Review on the quality short focal length refractor - Levenhuk Ra R90 ED Doublet OTA | Levenhuk - best optical equipment

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Levenhuk.com » Good to Know » Levenhuk Ra R90 ED Doublet OTA Review

Levenhuk Ra R90 ED Doublet OTA Review

A few days back I picked up (for testing, of course) a 90-mm F5.5 refractor with a short focal length, with nearly non-existent chromatic aberration, thanks to the extra-low dispersion glass of the objective lens. The OTA is manufactured by Levenhuk.

Here are the specifications of the optical tube, as given by the manufacturer:

Specifications
Optical design refractor
Optics coating fully multi-coated
Optics material ED glass (one of the lenses)
Objective lens diameter, mm 90
Focal length, mm 500
Focal ratio f/5.5
Eyepiece barrel diameter 2"
Focuser 2" dual-speed Crayford

The objective lens in this OTA consists of two lenses: a positive lens made of ED glass and a negative lens that diverges the light beam. These lenses work together to minimize the residual chromatic aberration of the objective lens. Unfortunately, the margin of error during the manufacturing process of APO lenses is very small, as even a slight defect significantly reduces the quality of produced views. APO lenses are, therefore, rather expensive to make.

Lenses in a doublet are arranged in a particular manner, to negate chromatic aberration and coma for a certain wavelength of the spectrum (generally green or yellow-green) and to bring two different wavelengths of the spectrum to a common focus (generally red and blue). However, focal points of other wavelengths do not lie on the same plane and, as a result, stars are surrounded by a purple or violet haze. This haze may be significantly reduced if the positive lens is made of ED glass - the halo will be three to four times smaller. There is a drawback, however. Apochromatic doublets - even the perfect APO doublet - suffer from spherical aberration at the edges of the spectrum (therefore, these lenses are also known as Semi-APO). Nevertheless, if you do not push the limits of your telescope's capabilities, you'd be amazed with the quality of produced views, practically free of chromatic aberration.

On the one hand, the f/5.5 focal ratio of the R90 ED OTA makes it perfect for wide-field photography of nebulae and star clusters, allowing for faster shutter speed. On the other hand, however, such focal ratio enhances possible problems with aberration correction. The question, therefore, is: Did the manufacturer manage to create a good Semi-APO refractor with a short focal length or not?

Packaging

The OTA is shipped in a regular cardboard box (23" x 14.6" x 8.5"). The label on the box says "44741 Levenhuk Ra 90R Doublet ED OTA S500-D Niki Optics & Electronics Co." The box contains a sturdy, black plastic case (22.4" x 12.6" x 7.9") with aluminum corners. An EPE block inside the case holds the OTA itself, which is roughly 15" long, 7" wide (at the widest point), and 5" high (with the mounting foot). The case has enough space left over to hold two or three eyepieces, a star diagonal, several filters and an aperture solar filter - the necessary cutouts are already in the EPE block.

Packaging
Packaging

Optical tube

The black telescope optical tube (∅4.6" x 15.4"/7.2") is made of anodized aluminum. It's relatively light at only 7 lbs (without a star diagonal or an eyepiece). You can easily extend the retractable dew cap (∅4.6" x 4.4"), which makes the OTA 2.6" longer. The Vixen-style dovetail foot (3.1" long and 1.8" across) is located in the middle of the tube. The foot is relatively short, so you might have to rotate it before you can properly balance the OTA with all the accessories. Both the dew cap and the tube are painted dark gray inside, and there are two diaphragms inside the optical tube - all this reduces the unwanted glare during observations. The focuser assembly allows you to rotate the focuser freely once you have attached all the necessary accessories, to provide access to focus knobs and find the most comfortable position for observations.

I easily attached the compact and lightweight tube to my altazimuth Porta/Vixen mount - but I'm quite certain that any other mount will do, as long as there's a dovetail shoe on it. However, I did have to reverse the position of the dovetail foot before the assembly, to make it simpler to balance the OTA. Personally, I wouldn't recommend using the OTA with a photo tripod, even though all the necessary ¼" threaded holes are already there.

Optical tube Optical tube

Objective lens

The objective lens of this telescope is a doublet. It consists of two lenses with a thin gap between them – the positive lens is made of ED glass and is located in the front. I did not notice any defects on the glass. The coating gives the outer surfaces of the objective lens (i.e., the first and the fourth surface of the two-lens array) a uniform green tint, and a blue tint to the inner surfaces (i.e., the second and third). A compression ring holds the lens in place. The front end of the optical tube is felted to allow the dew cap to slide back and forth. The felt cloth might be hiding adjustment screws - however, you'd have to tear it off, so I decided not to follow that assumption.

Objective lens

Focuser

The OTA is fitted with a Crayford focuser. By rotating the focus knob, you rotate a small shaft that sets the focuser tube in motion. Two pairs of linear bearings - hidden in the shell - transmit the motion from the shaft. You can change the focuser precision with the help of a hex-cap adjustment screw. During astrophotography sessions, you can lock the focus with a large thumbscrew on the bottom of the focuser. The thumbscrew on top of the focuser (where the focuser meets the optical tube) locks the focuser body in place - you can loosen it to rotate the focuser during observations. The thumbscrew on the focuser flange along with a brass compression ring is used to lock 2" accessories (eyepieces, Barlow lenses, CCD - you name it) in place. If you have a 1.25" accessory, you'd have to use a 2" to 1.25" adapter (as shown in the picture). The focuser drawtube has the travel length of 80 mm (~3.15"). Thanks to the scale (in mm) on the drawtube, you can adjust the focal plane location with an accuracy of 1 mm. The Levenhuk Ra logo and the specifications of the OTA are stamped on the focuser body. With a wide-field 2" eyepiece, the OTA yields up to 5-deg field of view and up to 3° with a long focus 1.25" eyepiece.

Focuser

Details

Here are the details of the OTA:

  • dew cap (∅4.6"/4.4" x 5.7") with a stiffener ring (∅4.6"/4.3" x 0.8") that keeps the dew cap from sliding off the optical tube - anodized aluminum, gloss-painted outside, flat-painted inside;
  • optical tube (∅4"/3.84" x 15.4") that holds the objective lens mount, also fitted with a Vixen-style dovetail foot - anodized aluminum, gloss-painted outside, flat-painted inside;
  • focuser (∅3.6" x 3.7") is attached to the optical tube through a PTFE ring and locked in place with an M5 thumbscrew - anodized aluminum, gloss-painted outside, flat-painted inside;
  • dovetail foot (3.1" x 1.8" x 0.9") with two threaded holes that accept M6-25 screws and two ¼" threaded holes that you can use to attach the OTA to a photo tripod - aluminum, black flat paint;
  • dew cap cover (∅4.8"/4.64" x 0.6") - aluminum, black flat paint;
  • 2"-1.25" adapter with a brass compression ring, tapered, 1.2" high, with a 0.3" flange and a 2" thread for filters - anodized aluminum, black gloss paint;
  • objective lens (∅4"/3.54" x 1.3") with a felted frame (to allow the dew cap to slide back and forth), with no signs of adjustment screws.
Details

Comparison

Naturally, the benchmark against which I was measuring this telescope has been my trustworthy SkyWatcher 80mm 1:7.5 ED Pro refractor.

Well, what can I say - the rate at which manufacturers are progressing is staggering. Levenhuk Ra R90 ED is half as long as my SW 80mm and has a slightly larger aperture. They were able to reduce the length by improving the focuser and dew cap design - even the weight was reduced by one-third. The apparent field of view of the R90 ED is 20% larger than the SW 80mm. The focuser is very practical. It has a fine focus knob; it allows you to adjust the position of the eyepiece or CCD camera with an accuracy of 1 mm; and it even allows you to change the position of the focus knobs! The OTA looks the part, too. It's a properly glamorous instrument, hell-bent on making it big in this world. The only fault that I can find is that the standard kit of my SW 80mm ED Pro included a 2" star diagonal, an 8x50 viewfinder and a pair of decent eyepieces, while the owners of the R90 ED will have to purchase all these things separately.

A comparison between Levenhuk Ra R90 ED Doublet OTA and SkyWatcher 80mm 1:7.5 ED Pro refractor

Testing

As usual, I've performed two tests of the tube: artificial star observation and regular visual observations of celestial objects. The results were later compared to my SW 80mm ED Pro and the Levenhuk R72 ED that I had at the time.

These are the results of out-of-focus observations in the city. The temperature was about 23°F:

  • There was a slight blue aura around Jupiter and along the lunar limb. Residual chromatic aberration was only noticeable in the coloration of diffraction rings (green in extra-focus and purple in intra-focus). This leads to a slight loss of contrast and detail on the surface of Jupiter. Overall, the R90-ED shows more residual chromatic aberration than the R72 ED and much more aberration than my long focus SW 80ED Pro;
  • Slight residual spherical aberration enhanced the image of the first and second diffraction rings on brighter stars, blurred the contrast of smaller details on the surface of Jupiter, but did not influence the separation of binary stars (within its resolution threshold of 1.4"-1.5"). Neither the R72 ED nor my SW 80ED Pro presented any residual spherical aberration;
  • There was no comatic aberration in the center of the field of view after misalignment. The lenses in the objective lens are very well aligned by the manufacturer;
  • Due to the relatively large focal ratio (f/5.5), all the accessories have to be perfectly aligned on the optical axis. The field curvature is noticeable even with a slight shift of 4-5 mm from the center of the field of view. Keep that in mind when you're attaching adapters or a star diagonal.
  • When trying to observe objects with the OTA before it fully adapted to the outside temperature, I noticed a relatively strong astigmatism of the optical system. It greatly reduced the contrast of produced images, especially at higher magnifications (over 100x). It may have been caused by compression of the objective lens in the frame. After about an hour in the cold, the astigmatism completely vanished, along with most of the residual chromatic aberration.
  • Images of star clusters, starfields and other deep-sky objects at low (16x-20x) and average (30x-50x) magnifications were of a very high quality. The field of view was comparable to R72 ED, albeit with more detail and resolution. Images of the Moon at medium magnifications (70x-100x) were exceptionally bright - I'd recommend using a lunar filter for that - and full of detail. Images of planetary discs and satellites at high magnifications (125x-160x) were not as impressive, due to the already mentioned problems with aberrations and long adaptation time.
  • Colors in the field of view were very natural; light scattering is minimal. I haven't noticed any glare from within the OTA.
  • The focusing is very smooth and precise, even during adaptation. Vibration fades much faster after slewing or focusing than in my SW 80ED Pro. During adaptation, the OTA is noticeably warmer than the icy-cold R72 ED - it might be the different paint.

Summary and recommendations

Levenhuk Ra 90mm ED Doublet is a strange beast indeed. On the one hand, a great combination of the aperture and the field of view makes this OTA a great instrument for observation of vast starfields (e.g., in Sagittarius or Cassiopeia), elongated nebulae (Veil Nebula, California Nebula, etc.) and comets. On the other hand, this isn't a good all-rounder, as it doesn't really deliver during planetary observations. On the other hand - I've got three hands... Well, let me just add to those hands that this OTA will make a great instrument for astrophotography, provided of course that you equip it with a decent coma corrector.

If you do decide to buy this OTA, I'd recommend stocking up on a 2" star diagonal, a couple of 2" eyepieces and UHC or O-III filters. These will make the best of its wide-field capabilities.