In the shops

September 2012 | Astronomy Now | 81

In the shops

▲ The Altair Astro 102. All images: Olly Penrice.

▲ The three-screw eyepiece holder.

A matter of speed: the Altair Astro 102Olly Penrice of Les Granges Astronomy Holidays in southern France puts the latest Altair Astro apochromatic telescope through its paces.

I thought that apochromatic refractors were supposed to be frighteningly expensive? I know

mine were! How times change: I’m looking at the Altair Astro Wave Series 102mm f/7 triplet apochromat in front of me and thinking, “Where have the corners been cut?” This telescope retails at £1,150, a price that includes an exceptional right angle finder of 60mm aperture, CNC tube rings with Vixen-style dovetail, a three-inch dual speed Crayford focuser with rotators in two places, a dedicated field flattener for the imagers, an excellent eyepiece holder with three locking screws and an aluminium flight/delivery case. No corners being cut there, then, so let’s have a closer look.

The triplet objective lens uses the top quality FPL-53 glass. The cell itself is collimatable, should this ever become necessary, and the beefy focuser threads into the back of the main tube rather than being pushed in and retained by radial screws as is sometimes seen on

budget designs. I am not a Crayford enthusiast but this one is highly adjustable and, most importantly, the drive roller bears upon a matte steel strip offering decent friction and not on the smooth anodised body of the drawtube itself, a notoriously poor idea. No, my closer inspection has found nothing that looks like a shortcut. The only concession to price lies in the choice of focal ratio and it is a judicious and well thought-out concession at that.

The Altair Wave 102 is f/7, which is not fast but nor is it unreasonably slow. By keeping the f-ratio down to f/7 the optical engineers have been able to assure excellent colour correction – this is a full apochromat, not a ‘semi-apo’ – at a moderate price. Speeding up the optics, especially in larger apertures, drives the price of a lens sky high. Altair Astro have decided, with this instrument, to make no compromise on colour correction but, instead, to accept a slightly slower f-ratio. I think this was a good decision.

As when reviewing the Altair Astro 115 triplet (see Astronomy Now, July 2011), I was able to draw on the expertise of visiting optical engineer Ralf Ottow when I gave the 102 its first light. A second magnitude star viewed near the zenith with a 9mm TeleVue Nagler eyepiece gave a pinpoint image right across the field, literally from edge to edge. Inside and outside focus the diffraction circles were concentric and were just as perfect at the very edge of the Nagler’s field as they were on axis. I know Ralf quite well by now and it’s easy to tell when he’s impressed. He was impressed! I was able to discern no difference between the intra- and extra-focal diffraction patterns. Once again this Altair Astro telescope had exquisite optics and the general views we enjoyed confirmed this both at high powers and low.

Taking on a fast challengeFor the imaging test I decided to give the telescope a difficult challenge. I also wanted to see how I’d feel I about working at f/7, although I have lots of experience of that focal ratio in our own TEC 140 triplet. Not far above the southern horizon of Les Granges lies the constellation of Ophiuchus, home to many of the dark Barnard objects, and I opted to try the Snake, B72. Dark nebulae respond best to very fast optics so would f/7 cut the mustard? Also, close to the Snake, lies a bright blue star that often seems poorly controlled in images of this region. How would the 102 handle it? The region

is so star-packed as to guarantee stars right into the corners to see how well the flattener would do its job. A final reason for choosing the Snake was that I have a reference image of the region in the same camera. Tom O’Donoghue used it in his own Takahashi FSQ106 quadruplet to image the Snake here a few years ago. You may think it unfair to test the Altair Astro 102 in a head to head against a telescope costing more than four times as much but I’m not here to be fair, I’m here to test for Astronomy Now!

In setting up the image I had to find some adapters to space the chip correctly and step down from the M48 thread on the rear of the flattener to the T-thread on my filterwheel. Even though I didn’t have quite the right ones and ended up with a chip distance several millimetres over length, I still found my corner stars perfect. I had adjusted the two camera rotators to be a little tight so as to avoid any sag and set about finding critical focus. This, at a tolerant f/7, was very easy and the two-speed focus was pleasant to use. I found I didn’t need to use the lock screw to secure it. What followed was a succession of frustrating nights as low horizon cloud teased me and produced a mixed bag of data. However, after a few nights I had collected 130 minutes of red, 70 of green and 120 of blue and I could head off into the processing room to work with it. You can see the result here.

In the shops

At a glance

Altair Astro 102 triplet apochromatAperture: 102mm (four-inch)F-ratio: f/7Resolution: 1.01 arcsecondsLimiting visual magnitude: +11.1Highest useful magnification: 230×Tube weight: ~5.5 kilogramsTube length (with dew shield retracted): 61.5cmAccessories: Three-inch field flattener, dual speed

three-inch Crayford focuser, 60mm right-angle finderscope, flight case

Price: £1,150Details at: www.altairastro.com, tel: 01263 731505.

I was very happy with the picture. The faint dark dust clouds show some modelling and structure so f/7 is perfectly workable and a small price to pay for that wonderful colour correction. For a dark object the 5.3 hours of integration that went into this could not be called generous but still gave a strong signal and a picture that seems to have held up very well against that taken in the far more expensive quadruplet. The difficult blue star is similar in both images.

The four-inch refractor has long been an amateur astronomer’s favourite, small enough to be portable, large enough to show the full Messier list and sharp enough to provide killer planetary views. The Altair Astro Wave 102 triplet sits proudly in that tradition offering real quality at a modest price. I found it very easy to like.

Olly Penrice is proprietor of Les Granges Astronomy Holidays – see www.sunstarfrance.com for more details.

▼ The three-inch Planostar field flattener works superbly.

▼ The steel strip of the Crayford focuser.

▼ The Snake Nebula, B72, in Ophiuchus, taken through the Altair Astro 102 triplet apochromat using RGB filters.

80 | Astronomy Now | September 2012