Levenhuk Ra 250N Dobsonian
Perfect for Observing Deep-Sky Objects
by Ernest Shekolyan, for
Recently, the Levenhuk Ra series has seen an addition of three Dobsonian telescopes in apertures of 8-inch, 10-inch and 12-inch. I managed to get my hands on one of these models for a test run. I opted for the 10-inch Dobsonian, as I figured this mid-model would tell me all I needed to know about the quality of manufacturing and optics.
Here are the specifications of the telescope from the manufacturer’s website: The primary mirror is made of BK-7 glass and the three-vane primary cell is fitted with a fan for additional cooling of the mirror. The azimuth axis of the Dobsonian mount is fitted with roller bearings. The optical tube features a 2-inch Crayford-style focuser with fine focus (1:10) and includes a 1.25-inch adapter. Reported accuracy of the primary mirror is 1/12th of the wavelength of visible light. Its focal length is 1250 mm resulting in a focal ratio of f/5. Included is a fully multi-coated 8x50 finderscope, two eyepieces (1.25-inch 9-mm Plössl and 2-inch 30-mm SuperView), a lunar filter and an accessory tray.
The Dob mount had already been assembled when I received the telescope. The mount is constructed of coated chipboard and weighs roughly 28 pounds, while the gloss-black optical tube weighs approximately 33 pounds. The standard kit of these telescopes also includes an optical viewfinder and a couple of decent eyepieces.The mount is rather bulky, but a practical handle allows you to carry it without too many problems. My 14-year-old son managed to do it, and so will you. Many of you might think that a 10-inch Dobsonian telescope is a behemoth, so I’ve included a picture (Image 1) of my son standing next to the telescope to better demonstrate its size. As you can see, it’s really not that big. During lengthy observations of objects near the horizon, you might even have to sit on a chair.
Image 1 - The author’s 14-year-old son poses with the
Levenhuk Ra 250N to lend scale to the telescope.
The day after receiving the scope, I took it for a test run. I already had a telescope and a few accessories in the trunk of my car, so I had to put the tube on the back seat – the mount went in front. I went alone, but still had no problems with the telescope during transportation and assembly. The side bearings slid into notches on the rocker box with ease. I had to spend some time getting used to the controls – adjusting just the right amount of pressure for declination slew. Controlling the scope was very easy: hold on to the front of the optical tube with your left hand and rotate the rocker box to slew it horizontally or to tilt the tube to adjust declination. The navigation is absolutely standard otherwise: insert the low-power eyepiece, use the finder scope to locate the desired object, confirm it’s centered in the eyepiece, change to a high-power eyepiece for observations.
I’d say that the most interesting thing about this telescope is the focuser (Image 2). It’s a dual-speed Crayford focuser, which does everything a regular focuser does, only significantly better. For one thing, it’s extremely precise. The full travel range is 35 mm (~1.4 inch), giving you plenty of flexibility with the focus. The main reason for its precision is the fine focus knob. With one turn of the white knob, you shift the focal plane by 13 mm (~0.5 inch), however, one turn of the smaller black knob is only a tenth of that.Therefore, the views it produces will always be clear (once you adjust the focus properly, of course). The movement is very smooth with absolutely no backlash. I have to warn you, though, that the fine focus mechanism is quite intricate – an inadvertent impact might affect its performance.
Image 2 - The Levenhuk Ra 250N is equipped with a quality
2-inch Crayford-style focuser with (10:1) fine focus.
Bottom line: The focuser is extremely good, simple to work with, and allows you to enjoy your observations without any difficulties. The 2-inch flange of the focuser has a brass compression ring that securely locks accessories in place. The standard kit includes a 1.25-inch adapter, which further increases the capabilities of your telescope. The adapter’s flange is 0.3 inch higher, so that you can change to a 1.25-inch adapter without having to refocus your view.
Secondary Mirror Assembly
The secondary mirror reflects light rays from the primary mirror into the eyepiece, which produces the final image. A four-vane spider holds the secondary mirror cell in place, and the secondary mirror itself lies at a 45-degree angle to the optical axis. The shadow of the secondary mirror’s frame is 66 mm (2.6 inch) in diameter, which produces a 26-percent central obstruction. The elliptical mirror itself is 87 x 66 mm (3.4 inch x 2.6 inch).
Collimation may be done with three adjustment screws, which isn’t very challenging. Simply put, during collimation you adjust the position of the primary and the secondary mirrors, so that there is no clipping of the image during observations.
Tube Front Ring
A silumin collar reinforces the front end of the tube. The collar is held in place by six M4x5 screws. The focuser lies roughly 5.5 inch away from that collar. The four-vane spider of the secondary mirror cell is held in place by four acorn nuts that should be tightened to keep the mirror cell in proper tension.
Among the most interesting features of this Dobsonian telescope is the ability to adjust the position of side bearings for perfect balance (Image 3). After you have aligned both side bearings so that they line up along an axis that goes through the tube’s center of gravity (make sure you attach the finderscope and the eyepiece beforehand), you can achieve even greater precision and accuracy in declination (altitude) slew.
Image 3 - Relative height of the two altitude bearings is easy to adjust
to balance the OTA to accommodate a variety of heavy accessories.
Tension knobs allow the user to fine-tune the action of declination slew
While the process of declination slew, or tube tilting, is extremely simple, the design of the individual altitude bearings themselves, however, is much more intricate. The construction consists of a shoe mount, a tension-adjustment mechanism with ball-bearing clutch, and a base with a scale (in inches).
Primary Mirror and Cell
If you look at the rear end of the tube, you will see the primary mirror cell (Image 4). Three black-capped adjustment screws allow you to shift the position of the primary mirror during collimation. Three white-capped locking screws lock the primary mirror in place after collimation. Before you can collimate the primary mirror, you need to loosen these locking screws (four or five turns should do). Afterward, you can use the adjustment screws to shift the tilt of the primary mirror within the optical tube until you achieve the perfect collimation. Do not forget to re-tighten the locking screws once you’re done, but don’t overtighten them.
Image 4 - Details of the primary mirror cell and OTA end ring.
There’s also a cooling fan at the rear end of the optical tube to speed bringing the primary mirror to ambient temperature before you begin your observations. The electric contact is located on the rear flange of the optical tube. The fan can be powered by a standard 12-volt source, or you can purchase an accessory battery pack (9 AA batteries).
Apart from the cooling fan, Image 4 shows the adjustment screws of the primary mirror cell. The adjustment range isn’t very large (about 4 mm in total), but I found this range entirely sufficient during my test run. Even loosening – or tightening – the locking screws has its effect on the mirror cell adjustment, so you might want to consider leaving a little leeway during collimation.
The rear flange, where the primary mirror cell is attached to the optical tube, is held in place by six M4x5 Phillips-head screws. If you try disassembling the mirror cell, note its alignment and position on the flange, as it can only be properly assembled if you maintain position and alignment of the elements.
The mirror cell is pretty standard: a three-vane frame with an open back to promote passive cooling. The primary mirror itself rests on three soft cork pads (1.3 inch x 0.7 inch x 0.06 inch), arranged in a circle. There are three more cork pads that reduce the side pressure on the mirror.
The primary mirror has an overall diameter of 249 mm, an effective diameter of 247 mm and a thickness of 33 mm. I measured the focal length myself, and it turned out to be 1255 mm, which means that the focal ratio is actually around f/5.1. The distance from the primary mirror to the center of the secondary is 1007 mm (39.6 inch); the distance to the focal point from the secondary mirror is about 250 mm (9.8 inch); the distance to the focal plane from the surface of the optical tube is roughly 100 mm (3.9 inch).
The primary mirror is held in place by three retaining clips made of rubber. Make sure there’s a hair-thin gap between the mirror surface and the clips; otherwise the pressure might deform the mirror with all the expected consequences.
Image 5 shows the primary mirror as seen from the focuser tube. I had to use a flashlight to capture it, so the out-of-focus shadow above the primary mirror is from the secondary mirror frame. The center of the mirror lies exactly at the intersection of spider vanes. The black spot in the center of the mirror is the shadow of the secondary mirror.
Image 5 - View of the primary mirror as seen from the focus tube.
Let’s take a look at the mount. The handle does not cut your fingers and provides for a comfortable grip. The mount itself is a bit bulky, but even if you’re not very strong, you’d still be able to carry it to your car.
The side boards are approximately 30 inches high, and the front board is 13 inches. There are a couple of wing nuts on the front board. These hold the handle in place. The screw on the bottom board connects the rocker box to the base. You can adjust the tension of the right ascension (RA), or azimuth, slew with this screw. This M8x50 screw has a ball bearing between two washers. I’d strongly recommend oiling the bearing from time to time.
There’s a roller bearing between the mount base and the rocker box. This bearing consists of two galvanized sheets, roughly 0.03-inch thick, and a plastic cage (12 inches in diameter) with 24 needle bearings (∅0.1 inch x 0.5 inch) arranged in a circle. Keep in mind that you have to service this roller bearing as well. You can use bicycle chain lubricants to grease the galvanized sheets. After some time, you may see signs of wear and tear on the sheets – simply flip them to extend their service life.
It was cloudy during my first test run of the telescope. I allowed about 8 hours for the instrument to fully adapt to the outside temperature (it was approximately 45°F at the time). Because of the clouds, I had to test it on an artificial star at a distance of 160 yards.
I had no problems assembling the telescope on my own. Configuring the declication slew wasn’t very difficult, either. However, due to the considerable weight of the accessories that I used, the optical tube was not very stable, so I had to tighten the adjustment knobs on the side bearings. Naturally, this had a negative impact on the precision of the declication slew. Of course, the design of the side altitude bearings makes it easy to adjust their heights to correct for balance, but it worked well enough without making that adjustment. The RA slew was very smooth and responsive.
The focuser supports the weight of heavier 2-inch accessories with ease. The focusing is easy, and I was really impressed with the fine focus feature. I collimated the instrument at home with a Cheshire EP and had no problems with mirror alignment during my observations. Most importantly, the mirrors maintained alignment throughout the full range of declication slew, even when observing near zenith or the horizon.
The views had good contrast, which is typical of Newtonian telescopes with such a small central obstruction. During out-of-focus observations of artificial stars, the optics produced slight astigmatism and showed mild signs of spherical aberration, although these cumulative aberrations do not exceed 1/4 to 1/5 of the wavelength. I think that bit of astigmatism is mostly due to the design of the secondary-mirror cell. The cell is made of plastic and may deform in the cold, putting additional pressure on the mirror. You can remove the secondary-mirror cell if you wish, but then you would have to modify the frame of the secondary mirror, so I do not recommend this if you’re an amateur stargazer. Mild aberrations aside, this instrument works wonders during observations of nebulae and other deep-sky objects, thanks to its limiting stellar magnitude of 14.7.
Summary and Recommendations
I really enjoyed observing the night skies with this telescope. The construction of the instrument is far more advanced than what I have seen on other manufactured Dobsonian telescopes. The optical tube is a bit bulky, but I’m not sure how much a 10-inch telescope would benefit from collapsible design – it wouldn’t be that much easier to transport, and obvious difficulties with optical elements and the rigidity of the construction are sure to arise. This instrument is perfect for observations of deep-sky objects (nebulae and star clusters) as is. It also performs well during observations of the Moon and the Sun (keep in mind that you have to use a filter for later, or observe the projection). I haven’t tested it on planetary observations, but I think that it would easily perform on par with views produced by quality 120-mm refractors, or larger.
Take the Levenhuk Ra 250N to the countryside, as far away from the city lights as possible, and I guarantee that you will be captivated by produced views of the celestial sphere.