A sturdy, stylish scope that works wonders for visual observing and astro imaging

WORDS: TIM JARDINE

VITAL STATS

• Price £1,449
• Optics f/6 apochromatic triplet
• Aperture 90mm
• Focal length 540mm
• Focuser 2.5-inch rack and pinion
• Extras Tube rings and a carry case
• OTA weight 3.3kg
• Supplier First Light Optics
• Email: questions@firstlightoptics.com
• www.firstlightoptics.com

The StellaMira 90mm ED telescope on test this month is a triplet lens apochromatic refractor introduced by First Light Optics. We were especially interested to receive our review model, as the company has always been upfront on its website blog about any issues with triplet telescopes, with the promise of only releasing new ones when production methods allow consistently good results. It was fair to say, given that stance, that we had high expectations for the StellaMira 90mm ED.

The packaging of the StellaMira 90mm ED is compact; it is supplied in a neat, lockable, black carry case. The telescope has a sturdy construction, with a glossy carbon-fibre tube and dew shield. It certainly looks and feels like a quality piece of equipment, with anodised red aluminium highlights, black tube rings and a carry handle. Although the carbon-fibre tube keeps the weight down to a mere 3.3kg, the StellaMira 90mm ED doesn’t feel flimsy or compromised in any way whatsoever.

The 90mm lens of the StellaMira telescope has a focal length of 540mm, making it a speedy f/6, and perfectly suited to astrophotography, especially with its focuser design. There’s no reason why it can’t also be used as a high-quality, wide-field visual instrument. Indeed, it’s fitted with a removable holder (for 2-inch and 1.25-inch eyepieces or a diagonal) with brass compression rings.

Sharp focus

To test the StellaMira 90mm ED’s astrophotography abilities we paired it with a StellaMira 0.8x reducer/ flattener. This threads firmly onto the focuser, after unscrewing the eyepiece clamp, and the M48 thread on the rear allows a camera to be attached with a spacing of 55mm, which is ideal for DSLR cameras.

With the reducer in place the telescope offers a coma-free image field up to APS-C sensor size. Test pictures were taken using a CCD camera and the back spacing was a few millimetres out, resulting in slightly skewed stars in the corners. But a selection of reference images (taken with a full-frame DSLR and the correct spacing) showed the true flat field on offer, which covers 36mm x 24mm. All the stars, except those on the outer edges of the full-frame image, appeared round and sharp.

When it comes to imaging, the focuser arrangement is particularly impressive. The whole focuser body rotates, allowing the control knobs to be conveniently positioned, while a separate camera rotator allows adjustment of the camera angle. Fast optical systems have a very shallow depth of focus, requiring precision movements; and in this regard we found the rack and pinion design to be smooth and impressively accurate, especially when making micro-adjustments for perfect focus

Testing the limits

Winter skies offer a host of targets, but the Pleiades, M45, and the Orion Nebula, M42, offer pretty much everything an astrophotographer might encounter, including bright dominating stars, faint reflection nebulosity, rich and thin emission nebulae, and tight star groupings that test the telescope’s limits.

The hot blue stars within M45 can produce unwanted artefacts (haloes around stars, irregular diffraction patterns or internal reflections) in poorly configured optics, while the tight star grouping within M42’s Trapezium Cluster can be mangled into an indistinct blob, but our images were clear of these issues. Instead, the StellaMira 90mm ED produced excellent results, which were consistent with the impressive 0.972 Strehl ratio rating given for this particular telescope (a measure of its optical quality). There were even tantalising hints of the Trapezium stars ‘E’ (Theta ¹ (θ ) Orionis E) and ‘F’ (Theta (θ ) Orionis F) in M42 during the camera focusing procedure.

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Briefly swapping the camera for an eyepiece was similarly rewarding and delivered excellent views of Jupiter, even using a Barlow lens to boost a 4.5mm eyepiece up to a boundary-stretching 240x magnification. This produced a sharp, detail-rich image, free of any unwanted colour aberrations, again testifying to the high-quality optics.

Overall, the StellaMira 90mm ED Triplet more than met our expectations. It’s a competitively priced, stylish telescope that offers excellent views and high-quality astro imaging results in a convenient, well thought out package.

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A superior objective triplet lens

The 90mm objective lens in the StellaMira 90 telescope is made from Extra Low Dispersion (ED) glass, equivalent to FPL-53, and the optics are fully multi-coated for high light transmission and better contrast at the eyepiece/camera.

The triplet lens design allows the telescope to bring the varying colours of light to the same point of focus. At the eyepiece, this results in sharp planetary or star images, but the good colour correction is more noticeable on bright targets such as the Moon or major planets, as inferior optics produce haloes or rings on the bright edges. When used for astrophotography, the StellaMira 90mm’s good colour correction is evidenced by sharp, tight star images. You can test this by photographing the bright, blue stars in the Pleiades and then comparing the pixel sizes of the resulting images, after splitting them into Red, Green and Blue channels. The different colour star layers we produced overlapped each other almost perfectly, pixel for pixel, demonstrating the true apochromatic fidelity of the StellaMira 90mm ED telescope.

KIT TO ADD

1. StellaMira 2-inch 0.8x reducer/ field-flattener

2. Astro Essentials 32mm f/4 mini guidescope

3. StellaMira 2-inch 90° Dielectric diagonal

VERDICT

ALL PICTURES : @THESHED/PHOTOSTUDIO