Focal Length: 1,624
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What telescope design might be best is a hotly debated topic among amateur astronomers.
For professional observatories, including the Hubble Space Telescope, the preferred design is the Ritchey-Chrétien (RC).
But the expense of constructing RC scopes typically rules this design out for amateurs.
Recent manufacturing advances have reduced that expense, however, and so the design forms the heart of the Omegon Telescope Pro RC 203/1624.
Aimed at experienced or advanced amateur astrophotographers, the only extras this scope comes with are focuser extension tubes.
We added a finderscope to help us star align the relatively long focal length telescope on our mount.
Although weighing a little over 6kg, the Omegon Pro RC feels quite hefty, owing to its short overall length and compact size.
It took us a little while to determine the correct spacers to use and how to adjust the balance positions accordingly so that our mount would perform at its best with the longish exposures required of an f/8 telescope.
With a clear sky forecast, we were keen to start taking pictures.
But our first session with the Omegon Pro RC was spent ensuring that its mirrors were properly collimated, as the design demands perfect mirror alignment for optimal results.
Amateur RC telescopes have a reputation for being difficult to collimate and the manual for this model, which we had to track down via an internet search, recommends you use additional accessories to make collimation easier, namely a laser and special eyepiece.
We breathed a sigh of relief when the review telescope proved to be nicely collimated and hadn’t become misaligned during delivery.
Working up to the whale
An ideal first target for this test was Messier 13, the Great Globular Cluster in Hercules.With a close-up view we hoped that the Omegon Pro RC’s optics would provide a sharp image right to the core of the cluster, and we weren’t disappointed.
We took a series of five- and 10-minute exposures, which revealed the full extent of the outer edges of the cluster, while maintaining distinct details and star colour at the bright core.
Encouraged by these results, our next targets included M27 (the Dumbbell Nebula), M51 (the Whirlpool Galaxy) and the distinctively shaped Whale Galaxy, NGC 4631.
When the first 15-minute exposure of the Whale appeared on screen it was one of those “Wow!” moments.
The galaxy has a bright core along with nicely defined dark dust lanes and is peppered with hints of star clusters and hydrogen-rich, star-forming regions, all of which were crisply captured by the precision optics of the Omegon Pro RC.
Acquiring and maintaining proper focus is a key requirement with astrophotography, especially so at longer focal lengths.
We were pleased to note that the quartz mirrors in the Omegon held tight focus for long periods, even during temperature shifts of several degrees.
The smooth Crayford focuser kept the target central during focusing and after locking it off.
We used our 8-inch Bahtinov mask to assist with the focusing process, but we could have used the diffraction spikes from the rigid secondary mirror holder to good effect too.
As mentioned, RC telescopes are designed for photography; the relatively large central obstruction caused by the secondary mirror makes for a loss of contrast when used as a visual instrument.
However, with Jupiter and the Moon situated nicely to the South, we did spend a couple of hours with eyepieces, enjoying some well-defined views, from 70x, up to 380x as the seeing allowed.
We also took the opportunity to perform a high-magnification star test to check the collimation after several nights of the scope being swung to positions all over the sky and were impressed to find that the optics were still properly aligned.
In summary, the Omegon Pro RC offers sharp, coma-free images at a focal length that allows the finer details of deep-sky objects to be revealed.
Outstanding feature: Hyperbolic mirrors
Although invented over a century ago by George W Ritchey and Henri Chrétien, difficulties in accurately manufacturing hyperbolic mirrors for RC telescopes have generally made them too expensive for amateur equipment.
But recent advances in optical measurement and manufacturing have meant that the mirror sets can be made much more economically today, allowing reasonably priced RC models to come onto the market.
The RC mirror design eliminates the optical problem of coma – when star shapes towards the edges of the view become elongated – so typical of Newtonian telescopes.
The Omegon Pro RC 203/1624 provides a flat-field of view over a full-frame (36x24mm) camera sensor and requires no front corrector plate like similar-looking Schmidt-Cassegrain telescopes do.
In addition, the low-expansion quartz crystal mirrors are fixed in place and avoid issues such as mirror flop that can spoil images from a Schmidt-Cassegrain.
Unlike some refracting telescopes, the Omegon Pro RC’s mirror arrangement avoids colour aberration problems, making it suitable for DSLR and one-shot colour astronomy cameras.
Such a pedigree perhaps makes the long-winded collimation process a worthwhile undertaking.
Solid build and construction
Solidly constructed from steel, with well-designed and reassuringly sturdy fittings inside and out, the tube measures just 580mm in length and 230mm in diameter.
Ideal for a permanent observatory situation, it’s also reasonably portable if required.
The Allen key collimation adjustment screws are conveniently located on the front and rear.
The inside of the tube assembly contains a line of circular ribs known as baffles.
These have the important role of blocking the passage of stray light into the imaging plane, without obscuring the primary mirror.
The effect they have is to improve image contrast by removing unwanted light artefacts andreflections.
Mounting rails and finder shoe
The top and bottom of the telescope is fited with a nicely machined dovetail bar, which fits both Vixen and Losmandy mounts.
These rigid bars add to the structural integrity of the telescope and allow you to attach extra accessories.
A standard Vixen/Sky-Watcher shoe is provided for a finderscope, with left- or right-side fitting.
The dual-speed 2-inch Crayford Focuser is rotatable and screws onto the tube or onto the extension pieces.
Its draw tube displays graduated markings and allows 50mm of travel, while the large knurled lock nut is easy to operate, holding cameras well; we found no trace of focuser slop.
The only additional items in the box are a pair of extension tubes, 1x50mm and 2x25mm.
These are necessary to extend the focuser position to suit a wide selection of cameras, filter wheels and accessories.
The internal diameter of these tubes is 80mm, avoiding any potential vignetting issues.
This review originally appeared in the August 2018 issue of BBC Sky at Night Magazine