What are the Differences Between a Powermate and a Barlow?




Powermate vs Barlow

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A Powermate lens consists of 4 optical elements: negative doublet followed by positive doublets. Meanwhile, a Barlow lens contains 2 or 3 optical elements: achromatic negative doublets. The negative doublets configure to give in diverging rays that stretch out the light cone of the Barlow, thus giving a double or more magnification. 

This article gives you complete information on both types of lenses. Despite both serving as magnification lenses, you will get an understanding of each, their benefits, and drawbacks to help you choose the best that suits your telescope.

But for you to understand what Powermate is, you need to first learn about Barlow.

What is Barlow Lens?

Barlow lens is a diverging lens named after Peter Barlow, a physicist and mathematician who innovated the first model of the Barlow lens in 1833. It is more of an auxiliary lens that mounts between the eyepiece and focal point of a telescope. 

A Barlow lens increases the power of the eyepiece using the telescope and magnifies the telescope focal length. It simply means, since magnification is equal to telescope focal length divided by eyepiece focal length, the longer the telescope focal length is, the higher power it is with any given eyepiece.

Despite Barlow lens increasing the eyepiece power, there are no more extended eye relief changes typical for low power eyepieces. That helps an eyeglass wearer observe a full field view at high intensity without removing the glasses to get closer to the eyepiece, just as it happens when only using a shorter focal length eyepiece. 

The majority of Barlow consist of two lenses (achromatic), while others are three lenses (Apochromatic). Since Barlow increases the effectiveness of focal length from fast to medium ratio, the diverging optical elements give affordable cheap eyepieces.

They are easier to handle, have lower astigmatism and create good color correction at the eyepiece edges. It is also valuable for improving the edge sharpness for a better quality wide-angle eyepiece that uses a fast focal ratio telescope

However, when using Barlow designed for long focal ratio telescopes, they may cut off the edges of an image using a low power wide angle eyepiece on a fast focal ratio scope. 

How can a Barlow lens increase the number of eyepiece collections?

There is the need to set several eyepieces with different magnifications. For example, you require a low-powered eyepiece that will offer a wider field of view on larger objects.

Meanwhile, it would be best to have a high-powered eyepiece for more magnifications to view more minor things. That is where a Barlow lens comes in handy. 

Barlow’s effectiveness increases the number of your eyepiece collection. For instance, you have a 32mm and a 20mm eyepiece.

Adding a 2X Barlow lens gives you the advantage of doubling the number of your eyepiece collection with magnification capabilities of up to 16mm and 10mm. On the contrary, if you only have 10mm, 20mm, and 40mm lenses, having 2X will be of no help.

The magnification may end up the same as that of the eyepieces.

Which is the best Barlow size?

Choosing the best Barlow size should go hand in hand with magnifications and lens abilities. For instance, you have a 40mm lens and later added 2X Barlow.

The result of your 40mm will be no different from that of a 20mm eyepiece. To make the magnification work correctly, you need a Barlow of about 1.5X or 2.5X.

Benefits of Barlow lens

  • Cost-effective

In any amateur astronomer accessory, you will not miss out on a good quality Barlow lens. When placed between a telescope objective lens mirror and an eyepiece, it is a cheap tool that magnifies the power by 1.5x to3x and increases the focal length. 

  • More comfortable eye relief at high magnifications
  • Saves you more

You can use a 2x Barlow with a 40mm eyepiece to get a 20mm eyepiece. Where you have a 1000mm focal length, your 40mm adds an advantage of getting up to 25x magnification and 50x magnification when paired with a 2x Barlow.

What is Powermate?

A Powermate or focal extender lens is an invention by AI Nagler, a designer and company head at TeleVue. It is a combination of negative doublets and positive doublets.

Additionally, a Powermate adds a positive doublet on the Barlow lens concept to collect the pupil exit. Therefore, it means that a Powermate will be invisible in a light train of a telescope. 

In the AI Nagler’s invention, a negative doublet comes first to intercept light rays. According to Steve White, a Tele Vue representative, the negative doublet works similar to the Barlow lens.

But instead of the rays converging to one point of the eyepiece, they head parallel before the positive doublet intercepting them more as they finally pass on to the eyepiece.

A positive doublet comes as an advantage to the standard negative doublet in Barlow by seeing incoming light from a much more comprehensive source. Its design avoids any vignetting that may result when a Barlow pairs an eyepiece.

The positive doublet also helps minimize various edge aberrations that may occur in the eyepieces. 

How can a Powermate lens increase the magnification without vignetting?

TeleVue added a positive doublet lens to add more magnification without vignetting. Therefore, Powermate offers several embellishments, with its highest being 5X.

It can make a 25mm focal length eyepiece focus like a 5mm eyepiece. 

However, your telescope focal length and sky conditions can play a significant role in any views that you might observe. Therefore, using 5X is ideal when there are favorable conditions for high magnification views. 

However, if you own a 24mm panoptic, you get a focal length eyepiece with 2.0X and 12mm of power, a 9.6mm match with 2.5X Powermate, while a 6mm goes hand in hand with a 4X Powermate. 

Which is the best Powermate lens?

There are several optional Powermate lenses for Astro-imagers. 

  • 1¼ Powermates: 2.5x and 5x

The model appears to be less than 3 out of your focuser, while the 5X takes 1¼ thread filters. Both are ideal for their 1¼ diagonals and prisms than regular Barlows that have 1¼ diagonals and reflectors.

The 5X Powermate is best in long focusing CCD and film photography. It magnifies about 7.7X with high performance and is an excellent alternative to eyepiece projection.

  • 2 powermates: 2x & 4x

These types of Powermate are better choices for two diagonals, for they are almost parfocal. Their performance is excellent for every 1¼ or two eyepieces with a 48mm threaded filter.

A 4x model offers an imaging quality of 35mm panoptic at 8.8mm and 25mm of eye relief. It is important to note that Powermate does not require a panoptic-Barlow interface.

A 2″ Powermate only requires minimal refocusing due to its design of fitting into 2-star diagonals as compared to a 2″ Barlow.

Benefits of Powermate

  • The introduction of Vignetting, pupil movement, and edge field aberrations when using long focal length eyepieces in Barlow becomes minimized when using Powermate.
  • Powermate gives a telecentric operation. It means that field rays leave parallel to the optical axis compared to diverging rays present in Barlow lenses.
  • More excellent performance in image amplification 
  • Powermate gives more flexibility in imaging and visualization on all types of telescopes and eyepieces. Powermate is parfocal and almost constant magnification despite the image distance. (an exception of the 5X model that magnifies 1x on every 35 mm of distance increase towards the image).
  • Instead of doubling up as it happens in Barlow, Powermate has higher magnifications and higher optical performances.

Final Word

A good magnification in a telescope requires a variety of different embellishments. However, the product comes in a wide range of prices.

Therefore, you need to plan on your budget. Barlow lenses are more affordable but functional when chosen carefully.

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