Barlows

[The Warthog]

I am curious about the use of Barlows with different types of eps. Barlows seem to come in pretty much one flavour, a single or complex lens at the bottom of a tube. If I wanted to get an orthoscopic ep, say, which has a reputation for having a flat FOV, and also for having miserable eye relief, would the use of a Barlow change the flatness of that FOV? I know the Barlow allows you to retain the eye relief of the longer ep inserted in it, but I wonder if it changes any other characteristics than the focal length?

[FLO]

I saw this and thought of you

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.

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.

http://www.astunit.com/tutorials/barlow.html

[orion f6.3]

strange thing barlows! with my 80mm refractor my 3x fine, no probs buy my 2x to focus you have to wind fully in, great on houses buy wont go i far enough to focus on planet? Orion

[dfronteras]

Today just payed for a 2 x Barlow as the group known that my Saturn Photo was not that big. So can anyone tell me if the Barlow help me get a better Photo of the Moon,Saturn,Mars as I have never used one yet???.

[GazOC]

Today just payed for a 2 x Barlow as the group known that my Saturn Photo was not that big. So can anyone tell me if the Barlow help me get a better Photo of the Moon,Saturn,Mars as I have never used one yet???.

Its trickier to focus and to keep the object in the FOV but it'll double the size of the image captured.