How to choose a Barlow lens?

A Barlow lens is an optical accessory that increases the focal length of a telescope. The Barlow lens is mounted on a focuser, visual back, or diagonal mirror. It can be used for both visual observations and astrophotography. The main photographic application is photographing planets, the Moon, the Sun, double stars, and in some cases, photographing small planetary nebulae.
A Barlow lens looks like a tube with glass in the front. It consists of a lens cell (lens in a metal frame), as well as a housing with a clamping screw.

Разобранная Барлоу: Линзоблок + юбка + корпус
Disassembled Barlow: lens cell + skirt + body

Basic parameters of Barlow lenses

Magnification is a multiplier of the focal length of the telescope. Indicated on the case or in the specifications. For example, 2x. There are also Barlow lenses with variable magnification (for example, NPZ PAG 3-5x)

Легендарная линза Барлоу НПЗ PAG 3-5x
Legendary Barlow Lens NPZ PAG 3-5x

The mounting diameter of the metal skirt is 1.25 or 2 inches. For most purposes, a 1.25-inch fit is sufficient.

The number of lenses is usually from 1 to 4. The simplest and cheapest Barlows are single-lens, they are often included with the telescope and noticeably worsen the image. A much better picture is provided by two-lens Barlows (achromatic), usually a gluing of two lenses (but looks like one thick lens with a groove). There are also three- and four-lens Barlows.

Открученный линзоблок
Unscrewed lens cell of a three-element Barlow lens SVBONY SV213 1.25″ 3x

Telecentricity is the ability of a Barlow lens not to change the inclination of off-axis rays. This is good for visual observations, especially with wide-angle eyepieces, and this Barlow does not change the eyepiece’s eye relief. There are two types of Barlow lenses – telecentric and non-telecentric. The telecentric Barlow cannot be “overclocked” — increasing the magnification using an additional extension tube between the Barlow and the eyepiece. A possible sign of Barlow’s telecentricity is an additional lens inside the body closer to the eyepiece. Also, if you look into the telecentric Barlow from the front, it will work like a small spyglass, while you won’t be able to see anything through the regular non-telecentric Barlow, since the image will be very blurry.

Ход лучей в обычной линзе Барлоу (справа)
Ray path in a conventional Barlow lens (right)
Ход лучей в телецентричной линзе Барлоу
Ray path in a telecentric Barlow lens
Разобранная телецентричная линза SVBONY SV216 1.25″ 2X.
Disassembled telecentric lens SVBONY SV216 1.25″ 2X.SVBONY SV216 1.25″ 2X.

Length – there are “short” Barlows and “long” ones. A short Barlow lens works better on the axis, a long one works better along the field.

Coating is a colored coating on lenses that reduces glare and increases light transmission. May not be available in cheap Barlows.

Передняя линза Svbony SV216
Front lens Svbony SV216

Built-in T-adapter – M42x0.75 thread on the body for installing a T-ring, astro camera, etc. Useful thing.

ED glass. Low dispersion glass designed to reduce distortion introduced by lenses. Most likely, there is no ED glass in Barlow with this marking, it’s just an inscription. And even if it were there, it would not reduce the chromatism of the lens telescope. For the calculated operation of most Barlow lenses, simple glasses without ED are sufficient.

Does a Barlow lens degrade the image?
Technically, yes – the Barlow lens introduces distortions both on the axis and across the field. However, from my own observation and shooting experience, I can say that a good short or telecentric Barlow lens does not degrade the on-axis image. But a long Barlow lens coupled with a fast telescope (f/4) can easily reduce sharpness in the center.
In addition, a simple single-lens Barlow, which usually comes with a telescope, noticeably degrades the image.

Please note that with a Barlow lens, the focal length of the telescope increases, and when using the same eyepiece, the resulting magnification increases. In this case, the image becomes darker as the exit pupil decreases. This is normal, but some people feel that the Barlow lens degrades the image this way. However, if you take 2x Barlow and a 10 mm eyepiece, as well as a separate 5 mm eyepiece, then the brightness of the image with a combination of 2x Barlow + 10 mm versus a separate 5 mm eyepiece will be approximately the SAME.

The barrel of the Barlow lens can also reflect from the inside, introducing light halos and stray reflections when shooting the Moon. In this case, either trying to use a Barlow lens from another manufacturer will help, or refusing to shoot with a Barlow lens and switching to an astronomical camera with a small pixel (1.45-2.4 microns, depending on the required scale). In addition, some completely “black” Barlows from the inside may reflect in the near-infrared due to the nature of the paint.

Is it better to buy a Barlow or a separate eyepiece?
It depends on what telescope and what eyepieces you already have.
With a Barlow lens, the number of eyepieces actually doubles. However, not all Barlow lenses work well with eyepieces. For visual observations, it is better to use a telecentric Barlow of low magnification (2x or 3x). I gradually switched to observing ONLY through eyepieces, but sometimes I use the Svbony telecentric Barlow lens for visual observations with a wide-angle eyepiece.

Which Barlow lens to choose for lunar-planetary astrophotography?
When photographing the Moon and planets in conjunction with a Barlow lens, I use an additional accessory – the ZWO ADC atmospheric dispersion corrector. It is a metal tube with two glass wedges inside, which can be rotated relative to each other to compensate for atmospheric dispersion. It is advisable to place the corrector between the telescope and the Barlow lens so that the wedges introduce less distortion into the image. However, if the Barlow lens is non-telecentric, then the corrector also works as an extension tube, adding magnification, and this is not always necessary. But this corrector has a filter thread in the front part, and there are Barlow lenses whose lens cell has the same thread (Sky-Watcher 2x with T-adapter, NPZ 2x, NPZ PAG 3-5x). Therefore, I simply unscrew the lens cell from the Barlow, screw it into the dispersion corrector and get two devices in one. In this case, the Barlow multiplicity almost does not change, since the length of the corrector and Barlow bodies is approximately the same. My personal recommendation is to buy a Barlow lens whose lens cell can be screwed into the dispersion corrector. Or just buy a good lens cell with the required magnification. With the dispersion corrector, I use lens cell from Barlow lenses Sky-Watcher 2x with T-adapter, NPZ 2x and NPZ PAG 3-5x.
If there is no dispersion corrector or it is not used (for example, when shooting in the red or infrared region of the spectrum), then you can use any Barlow lens of the required magnification.

Линзоблок + корректор дисперсии + IR-cut фильтр + Т-адаптер + астрокамера
Lens cell + atmospheric dispersion corrector + IR-cut filter + T-adapter + color astronomical camera
Lens cell + atmospheric dispersion corrector + T-adapter + T-ring + Canon 60D DSLR camera
Lens cell + atmospheric dispersion corrector + T-adapter + T-ring + Canon 60D DSLR camera

In addition, to photograph some planets (Mars, Venus), or with small telescopes, you can use a higher Barlow magnification. If Barlow is non-telecentric, then 1x can be easily added to the magnification using an extension tube. Don’t be afraid to experiment, try shooting at different magnifications and then compare the results. You can look at my photographs of planets on this site (Main menu > Astrophotos > Planets), I make sure to indicate all the used equipment.

What magnification of the Barlow lens is needed when shooting?
This depends on several factors – the focal ration of the telescope, the pixel size of the camera and the subject being photographed.
For photographing planets, my magnification formula is (6.66*pixel in microns)/(denominator of focal ratio).
For example, to photograph Jupiter through a Celestron C8 telescope with a relative aperture of 1:10 and a ZWO 183MC camera with a 2.4 micron pixel, you will need a Barlow with a magnification of (6.66*2.4)/10=1.59. The smaller the pixel and the lower the aperture of the telescope, the lower the Barlow lens magnification required. There are cameras with very small pixels (for example, QHY5III715C, 1.45 micron pixel), for which a Barlow lens may not be needed.

In general, the first number should be 5 (instead of 6.66), but I prefer to “expand” the scale a little in order to get the finest and smallest details in the processing. For the Moon, a value of 5 or 4.16 works well. Ideally, there should be 2-4 pixels of the matrix per unit of resolution of the telescope, then it will be possible to fully realize the capabilities of the telescope.

In addition, when photographing the Moon and the Sun, it is not necessary to use Barlow if, for example, it is necessary to photograph the entire disk or as much of it as possible.

Care
The Barlow lens is a very unpretentious accessory and does not require special care, but you should not drop it or wipe the lenses after each use. After use, the housing must be closed with protective covers. Avoid fogging the lenses and body. If the optical surface becomes dirty, you can wipe the outer part of the lens with a clean microfiber cloth (without fanaticism). If the Barlow lens is telecentric, I DO NOT RECOMMEND disassembling it, as you can easily catch dust on the inner surface of the lenses.

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