|Review of Revelation 12
305 mm f/5 Dobsonian from Telescope House
This Dobsonian telescope is remarkable for its low price – only £449 if you collect it from the importers, Telescope House in Tunbridge Wells. You could hardly buy the optics for a 300 mm telescope cheaper than this, so you might wonder what sort of a telescope you get for the money.
I tested the telescope in spring 2005 as delivered in flat-pack form. It so happens that I have also now written the assembly and use instructions for this telescope, but that was a separate job and I am not an employee of the importers and can say what I like about it (and them). The tube comes complete, but you have to assemble the Dobsonian base. This will take you an hour or two, and mostly requires just the single Allen key supplied.
The finish of the telescope is good and durable. It is made in China, but not by the well-known firm of Synta, who are possibly the largest telescope manufacturers in the world and make Sky-Watcher and Celestron telescopes.
I will be writing a fuller review for the October 2005 issue of Popular Astronomy, so these are really brief notes covering the key details.
For your money you get:
* Tube and 305 mm f/5 optics (1500 mm nominal focal length)
* Crayford focusing mount (2-inch) with locking and tensioning screws
* 32 mm eyepiece --2-inch fitting. I was not able to work out what type but it has about a 70º apparent field of view and from the internal reflections is definitely not a Kellner. Gives a nominal power of 47x but I did not check the focal length of either telescope or eyepiece.
* Adapter for 1.25 inch eyepieces
* Tube cap
* Battery-operated fan behind the mirror blows air up into the tube around the mirror cell.
* Fully adjustable mirror cell and flat mounting
* Eyepiece holders on side of Dobsonian mount
* Dobsonian mount of high-density chipboard, faced with black plastic
* Teflon pads on side bearings in classic Dobsonian style
* Roller bearings between base and rocker box
* Spring tensioners on side bearings
* Good-quality 8 x 50 inverting finder on quick-release shoe and with spring-loaded adjusters
The mirror quality is perfectly good enough for normal Dobsonian use. Good contrast – Earthshine was visible on the first-quarter Moon with the illuminated portion within the field of view. There is distinct astigmatism but not so bad as to be worrying when observing deep-sky objects. The telescope was good at low and medium powers but not at high powers eg x 400 on Jupiter. It was much better when stopped down to 250 mm. Tube currents are a problem. The fan starts the air inside the tube swirling in a circular motion which persists as long as it is on, but when you switch it off after a few minutes the images are improved. It may be that leaving the fan on for too long is a bad idea – it could cool the middle of the mirror down too much, leading to even worse images.
Collimation is tricky because of the thin metal plate used for the mirror cell, and weak springs. The metal bends as you tighten the lock screws.
The primary mirror has a central ring, which is very thoughtful touch and is a great aid to collimation. Details such as this show that the telescope has been designed with care and not just thrown together, and maybe in time some of the niggles will be sorted out.
All in all, the optics are well suited to Dob use, but don’t expect perfect planetary optics as well for the price. However, the test instrument did give pleasing lunar and planetary images. If you want a perfect all-round instrument spend a lot more money! The 12-inch mirror reveals many objects, even in moderate light pollution, which lesser apertures will not show.
The finder is of good quality and easy to adjust. A great improvement on the basic Meade rubbish
The Dobsonian mount is smooth until you try to use the highest powers, then you might encounter a little 'sticktion' when trying to follow an object smoothly. But it can be done, and again Dobs are not intended for this sort of thing. The roller bearing is a great help.
I found it quite easy to carry the telescope around once I had improvised brackets on the side bearings, but otherwise you have to grasp it by the rather small pins that hold the tensioning springs.
The tube was well balanced even without the tensioning springs which I didn't use in the end.
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