Gavin R. Putland, BE PhD

Monday, July 24, 2006

(Comment)

Not quite a department-store telescope

"Equipment" review:
Celestron PowerSeeker 50 refractor

The ubiquitous department-store telescope (DST) is a 60mm Chinese refractor with a focal length of 700mm. You know — the one with the 24.5mm rack-and-pinion focusing tube, the yoke-type altazimuth mount, the vertical slow-motion control (with plenty of backlash), the wooden tripod (with the underdamped high-frequency torsional vibration that spreads star images into shimmering horizontal lines), the "5×24" finderscope (internally stopped down to about 12mm to reduce chromatic aberration from the single-element plano-convex plastic objective) on the single plastic bracket (whose base flexes as you tighten the mounting screws, changing the alignment of the finder), the "H20MM" and "H12.5MM" eyepieces (which the labels suggest are of Huygens type, but which are actually Ramsdens with eye relief of about half the focal length, hence plenty of lateral color), the "SR4MM" eyepiece (which probably really is a symmetrical Ramsden), the 3× single-element Barlow lens (which adds to the lateral color of the eyepieces), the 1.5× terrestrial inverting lens (intolerable), the mirror-type star diagonal (in which you can see the ripples in the mirror by viewing reflections in it at arms' length), the amazingly compact packaging (which is impractical for storage or transportation because you must dismantle the instrument completely to make it fit in the box), and of course the trademark black dew cap with the two decorative silver rings. You can't focus with an eyepiece inserted directly in the focusing tube, because there's not enough back-travel, so you have to degrade the image by inserting something else in the optical path; the star diagonal, bad as it is, seems the least evil option. The main tube has only one baffle, which is insufficient to stop stray light from reflecting off the inside of the focusing tube and fogging the image, especially in terrestrial use. (Even indoors, the reflections can be seen simply by uncovering the objective and looking into the focusing tube.) The printing on the packaging trumpets a maximum magnifying power of 525×, which is obtained by using the 4mm eyepiece and the Barlow lens. (Here the marketers have sold themselves short; by adding the 1.5× terrestrial inverter, you can magnify the blur and the vibration and incredible 787.5 times!) In fact, the maximum useful power is 56× (given by the 12.5mm eyepiece).

Well, this review isn't about the DST. It's about something slightly better: the Celestron® PowerSeeker 50.

Basic specifications

Made in China, the PowerSeeker 50 is a 50mm f/12 refractor on the simplest imaginable altazimuth mount and a folding aluminum tripod, with a rack-and-pinion focuser accepting 24.5mm accessories. It comes with three parfocal eyepieces: "H20mm" (30×), "H12.5mm" (48×), and "SR4mm" (150×). Also included are a 3× Barlow lens, a mirror-type start diagonal (but no terrestrial inverter), a "5×24mm" finder,* and an accessory tray.

How the PowerSeeker 50 is better...

The stand-out feature of the PowerSeeker 50 is the internal baffling. There are three baffles: one in the lower half of the main tube, and two that move with the focusing tube. If you look into the focusing tube with the objective uncovered, the only single reflections that you can see are from the far end of the main tube, and only when you look from off-axis. The minimization of such reflections ("stray light") is critical not only for terrestrial viewing, but also for viewing faint celestial objects within an acute angle of a brighter object.

The 20mm eyepiece bears the Celestron logo and shows all the essential hallmarks of a genuine Huygens. The field stop is between the elements. The eye relief (estimating by projecting an image of an interior light) is about 30% of the focal length. The apparent field is about 30 degrees. The only contrary indication is that the outer surface of the eye lens is slightly convex, whereas in a classical Huygens it is flat; but this is of little or no consequence in terms of lateral chromatic aberration.

The star diagonal, although unbranded, seems to be a reasonable optical flat. Unlike its DST counterpart, it passes the "look through it at arms' length" test. (Yes, yes: that's not a very severe test. Which makes it all the more necessary to pass!) Actual use at high magnifications confirms that the star diagonal does not significantly degrade the image. Moreover, you are not forced to use the diagonal; the focusing tube (again unlike its DST counterpart) has enough back-travel to permit "straight through" viewing.

In terrestrial use, the star diagonal gives upright but mirror-reversed images, with little fogging (thanks to the three internal baffles), and good sharpness across the field at 30× (thanks to the Huygens eyepiece). With the addition of a 45-degree Amici prism (which however costs about as much as the rest of the telescope!), the PowerSeeker 50 becomes a passable 30×50 spotting scope, which is more than can be said for the DST.

To pack the PowerSeeker 50 in its original box, you need to undo only two screws and two nuts: one screw to separate the accessory tray from the tripod, one screw to separate the main tube from the tripod, and two nuts to separate the finder (with its bracket) from the main tube. The tripod then folds up much like a camera tripod, except that it has no central post. There's no need to dismantle the tripod further or to remove the finder from the bracket.

Included with the PowerSeeker 50 is a CD-ROM with Software Bisque's planetarium program TheSky (Level 1) for Windows 98 or above. I can't say much about that (sorry, Bill — I don't do Windows), except that it's advertised by Celestron as a $49 value, which it may be if you insist on using Windows. But if you use Linux or BSD or Mac OS X, you'll probably find that the best planetarium software is free. (Speaking of which, I must put in a plug for Fabien Chéreau's Stellarium; its free, open-source, easy to use, and available for almost any computer built in the present millennium. I know of some other planetarium programs that have more features, but none that are so accessible.)

...and how it isn't

The most obvious shortcoming of the PowerSeeker 50 is its aperture — only 50mm, compared with 60mm for the DST. That said, aperture won't protect you from stray light or a poor star diagonal.

If you close one eye and look at the reflections of the ceiling lights in the objective lens, you see two small bright white reflections and one large faint colored reflection, indicating a cemented doublet. You also see three rectangular shims measuring about 3mm×2mm, equally spaced around the edge of the objective, and apparently inserted between the elements prior to cementing. These protrude into the aperture. Further back, you also see the three screws that attach the objective cell to the main tube. Looking into the other end of the tube confirms that these also protrude into the aperture, giving a total of six protrusions at 60-degree angles. Not even the DST is that daft.

The mount of the PowerSeeker 50 is arguably better than the DST's in that it is lighter and vibrates at lower frequencies, and arguably worse in that it has no vertical slow-motion control. But yes, the mount does vibrate (and the frequency of the up-and-down vibration is about twice that of the side-to-side vibration). In astronomical observing at 150×, the vibration settles out before the object of interest drifts out of field, but only if there is no significant breeze.

While one would think that ‘H’ is for "Huygens", the "H12.5mm" eyepiece, with its exposed field stop and flat outer surfaces, is clearly a Ramsden. It has an apparent field of about 30 degrees and eye relief of about 5mm. The combination of an external field stop and positive eye relief indicates under-corrected lateral chromatic aberration, which indeed can be seen by looking at the edge of the field with the eyepiece out of the telescope. The "SR4mm" has an apparent field of about 38 degrees and eye relief of perhaps 2mm. The 12.5mm eyepiece, like the 20mm, bears the Celestron logo. The 4mm does not have room on its body for the logo, but otherwise has the same styling as the other two. Apart from branding and styling, there is little to distinguish the 12.5mm and 4mm eyepieces from their DST counterparts.

The unbranded "3× Barlow lens" supplied with the PowerSeeker 50, like its DST counterpart, is trash. The single element is apparently meant to be plano-concave, but the ripples in the "plano" side can be seen by viewing reflections of interior lights, even without the benefit of any straight lines on the lights! The "Quick Set-up Guide" shows the Barlow lens piggybacked on the star diagonal; but the focuser doesn't have enough inward travel for that. The full instruction manual correctly shows the reverse arrangement: the diagonal piggybacked on the Barlow. You can also omit the diagonal and insert an eyepiece directly in the Barlow. In the latter arrangement, the Barlow has a shorter back focal length and hence a lower power. So which arrangement gives the stated 3× amplification? Who cares?

The "5×24mm" single-bracket finderscope appears to be of the same design as the DST's — including the internal aperture stop — except that the eyepiece has slightly convex outer surfaces, whereas the DST's (or at least the sample that I have seen) has flat outer surfaces. Moreover, the mounting plate of the bracket appears to have the same radius of curvature as the DST's — that is, too small for the DST and too big for the PowerSeeker 50. In the latter case, the under-curved mounting plate gives more definite tightening of the mounting nuts, hence more stable alignment of the finder. But it also visibly tilts the finder. So, to permit correct alignment, the shim between the finder tube and the bracket tube must face the main telescope tube. (Any single-bracket finder with only one set of adjusting screws needs such a shim to stabilize the alignment; but in this case the positioning of the shim is critical.) Eye relief for the finder is about 10mm.*

In the PowerSeeker 50, as in the DST, and contrary to the statement on Celestron's web site (July 20, 2006), all lenses appear to be uncoated (at least in the sample that I have seen). Reflections of interior lights are similar in color and intensity to those from plain window glass, and brighter than those from my coated eyeglasses. Furthermore, the eyepiece bodies appear to be made of plastic. Apparently this is the reason why the instruction manual, in addition to the usual warnings against looking at the sun through any optical device, has a warning against projecting the sun onto a screen — a procedure which is normally considered safe (provided that no children can get near the eyepiece or finder), and for which a Huygens eyepiece is often recommended because of its lateral color correction and lack of cemented elements.

Performance

(Note: My location is Brisbane, Australia, where the scope for comment on deep-sky objects is restricted by city lights. Not that anyone would regard a 50mm refractor as a deep-sky instrument.)

Mercury: On March 28, 2006, according to published ephemerides, Mercury was just past inferior conjunction, 9.5 seconds in diameter, about 24 degrees west of the sun, and of magnitude 1.1. Before sunrise, I could see the crescent with the PowerSeeker 50 at 48× and 150×. The appearance was more like a banana than a crescent, but quite distinct.

Saturn: On the evening of April 3, when Saturn was about three weeks short of eastern quadrature, I could see the shadow of the planet on the rings at 48× and 150×. The part of the shadow not obscured by the planet would have been about 2 seconds wide, i.e. at about the theoretical limit of resolution for a 50mm instrument; yet its appearance was inky black. At 150× there was also a hint of non-uniformity in the rings. But there was not, and at this aperture nor should there have been, any sign of Cassini's division.

By the way, the observations of Mercury and Saturn were made using the included star diagonal.

Jupiter: On three successive evenings in early May, I observed Jupiter at opposition and never saw any surface detail beyond the two equatorial belts, although the earlier view of Saturn indicates that the raw resolution should have been more than adequate. This seems to indicate a lack of local contrast.

Alpha Centauri: The two components (current separation about 9 seconds) are clearly resolved, but the contrast is affected by localized glare. Perhaps the glare explains the lack of observable detail on the surface of Jupiter.

(Glare is also an issue with the DST. I concede that the DST, with its larger aperture, gives a better view of Jupiter than does the PowerSeeker 50, but only if the DST is used with an alternative star diagonal; the diagonal supplied with the DST blurs the image.)

Room for improvement

Three wrongs don't make a right: The 12.5mm Ramsden is a small step in magnification from the 20mm Huygens. The 4mm Ramsden gives too much magnification for the mounting, and ridiculously short eye relief. The Barlow lens is a throw-away. The two Ramsdens and the Barlow should be replaced by a 9mm symmetrical Plössl, which would be superior to the Ramsdens in every respect, including eye relief. Fully multi-coated 9mm symmetrical Plössls with 1.25-inch barrels are available for as little as AU$25. With 24.5mm barrels, plastic bodies, and some compromise with the coating, the price could be reduced. Moreover, I am proposing that three accessories, containing five optical elements including two matched pairs, be replaced by one accessory containing four optical elements in two matched pairs. The latter should be cheaper.

Tripods 101: The purpose of a tripod is to stabilize not only the position, but also — and more importantly — the orientation, of whatever is mounted on it. To preserve the orientation of the head, each leg must be triangulated; in the case of a symmetrical tripod, that means the top platform must form an equilateral triangle while each leg must form an inverted isosceles triangle whose top side meets one side of the equilateral triangle. The ground or floor between the feet then makes another triangle, and three more triangles fill in the gaps. The overall structure is not a tetrahedron, but an octahedron. Can a cheap tripod be made octahedral? Yes — by broadening the legs and pinning the top edge of each leg to the top platform at two widely separated points. If the legs must be made of plastic to contain costs, so be it; almost any sort of plastic has better vibration damping than aluminum.

Coatings: The Celestron web site specifies fully-coated optics. That would be good if it were true. Perhaps it is true of some samples. Consistency, please.

Duh!

In the "Quick Set-up Guide", and in the instruction manual, and on the Celestron web site, and in retailers' advertisements, the PowerSeeker 50 is invariably illustrated with the optical tube assembly mounted back-to-front on the altazimuth head, so that it can't be pointed at the zenith — but can be pointed at the ground.

Summary

Good points: Extensive baffling to reduce stray light; genuine Huygens 20mm eyepiece; decent star diagonal; adequate travel in the focuser; light weight; comparatively easy disassembly and storage; planetarium software.

Bad points: Small aperture; protrusions into aperture; no vertical slow-motion control.

Bad but no worse than the competition: Flimsy tripod; Ramsden eyepieces for medium and high power; useless Barlow lens; crude finder.*

Verdict

It's a toy; but there are (slightly) worse telescopic toys on the market.

In Australia, the Celestron® PowerSeeker 50 is available from Dick Smith Electronics (not quite a department store) for AU$59 (or occasionally less). U.S. retail prices range as low as US$40.*

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* Reposted with corrections, July 30, 2012. Specifications, price and availability are out of date. The current model offered by Dick Smith has a red-dot finder, which is superior for night-time use but cannot be used in sun-projection mode.

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