Levenhuk Ra R72 ED Doublet OTA Review
The objective lens of R72 ED Doublet refractor consists of a positive convex lens that converges the passing beam of light, and a negative concave lens that diverges the same light beam. As a result, this lens corrects chromatic, comatic and spherical aberrations, and produces a clear image. The positive lens of the doublet is made of ED glass with extra-low dispersion. This significantly enhances the contrast of produced views and reduces the residual chromatic aberration (i.e., secondary spectrum) by bringing the red and blue wavelengths to the common focus.
ED-doublets are considered to be semi-apochromatic: they produce much sharper views than a regular achromatic lens (with a crown-glass concave lens and a flint-glass convex lens), but are still not as effective as true apochromats that consist of three lenses with specific properties. ED glass contains a large amount of calcium fluoride (CaF2), which is extremely difficult to process but very effective in correcting chromatic aberration. This means, unfortunately, that the better the objective lens corrects aberration, the more expensive it is. It should also be noted that larger apertures and focal ratios are susceptible to higher levels of chromatic aberration.
These are the specifications of this OTA, as provided by the manufacturer:
|Optics coating||fully multi-coated|
|Optics material||ED glass|
|Objective lens diameter, mm||72|
|Focal length, mm||432|
|Eyepiece barrel diameter||1.25" (it's actually 2",
but it still accepts 1.25"
eyepieces with an adapter)
|Focuser||2" dual-speed Crayford|
These are just numbers, though. Let's open the box, shall we?
The instrument is shipped in a regular cardboard box that contains a metal case (approximately 15" x 10" x 7") – wrapped in bubble wrap – with the OTA itself.
The extremely compact optical tube rests safely in an EPE block within the case. However, the slot is such a tight fit that it might be difficult to remove the optical tube from the case. You might want to consider making it a bit wider, which can be done with a regular utility knife. Don't make it too wide though, as this would make it rather pointless. The kit is pretty basic: the tube itself with a 2" focuser and a retractable dew cap, a 2"–1.25" adapter, a white 1.25" plastic cap, a metal cap for the dew cap, a cleaning wipe for chrome parts and a set of keys for the case. You can store a star diagonal (a must-have for celestial observations) and a few eyepieces in the case – a few holes are already cut in the EPE block for these accessories.
The optical tube is quite small (∅100 x 304 mm) and light (5 pounds), and it looks amazing. You can see the tube standing upright in the pictures below. The objective lens is at the bottom; the dew cap is retracted and the focuser knobs are at the top.
The markings on the objective lens indicate the most important information about the optical tube: ED stands for extra-low dispersion glass used in the objective lens; F/6.0 is the focal ratio of the optical tube; D:72 is the aperture; F:430 is the focal length; FULLY MULTI-COATED is pretty self-explanatory. When looking directly at the objective lens, you can see the purple tint of the optics coating and notice the diaphragms inside the tube that are meant to further reduce the light pollution during observations.
The image below gives a good idea of what the optical tube looks like up close. The fully extended dew cap (to the left) increases the length of the tube by 55 mm and protects the objective lens from fogging during early morning observations. The dual-speed Crayford focuser (to the right) may be rotated for better access to focus knobs (the silver thumbscrew at the top of the focuser locks it in place). The focuser tube may be extended up to 80 mm; however, the focus is reached @ 150 mm away from the flange, when the focuser tube is retracted; and @ 70 mm when it is fully extended. This means that you would have to use a star diagonal or a 70-mm extender to achieve focus. The dovetail mounting bracket is attached to the tube with two M6 bolts. You can use it to attach the OTA to a photo tripod (which, personally, I wouldn't recommend) or to a telescope mount.
I was able to quickly attach the OTA to my trusty Vixen Porta altazimuth mount. Once I had attached several heavier accessories (a star diagonal and a large eyepiece), I had to rotate the mounting bracket so that it faced the focuser, to balance the assembly. I also had to remove the focus lock screw (you don't really need that for visual observations) to allow for focuser rotation.
The OTA's focuser is quite advanced – it's a Crayford design with four linear bearings, a focus precision adjustment screw, a focus lock screw and a fine focus knob. The retractable 2" focuser tube may be extended up to 80 mm; its interior is blackened and grooved to reduce glare during observations. One full rotation of the focus knob shifts the eyepiece by 12.5 mm, while the fine focus knob shifts it by approximately 1.14 mm. In the image below you can see three thumbscrews: the one on the left locks the focuser body in place – you can loosen it to rotate the focuser; the lower thumbscrew on the right is a part of the 2"–1.25" adapter and it locks 1.25" accessories in place; the other thumbscrew is the locking screw for 2" accessories (in this case it holds the adapter).
In the image below you can see the bottom of the focuser. The thumbscrew on the left is the focus lock screw; the one on the right is the locking screw for 2" accessories. The larger hole to the right of the focus lock screw hides the hex cap screw that allows you to adjust focus precision. The mounting bracket with cork pads has been disassembled and lies separately (two threaded holes on the optical tube are used to attach it in place). The 45-mm dovetail plate will fit almost any mount, and you can even attach it to a photo tripod with two ¼" screws.
Both the 2" flange of the focuser and the 2"–1.25" adapter have brass compression rings that safely lock eyepieces and other accessories in place without damaging their barrels. The 2"–1.25" adapter is 30 mm long, and fits approximately 18 mm (0.7") off the length of 1.25" accessories. The focal length difference between the 2" and the 1.25" flange is 7.5 mm (0.3").
The objective lens frame does not allow for user adjustments. The positive front lens seems to be made of ED glass, judging by the dispersion of a laser pointer beam. The effective diameter of the objective lens is exactly 72 mm.
I have performed two sessions of visual observations with this OTA to test its capabilities and to see how it performs against my good old SW 80ED Pro: first, observing an artificial star; and second, observing Jupiter, C/2011 L4 PANSTARRS comet, Castor (α Gem), and other real objects.
Long story short, the OTA turned out to be near perfect. I didn't notice any residual spherical aberration or coma, or astigmatism. The only issue was a slight residual chromatic aberration (green extra-focus diffraction ring and purple intra-focus diffraction ring) that didn't really bother me during observations of real objects – including brighter ones, like Jupiter. In my SW 80ED Pro, residual chromatic aberration is far more noticeable – so much so, in fact, that I'd consider the tested OTA to be a proper apochromat. It took the optics about 25 minutes to completely adapt to the outside temperature (at 23°F). Afterwards, there were no artifacts in finer details at high magnifications. For instance, at 100x magnification I was able to see two equatorial cloud belts on a 36" disk of Jupiter, as well as discern the gap in the SEB, where the Great Red Spot is supposed to be, and two thin belts of the North and South Tropical zones. A textbook image of Castor (α Gem), a bright binary star, was beautiful: two Airy discs, surrounded by an intricate diffraction pattern.
Observation sessions might have been easier with a viewfinder, but I was still able to find desired objects with ease, thanks to the 5-deg field of view (with a 30mm, 82° eyepiece). PANSTARRS comet, for instance, showed a magnificent tail of 40-45 arcminutes. I was even able to see the entire Hyades star cluster without slewing the OTA. The optical tube produced a wonderful view of regions around α and δ Persei, and I spent some time enjoying the view of California Nebula (NGC 1499) and Rosette Nebula (Caldwell 49) with a UHC filter. My son was captivated with Pleiades star cluster – as seen with a 16mm, 82° eyepiece. Three bright stars of Orion's belt, surrounded by fainter stars; the sword, including the M42/M43 Nebula; the faint Beehive cluster – everything looked spectacular in the eyepiece.
Summary and recommendations
Levenhuk Ra R72 ED Doublet OTA is a wonderful example of a small-aperture telescope. Great resolution threshold (to resolve binary star elements) and limiting stellar magnitude (to see fainter stars), coupled with wonderful optics, guarantee that your images will be sharp and full of contrast all the time. Wonderful design means you can concentrate on observations without wasting time on adjustments, alterations and upgrades. The OTA might look flashy, sure, but it would definitely make a wonderful present to anyone interested in the mysteries of the night sky. The tube is very compact and lightweight, so you can easily take it with you, wherever you're going. Besides, you can store all your accessories (eyepieces, stargazers, adapters – you name it) in the metal case, so nothing gets damaged along the way.
This OTA should be used with a lightweight altazimuth mount (preferably with slow-motion controls). I'd also recommend using the following accessories:
- A solar filter, if you want to see all the flares and spots on the solar surface;
- A good 2" star diagonal or an erecting prism;
- 2" eyepiece with a 30–40-mm focal length to maximize the wide-angle potential of the OTA;
- 15–18mm UWA eyepiece for observations of diffuse nebulae and loose star clusters;
- 10–12mm UWA eyepiece for observations of planetary nebulae and dense star clusters;
- 5–7mm eyepiece with an average field of view for observations of the Moon and the Sun;
- 3–4mm eyepiece with an average field of view for planetary observations and maximum detail on lunar and solar surfaces;
- UHC filter for enhanced views of various nebulae.