The ability to easily capture images with a minimum of fuss is of great interest to many astronomers, and the Tecnosky AG70 astrograph goes a long way to letting you do just that. With an optical system designed to eliminate two of the major issues that can spoil a photograph – colour problems and star shape issues – the AG70 removes the need for the likes of coma correctors and relieves the associated headache of achieving optimal spacing that tends to come with them.
Setting up the telescope for use involved little more than removing it from the box and putting it on a mount. The supplied tube rings and dovetail offer a little movement to help with balancing, and the scope is light and compact. It should sit comfortably on portable travel mounts.
The AG70 is geared towards use with DSLR and Micro 4/3 cameras, though because of weather conditions and Moon interference at the time of review we resorted to using a CCD camera and narrowband filters for the accompanying images – to do so we had to limit the camera and filter wheel width to around 60mm to achieve focus.
With a focal length of 350mm, this astrograph is ideal for imaging extended objects and large nebulae. The fast, f/5 focal ratio allows for short exposures, another advantage for DSLR users.
Our review telescope had been checked for collimation and field flatness using an APS-sized sensor. Our own camera provided a larger full frame field of view, which confirmed that the star shapes were very good out to beyond the range of an APS sensor.
Only at the edges and corners of our full frame camera view did the star shapes start to suffer.
At this stage we also tested for colour focus across the field and found that red, green and blue wavelengths are dealt with well, achieving uniform focus, an important factor for tight star shapes and colour reproduction.
Sheer simplicity of use made the little astrograph a pleasure to operate, as once the camera was focused we were able to just get on with the job of taking photographs.
We did find that even when tightly locked, the fine-tuning focuser knob allowed the drawtube to be adjusted. Whether this is by design or happy accident is not clear, but it did make for a useful feature, allowing us to obtain perfect focus without the need to subsequently turn the locking knob and potentially alter the focus position.
With nighttime temperatures dropping rapidly, we kept a regular eye on the focus, checking it every 20 minutes, but eventually determined that these checks could be much farther apart. In fact, the AG70 held tight focus extremely well, even over subsequent nights.
The focusing process revealed one of two niggles we found with the telescope, arising when we placed a Bahtinov mask on the end of the retractable dew shield. Even the featherweight mask was heavy enough to cause the shield to slide down, as there is no means of locking it in place.
In the end we resorted to using our elasticated dew bands to keep the shield extended.
The other issue on our review scope was with the camera rotator, which was stiff and clunky. This made it a little difficult to determine whether the camera was rotating or unscrewing.
Our favoured 10mm eyepiece, working at 35x magnification, gave sharp contextual views of large objects such the Andromeda Galaxy, the Orion Nebula and the Double Cluster in Perseus.
Offering flat, fringe-free images right out of the box, with visual observing capability thrown in, the AG70 is an instrument with enormous potential.
Quadruplet lens system
Over the past few years various attempts have been made at producing fast, flat-field refractors with good colour correction using four or five lenses, with varying degrees of success. The Tecnosky AG70 uses four.
It has a triplet objective lens at the front made of low-dispersion glass, including FPL-53 and Lanthanum elements. These ensure that chromatic correction is good and remove bright colour fringes from stars.
The fourth element is at the rear and provides the flat field of view, correcting for lens aberrations. The telescope is fully baffled.
Astrograph Ltd, the UK dealer for Tecnosky, states that it inspects each AG70 before shipping to ensure that the alignment of the lenses is correct, a quality control step that should help to reduce the issues typically associated with multiple lens astrograph designs.
Adding to the ‘ready to go’ nature of this telescope are the included standard accessories, dedicated black tube rings, 6-inch Vixen-style dovetail, and adjustable finderscope shoe (finder not included). Additional accessories can be attached to the top of the tube rings via M6 threaded bolts as desired.
Weighing in at a shade over 2kg and being only 310mm long, the compact AG70 is extremely portable. Since cameras simply screw onto the focuser thread, set up time is kept to a minimum.
With no additional field-flattening lens required for this scope, cameras and other accessories attach straight to the standard M48 male thread on the focus tube. Although designed primarily to fit DSLR cameras, we did manage to squeeze a slim-line filter wheel and CCD camera into focus.
The rack and pinion construction offers smooth and precise focusing, with a camera rotator and tilt adjustment built in. Even when locked the focuser still allows fine-tuning movement, although we found the camera rotator a little stiff to use. The focus tube is incrementally marked, allowing for repeatable focusing.
A 1.25-inch eyepiece adaptor is the perfect complement for this travel-ready telescope. Simply unscrewing the end of the focuser allows the 1.25-inch dielectric diagonal to fit into place, quickly converting the astrograph into a capable visual observing instrument.
- Price £1,079
- Optics Flat-field quadruplet with Lanthanum and FPL-53 elements
- Aperture 70mm (2.75 inches)
- Focal length 350mm (f/5)
- Focuser Dual-speed rack and pinion with tilt adjustment
- Extras Eyepiece adaptor, tube rings, dovetail, finder shoe
- Weight 2.1kg
- Supplier Astrograph Ltd
- Tel 0843 330 4988
This review originally appeared in the February 2017 issue of BBC Sky at Night Magazine.