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Tele Vue NP101 Review

I have an old 1960s book about Mars with illustrations by Chesley Bonestell, my favourite space artist. It also comedic text that makes the 1960s sound more like the 1860s. For example, the author devotes an entire chapter to whether the canals were constructed by intelligent beings and another on why the Martians might have problems with their electrical power supply! In 1964 the author clearly still believed that Percival Lowell’s writings about Mars were factual and he reports having seen the Martian canals himself with the 60” Hale at Mount Wilson (I’ve spent hours viewing Mars through the 60 inch Hale and never seen anything like a canal).

What relevance does this have to Tele Vue’s premium 4” refractor – the NP101? The book in question derides Beer and Madler (producers of the first Mars Map) for having used only a four-inch refractor, which it describes as “about the same aperture as the coin telescopes set up in public parks”. Now I used to own a 1960s Unitron. You can find it reviewed on this site. The Unitron was a lovely looking telescope with quality construction, but the optics were frankly mediocre. So perhaps we shouldn’t blame that ‘60s astronomer for doubting a 4 inch refractor as a planetary performer; extrapolating from mine, a 1960s four inch Unitron would not be.

It turns out that almost everything written in my charming old Mars book is wrong. Nowadays, we know there are no canals on Mars and contrary to the author’s prejudices, a four inch APO refractor can show a surprising amount of planetary detail, if it’s a good one … like the NP101.

At A Glance

Telescope

Tele Vue NP-101

Aperture

101mm

Focal Length

540mm

Focal Ratio

F5.4

Length

~650mm

Weight

~4.5 Kg

 Data from tele Vue

What’s in the Box?

Design and Build

The NP-101 looks like most other Tele Vue telescopes. Internally, though, the NP-101 is unlike most other Tele Vues (except the Genesis and Renaissance) because it is a (once unusual, but now less so) optical design called a Petzval.

Petzval lens, seen through the objective, nestling at the font of the focuser.

Optics

Like other Tele Vues, the NP101 objective lens sits in a slim cell and is almost as wide as the tube. As you can see, the curves on the glass are gentle – not what we might expect with a fast lens; that’s because really it’s an F11, not an F5.4 at all – I’ll explain why in a moment. As with all modern Tele Vues, collimation is not adjustable: the cell is held to the tube by three dome-headed bolts which are tightened at the factory once the cell has been nudged into perfect alignment.

The NP101 is Tele Vue’s current premium four-inch that uses the Petzval design. Tele Vue have been producing Petzval refractors for the last quarter century and only in recent years have other manufacturers started to catch up.

The photo below shows the Petzval lens nestling at the front of the NP101’s focuser. So what exactly is a Petzval anyway?

A Petzval combines a long focal length (in this case doublet) objective with a smaller doublet field flattener/reducer in front of the focuser. The Petzval lens corrects many of the aberrations inherent in the objective, along with halving the focal length and reducing the physical length. So a Petzval is a compact, highly-corrected, short focal-length four-element refractor.

The NP101 is an apochromat, but not all Petzvals are, because the Petzval lens doesn’t correct for chromatic aberration (false colour), just for other aberrations, like off-axis astigmatism and coma. The level of false colour is determined by the objective. That’s why the original Petzval, the ‘Fluorite’ Genesis, was really a semi-APO: the fluorite was used in the Petzval lens, not in the (admittedly ~F10 and so better corrected than most) achromatic objective. In the case of the NP101, the objective is an ED glass doublet apochromat of about F11. That’s a large focal ratio for a four-inch APO (even four-inch F9 doublet APOs are pretty good), so correction for CA should be virtually perfect and it is. You can see the high-level of colour correction achieved by the NP101 in this ronchi-test (chromatic aberration shows up as strong colour in the test):

Ronchi-tests of the NP101 (copied from: http://homepage3.nifty.com/cz_telesco/refracter_test.htm)

The management summary is that the NP101’s Petzval design allows a short focal ratio (F5.4) and a wide field of view in which stars are sharp pinpoints to the very edge with virtually no false colour and all in a relatively short tube.

Tube

The NP101 looks very much like Tele Vue’s other refractors, past and present. In fact the original NP101 was almost identical in appearance to the older Genesis. So the tube is powder coated in a durable pebble finish, whilst the sliding dew-shield and the focuser are in satin black. You either love or loathe Tele Vue’s standard metal screw-on lens cover; at least the NP101 version seems easier to thread and less screechy than my old TV76’s

The Petzval design makes the NP101 shorter than the (now defunct) TV-102, but a little longer (at 64 cm, 25.5 inches) than its 540mm focal length would suggest. The NP101 is thus one of the shorter 4” refractors, but not amongst the very few that are truly airline portable.

Tele Vue claim airline portability for the NP101 in its short soft case, but for most airlines 22” is the maximum, so whilst you might get away with the NP101, I personally wouldn’t like to risk being told it has to go in the hold.

Weight, at just over 4kg, is very modest – the NP101 is among the premier league in this respect, though the ‘i.s’ imaging version (see below) is a bit heavier. The typical Tele Vue flocking material (it looks like matt black sandpaper) in place of baffles makes the tube compact, at a bare 4” diameter. The Petzval lens is well recessed at the front of the focuser, so no risk of damaging it with a diagonal.

Tele Vue’s flocking material looks like sandpaper, but it works.

Focuser

The focuser is a quality rack and pinion with the traditional TeleVue “mag wheels”; a dual-speed pinion assembly is available for retrofit, though the latest version has a FeatherTouch dual-speed pinion as standard. The oldest versions have a simple chrome focuser tube with a single tension knob, whilst modern ones have a satin-finish CNC focuser of greater precision, with two tensioners and two locking set-screws. The new design is supposed to be more stable with heavy loads and to have less image shift when locked-up. Some people prefer the older focuser, but I found the newer unit is indeed more stable under load, if slightly less fluid in action.

Original focuser with single lock-screw.

Later (non-IS) focuser with twin lock-screws.

NP101 or NP101is?

The two NP101s I have owned have been the standard visual versions. These differ from the NP101is in two key areas

1)     The NP101is has a larger focuser with features suited to CCD work: a larger draw-tube, a built-in micro-focuser and a visual back with four set-screws for perfect centring and a plate that allows small adjustments of tilt so you can get your camera perfectly aligned to the optical axis.

2)     Perhaps more significantly, the imaging system version has a larger Petzval lens to minimise vignetting (loss of brightness at the field edge), which is otherwise a problem with Petzvals.

If you want to image with a full-frame chip, then you will need the i.s. version. For visual use, or smaller sensors, the standard version will likely be just fine.

The imaging-system focuser on an NP-127.

Mounting

Like other smaller Tele Vues, the NP101 has a clamshell ring which is slim and attractive, but doesn’t hinge like usual (you need an Allen key to remove it). The clamshell has two ¼-20 threads on the base set at a standard spacing that fits Tele Vue’s mounts. If you splash out for Tele Vue’s own dovetail, the NP101 can be quickly swapped between a Panoramic or Gibraltar and a Vixen or Sky-Watcher mount, because the dovetail replicates the ¼-20 threads of the clamshell base, so that you don’t have to remove it to swap.

The Gibraltar is the better of Tele Vue’s mounts for the NP101, but the lighter Panoramic is workable, if a bit vibey. Don’t try the NP-101 on a Telepod!

The TV-Vixen dovetail replicates the TV ¼-20 threads of the clamshell, so it goes straight on a Gibraltar head.

 

Accessories

The NP101 ‘package’ includes clamshell, a dielectric 2” diagonal and a 20mm Plossl Eyepiece. A compact soft case, like the ones provided with the TV 76, is an optional extra that makes the NP101 “just about” airline portable, as I’ve said. Other accessories include finders, a reducer and (of course) a huge range of eyepieces that are guaranteed to work with the NP101’s fast focal ratio.

Older NP101s have a big hard case like the NP127s (shown above), but more recent ones have a more compact version.

In Use – Daytime

This is purely an astronomical ‘scope, right? Wrong! The NP101 makes the finest terrestrial ‘scope I have ever used, albeit too large to take into the field. With a good wide field eyepiece (a Pentax XW 20mm or Nagler 17mm T4 come to mind), the NP101 gives views of birds and wildlife that have to be seen to be believed. I recall a memorable morning watching a squirrel eating nuts in a tree about 100m away, it was as if I was right there sitting on the branch with him. The reason for this brilliant (literally) terrestrial performance is the combination of high optical quality, perfectly flat and coma-free field and complete absence of chromatic aberration. For all these reasons, the NP101 also makes a superb telephoto lens for wildlife photography as well (note the swirly bokeh, typical of Petzval camera lenses).

Dusk snap of deer through NP101.

My local Jackdaw colony, silhouetted at dusk, usually cause false colour problems, but not with the NP-101.

In Use – Astrophotography

So the NP101 is a superb all-round visual instrument, but it doesn’t stop there. This is not the imaging-system version, but even so, put a DSLR in the back and the wide flat field and fast f-ratio give superb results on bright DSOs. No flattener or reducer is required – the NP101 already has one built in (though you can fit another to give an even faster f-ratio). The arguments about whether built-in or add-on reducers are better will continue, but if you’ve spent any time messing with adapters trying to get a reducer to work right, you’ll appreciate the hassle-free NP-101.

If you want to image the Moon or planets you really need a much longer focal length and Tele Vue’s own ‘Powermate’ extenders make an excellent and aberration-free way of doing so.

The following images were taken at prime focus and are unprocessed, apart from cropping (note the vignetting evident in the deep sky shot may have been due to a too-small adapter in the light path).

In Use – The Night Sky

Cool-down

You might think that having the airmass in the tube constrained between two lenses would be a cool-down problem. In fact, the larger NP127 is a little slow to cool, but the NP101 and Genesis seem like typical doublets: cool-down is quick and benign (i.e. largely free from transient pinching, astigmatism, spherical aberration etc).

Star-Test

Like most Tele Vues I have tried, the NP101 exhibits a little under-correction, but is otherwise very good. It is not, however, quite at the level of a Takahashi TSA-102 or TMB 100/800. Tele Vue seem to have adopted a ‘more than adequate, better than diffraction limited’ approach to optical quality. I suspect that to retain a premium brand position long-term they may eventually have to step up to the 98% Strehl, near-perfection level of Takahashi, LZOS, TEC and Astro-Physics; whether it’s a difference you could discern is a moot point.

Deep Sky

Al Nagler’s particular interest in astronomy must surely be sweeping for star fields and bright nebulae, since all his products seem optimised for that purpose. So it goes without saying that the NP101 gives beautiful views of clusters and bright nebulae, like Orion. Again, it is that perfectly flat and very wide (due to the short focal length) field that is the key here: diamonds on black velvet, edge to edge, with superb contrast. Wide-field eyepieces do give that “space-walk” feel that is a stated Tele Vue aim.

I really enjoyed sweeping the Milky Way for clusters using a Nagler 17mm giving 32x magnification, but the NP-101 will do small planetaries and double stars at high power too – I split Rigel without trouble and the Double Double was easy too.

The Moon

The NP-101’s forerunner, the Genesis, had sufficient chromatic aberration to make it unpleasantly colourful at much over 100x; not so the NP-101. With a focal length of just 540mm it is true that you need barlows or fancy eyepieces to achieve planetary and lunar powers, but the results are worth it. Given the right seeing, the NP-101 will happily take a 2.5mm Nagler to give 216x, though usually the 3.5mm or 5mm are better choices. Given that level of magnification, the NP-101 is much like any other top-flight 4” APO on the Moon – i.e. absolutely superb, with sharp, high-contrast views in stark greys and whites. As I’ve said before, a 4” APO shows a lot more fine Lunar detail than a 3” and on many seeing-limited nights the NP-101 will show as much Lunar detail – rilles, domes, subtle rays and crater chains – as any larger telescope.

Jupiter

Most surprising, given the astrograph-like spec’, is the planetary resolution on offer with the NP101. The NP101 has that “snap-to-focus” typical of fine optics and necessary for high-power planetary viewing, but due to the short focal length the sweet spot is tiny, so versions with a micro-focuser are easier to use for planets.

It so happens that my best views of Jupiter ever were had with the NP101 from my balcony in superb seeing at the very end of Summer. My big ‘scopes were out of action due to a fault in their mount and I nearly didn’t observe at all. It was late and I was about to turn in, but decided to have a peek at Jupiter, so I plonked the NP101 on the balcony and let it cool whilst I cleaned my teeth ready for bed. Stepping up to the low power eyepiece, I immediately noticed something was unusual. Jupiter, even at just 25x, was like an image from a book. At higher powers, a wealth of belts and swirls and spots and festoons were visible, along with the red spot, like a mini space-probe view.

Mars

When it comes to the Red Planet, the polar caps and major albedo markings – Syrtis Major, Mare Acidalium -are readily visible with the NP101, refuting that ‘60s astronomer’s “coin telescope” comparison. Many doublets are poorly corrected in the red, which can lead to problems when it comes to Mars. But the NP101 gave sharp views, free of the softness and stray-light that can beset other APOs on the Red Planet.

Despite being an ED doublet, the NP101 has minimal chromatic aberration due to the long native focal length of the objective (it’s an F11, you will recall, which is much longer than a 4” ED doublet needs to be for merely good correction). Consequently, overall false colour is at ‘Super APO’ levels, so that even for imaging it gives little away to the finest triplets - confirmed by independent bench-tests.

Summary

The NP101 is quite competitively priced in the USA – comparable to other premium 4” APOs; here it is expensive, which is a shame.

Cost aside, it is very hard to fault the NP101. Indeed, if someone were to put a gun to my head and force me to own just a single telescope, this would be high on the list, because it does everything. Not only does it perform well for visual or imaging use on all types of target, astro’ and terrestrial, it is compact and highly portable: light, quick to cool and easy to mount. The icing on the cake is guaranteed quality and a huge range of Tele Vue accessories to go with it.

The other main competitor Petzval – the Takahashi FSQ106 – may be a slightly better dedicated imaging scope, but it is also heavier, more expensive and less suited to higher images scales and visual use. A Takahashi FC-100D (or TSA-102, TMB 100-8 etc) might well be just a tiny fraction sharper on planets at high power, but is typically less portable and lacks the wide, flat field for star-sweeping and imaging.

So is the NP101 the best do-everything small telescope currently available? I think it just might be: highly recommended.

 

 

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