You can focus with just your camera, but as soon as you insert a 2x Barlow lens, you can no longer get a sharp image. The focuser hits its stop, and the image remains blurry. This isn’t a defect in your equipment. It’s the most counterintuitive optical effect of Barlow lenses: they shift the focal plane—and not in the way you might think.
This is a problem I ran into: I couldn't map the sky with my camera once I added the x2 Barlow lens, even though everything worked fine without it. Here's why, and how to fix it.
A Barlow lens is a diverging lens. When placed in the optical path, it spreads apart the light rays that converge at the focal point. As a result, the light converges farther away, farther behind the telescope. The focal plane shifts outward from the focuser.
To understand: Without a Barlow lens, your telescope’s focal plane is located at a specific position in the eyepiece holder. Your camera is placed there, and you focus it. When you insert a Barlow lens between the telescope and the camera, the Barlow lens «pushes» the focal plane backward. The light needs a greater distance to converge.
That’s where things get tricky. To compensate for this offset, the focuser must be moved inward toward the telescope. But on many telescopes, especially Newtonians and compact refractors, the focuser doesn’t have enough inward travel. It hits the stop before the shifted focal plane reaches the sensor.
The basic formula for a Barlow lens is simple. If FB is the focal length of the Barlow lens and d2 is the distance between the Barlow lens and the focal plane of the eyepiece or sensor, the magnification factor MB is:
MB = 1 + d² / FB
A typical 2x Barlow lens has a focal length of about 75 mm. To achieve 2x magnification, d2 = 75 mm; that is, the sensor is located 75 mm behind the Barlow lens. This is the position for which it was designed.
But there is another parameter: d1, the distance between the Barlow lens and the telescope's original focal plane. The relationship is:
d1 = d2 / MB
For a 2x Barlow lens (FB = 75 mm, d2 = 75 mm, MB = 2), we get d1 = 37.5 mm. The Barlow lens must be positioned 37.5 mm in front of the original focal plane. To compensate for this, the focuser must be retracted by 37.5 mm from its position when mapping without the Barlow lens.
On an f/4 or f/5 Newtonian, the eyepiece holder sometimes has less than 30 mm of inward travel. That’s enough for a single eyepiece, but not enough to make up the 37.5 mm required by the Barlow lens. The focuser hits a stop, and you can’t focus.
Here’s a typical scenario. You’re using an astronomical camera (ZWO, Player One, ToupTek) with a telescope. Without a Barlow lens, the image captures just fine. You add a 2x Barlow lens to magnify the image of a planet or the Moon. The focuser is extended as far as it will go, but the image remains blurry. What happened?
The Barlow lens has shifted the focal plane backward by approximately 37.5 mm. Your camera is now at the correct distance d2 from the Barlow lens (it is at the end of the tube), but the focuser does not have enough travel to compensate for the offset d1. The sensor is too far from the new focal plane.
There are three solutions, listed in order of preference.
Most Barlow lenses have a screw-off lens element. Instead of using the entire tube, unscrew the lens element and screw it directly onto the camera’s eyepiece holder (using the M42 thread or the 1.25″ thread). The result: the distance d2 decreases, so the magnification factor also decreases (to about 1.5x instead of 2x), but more importantly, the distance d1 decreases. The lens is closer to the original focal plane, and the focuser has less travel to make up.
This is the simplest and most economical solution. You lose a little magnification, but you gain the map.
Compact Barlow lenses («Shorty») have a shorter tube and a stronger lens (shorter focal length). A typical 2x “Shorty” Barlow is about 78 mm long, compared to 97 mm for a standard 2x TeleVue. The lens is located closer to the top of the tube, so d2 is shorter, and so is d1. The focuser has less travel to make up.
This is a compromise: you keep the nominal x2 magnification, but with less shift in the focal plane.
TeleVue Powermates are telecentric 4-element Barlow lenses. Unlike a conventional Barlow lens, a Powermate does not significantly shift the focal plane. The rays emerge parallel to the optical axis, which means that the focal position remains virtually the same with or without a Powermate. This is the fundamental difference between a Barlow lens and a Powermate.
Al Nagler, founder of TeleVue, explains: The classic negative Barlow lens shifts the exit pupil and the focal plane. The Powermate adds a positive doublet that redirects the rays back to their original path, as if the Powermate weren’t there. As a result, the focuser has virtually no additional travel to make up.
It's the premium solution, but it's also the cleanest. Powermates are available in 2x, 2.5x, 3x, 4x, and 5x.
TeleVue offers two product lines: the classic Barlow lenses (2-element, 2x and 3x) and the Powermate lenses (4-element, 2x to 5x).
The TeleVue 2x Barlow (1.25″) has a total length of 97 mm, with a 56 mm barrel. It is parfocal with TeleVue Plössl eyepieces shorter than 32 mm. It is a high-quality Barlow lens, featuring high-refractive-index lenses and a multi-layer anti-reflective coating. Terence Dickinson, in *Sky and Telescope* (July 1997), writes: «Technology has eliminated the old objections. A modern Barlow lens does not degrade your telescope’s optics.»
The TeleVue 3x Barlow (1.25″) is longer, with a total length of 127 mm. It is designed for short eyepieces and planetary imaging. It offers the same optical quality but is intended for high magnifications.
The Powermate 2x (1.25″) is more compact than the 2x Barlow lens and does not shift the focal plane. It’s the ideal choice if you have a focuser travel issue. The Powermate 2.5x (2″) is the large-format version for 2″ eyepieces and wide-field imaging. The Powermate 5x is designed for very high-magnification planetary imaging.
The technical difference is significant. A conventional Barlow lens with a single negative element causes the rays to diverge. The rays emerge in a fan-shaped pattern, which shifts the focal plane. A Powermate adds a positive doublet after the negative element. This doublet «straightens» the rays, bringing them back into alignment with the optical axis. The result: the focal position remains virtually unchanged.
In practical terms, if your camera maps without an accessory at a given position on the focuser, with a 2x Powermate it will map within a few millimeters of that same position. With a standard 2x Barlow lens, it will have to compensate for a 37.5 mm offset. This difference explains why the Powermate solves the backfocus problem caused by a standard Barlow lens.
Schmidt-Cassegrain (SCT) telescopes have a mirror-shift focuser. Focusing is achieved by moving the primary mirror, which provides a very large range of travel. On an SCT, backfocus issues with a Barlow lens are rare: the mirror can move over a wide range, and the focal plane follows. The 37.5-mm shift caused by a 2x Barlow lens is easily accommodated.
But on a refractor, a Newtonian, or a Maksutov, the focuser is mechanical and has limited travel. That's where the problem arises.
A Barlow lens shifts the focal plane backward by about 37.5 mm for a 2x magnification. If your focuser doesn’t have enough inward travel, you won’t be able to create the map. There are three solutions: unscrew the lens and screw it directly onto the camera (this reduces the magnification to about 1.5x), use a shorty Barlow lens (shorter tube, less backfocus), or switch to a TeleVue Powermate (no shift in the focal plane). TeleVue units are my favorites for optical quality, and Powermates are the cleanest solution to the backfocus problem.
Sources : TeleVue – Powermates ; Agena Astro – Barlow Lens Guide ; Agena Astro – Back Focus in Astronomy ; Cloudy Nights – Focus with Barlow.
See also on Deep Space Astronomy: Retrieving Raw Data from Your Smart Telescope – Seestar, DWARFLAB, and Vespera
Signature: CBurkhalter
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