Vixen A70LF Review
As you can see, I bought this, ready mounted on Vixen’s Porta mount, for a certain small child. It’s worth a review, because it is an example of a (til recently) much overlooked class of scope – the long focal length small achromatic refractor. Such refractors were the only ones available in my own youth, my first ‘proper’ refractor being a Tasco 3” F15 that was very expensive (£300 in 1977) and not very good.
When I say ‘overlooked’, of course there are numerous such refractors available from toy stores and Ebay. Reviewers like me shout ourselves hoarse at Christmas trying to persuade parents not to put one under the tree. But such ‘department store scopes’ tend to be marketing exercises with poor optics and crummy accessories. Serious (whatever that means) astronomers tend to shun long-f refractors (even good ones) for other reasons – mainly because they are slow photographically and so limited for astrophotography.
However, a good long focus achromat (as opposed to a useless toy store device with plastic lenses) has some key advantages for visual use:
· Much lower chromatic aberration (false colour). An F12 design like this one should be virtually CA-free for visual use – APO not required!
· Long focal lengths are less collimation sensitive.
· Long-f lenses are easy to make well due to the less pronounced curves.
· High magnifications for the Moon and planets don’t require expensive specialist eyepieces, simple plossls or orthos will do fine.
Now given that Vixen has a fairly illustrious name, I hoped that the A70LF would be much better than your average toy store refractor, whilst delivering the advantages described. Let’s see if it does …
Design and Build
This is a simple, entry-level, longish focal length (900mm, F 12.9) achromat from Vixen’s current (white and black) range of refractors. The A70LF is probably Synta made, with a basic looking foil spaced objective and a plastic push-on dewshield.
Though long at 34”/860mm, the OTA is very light weight (1.9kg) – great for kids or as a budget guide-scope which won’t strain a small mount. To this end, the 1.25” focuser has a lightweight, but high-quality, plastic body with a metal draw tube. Focusing is well-weighted and precise, however – better than the Synta all-metal 1.25” unit found on the ST80.
Overall fit and finish is good and the OTA interior well baffled and matte-painted. Light weight but good quality cast rings are provided with a Vixen dovetail.
The accessories are a mixed bag. The eyepieces – 20mm and 6.3mm giving 45x and an optimistic 142x respectively - are silver-top Plossls, are very good indeed and would stand a scope-upgrade later on. The diagonal is a good-quality prism type. However, the finder is a toy-scope item: plastic singlet lenses with the most horrible, dimmest view imaginable and a plastic finder-mount which won’t hold alignment. Unfortunately, the fitting is non-standard so there is no easy way to fit an RDF.
It annoys me when manufacturers sell short-tube refractors with optical finders – the thing is a finder! However the same comment doesn’t apply to a 900mm focal length like the A70LF. The reason is that a longer focal length gives a smaller field of view. That means the A70LF delivers a maximum (with a 32mm Plossl) field of just 1.7 degrees. Compare that to a 55mm plossl in an ST102 giving a whopping 5.3 degrees. However in this case the 20mm plossl supplied yields just 1.1 degrees and here’s the thing – with that you really need a decent finder, but effectively the A70 has no finder at all.
What all that means is big frustration for a child. They just can’t find stuff. Even the Moon is a struggle.
Once you find things, though (which means Dad helping out) the view is very good indeed. As expected, there is no chromatic aberration worth mentioning and the Moon looks fantastic. That 6.3mm plossl seems like a crazy power for a beginner’s scope, but the A70 takes it and delivers wonderful Lunar views.
The planets are good too with the quality optics ensuring you get to see Jupiter’s cloud belts, saturn’s rings and Venus’ phases.
The small aperture and long f-ratio aren’t ideal for deep sky, but again the A70 works surprisingly well.
Make no mistake, on many targets this delivers performance on a par with a TeleVue Pronto, also a 70mm achromat. However, there is one purpose for which the A70 is particularly well suited: solar projection. The long focal length and modest aperture work well for this application (and the end-cap has a little aperture stopped with a plastic cap for the purpose). What’s more, at this price there’s little worry about getting it wrong and burning some part of the OTA innards! I’ve had many good views of sun-spots (projected onto a sheet of paper) with the A70 and it’s one of the best and cheapest ways I know of getting into solar viewing.
Please Note: (!!! Never, EVER look at the Sun through the eyepiece!!!).
The A70LF I had performed surprisingly well and had excellent optics. Unfortunately the finder problem spoils the party as a child’s scope: The A70LF is not suitable for children for this reason. I ended up selling it because my daughter couldn’t learn to find things without a finder and we couldn’t easily fit an RDF.
The basic concept of small aperture and long focal length certainly works though, delivering Solar System views close to that of a much more expensive 3” semi-APO.
The little Vixen makes an interesting comparison is to another long focal length refractor: my 1960s Unitron. Mechanically the Unitron is better of course (it was, relatively speaking, a much more expensive telescope when new), but optically the Vixen is far better. The A70 is also much better than that ‘70s Tasco I mentioned in the introduction. So in some ways it’s reassuring to see that cheap telescopes have improved! Talking of cheap, a friend picked one of these up brand new for £25 on Ebay, which must be the bargain of the century for a lightweight guide scope.
Recommended as a grab-and-go, guider or a knock-about solar scope, but not for children due to the small FOV combined with useless finder.