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View from a Scope | November 20, 2017

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How a Barlow Lens Works?

How a Barlow Lens Works?

What is a Barlow Lens?

A Barlow is a negative (diverging) lens that is placed between the objective lens (or primary mirror — from now on these words will be used interchangeably) and the eyepiece of a telescope. It increases the effective focal length of an objective lens, thereby increasing the magnification. The idea is that 2 eyepieces and a Barlow will give you the flexibility of magnification of 4 eyepieces, and will give higher magnifications with less powerful eyepieces.

What are its Advantages and Disadvantages?

Assuming that the Barlow is a good one, the only disadvantage is a slight loss of light throughput — this is of the order of 3%. The advantages are numerous:

  • Higher magnifications can be attained with longer focal-length eyepieces than would be possible without the Barlow. Short focal length eyepieces necessarily have optical surfaces that are more curved and therefore are likely to introduce more aberrations.
  • A Barlow increases the effective focal ratio of the objective. This gives a more acute light cone, which is less demanding of eyepiece quality because:
    1. Rays at the periphery of the cone are closer to being paraxial and thus are less subject to aberration.
    2. A smaller area of the field lens is used.
  • Many eyepieces have an eye relief (distance of exit pupil from eye lens) that is directly related to its focal length. For example, the eye relief of a Plössl is 0.73 × its focal length. Thus, with these eyepieces, for a given magnification there will be greater eye relief with a barlow than without.
  • Many eyepiece types do not work well with short focal-ratio objectives. The Barlow effectively increases the focal ratio, allowing the eyepiece to work well.

How does a Barlow work?

Barlow

Barlow Amplification

The amplification factor of a Barlow is a function of its position in relation to the eyepiece and the objective lens (or primary mirror). For any given eyepiece and objective, the Barlow-eyepiece separation and the Barlow-objective separation are related because the focal plane of the eyepiece is the same as the focal plane of the objective-Barlow combination; as the separation between the eyepiece and the Barlow increases, the separation of the Barlow and objective decreases.
The amplification factor of a Barlow can be increased by increasing its separation from the eyepiece using an extension tube — it must simultaneously be brought closer to the objective.
One thing that you need to watch for with Barlows used outside their design amplification factor is spherical aberration. SA will be minimised at the design factor, but will almost certainly be present outside this, although it may not be discernible. (But visually, using the old trick of shifting the Barlow to the “other” side of the star diagonal or of using extension tubes, this may be compensated by reduced SA in the eyepiece, as a consequence of a more acute light cone.)

Eyepiece Choice

If you use a Barlow with fixed-focus eyepieces, you need to give some thought to a suitable choice. If, for example, you have a x2 Barlow and a 25mm eyepiece, there is little point in acquiring a 12.5mm; it will mimic the 25mm + Barlow. A suitable choice might be 32mm, 18mm, 12mm.

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