Istar Perseus AT150-12 achromatic refractor with Moonlight focuser

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Istar Perseus AT150-12 achromatic refractor with Moonlight focuser

Vital Stats

150mm (6 inches)
Focal Length: 
1,800mm (f/12)
2.5-inch, two-speed Crayford
17.6kg, including focuser
Peak 2 Valley Instruments
01298 732440

Taking delivery of a telescope like the Istar Perseus AT150-12, you really start to grasp the significance of the numbers 150mm and f/12. This is a very big refractor, and an impressive sight to say the least.

The Istar is designed and manufactured mainly in the Czech Republic. Although the achromatic doublet optics were designed in-house by master optician Zdenek Rehor, the lenses are actually manufactured in China and then shipped back to the Czech Republic for testing and quality control before final assembly.

Indeed, each scope comes with a test report that includes a ‘Strehl’ measurement. This is the ratio of the amount of light contained within the telescope’s ‘Airy disc’ as a percentage of the maximum theoretical value that would be expected from a ‘perfect’ optical system. With a Strehl of 0.92, the Istar’s lens places 92 per cent of the light correctly – only 8 per cent is dispersed into the surrounding rings of the Airy disc. Such a figure translates to bright views with plenty of contrast.

The Istar is supplied as a bare tube with an attractive matt black, powder-coated finish and chrome trim, leaving the choice of focuser for you to decide. For this review a MoonLite 2.5-inch, two-speed focuser was supplied, but for visual use when you don’t have to support a heavy camera, Istar’s own steel-tracked Crayford costs £220 less.

A true heavyweight

Substantially built from aerospace-grade magnesium and aluminium alloys, with many parts machined from solid block, the telescope exudes quality. The optional tube rings were also CNC-machined from aluminium and were attached to a Vixen-style dovetail bar, with a metal insert at its centre for a mount’s clamping bolts to bear against.

Assembly on our mount was really a two-person task: one to cradle the tube in their arms and the other to locate the dovetail bar in the mount’s clamp. Although we tried an HEQ5 mount, pictured right, an NEQ6 should be considered the absolute minimum, and then for visual use only. Our EQ6 required four 5kg weights on the counterbalance bar to achieve a perfect balance. It should be noted that it is not so much the total weight of the telescope but the torque generated when slewing with such a long tube that necessitates such a substantial mount. We also had to fully extend the tripod legs to view objects above 60˚ altitude, otherwise we would have had to lie on the ground to get eye to eyepiece!

Having examined the superb build quality, we were keen to see how the Istar would fare on the night sky. Bear in mind that this telescope is designed for serious observing and wasn’t really intended for astrophotography – imagers would be better served by a more compact design that would be less prone to buffeting by the wind and put less demand on the mount.  

Quality viewing

Even our first alignment star, Vega, was a wonderful sight through our 28mm 2-inch eyepiece, remaining sharp to over 95 per cent of the field of view. Views through the Istar were bright and full of contrast, helped no doubt by the excellent multicoating on the lens surfaces and the four knife-edge baffles, which are strategically positioned within the tube and bolted solidly in place from the outside.

Focus was wonderfully simple thanks to the very smooth action of the MoonLite focuser and a generous sweet spot. However, we did notice that the view danced around when we touched the focuser, underpinning the requirement for a very substantial mount to dampen down this movement.

Double stars would be a typical target for an instrument like this, and both pairs of double stars in Epsilon Lyrae, the Double Double, were cleanly separated using our 10mm eyepiece. Albireo, with its contrasting blue and gold colours set against a dark background, made for a truly gorgeous sight, but with the clearest night we’d had for some time we stayed up until 3.30am to enjoy a wonderfully crisp view of Jupiter and its attendant moons.

The Istar Perseus would make an excellent upgrade for lunar, planetary and double star observations, and its high-contrast views would also come in useful when looking at many deep-sky objects, such as planetary nebulae and globular clusters. But don’t forget that you’ll need a very substantial mount for this substantial tube.

Why bigger is better

Why would you want a simple achromatic telescope over six feet long, when there is such a wide choice of much more manageable instruments? The answer is to do with contrast, chromatic aberration and focus.

The unobstructed light path of a traditional refractor yields higher-contrast views than reflectors (telescopes that use mirrors instead of lenses) and catadioptrics (telescopes using a combination of mirrors and lenses). However, most current achromatic refractors have small focal ratios and this means views show chromatic aberration – colour fringing around bright objects.

High focal-ratio telescopes like the Istar Perseus don’t suffer from chromatic aberration to the same extent because their long focal-length enables different-coloured light to reach a convergence point. The modified achromatic design used in the Istar is also far easier to manufacture and figure accurately than its shorter focal length cousins.

Small focal-ratio scopes can also be quite tricky to focus perfectly as the sharpness sweet spot is very small. The Istar’s higher focal ratio gives a greater depth of field, however, making accurate focus easier to obtain and maintain.

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This article appeared in the November 2011 issue of Sky at Night Magazine

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