Author: Ravi Kailas
|The rather imposing size of the set-up, belies the simplicity of the Dobsonian telescope design|
For observing the night sky, I primarily use a GSO Star Tracker, a 8\” Dobsonian telescope (a type of telescope, which despite its bulk, is the best value for money for light gathering ability), manufactured by GSO (Guan Sheng Optical, a Taiwanese Manufacturer). I purchased this from Tejaraj & Sons, Mumbai, a reputed dealer, probably the only one of its kind in India, of astronomy related equipment, relying on their advice to buy this model (as against similar from well known brands like Orion), for its better value for money among identical optics in the alternatives.
The telescope comes supplied with with 2 eyepieces – a 9mm 1.25″ and a 30mm 2″, among a handful of other accessories (see below). I also have a Pentax AMC XW 14mm, 1.25″, which works for both wildlife watching with a spotting scope, as well as for astronomy with this telescope. I find both the Pentax and the 30mm 2″ eyepieces provide very satisfactory views, with ample eye-relief, field of view and a wholesome image circle, but the Pentax is significantly more refined with excellent edge to edge sharpness. The 9mm is not as satisfactory to look through, with a poor eye-relief and a small, incomplete image circle, but centre sharpness and resolution is adequate and provides good views of difficult subjects like Epsilon Lyrae. The eye-pieces give me magnifications of 133x (9mm), 40x (30mm) and 85x (14mm) (magnifications are calculated by dividing the focal length of the telescope, in this case 1200mm, with the focal length of the eyepiece). Higher magnifications are better for planets, features on the moon, globular clusters and double stars and wider views for diffuse subjects like nebulae, star clusters, star fields and the like.
Sadly, however, the light gathering ability of this telescope, with its 8\” aperture (a size, on the cusp of what are called \’light buckets\’, for the amount of light that they allow inside), is wasted on Chennai\’s skies, where some of the more diffuse subjects are often hidden by light and atmospheric pollution. Also, the highly magnified perspective offered by a telescope, might not be ideal to observe large (apparent size) celestial objects, such as the Pleiades Star Cluster, and one might be better off using lower power optics typical in Binoculars (please see article for binocular basics as relevant to nature observation) to enjoy a more satisfactory view of these.
|Sometimes, all it takes is a good pair of binoculars|
Other supplied accessories:
The telescope (tube, mirrors, front cover and focussing mechanism) is supplied with:
– guide scope,
– AA Battery Holder to run the cooling fan,
– Wooden base, which works surprisingly smoothly (at least until it was left out in overnight rain) to move the telescope around to point at the desired portion of the sky,
– an adapter for 1.25″ eyepieces,
– a Neutral Density Filter (decreases brightness and increases contrast – essential for lunar features to pop’ into view) that screws onto to the 1.25″ eyepiece for observing the moon
– A 35mm extension tube
– some collimation tools
– An Allen Key, size as appropriate for assembling the equipment
– A silica gel holder, which can be used in place of the eyepiece, while stored
There is a bit of DIY to get things up and running, much of the work to do with putting the base together. The instructions are fairly clear though, and if I remember correctly, I could get everything functional within 2 hrs (altogether another matter that I had to wait quite a while longer for clear skies – Murphy’s Law governing new equipment, the law’s effects, inevitably, directly proportional to the enthusiasm with which you want to try the equipment out!).
Mobility & Ease of Use
This telescope is large, as all Dobsonians tend to be. As such it is best suited for situations where clear skies are close at hand (unlike in my case, where I live under extremely polluted city skies – but I was sorely tempted by the cost-to-brightness ration of the set-up, just in case I managed to move it to a dark sky location for any decent length of time). However, if you envisage moving your device around a lot, then a Newtonian or Refractor telescope could be a wiser investment.
One reason Dobsonians tend to be cheaper are due to their simplistic design. Most (if not all) are manually operated, with no tracking or star location aids. For a relative newbie, when I invested in this telescope (my first one), I found the necessity to manually train the scope on the celestial object of choice, to be an educative experience, allowing me to \’learn the sky\’ in the process, but a tracking feature would have been nice all the same. Initially, the base was excellent for manipulating the scope, but later some play developed in the altitude adjustment, that has made it harder to hold highly magnified views in place.
A slight nuisance of the Dobsonian design (or any reflector telescope) is the need to collimate its components, before first use, and at intervals determined by how much ‘travelling’ the telescope has done. This is essentially a technique to ensure that all the optical and mechanical parts of the device are proper are properly aligned. I collimated the mirrors just once, given the limited travel my telescope has done (between my home interiors and the terrace on a good day or night), and this was easy enough to perform on this telescope, with which I had only ever enjoyed the optics so far.
Given the lack of mechanised tracking ability, this telescope has a fairly limited use for long exposure astrophotography. However, with the right adapters and filters, the set-up should be adequate, albeit with limitations, for images of our Moon and Sun. I have only ever tried photographing the Moon, by awkwardly manipulating my phone camera on the eye-piece, for modest results, not commensurate with the effort required to time the shutter release, in that fleeting, unpredictable moment that a decent image appears on the screen!
For some knowhow on astrophotography, and it’s nuances, from yours truly, please follow links for basics and deep sky subjects, respectively.