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CHOOSING EYEPIECES AND USING BARLOW LENSES

John Lindsay-Smith

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HOW DOES AN EYEPIECE WORK?

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The objective lens forms an image at the focal plane.
The eyepiece magnifies this image and directs it to our eye.
The shorter the eyepiece focal length, the smaller the portion of the image that is magnified and directed to our eye.
So effectively, the shorter the eyepiece focal length, the more magnified is the image that we see.

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The objective lens forms an image at the focal plane.

As the eyepiece focal length gets shorter, a smaller and smaller portion of this image is magnified

The quality and resolution of this image is limited by the telescope and at a certain point will start to be over-magnified.

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EXAMPLE

A formula to calculate magnification is:

Telescope focal length / Eyepiece Focal length

24mm Eyepiece gives 2000 / 24 = 83x magnification

20mm Eyepiece gives 2000 / 20 = 100x magnification

16mm Eyepiece gives 2000 / 16 = 125x magnification

13mm Eyepiece gives 2000 / 13 = 154x magnification

10mm Eyepiece gives 2000 / 10 = 200x magnification

5mm Eyepiece gives 2000 / 5 = 400x magnification

If we take an 8”SCT with a focal length of 2000mm:

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EYEPIECE COMPONENTS AND TERMS

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EYEPIECE BARREL SIZES

Modern eyepieces come in either a 1¼” or a 2”Barrel Size.

Older eyepieces can be found with a 0.965”barrel size and can still be used with an adapter sleeve to bring them to 1¼”

1 ¼” 2” Combined

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EYEPIECE BARREL SIZES

2” Barrels are used for wide angle long focal length eyepieces where a 1¼” barrel would cause vignetting or limit the field of view.

1¼” Barrels are used for shorter focal length, higher magnification eyepieces.

The field stop in an eyepiece cannot exceed the internal barrel diameter, so the maximum possible field of view using a 1¼” eyepiece is achieved using either:

A 32mm eyepiece with an apparent 50° field of view, or a 24mm eyepiece with an apparent 68° field of view, or

an 18mm eyepiece with an apparent 82° field of view.

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APPARENT FIELD OF VIEW

The Pleiades, M45 seen through a 1000mm focal length telescope and 3 eyepieces of 24mm focal length.

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HOW IS GREATER FIELD OF VIEW ACHIEVED?

In short, more lens elements are required.

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HOW IS GREATER FIELD OF VIEW ACHIEVED?

Use of a Smyth lens, effectively a built-in Barlow lens

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WHAT DO WE NEED OR WANT FROM AN EYEPIECE?

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Sharp image across the entire field of view.
Flat field so that the whole field of view appears sharp without having to refocus for centre or edge.
Low barrel or pincushion distortion
Low or no chromatism or colour fringing of bright objects.
Comfortable eye relief.
Pleasing apparent field of view.

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THE HOLY TRINITY

Wide apparent field of view

Sharp, lack of aberrations

Low Cost

The law of diminishing returns applies here – you pay a lot more for a small improvement in visual quality

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THE LAW OF DIMINISHING RETURNS

Baader Hyperion 24mm 68º, ER 17mm - $ 169
Explore Scientific 24mm 68º, ER 18.4mm - $ 270
Televue Panoptic 24mm 68º, ER 15mm - $ 352

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EYE RELIEF

12mm Plus is considered comfortable without glasses
18-20mm is considered comfortable wearing glasses.
Very long eye relief can cause viewing problems.

Some eyepiece designs have scalable eye relief.
Some eyepiece designs have constant eye relief.

Plossl = 0.6 x focal length – a 20mm Plossl will have 12mm eye relief
Ortho = 0.8 x focal length – a 20mm Ortho will have 16mm eye relief
Televue Delos, Pentax SMC – 20mm constant eye relief
Baader Hyperion – 20mm constant eye relief. (17mm for the 24mm)

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SO WHAT TO BUY?

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FIRST, WHAT IS NOT A GOOD BUY?

Very cheap eyepieces – they are cheap for a reason, buy at your own peril.

Low end eyepiece sets – they are generally filled with low grade eyepieces and filters that you will never use.

High end eyepiece sets unless you have deep pockets and really want to own the whole range – not strictly necessary .

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CONSIDER BUYING A MINIMALIST SET

A time honoured strategy is to buy eyepieces to give magnifications of 50x, 100x and 150x.

Adding a 2x Barlow will give you 200x and 300x. Seeing conditions rarely allow magnifications above 300x.

Taking for example a 6” f8 Newtonian telescope of 1200mm focal length:

A 24mm 68° eyepiece will give 50x and maximum FOV
A 12mm eyepiece will give 100x
An 8mm eyepiece will give 150x
The 12mm eyepiece plus Barlow will give an effective 6mm = 200x
The 8mm eyepiece plus Barlow will give an effective 4mm = 300x

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AN ALTERNATIVE MINIMALIST SET

Buy a 24mm 68° Eyepiece for Maximum FOV
Buy a 24 to 8mm Zoom eyepiece
Add a 2x Barlow if you want higher magnification.

Why a 24mm 68° eyepiece (or a 32mm Plossl)?

The zoom eyepiece only has a 48° AFV at 24mm
The AFV opens up progressively to up to 68° at 8mm

Cheap zooms do not deliver good quality views – the Baader Hyperion Zoom is very good and can be recommended. Currently costs about US$ 310 but you get the quality and versatility.

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RETICLE EYEPIECES

A reticle eyepiece has crosshairs visible in the field of view – usually these crosshairs can be illuminated.

Uses:

Manual guiding whilst imaging

Accurately and quickly aligning a go-to mount

Accurately aligning the finder scope with the main scope

Visually drift aligning a polar mount.

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POPULAR ALTERNATIVE STRATEGIES FOR CHOOSING AN EYEPIECE SET

Have the eyepieces provide jumps in magnification in approximately 1.4x or 1.5x increments.

So choose the first eyepiece to give 50x, next 1.4 x 50 = 70x or 1.5 x 50 = 75x etc. Some eyepiece lines follow this rule, e.g 20mm, 14mm, 10mm, 7mm, 5mm

Another strategy is to choose eyepieces is to select them based on the specific exit pupils that are considered optimal for the basic observing situations:
Maximum brightness 6mm
General observing 4mm to 5mm
Optimum deep sky object 2mm to 3mm
Optimum planetary 0.75mm, 1mm and 1.5mm exit pupils

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SOME GENERAL GUIDELINES

Any eyepiece made by Televue, Pentax, Nikon will be superb, but at premium prices.

Next category, very good quality would be Explore Scientific, Baader Planetarium, Meade, Takahashi, William Optics, Vixen.

Generally good quality with some cheaper lines offered, Celestron, Orion

Please note that the above is a broad generalisation, and is not an exhaustive list of good eyepiece brands!

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I’M STARTING OUT AND DON’T WANT TO MORTGAGE MY HOUSE TO BUY EYEPIECES

For general viewing it is hard to go wrong with decent quality Plossls.

Televue Plossls, arguably the benchmark range from $155 to $101 – factor in VAT plus courier and import costs.

Celestron Omni – $ 30-50 + VAT, courier etc or locally available for around R 1500 – usually works out slightly cheaper if you can buy locally.

For planetary viewing the Baader Classic Orthoscopic are excellent - $84 + VAT, courier etc. Eye relief is tight, but clarity and resolution of fine detail is superb. Available in 6mm, 10mm and 18mm focal lengths.

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BARLOWS AND FOCAL EXTENDERS

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WHAT IS A BARLOW?

A Barlow or focal extender is a lens that increases the effective focal length of a telescope and therefore increase the magnification of a given eyepiece used with that telescope.

Alternatively, we can think in terms of eyepiece focal length – a Barlow will effectively shorten the focal length of a given eyepiece by the magnification factor of the Barlow.

The end result is to get higher magnification from a given eyepiece – a 20mm eyepiece on a 2x Barlow is the same as using a 10mm eyepiece.

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HOW DOES IT WORK?

A Barlow has a negative lens that stretches out the light cone so that the eyepiece sees a smaller part of the projected image, effectively increasing the magnification

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CONSTRUCTION

Most modern Barlows have 2 element lenses to minimise chromatic and spherical aberrations.
Some premium Barlows have triplet lenses incorporating an ED glass element to further reduce chromatic aberration.
Most Barlows are well corrected and the lenses are multicoated to minimise reflections so that there is very little image degradation using a good quality Barlow
Telecentric focal extenders have 4 lens elements to eliminate edge aberrations and vignetting.

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MAGNIFICATION

Barlows are designed to deliver their nominal magnification when fitted directly in front of the eyepiece.

Barlows are available in magnifications from 2x to 5x

If the Barlow is placed in front of the diagonal or an extension tube is used between the Barlow and the eyepiece, it will deliver more than the nominal magnification. However the focuser has to be racked in to achieve focus and there may be insufficient travel to achieve focus.

A ‘Shorty” Barlow has a stronger lens element than a long Barlow and will give a greater difference in magnification for a given distance moved.

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MAGNIFICATION

If the front Barlow element is unscrewed and screwed directly into the front of the eyepiece like a filter, a 2x Barlow element will give 1.5x magnification.

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POWERMATES OR FOCAL EXTENDERS

These are Barlows with 4 lens elements, made by Televue and Explore Scientific.

Televue call them Powermates, Explore Scientific call them Focal Extenders.

Also available in magnifications from 2x to 5x

Barlow lenses have divergent exit rays. Focal extenders have parallel exit rays which eliminates vignetting.

With the exception of the 5x extenders, magnification is constant irrespective of the position in the imaging train.

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EFFECT ON EYE RELIEF

Using a Barlow retains or increases the eye relief of the eyepiece being used. A 10mm Plossl has 6mm eye relief. A 20mm Plossl on a Barlow will retain or increase the 12mm eye relief of the 20mm Plossl.

This increase in eye relief is more pronounced with longer focal length eyepieces.

This can become a problem when the eye relief of long focal length eyepieces is increased too much, as it becomes difficult to find and hold the exit pupil, causing blackouts.

A Powermate will retain but not increase the eye relief of the original eyepiece.

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STACKING BARLOWS

Barlows can be stacked or combined in series.

They are not designed to minimise aberrations when used this way so image quality may suffer.

Stacking for example a 2x and another 2x will not necessarily give 4x magnification depending on the distance between the two lenses and the distance to the focal plane.

Better image quality will be achieved by using a single Barlow of the desired magnification.

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FILTER THREADS

Not many Barlows have filter threads – filters will need to be attached to the eyepiece or camera adaptor.

In a few examples with removable lens cells a filter can be attached between the tube and the lens cell – e.g. the GSO 1¼” and 2”shorty Barlows.

The Explore Scientific extenders have filter threads at the bottom of the barrel.

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INTEGRATED T- THREADS

A few Barlows such as the GSO 1¼” 2x T2 Barlowand the Celestron Universal Barlow have an integrated M42 T-thread for direct attachment of an astronomy camera or a DSLR adapter.

This means that you don’t need an extra T-threaded prime focus adapter.

If you unscrew the lens cell you can use the tube for other imaging applications.

Obviously, these Barlows can also be used with eyepieces

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PLANETARY IMAGING

A planetary camera (or DSLR) is attached to a Barlow and inserted into the back of the telescope (prime focus)

You will need to magnify the image so that it is large enough to be sufficiently sampled by the pixel size in your camera to record fine details.

The amount of magnification used should be based on the seeing and the pixel size of your camera.

A simple rule of thumb for high-resolution work is to multiply your pixel size by 3x to 7x to get the focal ratio at which you should work.

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