William Optics produces two versions of its 4-inch apo refractor: one a conventional triplet lens and the other a five-element astrograph – it’s the latter we’re reviewing here. The word astrograph indicates that the telescope is designed with astrophotography in mind.
William Optics has recently changed from using Crayford focusers to rack and pinion alternatives, and the 2.5-inch unit supplied with the review telescope worked smoothly and flawlessly.
With a maximum extension of 80mm there is plenty of focus travel, but not knowing exactly where the sensor would be when we achieved focus we inserted a 50.8mm extension tube in front of our camera and off-axis guider.
This turned out to be a pretty good guess – we achieved crisp focus at an extension of just 18mm, which left plenty of the focus tube within the focuser for stability.
The focuser easily handled the weight of our imaging system using both the coarse and fine focus knobs, and there was only the smallest amount of image shift when we tightened up the locking bolt once we had achieved focus.
The telescope produced a field of view 1° and 54 arcminutes wide by 1° and 16 arcminutes deep with our test camera, a one-shot-colour CCD with an APS-C sensor.
This area is just large enough to capture the Rosette Nebula and some surrounding stars.
With a focal ratio of f/6.9, this telescope is relatively slow compared to many astrographs, but this does have some advantages – chromatic aberration and field curvature are slightly easier to control with longer focal length lenses.
As the CCD camera we used for this review had an APS-C sensor, the star shapes in our images are a good indicator for what to expect with a typical DSLR camera, which has a very similarly sized sensor.
We were very pleasantly surprised by the shape of the stars across the field of view and right into the corners. The two-element internal field flattener was certainly doing its job and so too was the triplet lens, as chromatic aberration was also well controlled.
There was some vignetting, which caused the edges of the field of view to be darker than the centre, but this was easily corrected by calibrating the images with suitable flat frames.
One problem that can occur with multi-element designs like this is that of focus shift with temperature change. However, we didn’t find it necessary to adjust focus during our 4.5-hour imaging session despite a thick frost forming on the inside of our domed observatory.
Although the GTF 102 is designed as an astrograph, we couldn’t resist using it for some simple observing and we weren’t disappointed. Using our own 2-inch dielectric diagonal and eyepieces we enjoyed some glorious views of Jupiter and the gibbous Moon.
The lack of false colour in our imaging tests was corroborated through the eyepiece. Confirmed too were excellent star shapes right out to the edge of the field of view.
We couldn’t ignore the presence of the Orion Nebula and the large swathe of nebulosity observed through our 17mm eyepiece was particularly memorable, as was the crisp, sparkly view of the Pleiades star cluster.
We thoroughly enjoyed our experiences with the GTF 102 and would recommend the telescope to any astrophotographer searching for an imaging instrument that removes the pain of achieving a flat field with stars pin sharp to the edge.
Quintet of quality
The GTF 102 astrograph has an unusual optical system, comprising five elements arranged in two groups. The primary lens is an air-spaced triplet that uses FPL53 extra-low dispersion glass to bring all the colours of light to the same focus point. This is important to avoid chromatic aberration, which results in unwanted coloured halos around bright objects.
The three lens elements are housed in a fully adjustable, CNC-machined lens cell. The second group is made up of two lens elements (also made from extra-low dispersion glass), which are set inside the telescope tube. They act as a field flattener to combat the field curvature that is normally present in a refractor.
Field curvature causes stars at the edges of the field of view to appear elongated, which is a particular problem in deep-sky imaging. All of the lens surfaces are fully multicoated to increase light transmission and reduce unwanted reflections.
Since the field flattener is built in, there is no critical spacing issue when using a camera, so installing a filter wheel and an off-axis guider is straightforward.
The construction of the telescope tube, which is machined from aluminium, underpins the optical system’s stability – the whole telescope felt very solid indeed. Internally the tube is finished in matt black and has three knife-edge baffles, which together help to reduce internal reflections and increase the contrast of the view.
The 105mm diameter, CNC-machined tube rings have a crisp, black crackle finish. With a choice of mounting holes in both upper and lower mountings, they accommodate various types of dovetail bar and accessories. Nylon spacers on the locking bolts help to protect the clamp surfaces.
Rack and Pinion Focuser
The 2.5-inch rack and pinion focuser has anti-marring brass compression rings, can be rotated through 360° and is very smooth in operation, with minimal image shift when applying the simple lock mechanism. The 10:1 two-speed mechanism handled the weight of our imaging camera, off-axis guider and guide camera with ease.
Retractable Dew Shield
The 127mm-long dew shield does an excellent job of deflecting extraneous light and keeping the dew at bay. The front is capped with an aluminium dust cover that simply and safely slips on and off. A convenient thumbscrew locks the shield in its extended position.
The GTF 102 is supplied with a high-quality soft padded case. It is very substantial and protects the telescope in transit very well indeed. With both carry handles and a shoulder strap, transporting the scope is easy.
Aperture 102mm (4 inches)
Focal Length 703mm, f6.9
Weight 5kg excluding tube rings
Supplier The Widescreen Centre
Telephone 020 7935 2580
This review originally appeared in the April 2014 issue of Sky at Night Magazine.