass, the prismatic effects of a single lens could be brought under some control by a second lens of different refractive properties. Crown and flint glass was used then and still today for the common two element achromatic telescope.

With the creation of the two element objective, focal lengths for small telescopes at least, could (and can) be made of only a few feet to give good results. Look at the Chromatic Error vs Focal Length chart and you can get the whole picture for the classic two element objective telescope.

From the chart, you can see how as the diameter of the telescope goes up, the focal length must go up as well to get acceptable chromatic error. The Sidgwick focal ratio standard indicates that for acceptable chromatic performance, the focal ratio of a lens divided by the lens diameter (in inches) should be 3 or greater.

The standard is commonly accepted as the ratio where negligible chromatic aberration will be seen, and is easy enough to achieve with a telescope of 2 to 3.5 inches. A 3.5 inch diameter telescope, as indicated by the table, must have an f ratio of f/11 or higher, giving a focal length of 38.5 inches or better. That diameter, weight, and length makes a manageable telescope.

But for a 5 inch telescope, the ratio moves to 15, giving a resulting focal length of 75 inches. That's over 6 feet, and for a much heavier 5 inch telescope, being of such a long length makes a package significantly more difficult to manage. That's way most 5 inch refractors don't meet that strigent level of chromatic aberration.

Another way around the problem is to use the new low dispersion lenses, which are used in the ED telescopes, the ED standing for Extra-low Dispersion. The exotic glass objectives, while expensive, allow telescopes to be shorter and still have very low chromatic aberration. You'll know if the telescope you are considering is an ED telescope -- it will be expensive.

For a 70mm telescope, like the Vixen A70LF Telescope, the Sidgwick standard suggests a focal ratio of f/8 or better. The Vixen has a diameter of 70mm, or 2.76 inches. It has a focal length of 900mm, giving a focal ratio of f/12.9. So for the Vixen, the Sidgwick ratio is 12.9 divided by 2.76, yielding 4.67, far in excess of the required ratio of 3.

Translation, the Vixen A70lf telescope has a long enough focal length to produce virtually chromatic aberration free images. Super images, in fact. Given that in spite of its overall built to a price point construction, it has a quality glass, two element objective of a fairly optimal focal length. Optimal in the sense that it is long enough to have virtually no chromatic aberration, and short enough (3 feet) to have a moderately large field of view.

Final Thoughts

My impression of the Vixen A70lf telescope is a very favorable one. I see a couple of components that I would prefer to be made of metal and not plastic, but those components do their job adequately. The lens is something I can't criticize, for it delivers great images. At 70mm, the telescope delivers a wide range of targets, from planets to double starts to sweeping star clusters, with a few nebula and galaxies for good measure.

If galaxy hunting is your desire, then the Vixen falls short, but for general viewing, it is very good. It is surprisingly light, easy to mount if you don't buy it with tripod, affordable if you do buy it with tripod, and it cools down quickly. You may want to look into a better finder, as that is the only letdown of note. It comes with a 24mm finder that you might expect on a department store telescope. But nothing else about it, including the eyepieces and star diagonal, give anything but a favorable impression.