Hack46.Use a Barlow

Hack 46. Use a Barlow

Double your eyepiece collection on the cheap.

A Barlow lens (named for the 19th-century British mathematician and optician Peter Barlow) is one of the most useful accessories you can have in your eyepiece case. A Barlow fits between the telescope's focuser and the eyepiece, where it increases the magnification provided by the eyepiece [Hack #47]. A Barlow lens has negative focal length, which means it reduces the convergence of the light cone arriving from the primary mirror or objective lens, effectively increasing the focal length of the scope. Because the magnification provided by an eyepiece is directly proportional to the focal length of the scope it is used in, using a Barlow has the effect of increasing magnification.

Although it's possible to describe a Barlow by its negative focal lengthjust as an eyepiece is described by its positive focal lengthit's more convenient simply to describe it by its strength or amplification factor. Barlow amplification factors range from 1.5X to 5X or more. A 2X Barlow, for example, effectively doubles the magnification of any eyepiece it is used with. (So-called "zoom Barlows" with variable amplification factors exist, but most of these are of low quality.) Figure 4-4 shows a selection of high-quality Barlows. From left to right are a Tele Vue 3X, an Orion Ultrascopic 2X, and a Tele Vue Powermate 2.5X.

Figure 4-4. Tele Vue 3X, Orion Ultrascopic 2X, and Tele Vue Powermate 2.5X Barlows

Barlows are available in 1.25" models, which accept 1.25" eyepieces and fit 1.25" focusers, and in 2" models, which accept 1.25" and 2" eyepieces and fit 2" focusers. 1.25" models are far more popular than 2" models. This is true because most astronomers use Barlows with mid-power and high-power eyepieceswhich are nearly all 1.25" modelsto reach the high magnifications needed for Lunar and planetary observing, as well as observing small DSOs.

It is possible to use a Barlow with 2" eyepiecesfor example, a 27mm Tele Vue Panoptic used with a 2" 4X Powermate yields the equivalent of a 6.75mm Panopticbut few astronomers choose to do so. For various reasons, it is generally preferable to use a 1.25" Barlow with a shorter 1.25" eyepiece, not least because 1.25" eyepieces and Barlows can also be used with secondary scopes that have only 1.25" focusers.

Although image amplification is the most obvious characteristic of a Barlow, it is not the only effect, nor necessarily even the most important one. Here are other good reasons to use a Barlow:

Double your eyepiece selection

The most compelling reason to buy a Barlow is that it effectively doubles the number of eyepieces you have available. For example, if you have 32mm, 20mm, and 12mm eyepieces, using a 2X Barlow adds the equivalent of 16mm, 10mm, and 6mm eyepieces to your collection. Because a good Barlow costs no more than a mid-range eyepiece, this is an extremely efficient way of adding to your eyepiece collection.

Reach high magnifications with short focal length scopes

Short focal length scopes, such as short-tube refractors and many Dobs, require very short focal length eyepieces to reach high magnifications. For example, you need a 2.5mm eyepiece to reach 160X with an 80mm f/5 [Hack #9] short-tube refractor or a 3mm eyepiece to reach 400X with an 8" f/6 Dob. Inexpensive eyepieces in these focal lengths are nearly unusable because of their tiny eye lenses and nonexistent eye relief. Using a Barlow with a longer focal length eyepiece allows you to reach high magnifications more conveniently.

Increase the focal ratio of the scope

By increasing the focal length of a telescope, a Barlow also increases its focal ratio. For example, using a 2X Barlow with a 250mm f/5 telescope (focal length 1,250mm) effectively converts that scope to a 250mm f/10 scope (focal length 2,500mm). If you are imaging, this increase in focal ratio increases exposure times by a factor of four, albeit at a larger image scale.

More important to visual observers is the effect of focal ratio on eyepieces. The obtuse converging light cone from a fast scope (such as an f/5 model) is very difficult for eyepieces to handle without showing obvious aberrations, particularly near the edge of the field with wide-field eyepieces. The acute converging light cone from a slower scope is much easier on eyepieces. Only expensive premium eyepieces handle the light cone from f/6 or faster scopes well, but even inexpensive eyepieces handle an f/8 or slower light cone well.

So, for example, to view at 125X with the 250mm f/5 scope, you need a 10mm eyepiece or its Barlowed equivalent. If you want a 10mm wide-field eyepiece that you can use natively with good edge performance in an f/5 scope, you're in premium eyepiece territorya $240 Tele Vue Radian or a $310 Pentax XW. If, instead, you use a 2X Barlow, you can mate it to a less expensive 20mm wide-field eyepiece and still get excellent image quality.

Preserve eye relief

Other than premium eyepieces, many of which feature fixed 20mm eye relief regardless of their focal lengths [Hack #49], most eyepiece designs have eye relief that is some fraction of the focal length. Plössls, for example, typically have eye relief of at most 60% to 70% of their focal lengths, and even that may be reduced by the physical design of the eyepiece. A 32mm Plössl might have eye relief of 20mm, a 20mm Plössl 12mm, and a 11mm Plössl only 6mm.

Most observers find an eyepiece with eye relief shorter than 12mm uncomfortable to use. (Those who wear eyeglasses require 20mm or so.) But a Barlowed eyepiece retains its original eye relief despite its effectively shorter focal length and higher power. So, for example, rather than use an 11mm Plössl natively with its short 6mm eye relief, you can use a 3X Barlow to effectively convert a 32mm Plössl to an 11mm Plössl while maintaining the 20mm eye relief of the 32mm Plössl.

Using a Barlow actually extends the eye relief of most eyepiece designs, including Plössls. The amount of extension varies with eyepiece design and the type and power of the Barlow. Unfortunately, the amount of extension varies proportionately to the focal length, which is the exact opposite of what we'd prefer.

Short focal length eyepieces, which need more eye relief, show little increase. Long focal length eyepieces, which already have lots of eye relief, gain lots of eye relief when used with a Barlow. In fact, Barlowing a long focal length eyepiece can make the eye relief uncomfortably long. It becomes difficult to find and hold the exit pupil of the eyepiece, which causes blackouts.

Improve filter performance

Interference filters, such as narrowband and line filters [Hack #59] work best at longer focal ratios, where the acute light cone from the primary mirror or objective strikes the filter nearly perpendicularly. The obtuse light cone from a fast scope, such as an f/5 model, strikes the interference layers of the filter at a more oblique angle, which effectively increases the thickness of the layers, degrading filter performance and shifting its transmission curve slightly. Line filters in particular work better with slower focal ratios, and the easy way to accommodate their needs in a fast scope is to use a Barlow between the scope and the filter.

Broadly speaking, there are two types of Barlows:

  • Standard Barlows, such as the Orion Ultrascopic 2X and Tele Vue 3X models shown in Figure 4-4, are 5" to 6" long, and are used primarily in Newtonian reflectors, including Dobs. You can use a standard Barlow in a refractor, SCT, or other scope that uses a diagonal, either by inserting it between the telescope and diagonal or by (carefully) inserting it between the diagonal and the eyepiece. (The danger is that the long Barlow may protrude too far into the diagonal, damaging the mirror.)

  • Short Barlows are about half the length of standard Barlows and may be used in any type of scope, including Newtonian reflectors with very lowprofile focusers, in which a standard Barlow may protrude into the light path.

Tele Vue makes a series of Barlow-like devices called Power-mates. In effect, a Powermate is a standard 2-element Barlow with a second doublet lens added to minimize vignetting (darkening the edge of the image) and excessive extension of eye relief when used with long focal length eyepieces. Power-mates are available in 1.25" 2.5X and 5X models ($190) and 2" 2X and 4X models ($295). We don't doubt that Power-mates are excellent products, but we've never been able to tell any difference in image quality between a Powermate and a high-quality standard Barlow.

Standard and short Barlows of comparable quality sell for similar prices, but there are significant optical differences. The shorter tube of a short Barlow means it must use a stronger negative lens to achieve the same level of amplification as a longer Barlow. That has three disadvantages:

  • Inferior image quality. Although the best short Barlows are very good indeed, the laws of physics dictate that they must be inferior optically to a full-length Barlow that uses lenses of similar quality. This inferiority most commonly manifests as lateral color (fringing), particularly near the edge of the field.

  • Vignetting. Because a short Barlow must bend light much more sharply than a full-length Barlow, short Barlows are subject to vignetting, particularly when used with longer focal length eyepieces.

  • Excessive eye relief. Although any Barlow extends the eye relief of most eyepiece designs, the stronger negative lens of a short Barlow exaggerates this effect. For example, we have no problem using our Orion Ultrascopic 30mm eyepiece with either our Orion Ultrascopic 2X Barlow or our Tele Vue 3X Barlow. But the Ultrascopic 30mm used with a short Barlow has its eye relief extended so far that we have trouble holding the exit pupil.

As you might have guessed, we're not fans of short Barlows. In fact, we don't own one. We use only full-length Barlows in our scopes, including our refractor. We freely confess, though, that many very experienced observers use and recommend short Barlows such as the Orion Shorty Plus Barlow and the Celestron Ultima Barlow (which are identical except for the brand name).

There are many cheap Barlows available, but we suggest you avoid them. A Barlow is a lifetime investment, and the difference in price between a mediocre model and an excellent one is not great. For a full-length Barlow, we recommend the $85 Orion Ultrascopic 2X modelwhich is often on sale for $75and the $105 Tele Vue 3X model. (Tele Vue also makes a superb 2X Barlow, but it sells for $20 or $30 more than the Orion Ultrascopic, and we can discern no difference in image quality between them.) If you must have a short Barlow, get the $80 Celestron Ultima or the identical $70 Orion Shorty Plus, which is often on sale for $60.

Ignore the marketing hype. It doesn't matter if a Barlow has two or three elements or is described as "apochromatic" (which is marketing-speak for a 3-element Barlow). What matters is the figure and polish level of the lenses and their coatings and the mechanical quality of the Barlow. There are superb 2-element Barlows, including both Tele Vue models, and very poor 3-element Barlows.

Choose the amplification factor of your Barlow with your current eyepiece collection in mind, as well as any plans you have for expanding it. Avoid duplication between Barlowed focal lengths and native focal lengths. For example, if you have 32mm and 16mm eyepieces, using a 2X Barlow with your 32mm eyepiece effectively duplicates your 16mm eyepiece. Using a 3X Barlow instead provides the equivalent of 10.7mm and 5.3mm eyepieces, both of which are useful extensions to your arsenal. Conversely, if you have the 25mm and 10mm eyepieces commonly bundled with inexpensive scopes, a 2X Barlow adds the equivalent of 12.5mm and 5mm eyepieces, again a useful expansion of your selection.

Don't hesitate to buy more than one Barlow. Just as having one Barlow can effectively double your eyepiece collection, having two Barlows can effectively triple it, if you choose your native eyepiece focal lengths carefully to avoid overlap. We carry 2X and 3X Barlows in our eyepiece case, so we're always prepared for any eventuality.

If you have two Barlows, you can stack them to achieve "stupid high powers" on those extraordinary nights when the atmosphere is stable enough to support them. It doesn't happen often, but when it does, stacking two Barlows has allowed us to view Jupiter at 800X and Luna at 1,200X.