I tested a 3x Barlow SVBONY SV213 lens with a 1.25″ barrel. The manufacturer claims a three-element optical design and blackening of the lens edges. Thanks to the three-lens design, the image quality across the field should be better than that of simple two-lens Barlows. At the same time, the SVBONY SV213 is not telecentric:
– magnification can be increased using an additional extender tube
– increases the eyepiece eye relief
– works worse with eyepieces (especially with wide-angle ) than a telecentric lens like Svbony SV216 2x.

SPECS

Brand	SVBONY
Model	SV213
Barrel Size	1.25 inch
Magnification Factor	3X
Number of Lens Elements	3- Elements
Lens Coatings	Fully multicoated
Outside Length of Upper Body	2.30inch / 58.5mm
Insertion Depth	1.02inch /26mm
Max Diameter Excluding Thumbscrew	1.57inch / 40mm
Net Weight	3.14oz / 93g
Filter Threads	M28.5×0.6
Barrel Material	Aluminum Alloy
Number of Thumbscrew	2PCS
Thumbscrew Thread Specification	M4x8
Compression Band/Ring in Eyepiece Holder	Yes

GENERAL VIEW. WHAT IN THE BOX
The Barlow lens comes in a plastic case. Inside the bag is a Barlow lens, two screws and protective covers. Metal body, lightweight. The skirt is cylindrical, without grooves and cones. There is no T-thread on the outlet. The screws, when tightened, rest against a metal ring that holds the eyepiece or astronomical camera. There are no complaints about the quality of workmanship.

RESULTS OF MY MEASUREMENTS

Weight in can: 131 grams
Weight with caps and screws: 102.4 grams

Length without caps: 84.5 mm
Body length (without skirt) 58.5 mm
Skirt Diameter: 31.76mm
Skirt length: 26.2mm
Front thread diameter: 27.9mm (female)
Thread for unscrewing the skirt with lens block: 29.5 mm (male)
The front lens is recessed by 7 mm, the coating is dull bluish-blue, the glare is greenish inside
Front lens diameter: 21.93 mm
Light diameter of the lens component at the exit: 22.89 mm (highly convex shape)
Outlet case light diameter: 31.76 mm
Case diameter (without screws): 40.3 mm

The optics are clean and undamaged. Inside there are glare surfaces.

TEST
For visual testing of the SVBONY SV213 1.25″ 3x Barlow lens, I used a 150 mm Newton telescope with a parabolic mirror (f\5) on an azimuth mount, as well as two wide-angle eyepieces – Explore Scientific 24 mm 68 ° 1.25″ and Explore Scientific 11 mm 82 ° 1.25″. I also found a non-telecentric 2x lens from Sky-Watcher with a T-adapter for comparison. The ES 24mm eyepiece is known for fully exploiting the capabilities of the 1.25″ format while being wide-angle. The ES 11mm eyepiece works very poorly on the Moon with simple Barlow lenses due to the strongest bean vignetting and increased eye relief.

Visual observations of the moon
Weather conditions: partly cloudy, hazy.
1) Sky-Watcher 2x + ES 24mm. The eyepiece eye relief noticeably increased, vignetting (darkening at the edges of the field of view) appeared. Approximately half of the field of view with good quality, then the image becomes blurry.
2) SVBONY SV213 3x + ES 24mm. The eye relief has noticeably increased, the image quality across the field is good, but the field of view is clipped at the edges.
3) Sky-Watcher 2x + ES 11mm. Increased eye relief. If you bring the eye closer, something dark in the center of the field of view interferes with observation. At a distance of the eye from the eyepiece, observations are possible, but only about half of the field of view is visible.
4) SVBONY SV213 3x + ES 11mm. Eye relief increased. When trying to bring the eye closer and cover the entire field of view, annoying bean vignetting interferes. When the eye is moved away, observations are possible, but then the entire field of view does not fit. The image quality across the field is excellent.

For visual observations of the Moon with wide-angle eyepieces, especially those with strong spherical aberration in the exit pupil (bean vignetting), the SVBONY SV213 3x Barlow lens is not well suited. With 20mm to 10mm plossls, this lunar observations this Barlow lens should work flawlessly. For deep-sky objects, bean vignetting should not interfere, since the pupil will open wider in the dark, but specific combinations of eyepieces and Barlow lenses should be studied separately. If you need a Barlow lens specifically for visual observations, I recommend taking the SVBONY SV216 1.25″ 2x – it is telecentric, does not change eye relief and is guaranteed to work better with eyepieces, including long-focus wide-angle ones.

Lunar test with 150mm f/5 SLR telescope and Canon 550Da SLR camera

Measured magnification: 4.26x

The increased magnification was due to the fact that the Canon 550Da camera has a working length (the distance from the bayonet to the matrix) is 44 mm, plus a T-ring (11 mm), resulting in a kind of accelerating sleeve. Most likely, the calculated magnification is achieved during visual observations through the eyepiece, or when shooting with astrocameras in the 1.25″ form factor (for example, Svbony SV905C).

Lunar test with Meade APO quadruplet (D=70 mm, f=350 mm), astronomical camera ZWO 183MC

Measured magnification on the image of the Moon: 3.34x

SUMMARY
SVBONY SV213 1.25″ 3x –  good Barlow lens with a sharp image. Glare during visual observations of the Moon and astronomical shooting was not detected, but just in case, I would advise the manufacturer to improve the blackening of the inner side. Non-telecentric Barlow lens imposes certain restrictions on the use of wide-angle eyepieces due to the change in the inclination of the rays coming from the lens, the shift in eye relief and the potential clipping of the field of view. For astrophotographers, the problem may be that the lens block has a non-standard outlet thread (29.5 mm, male), so screwing the lens block into a dispersion corrector or a regular short T-adapter will not work. And this is a shame, since the difference is only 0.5 mm. I recommend that Svbony make the thread the same as on the filters, or at least make a thin adapter for the M28x0.6 thread. Of course, you can place the atmospheric dispersion corrector after the Barlow lens, but in this case, the magnification may turn out to be excessive (4x or more) due to overclocking. However, for owners of fast newtons (f\4-f\5) and cameras with a large pixel (2.9 – 3.75 microns), such “acceleration” will even be useful for planetary shooting. If you need a Barlow lens with a constant magnification, I recommend look at SVBONY SV216 1.25″ 2X Barlow lens

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