A barlow lens is an additional lens system that when connected to the eyepiece will serve to increase the magnification of the telescopic system. The barlow can be used in both observational astronomy and astrophotography. A Barlow lens is essentially just a negative lens. The effect of a negative lens is to disperse light. By dispersing the converging beam from a telescope, a barlow increases the distance to the focus point, thereby increasing the effective focal length. 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. A well made barlow can even compensate for eyepiece aberrations and also improve eyepieces that have less comfortable, short eye relief. This allows eyeglass wearers to see a full field of view at high power without having to remove their glasses to get close to the eyepiece, as they would have to with a shorter focal length eyepiece alone.
The barlow effectively extends the magnification of your eyepiece collection. As an example, a 20 mm eyepiece giving a 50x magnification in a telescope system would give a magnification of 100x using a 2x barlow. The essential result of using a barlow lens is to increase the focal length of the telescope. As an example if the focal length of the telescope is 1000 mm, using a 2x barlow would effectively change the system to a focal length of 2,000 mm. The focal ratio is also essentially doubled so that a telescope with a focal ratio of F5 will now function as an F10. It will thus become a “slower” scope for imaging. The result will be a reduction of incoming light so that for the purposes of imaging it will take 4x longer for exposures. The field of view will be reduced by a factor of 2 depending on the lens system. A barlow can improve the eye relief of certain less comfortable eyepieces like the standard Plossls or Orthoscopics.
There are additional benefits of using a barlow besides those mentioned above:
Another use for a Barlow is in negative projection astrophotography. In this configuration, you use a Barlow in the telescope, but without an eyepiece and without a lens in the camera. This magnifies the image beyond what you would get in prime focus mode, but not as much as with eyepiece projection photography.
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. By increasing the effective focal length of a fast to medium focal ratio telescope, the diverging lens optics of a Barlow give inexpensive or moderately priced eyepieces a slower-converging and easier-to-handle light cone to deal with. This results in lower astigmatism and better color correction at the edge of the eyepiece field.
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 the periphery of the cone are closer to being paraxial and thus are less subject to aberration and a smaller area of the field lens is used.
A Barlow can similarly improve the edge sharpness of good quality wide angle eyepieces used with fast focal ratio telescopes. It should be considered in lieu of higher power eyepieces alone for star cluster and planetary nebula viewing with such a scope.
Magnifications specified for Barlows only hold true if the eyepiece is mounted directly in the Barlow and the Barlow is used with a reflector, with a refractor used without a star diagonal, or mounted in the star diagonal of a catadioptric or refractor scope. Generally, if the Barlow is installed between the visual back (or focuser drawtube) and the star diagonal of a refractor or catadioptric (in other words in front of the diagonal), the projection distance from the Barlow’s lens to the eyepiece will be increased and the resulting magnification can be as much as 50% higher than that specified on the Barlow. However, some Barlows are inconvenient to use in a star diagonal because of their length, or are physically incompatible, and are so noted in the individual Barlow descriptions.
Barlow amplification factors range from 1.5X to 5X or more. 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.When choosing a barlow make sure that the resultant magnification will be appropriate for your telescope. A rule of thumb would be that useful magnification is 50 or 60 times the aperature in inches of the telescope. Rarely does the atmospheric conditions allow magnifications greater than 300x.
Not all barlows work well with all eyepieces. Choose carefully. When buying a barlow, make sure that the barlow exit pupil is less than that of the eyepiece or vignetting may occur. When using a barlow with long focal length eyepieces, the eye relief can be made to increase too much that vignetting can occur. This case will be worsened when using shorter barlows. Besides matching the eye relief with the eyepiece, a good quality barlow should be chosen so as not to adversely contribute to the degradation of the optical system by the added lenses.
When selecting, it is critical that you choose one with a barrel size that will fit the eyepieces you have. The barrel size is the diameter of the eyepiece tube that fits into the focuser. Barlows come in .965″, 11/4″ and 2″ formats, and there are two types of standard 11/4″ format Barlow lenses – the full-length (long) type and those commonly referred to as “shorty” Barlows. Most full-length Barlows (such as the Orion Ultrascopic) can only be used in a Newtonian telescope because they extend deep into the focuser tube. In a refractor or SCT they could hit the mirror or prism in the star diagonal. Because of their shorter focal lengths, the short versions (e.g.: the Orion Shorty Plus) are more sensitive to the placement of the eyepiece, and will deliver a variety of magnifications with different eyepieces. A full-length version tends to give the stated magnification factor more consistently.
Note that Barlows that are designed for long focal ratio telescopes may vignette (cut off) the edge of the image when used with low power wide angle eyepieces on a fast focal ratio scope. Also, an eyepiece and Barlow combination will have a slightly (2%-4%) dimmer image and slightly lower contrast than the short eye relief/short focal length eyepiece alone that is needed to get the same power as the eyepiece/Barlow combination.
Certain barlows can be stacked one atop another or with hollow tube extenders which serve to increase the magnification. The amount of magnification is one more than the distance between the Barlow lens and the eyepiece lens, when the distance is measured in units of the focal length of the Barlow lens. When calculating the resultant magnification to stacking consider the following: A standard Barlow lens is housed in a tube that is one Barlow focal-length long, so that a focusing lens inserted into the end of the tube will be separated from the Barlow lens at the other end by one Barlow focal-length, and hence produce a 2x magnification over and above what the eyepiece would have produced alone. If the length of a standard 2x Barlow lens’ tube is doubled, then the lenses are separated by 2 Barlow focal lengths and it becomes a 3x Barlow. Similarly, if the tube length is tripled, then the lenses are separated by 3 Barlow focal lengths and it becomes a 4x Barlow, and so on.
By placing a hollow extension tube between the Barlow lens and the eyepiece, you will increase the magnification of a telescope by two three or more times, depending on the size of the extension tube. The idea is that as you increase the distance between the Barlow lens and the eyepiece, you reduce the eyepiece´s focal length, thus increasing the magnification of the telescope.
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.)
In general, unless you decide to invest in a high quality barlow, more glass has the potential to worsen the image. (In fact, some observational astronomers choose not to use barlows for this reason as well as being cumbersome to use in the field.) A Barlow lens should not be used as a substitute for a good eyepiece if at all possible. Of course, there are now really high quality Barlows, such as the Telvue Powermate that provide a good image for observational purposes and in astrophotography. However, these Barlows are usually in higher price range. Look for a Barlow with lenses that are fully multi-coated; this will enhance light throughput and prevent scattered light. A Barlow that is simply “multi-coated” has coatings usually on the two outside lens surfaces. One that is listed as “fully multi-coated” has all the lenses coated on all air-to-glass surfaces. The Orion, Celestron Ultima, and Tele Vue models are fully multi-coated.
The Telvue Powermate
Strictly speaking according to Al Nagler the Powermate is not a barlow. While the Powermate acts to magnify as a barlow does it also incorporates additional lenses which correct for eye relief, unlike a barlow. This has its greatest benefit for use with eyepieces of longer focal length. With longer focal length eyepieces the barlow can increase the eye relief to an extent sometimes unintended by the manufacturer while the Powermate causes no change to the eye relief at all. The Powermate can also be stacked like a barlow to increase magnification and renders a high quality in viewing and imaging.
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