Starwave 152mm f/5.9 achromatic refractor — Part 1
Altair Astro — www.altairastro.com — £695
Written: October 2013

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Short focal ratio six-inch achromats are currently very popular and it's easy to see why. A refractor, unlike a fast Newtonian, is very robust and largely immune to optical misalignment. An unobstructed optical system and modern high-transmission lens coatings ensure little scattered light and high contrast images. At f/5 to f/6, tube lengths are very manageable too. However, 6-inch achromats shorter than f/8 are generally perceived to be low-to-medium power 'rich field' instruments primarily for deep-sky observing since the level of chromatic aberration is automatically assumed to be intolerable for lunar and planetary observing. But is this still true of premium achromats like the Starwave 152mm f/5.9? I was very keen to find out. (Jump to Part 2 if you can't wait!)

First impressions

Two things immediately strike you about the Starwave 152mm optical tube assembly: it's reassuringly solid (tipping the scales in the 11kg range) and extremely well crafted throughout. It's a strikingly beautiful telescope, the tube and dewshield having a nacreous lustre in a certain light. With the dewshield extended and a 2-inch star diagonal in the two-speed Crayford it's about 100cm long, the tube being 176mm in diameter. The dewshield and threaded metal dust cap are 20cm in diameter. The CNC tube rings sport a secure carrying handle, while the rings themselves are pre-drilled and threaded for mounting a finder or guidescope instead of the handle should you so wish. On the underside of the rings you will find a Vixen-style dovetail bar. The OTA is slightly front heavy owing to the massive lens and cell (more on that in a moment), but the tube looks proportioned on a mount when the dewshield is extended.

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The objective end
With the dewshield retracted one can really see the quality of both the optical and mechanical engineering, plus the attention to fine detail. The objective cell attaches to the end of the tube and is collimated (should this ever be necessary) by three pairs of push-pull hex-head screws, while six pairs of inset screws around the circumference permit microfine lateral adjustments of each glass element for ultra precise lens registration within the cell. A large threaded retaining ring with sprung leaves holds the doublet gently but securely in place.

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The four lens surfaces have blemish-free deep blue coatings and the view down the barrel shows three precisely positioned knife-edge baffles within the flat black tube. With the star diagonal removed and the Crayford focuser fully extended, it appears that the fully illuminated focal plane is in the region of 3cm. At the 'scope's 900mm focal length, this corresponds to about 2°. And in case you were wondering, the unobstructed aperture of the lens is indeed a full 6" (152~mm).

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The focuser end
Altair Astro's 2-inch steel-tracked linear Crayford focuser is another superb piece of engineering — silky smooth and ultra precise. Unlike simpler Crayfords where the focuser shaft bears directly against a machined anodised drawtube, this Crayford's shaft turns against a machined steel track, thereby reducing the likelihood of slippage with heavy eyepieces and cameras. The tension on the focuser shaft is adjustable and the chromed lock-down knob (pictured right) is very positive. Each turn of the coarse focus wheel advances the drawtube 1.8cm (the fine focus wheel is on a 1:10 reduction gearing) over a focuser travel of 8.5cm. With the drawtube fully extended the focal plane lies some 4cm beyond the end of the 2-inch eyepiece adaptor, so an extension tube is required to focus eyepieces without a star diagonal, but DSLRs with sufficient flange back distance may possibly come to focus without one. The drawtube 2-inch adapter has three clamp screws with an internal brass compression ring for secure attachment of accessories. A standard 2-inch mirror star diagonal permitted every 2- and 1¼-inch eyepiece in my collection to focus. Finally, the entire focuser unlocks to rotate smoothly through 360° to place the eyepiece at any desired orientation.

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A suitable mount
During ongoing testing of the Starwave 152mm, my mount of choice is a Celestron CG5-GT (a.k.a. Advanced Series GT). This is a NexStar-driven GoTo German equatorial, one of the more robust EQ5 mounts owing to its 2-inch tubular steel legs, but any GEM in the same class will easily carry this instrument. Two 5kg counterweights placed mid-shaft perfectly balances the 6-inch f/5.9 optical tube and one never senses that the mount is overtaxed. With the tripod legs extended 45cm, vibration damping time is about two seconds. I haven't required a finder during testing since the 'scope delivers a 2.5° field with my 2-inch 32mm 70°AF Antares Erfle — ample sky to find alignment stars during setup.

Optical performance — low power
Most of my tests thus far were conducted on the nights of September 30th and October 7th, the latter under a clear, transparent sky with medium-to-good seeing. I'll cut to the chase: the Starwave 152mm is an exemplary deep-sky instrument. It delivered a jaw-dropping 'diamond dust on velvet' effect with the Perseus Double Cluster and rich Milky Way star fields with no obtrusive chromatic aberration on all but the brightest stars at 30x. At such powers it's tack sharp and delivers exceptionally high contrast images over a wide, flat field — but you do need 6-element (or more) eyepieces designed to work at f/6 to see the full benefit. The Triangulum Galaxy (M33) is a challenging low surface brightness object under perfect skies, yet this instrument revealed its faint but unmistakable oval glow to averted vision in a locally very dark location, but 3 miles from the lights of a mid-sized town. The Dumbbell Nebula (M27), bright and full of gauzy detail, seemed to float against the star fields of Vulpecula — a 3-D illusion that I can't recall seeing before.

Optical performance — high power
With no Moon or accessible bright evening planets to view, I used 7mm and 4mm TMB Planetary Series eyepieces delivering 129x and 225x, respectively, to examine the Starwave 152mm's collimation and colour correction on stars Altair and Deneb. The instrument had at least 1½ hours cool-down time. Altair revealed that the system was perfectly collimated with no sign of any mechanical distortion of the objective. At these higher magnifications the achromat began to show chromatic aberration, but I was pleasantly surprised how well it was contained. Maybe it's my 50-year-old eyes, but at best focus the faintly yellowish-white Airy disc and diffraction rings were surrounded by an unobjectionable deep violet halo. Altair presented a yellowish-green diffraction pattern with a purple centre when defocused a few waves extra-focally, while the intra-focal diffraction pattern had a similar yellow-green hue with a purple-red periphery surrounded by a diffuse violet halo. There was slight asymmetry in the intra- and extra-focal images that I ascribe to minor spherical overcorrection, but there were no discernible zones or signs of lens surface roughness.

Baader Fringe Killer, or not?
Turning to dazzling Vega, I thought it would be interesting to try a filter designed to suppress the purple/violet achromat overspill promising to turn such an instrument into a 'semi-apo'. I have mixed feelings about such filters. Yes, Vega did sit in a prominent but well confined violet halo without it, but it seems that the designers of the Starwave 152mm f/5.9 have optimised the lens to perform at mid-visual wavelengths, so yellow does predominate. I found that the Baader Fringe Killer did make a difference, but at the cost of making the star appear too warm. I preferred the unfiltered stellar view, so I'll revisit this when I come to examine the Moon and Jupiter in Part 2.

Double stars
Whenever I have an unobstructed aperture instrument and the seeing is reasonably good, I like to seek out some of my favourite double and multiple stars. Almach's golden primary and blue-ish companion separated by 10 arcseconds were rendered in faithful hues in the Starwave 152mm. Iota Cassiopeiae, a wonderful triple star, looked fantastic at 225x with clean diffraction rings around all three components. Saving the best until last, the 'Double-Double' epsilon Lyrae was riding high and delivered the crispest views under optimal conditions. At 225x, the 4mm TMB eyepiece showed an unobjectionable faint halo of chromatic aberration around all four components and a slightly yellowish cast to the images, but this was overshadowed by the pleasure at seeing tight Airy discs, dark sky between both pairs and two to three diffraction rings around each of the four stars when seeing permitted.

So, a particularly auspicious start for the Starwave 152mm on the deep sky. See Part 2 where I take a critical look at its lunar and planetary prowess.

v1.3 — 21st October 2013