reports-barlow

Good to know about Barlow

A Barlow lens is an optical device constituted between 2 and 4 element-lenses used to diverge the incident light beam in order to increase the focal length of the primary objective. The magnification power of such a Barlow is usually between 1.5 and 4 times.

In this way your f/10 scope may become a f/40 (+15%) at its prime focus. But this combo presents other advantages too.

A barlow diverges the light beam to increase the focal lenght of the primary objective. The amplification factor is the ratio between the distance from the lens to the focal plane and the focal length of the barlow. We shall explain below how to calculate this value.

Aberrations are reduced with short focal eyepieces

For high-end wide fields EP this improvement is not always valid but using Orthos or Plossls, aberrations will be reduced; the edge field will be sharper. Of course with both short and long focals EP the combo will get a longer eye-relief too.

Another way to enhance Plossls EP is to design them much thicker. This will avoid vignetting with Barlows too but the bad point is a decreasing in eye-relief, not really welcome !

Barlow on fast Newtonians : better sharpness too

Idem using the Tele-Vue coma corrector, among other brands, which is a Barlow/corrector for Newtonians called Paracorr. Contrary to the SCT Reducer/Corrector, this device increases the scope magnification of 15% (a f/4 becomes an f/4.6) BUT the positive point is this Paracorr in fact increases the sharpness of the EP, even using a Nagler or a Panoptic !

Tele-Vue 2x Barlow.

Nothing better corrected than Radians ?

Currently probably nothing. According the optical designer Al Nagler himself, the optical quality of Tele-Vue Radians can probably not be reached by any EP/Barlow combination, dealing with contrast, sharpness, eye-relief or even convenience.

But nobody states the TV brand cannot be approachd by Zeiss optics, Lichtenknecker Optics, and other manufacturers. Don’t say what I don’t write !

The eye-relief is increased (sometimes)

As a Barlow diverges the light beam, your image, in fact the exit pupil, will be projected further out, which means the EP focal length will also be through out according its focal (long focal EP, greater throw out of exit pupil). This is the only known drawback of a barlow.

NB.Tele-Vue Panoptics are "insensible" to this effect because they include a Barlow by design. Moreover, the new Tele-Vue Barlow’s called "Powermate" (2.5x, 5x, etc) prevent this eye-relief effect.

Eyepieces are not all tolerants to Barlow : vignetting

In the Tele-Vue series of EP, Radians are no as tolerant to diverging light beam as Nagler’s. For all these "intolerant" EP, TV developed the Powermate, a special concept which recollimates the diverging light beam. Otherwise the effect is well-known, this is vignetting. The Powermate is a telecentric optics and yields sharper images than a Barlow.

Barlow apochromatic ?

Do you call an eyepiece "Apo" ? Never heard, never read. But the designers will. Idem for Barlows. There is no reason to call a 2 elements Barlow "APO" because they musn't introduce colors; if it does do not buy it ! So if you find a 3 elements Barlow somewhere using the prefix "APO", ask you why and ask to read its specifications…

Tele Vue Powemates

How to calculate the focal length of a barlow ?

Various formulae can be used to calculate the barlow focal length and magnification. The best sources are ATM and ASTUNIT websites that provide several articles on this subject.

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