Aperture: 107mm; 4 inches
Focal Length: 700mm; f/6.5
Weight: 5.45kg with tube rings
Supplier: Harrison Telescopes
Refracting telescopes may have spurred on the scientific revolution, but the simple lenses that Galileo used to observe Jupiter’s moons had a drawback – they suffered from an effect known as chromatic aberration.
This optical defect is still found in simple refractors today.
When a lens made of a single piece of glass doesn’t bring light of different colours to the same focus, bright objects get colour fringes around them.
The effect can be reduced by adding a second piece of glass – or element – to the objective lens, one to focus red light, the other to take care of the blue.
The APM 107 we’re looking at this month goes one step further by including a third element in its 107mm (4-inch) diameter objective lens, which makes colour focus even better.
This air-spaced lens is known as a triplet and is particularly popular with astrophotographers because of its ability to capture superb deep-sky vistas, rich in vibrant colour, with excellent contrast.
This ‘colour-corrected’ refractor, which brings different colours of light to the same focus, is known as an apochromat – apo for short – and during our tests we were really very impressed by the crispness of the views it delivered.
Everything we looked at, or imaged, snapped to a really sharp focus with no evidence of colour fringing.
It’s also a very portable telescope thanks to an extendable, lockable dew shield which, when fully retracted, reduces the tube length to just over 50cm (19.5 inches).
At the eyepiece end, we had an impressively solid Crayford-style focuser from Baader, though there is the option of going for a Starlight Instruments Feather Touch focuser for no extra cost.
The Baader’s Steel Track focus gearing includes an 11:1 fine-focus adjustment, which was very responsive and accurate.
The focuser held our DSLR camera very firmly with no slippage, but it wasn’t possible to rotate the camera to frame an object well once it was in focus – something that you can do with many other apos of this quality.
Attaching a DSLR at prime focus, we found we needed an extension tube to get the camera to the point of focus.
We needed the extension tube again when we attached a planetary camera and when we used our Barlow lens, which pushed the focus even further back – thank goodness for the focuser’s firm hold.
Moon with a view
Our first imaging target was the Moon, which looked crisp and highly detailed in our camera’s viewfinder.
There was no hint of false colour anywhere and the contrast between the sunlit lunar landscape and the inky black shadow features was fantastic.
Switching to an eyepiece view, we were able to confidently raise the magnification to 120x – as high as the slightly wobbly atmospheric conditions could stand without compromising the stunningly beautiful view.
The shadows running through the Sinus Iridium region on the northwestern shores of Mare Imbrium created exquisite detail, and we felt we could go visually rambling through the Jura Mountains for hours.
Visually, the Double Double (Epsilon Lyrae) was beautifully delivered too and, despite the star being low at the time, we were able to split the two tighter components at 70x magnification, although the separation was more convincing at 120x.
The Orion Nebula was up, so we switched back to our DSLR and took some shots.
The results were excellent, with the swept-back regions of the nebula looking very impressive.
The embedded stars, like the Trapezium Cluster, looked sharp across most of the field.
Visually, the high-contrast optics really delivered the goods in this region and we loved the detail in the nebula gas around the Trapezium we got through the eyepiece.
We needed to add an extension tube to achieve focus visually (just as we had to when imaging), even with a diagonal inserted.
Having to insert an extension tube so often made it less easy to use.
However, this really is a lovely scope and well worth its price tag. Its colour correction is excellent and the field of view is wide enough to fully illuminate even a full-frame sensor chip.
The quality of the stars was very good almost to the edge of the field of view, even without the optional field flattener fitted (at a cost of £140).
With this accessory in place it would produce some seriously beautiful deep-sky shots.
Without doubt, the outstanding feature of the APM 107 is its superb, colour-corrected air-spaced triplet objective lens. The quality of the view it delivered really impressed us.
The 107mm-diameter lens is formed by three separate pieces of glass held in place around their periphery, rather than being cemented together – hence the term ‘air-spaced’.
The central element is made of high-quality FPL53 low-dispersion glass from Ohara in Japan.
This glass comes close in performance to the now rarely found fluorite glass, from which many high-end and highly regarded apos’ objective lenses were crafted.
With a very acceptable 50mm diameter field for imaging, adding the field flattener for £140 delivers pinpoint stars right to the edge of the field of view.
This means that cameras with large format, full-frame sensors can make the most of the superb views supplied by the APM 107’s impressive triplet objective.
Visually too, the lens delivers some stunning views which are free from colour fringing, even at very high magnification.
There are three cooling fans built into the rear of the telescope tube, all powered by the supplied battery pack.
These help to bring the telescope to thermal equilibrium with the surrounding air, which is vital for getting rid of air currents within the tube that could otherwise disturb the light path and result in fuzzy images.
The 3-inch diameter Crayford focuser is finely crafted. Its 2-inch adaptor, graduated focus tube and smooth, dual-speed, 1:10 operation make it a joy to use.
The large diameter of the focus tube produced a well-illuminated frame, while there was a negligible amount of image shift when focusing and finally locking the focus tube.
The relatively large, central obstruction of the secondary mirror could sap some of the contrast from the views through this telescope.
However, the optical baffles in the main tube, together with those inside the mirror tube at the centre of the primary mirror, helped to restore brightness and resulted in sharp, high-contrast images.
Losmandy dovetail bar
There are two Losmandy-style dovetail bars installed, one above and one below the tube.
This type of bar is the preferred kind for the sturdiest method of mounting telescopes.
With the right clamp on a sufficiently substantial mount, it results in a very solid installation.
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This review first appeared in the March 2011 issue of BBC Sky at Night Magazine.