Barlow lenses, their magnification factors and working distances

 

Again and again the question arises whether a barlow provides the correct magnification at all. It is often overlooked that the magnification factor always depends on the correct working distance - you can read more about this in this PDF using the VIP Barlow as an example: Different Magnifications with the VIP-Barlow # 2406101

To settle this question once and for all, we took a few comparative shots on an ED80/600 telescope with a monochrome camera. Due to the seeing on a hot summer at an outside temperature of  30 °C, the shots of a neighbouring roof were somewhat unsteady, so that the sharpness of the individual frames is only of limited use for judging the quality of the Barlow lenses, but here we are only concerned with the scale of the image.

On this occasion, the focus shift was also measured: A barlow moves the focus point and is often used to come into focus when there is not enough back focus. To do this, the actual barlow lens element is placed deeper in the focuser and the focus moves outwards.

The reference shot: Fokal, without an additional barlow lens

At first, a shot without an additional barlow lens:


The sensor of the test camera is inside of the camera’s body. Using a caliper, it was placed at the top of the 1.25" eyepiece clamp (i.e. where the field stop of 1.25" eyepieces is usually located), behind a T-2 prism diagonal.

In Photoshop, the distance between two conspicuous spots on the roof tiles was measured: About 416 pixels without a barlow lens.

For a sharp image, the focuser had to be extended 7.7 cm.

All values are approximate values and within the limits of measurement accuracy.

 

The Baader Q-Barlow 1.3x/ 2.25xBaader Q-Barlow 1.3x/ 2.25x Baader Q-Barlow 1.3x/ 2.25x (#2956185, € 55,-)

The Q-Barlow lens has a special feature: It is designed to deliver its 2.25x magnification in combination with the Baader Classic Plössl/Ortho eyepieces, whose field stop is 2 mm deeper in the nose piece than usual. Therefore, the camera also had to be inserted 2 mm deeper into the eyepiece clamp than usual.

On the test image, the two reference points on the roof are about 917 pixels apart. This corresponds to a magnification of 2.2x - and is close enough to the theoretically expected distance of 936 pixel and thus within the range of the measuring accuracy (limited by no exact stop/index mark for the position of the camera sensor, identifying the reference points in the image etc).

As is well known, a Barlow lens shifts the focal point and thus saves backfocus; with the Q-Barlow, the focuser still had to be extended 7.3 cm - just 4 mm less than without the Q-Barlow.

The Hyperion Zoom 2.25x Barlow lensHyperion Zoom 2.25x Barlow lens Hyperion Zoom 2.25x Barlow lens (#2956180, € 133,-)

The Hyperion Barlow is also a special design: it was originally developed especially for the Baader Hyperion Universal Zoom Mark IV, 8-24mm eyepiece (1¼" / 2") (#2454826 , € 275,-) and can be screwed directly into the filter thread of this eyepiece with the 1.25" adapter "A" (and into any other filter thread of an eyepiece – the exact magnification factor then again depends on the length of the eyepiece sleeve and the distance to the field stop. As is so often the case in astronomy, there are no standards here either).


With T-2 extensions and a T-2 eyepiece clamp, the camera was positioned at the same 55mm distance that would also be used with a DSLR and a T-2 adapter. The result: the two reference points are about 1133 pixels apart, which corresponds to a magnification of about 2.7x - considerably more than the 2.25x specified.

The new focus was at 6.6 cm, i.e. 1.1 cm further inwards compared to the focus without the Barlow lens.

Since the Hyperion Barlow is designed to deliver a factor of 2.25x when used with a Hyperion 8-24mm zoom eyepiece (where it is mounted directly in front of the first lens group) and the magnification factor increases with the distance, the higher magnification due to the long working distance of a DSLR is no surprise.

To determine the magnification factor at different distances, more test images were taken a little later. This resulted in the following factors for a distance from the beginning of the T-2 thread of the B-adapter to the camera sensor within the measurement accuracy:

  • 25 mm distance: 2,3x
  • 36 mm distance: 2,5x
  • 55 mm distance: 2,7x

The 36 mm distance is particularly interesting, as it corresponds to the flange focal length of a Micro Four Thirds camera with the Baader T-Ring Micro Four Thirds (m4/3) to T-2 + 19mm expansion (#2408330 , € 31,-) (without the 19 mm extension supplied).

Just under 25 mm thus also roughly corresponds to the focal length of the Barlow.

 

Without Barlow

Without Barlow

with a working distance of 25mm.

with a working distance of 25mm,


36mm working distance (MFT),

55mm working distance (T-2-Standard).


The VIP 2x modular barlow lens, visual and photographicVIP 2x modular barlow lens, visual and photographic VIP 2x modular barlow lens, visual and photographic (#2406101, € 228,-)

Finally, we tested the VIP barlow. It is a "classic" barlow that should deliver 2x on all eyepieces where the field stop is at the transition to the wider part of the eyepiece. By removing the T-2 extensions, it can also be adjusted for cameras with a different back focus, such as a DSLR. For this test, the camera sensor was placed as close as possible to the top of the eyepiece clamp.

The result was as expected: At 853 pixels, the extension corresponds to 2.05x, which is more than accurate within the measurement accuracy. The new focus point was 4.4 cm, so an impressive 3.3cm of path length was saved.


The Baader Barlow lenses at a glance

We have not compared two other Barlow lenses: Both the FFC and the Zeiss Abbe Barlow lens are already well documented and have been in use for years, so no surprises are to be expected here.

With currently five different barlow lenses in Baader Planetarium's product range, it is not easy to keep track of which model fulfils which function best.

90mm field of view also for medium format cameras - uncompromising quality even for extreme enlargements.

Focus gain: depending on configuration about 1.5-2.6cm

Carl Zeiss 1¼" Abbe Barlow lens 2x (#1603321 , € 477,-)

Legendary Zeiss quality for the common magnification range between 2x and 3x (with additional T-2 extensions). With T-2 connection for cameras or eyepiece clamps. The qualitatively equivalent solution to the Fluorite FFC when smaller field size and smaller post-magnification are desired.

Focus gain: About 3 cm

Versatile barlow lens for magnifications from 2x, for use with cameras (via T-2 thread) and eyepieces. Calculated for 35mm image diagonal (full format). With additional T-2 extensions it can also be used for slightly higher magnification factors.

Focus gain at 2x: Approx. 3.3 cm

Baader Q-Barlow 1.3x/ 2.25x (#2956185 , € 55,-)

Inexpensive and amazingly high quality Barlow lens for the lower magnification range: In addition to being used as a normal Barlow lens with eyepiece clamp (with factor 2.25x), the Barlow element can also be used as a negative lens group when screwed directly into (almost) all eyepiece sleeves or directly in front of a camera with 1.25" filter thread and then provides a lower magnification of approximately 1.3x. Ideal for adapting a modern planetary camera to the resolution of an already long focal length telescope. Prerequisite: There must be no other elements in the nose piece that would prevent the approximeately 13 mm long Q-Barlow lens group from being screwed in.

Focus gain at 2.25x: About 0.4 cm

Hyperion Zoom 2.25x Barlow lens (#2956180 , € 133,-)

Specially designed for the Hyperion Zoom eyepiece, also fits the filter thread of other eyepieces.

With the included T-2 adapter, it can also be used directly on cameras; provides about 2.7x at 55mm working distance and about 2.3x at 25mm.

Focus gain: About 1.1 cm


 

Excursus: Telecentric System or Barlow Lens?

A Barlow lens is the best-known way to change the focal length and thus the focal ratio of a telescope. A telecentric system is similar to a barlow, but also contains an additional positive lens element. This makes it possible to change the focal length of the telescope and to achieve a parallel beam of light. For most applications, therefore, a barlow lens is fine; but especially in connection with narrow-band interference filters (as for solar observation in H-alpha), the more complex construction of a telecentric system is absolutely necessary. However, both systems have their own advantages.

Read more about this in our blog: The benefits of Telecentric Systems


About the author: Alexander Kerste

Alexander Kerste

Alex is a studied biologist and works as a freelancer as an author, consultant and translator. After his studies and the publication of the Kosmos Starchart-Set in 2004, he was a regular freelancer for Astronomie Heute and the yearbook Der Himmel for the Spektrum-Verlag in Heidelberg. He is in charge of the Beginner courses on www.Astronomie.de and is a voluntary active member in the Robert-Mayer-Observatory since 1993. Since then, he has published a number of books on Celestron-Telescopes as well as Digiscoping and Astrophotography. One of his books on Astronomy with binoculars is also freely available at freebook.fernglasastronomie.de. In addition he supervises the Northern lights and star tours from Hurtigrute – these were also published in a travel guide, further articles can also be found on his blog kerste.de.


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