OD150L De-luxe planetary Dobsonian
Orion Optics Ltd. — +44 (0)1270 500089/251559 — was £499
Written: ~5 years ago

OD150L
Despite appearances, this isn't an anorexic Dobsonian. This rather fetching little beauty is meant to be this thin — or rather, long. (154cm, or nearly 61 inches in length and 17cm in diameter, to be precise.) It may look unconventional, but this 6-inch f/11 instrument from renowned U.K. telescope maker Orion Optics looks the way it does because it's optimised for high power, high resolution viewing of the Moon and planets. It sports a tiny 1-inch elliptical secondary mirror (17% obstruction) in order to keep performance-robbing diffraction effects under control.

The De-luxe model has a number of enhancements compared to the standard production OD150L. The 1/6 PV wavefront optics come with Hilux coatings (better surface accuracy mirrors are available at extra cost) and the focuser is a very nice 50.8mm Crayford design with a 31.7mm adaptor (focusing range: 63mm). Two Plössl eyepieces, 10mm (160x) and 25mm (64x), plus an 8x50 straight-through finder completes the set.

Like all Orion Optics Dobsonians, the OD150L features a substantial yet lightweight altazimuth mount made from aircraft-grade aluminium with CNC-machined bearing rings acting on Teflon blocks. It's very smooth and precise in use. Given its height and staggeringly small footprint of 24cm x 22cm, you can store this 'scope in an upright position in the corner of just about any room. Since the altitude axis lies some 78cm above the ground, this does mean that the setup is not quite as stable as a conventional Dobsonian. Even so, when used in a keen wind on the evening of March 20th 2007 the vibration damping time was no more than 4 seconds.

vented_cell
Basic OD150L models come with unventilated primary mirror cells that mean long cooldown times. The De-luxe model does have passive ventilation which is quite effective; I found that I could get passable images in about 40 minutes after bringing the 'scope out of a warm room. Once the image had truly stabilised I could see the immediate benefit of having such a small secondary mirror — you see little sign of it in an extra-focal image of a star! The characteristic silhouette of the secondary almost disappears in an out of focus stellar image. The one thing you do see that reminds you that you're using a Newtonian is the cross-shaped diffraction pattern emanating from the secondary mirror holder.

Optical shoot out: The next opportunity I had to put the OD150L De-luxe through its paces under decent seeing was on March 29th when I had the long Dob pitted against some stiff competition — a Russian Intes MN61 6-inch f/6 Maksutov-Newtonian. The Intes has a formidable planetary performance owing to its high-contrast optical system (<1/7 PV wavefront) and a tiny central obstruction of 18%. I had some good seeing around 21:30 BST when both instruments had been cooling for about 90 minutes, so they were close to thermal equilibrium. A high waxing gibbous Moon provided plenty of detail for a side-by-side comparison.

The OD150L delivered a brighter, more faithfully colour-balanced view of the Moon, which is not particularly surprising given its Hilux coatings compared to the standard aluminium of the Intes. However, the interior of crater Gassendi revealed virtually indistinguishable craterlet and rimae detail in both instruments. It was hard to choose between the two at times, but as the evening wore on I found myself preferring the view through the Orion instrument, even though its secondary mirror support introduced an element of diffraction noise that was entirely absent in the MN61.

OD150L_spider
Verdict: I have to say that I was quite surprised that the OD150L held out so well against the Intes MN61, but the effect of Orion's Hilux coatings really did make a dramatic difference. Having used the long Dob for a few weeks, I find myself enjoying the fact that it's incredibly easy to store when not in use and takes no time to setup (even less if you have outside storage at ambient temperature). For visual observations of the Moon, Jupiter and Saturn (plus Mars near opposition) it maybe the only 'scope you'll ever need, though do bear in mind that if you remount it equatorially it won't be so convenient to use owing to its length.

The more I use it, the more I'm convinced that it will outperform an apochromatic refractor up to about 5-inches aperture. My only recommendations to Orion would be to provide the option of a curved secondary mirror support to diffuse the spider's diffraction spikes and a low profile focuser. If you're looking for apo performance on a budget, the OD150L De-luxe certainly delivers. Highly recommended.

Update
This instrument has been further improved by the used of a three-vane curved secondary mirror support from 1800 Destiny. You can read more about what I thought of these wonderful spiders by clicking here.