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CN Report: The Tele Vue NP101
Feb 04 2008 11:34 AM |
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The Tele Vue NP101
Sometimes things are black and white Tom Trusock 2/2008  I've looked at a lot of telescopes over the years. Like many of us, I've been looking for the "perfect" telescope. Superb optics, top notch fit and finish, portable and (here's the kicker) affordable. Yeah, I know, the perfect scope (at least by my requirements) is probably a bit of a lost cause. Nothing is perfect. But some scopes are better than others, and there are a lot of good telescopes out there today. So for those with limited time, I've prepared an executive summary: On the pile of 4" apos out there today, TeleVue's NP101 is at the top of the heap. Ok, that's pretty much it. Y'all can quit reading now. Readers who are willing to subject themselves to more of my ravings, feel free to continue. For my money, the best telescope design - at least in the smaller range (say 5" and under) - is the apochromatic telescope. Every design has its drawbacks, but these instruments generally seem to have less than others. While I won't go into a detailed analysis of the design here, suffice it to say that given a good design, for the visual observer, two of the more significant drawbacks in today's instruments are false color, and field curvature. The Nagler Petzval design, while not without its own characteristics, eliminates both of these. The first "Petzval" lens was developed by Joseph Petzval, a Hungarian born in 1807, as a "distortionless" camera lens, and has since found applications in astronomy as well as photography. Joseph's original design is a bit different from TeleVue's Nagler-Petzval 101/127 and its brother the NP101/127IS, which were designed by Al Nagler specifically for the needs of today's amateur astronomers. The Nagler Petzval objective can be thought of as a 4 element lens system consisting of two sets of doublets - a front and a rear grouping. In contrast to more common optical designs which the amateur may be familiar with, the Petzval is longer in length than its focal length would suggest. In the case of the NP101 it's about 40% longer or equivalent to an f/7.5 doublet. For references, this is 5" shorter than the f/8.6 Tele Vue-102. Other characteristics of the design include: improved color correction (in a Petzval, color is reduced from that of a comparable focal ratio doublet by some 30%), and flat fields. As implemented by TeleVue, the NP series allows for stunning widefield views with no field curvature. False color is nonexistent in the NP series. These things coupled with the fast focal ratios of the NPs result in telescopes that, as either an astrograph or visual instrument, are very hard to beat.  The Tele Vue NP101 has a long distinguished lineage, starting with the Multi-Purpose Telescope designed and patented by Al Nagler in the late 70's / early 80's, and offered for sale in 1982. This 5" scope had a focal length of 509mm, and sported a unique internal diaphragm which allowed the optic to operate at focal ratios from f4 to f20. The one shown above is still in use at Tele Vue today for QA on their eyepieces. The lineage continued with the Renaissance, Genesis, Genesis SDF, TV-101, TV140, and currently is capped with the NP101/101IS and NP127IS. Fit and Finish The TeleVue NP101 comes in a classic white crinkle powder coat finish, with satin black trim. The TV powder coat is one of the most durable finishes on the market today. I've owned 8 or 9 different Tele Vue scopes over the years, and let me say this - I don't baby my personal equipment. They are tools that enable to me experience the cosmos. While the gear is important to me, it's a means to an end. I use my gear. Like most amateurs, I try to take good care of my equipment, but things happen. Where other scopes have scratched or chipped, the TV finish has resisted all damage. And that's important. True story: Late one evening after some planetary viewing I was breaking down one of my favorite apos, a unit with superb optics and a beautiful, white (painted) OTA. As is pretty normal at the end of a late night session, I was tired. For some reason my brain thought I was using a tip out saddle plate. I wasn't. Further, I found out the knobs weren't captive when I loosened them a bit too far. One popped off and dropped 2" onto the OTA and chipping the paint. The predictable stream of invective followed. And then I got really stupid. With a firm belief that the Tele Vue finish wouldn't have chipped after such a minor ding, I took the knob into the house, opened the TV case and proceeded to drop the knob on my TV102 from an even greater distance. No chip, no mark, nothing. It wasn't till the next morning I'd realized what an idiot I'd been. For obvious reasons, I don't recommend this method of torture testing the finish on your scope, but the results were instructive. So - I'm quite sure it's possible to damage the TV finish, but after more than a decade of owning Tele Vue telescopes I've never managed to... even when in the grip of sheer stupidity. All TV scopes use a proven rack and pinion focuser, unique to TeleVue products. Frankly, I think it's one of the best focusers on the market. For telescopes that have a greater depth of field it works wonderfully. On the faster scopes however (like the NP series) a two speed focuser is definitely appreciated. Enter the Focusmate. The Focusmate is an add on fine focuser for, well, nearly any of the telescopes sold by TeleVue in recent history. (This is just one of the things there is to like about TV. When they design their equipment they do it with several goals in mind, including - but not limited to - backwards compatibility, as well as with an eye to future products.)  The Focusmate sports a 6:1 planetary reduction gearing and is easily installed by the end user. While perhaps not as aesthetically pleasing as the a StarLight Instruments FeatherTouch, it was designed to fit with other accessories within Tele Vue’s Imaging System line. The Focusmate works and it works well. It mates up to an optional motor drive which can be controlled by hand or computer. One of the definite pluses to the Tele Vue system is that the motor is quickly and easily disengaged (without tools) if one wishes to focus manually - you simply loosen a thumbscrew and rotate the motor out of the way. (If you are interested in imaging, this is but one small part of the IS system - a system that is definitely worth your consideration.) I've played a bit with imaging and IMO, motorized focus is a must. I did note a small amount of "wobble" in the Focusmate on my sample. In use it's not objectionable and frankly, unnoticeable. However, if you sit back and spin the focus knob, there is a little movement of the Focusmate from side to side. (I contacted Tele Vue regarding this and was told that the slight eccentricity is normal. The "torsion spring" which connects the Focusmate to the body of the scope is made of Delrin. It is designed to flex if such an eccentricity exists. Were it rigid, the focuser would bind.) It's not perfect, but it works quite well and it's a non-invasive approach to an add on focuser.  The NP101 sports a highly textured, flat back anti-reflective lining inside the OTA instead of more traditional knife edged baffles. These do a completely adequate job of suppressing glare and have the side benefit of allowing a smaller diameter OTA which results in both a more compact and lighter telescope. The telescope also comes equipped with the TeleVue Everbrite - a 2" dielectric diagonal mirror with a body machined out of one piece of aluminum. As such, you'll never have to worry about it accidentally unscrewing and dumping your gear on the ground during an observation session.  With a 4.9 degree max true field of view, this scope can easily serve as its own finder. I'm still fond of red dot finders as they make getting in the area a bit easier. If you want a RDF, they don't get any better than Tele Vue's Starbeam. While it's quite expensive for an RDF, it's unquestionably the best one on the market. The deeply recessed window allows the optic to remain dew free longer than its competitors, and at 40mm, it's quite large and easy to look through. Unlike other products, the optical window is not tinted. As per the red dot itself; well, it's one of the only RDF units that I've used that actually gets dim enough. Many others are just too bright from my semi-rural observing site, let alone a truly dark location. Finally, the Starbeam has one other trick up its sleeve; the flip mirror. The mirror allows you to site on stars high in the night sky without having to go through a series of contortions. The older (and fatter) I get, the more I appreciate that. Tele Vue's bat-handle clamshell is another little touch I appreciate. If you're using the scope on an alt/az mount (other than Tom Peter's Discmount), you'll probably have to rebalance at some point. If you're using rings, that typically means a 10 minute procedure that starts with finding the appropriate allen key. With the TV clamshell, rebalancing is a simple, no tools process. The only downside to the clamshell is that it might be a bit more prone to flexure. While not an issue for visual use, if you're planning on imaging, you might want to consider Tele Vue's optional set of tube rings (MRS-4000) to eliminate any potential sources of flexure. The tube ring set also offers the convenient bat-handle feature. I'd be remiss if I didn't point out that fact that with Tele Vue you're not just buying a telescope. It's more like choosing Nikon or Cannon for your DSLR - you're buying into a system. Like those, there's a lot of prior planning that goes into Tele Vue gear. Amateurs have complained that TV does not seem to refresh their line up with as much frequency as other companies. Why? In the words of my woodworking friends - measure twice, cut once. Tele Vue designs for the future and looks to backwards compatibility whenever they can in order to give their users as much support as possible. Further, Tele Vue has exclusive contracts with their subcontractors that assure the availability of their equipment, ensuring that everything is designed to their specifications alone and they never need to buy "off the shelf". Partly because of this, when gear is discontinued Tele Vue retains parts for years. And what's more because of the thought to what goes into the equipment as a whole, their systems are amazingly complete - even when connecting non-TV gear. Need an adapter to connect your doohicky to your whatsis? Tele Vue makes one. The only real gripe I have with the NP101 is the fact that at 26 inches with the dew shield retracted, it's about 4" longer than standard carry on permits. In these post 9/11 days, that can be a concern. While it would give me pause, I do know several folks who have flown with it. The NP is supplied with a hard case (the same one as ships with the TV102), and has an optional softcase that gets the package closer to most airlines "official" carry on size. Mounts Another beautiful thing about the NP101 is that at 10lbs, it's light enough to make it the perfect grab and go telescope and rides well on many Alt/Az mounts. I've had most of them at one point or another, and if cost is no object, my personal recommendation would be Tom Peters Discmount. If you need to save a couple of dollars, then you might want to investigate the Universal Astronomics Unistar Deluxe. While not a Discmount, it does an extremely capable job. Those inclined towards tracking may wish to look at either the Losmandy GM8 or G11. I have the NP mounted on a GM8 in my observatory and I'm very fond of the Losmandy clutchless drive system. For star hoppers, it's a match made in heaven. It also works well for photography but if you're serious, you'll probably want to mount it on a G11 instead.
I've been using the scope for a while now, both as an eyepiece test bed (the flat field presented by the NP yields the best test platform available) and for general observations. The star test on my sample is quite good, and I am particularly impressed with the color correction. Chromatic aberration isn't a problem simply because there isn't any. Period. Either LCA or spherochromatism. At least that I can see, and I consider myself pretty sensitive. Out of all the apos I've looked through over the years (and the articles that many of you have read are just the tip of the iceberg) this is a first for me. In focus, the airy disk is very tight and there's just a hint of a first ring with little to no junk floating around the star itself. This is exactly what you'd want in a first class apo. In a small scope, if the rings are too well defined or to easily visible it's a sign of a correction error. That energy is thrown out of the airy disk, and that isn't a good thing. While color control isn't the end all be all of a refractor, it is important. When all the wavelengths are where they are supposed to be, images are brighter and have better contrast. I've actually seen where color correction makes a difference in the reach (how deep one can go) between two apochromatic refractors. There is no sign of coma or astigmatism. And the field is flat - one of the major advantages to the Nagler-Petzval design. Apo-nuts who are fond of refractors for their sharp fields are doing themselves a disservice if they've not looked through an NP-101. These scopes, coupled with well corrected eyepieces like the Tele Vue Naglers, are the quintessential definition of "sharp to the edge".  One of the considerations of this particular scope is that there is some vignetting due to the size of the focuser and rear elements. In the standard (non-IS) model, this is most noticeable in long term 35mm CCD exposures (smaller chip sizes pose far less of an issue). While vignetting can be compensated for by several different methods, it's not ideal, ergo the IS scopes have a larger set of rear elements and a larger internal diameter focuser draw tube to increase the field illumination, further optimizing them for imaging. Visually, this is a non-issue. If there's a slight dimming of the edge of the field, I can't see it. FWIW, this is similar to a trade off that's often made in Newtonian design as well. There designers opt for an undersized secondary. This is a trade off in that the field isn't fully illuminated, but the smaller secondary results in increased contrast. Most of my observing life I've been conditioned to believe that "longer focal lengths are for the visual observer", and "fast scopes are for astrophotographers". Well, that's not entirely correct. In general, a longer focal length is perceived to have a few advantages, that's true. Slower optics are easier to make, and in the past short focal length eyepieces have had extremely short eye relief, so it's been easier to get to high magnification in a slow scope (barring use of a Barlow or Powermate). With a scope like the NP101, the first isn't even a consideration for the end user (TV manufacturing and quality control ensures the products are the best they possibly can be), and the second - well, even if it held a grain of truth at one time, with the new short focal length eyepieces available today it shouldn't be a factor. I'm sure you've also heard "the fewer elements the better" argument, but that's an oversimplification. Especially if you're arguing about light throughput: Today's coating technology is vastly superior to coatings of yesteryear. In short, you can sit and argue about how you think it's going to behave under the stars, but the acid test of any implemented design is in its performance under the stars. Performance  The NP101 makes a stunning rich field instrument. As shown above, the 55 plossl provides around 10x, and just over a 5 degree true field of view, while a a 31mm Nagler yeilds 17x, an exit pupil of 6mm, and a true field of 4.5 degrees! This field means the scope is capable of taking in targets not typically seen in an optic this size. The southern summer Milky Way is an awe inspiring view. The North American Nebula fills a low power field, and the entire Veil is easily seen from a dark site. It's an interesting contrast to set this scope up next to a giant 20" Dobsonian pointed at Pickering's Wisp, the Finger of God or some other portion of the Veil and have the ability to see the entire thing in the NP. In the fall, I'm very fond of M31 and M45. When winter rolls around I spend a fair amount of time staring at CR 70 (Orion's belt). And then spring means galaxies. It's always fun to see just how much of Markarians Chain you can pick out in a small scope. I also know that Al Nagler is partial to the pairing of M46 and M47 in Puppis and the false comet region in Scorpius. And aside from wide fields aside, 4" of aperture makes for surprisingly satisfying visual observations of a large number of targets. The Leo triplets present themselves surprisingly well to this amount of aperture. Another great target is the open cluster/galaxy pair 6939/6946. And there are many other marvelous targets as well - The Beehive, the Pleiades, Mel 111 - just too many to mention. Planning observing sessions can be quite interesting with this telescope. The typical observing guides don't match the flexibility the wide field gives you. One resource I'd recommend for the library - Binocular Astronomy by Crossen and Tirion. Lunar and planetary performance is excellent too. Contrast is extremely high, and the scope out performed my expectations, particularly while viewing Luna (one of my favorite targets for a small apochromatic refractor). While pretty much any short focal length eyepiece works well with the scope, I'd recommend the 2.5mm (216x), and 3.5mm (154x) Tele Vue T6 Naglers or the 2-4mm (270-135x) zoom for these applications. The choice will depend on how much you value top end magnification and the ability to zoom v.s. your desire for immersion and an ultrawide field. Under good seeing 75x per inch or higher on Luna is certainly doable, and I've had this scope up to 270x and above numerous times. (The mark of an excellent optic is usually 50x - 60x per inch.) The NP101 has an extremely diverse magnification range: from 9x to 270+, this is one of the major appeals. An illustration of this came clearly with the outburst of Comet 17p Holmes. In its early stages, seeing detail required a fair amount of power. As time passed, the expanding coma necessitated a larger and larger TFOV - something the NP101 was fully capable of supplying. My sample has a very definite "snap" to focus. Excellent optics compounded with the fact that it's an f5.4 (and thus the depth of focus is rather shallow) leads me to state that I feel either the Focusmate or some other fine focus accessory is a must have addition.  Summary Optically, the NP101 is superb. It's fast focal ratio and flat field combine to create a tour de force for wide field aficionados, while its excellent correction for chromatic aberration and control of other aberrations make it a superlative lunar and planetary choice. Accessories aside, the IS version would be a better choice for the astrophotographer wishing to shoot larger formats because of its larger focuser and rear elements. In terms of fit and finish, the scope is top drawer all the way. Tele Vue's powder coating is one of the best finishes out there. I find it very durable and easy to care for. The black and white of the telescope is a classic look that resonates well many, myself included. This is truly one optic that can do it all, albeit with one penalty - on your checkbook. However having been raised on the philosophy, of "buy the best, and cry once", I must admit that I don't even find that too daunting. If you can afford it, it's an heirloom quality piece of gear. In my opinion, the NP101 sets the standard for the 4" apochromatic refractor. |
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