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Cleaning an eyepiece

Technical Insights Posted on Fri, May 23, 2014 17:42:07

You will need a window cleaner solution with alcohol (mostly colored in blue), some cotton swabs, a bulb-type puffer (available in most photo shops) and some Kleenex tissues. Avoid tissues with perfumes or lotions, as they will leave a film on the lens. Prepare a clean work area, such as a freshly washed kitchen table.

First, use the bulb-type puffer to blow off any dust or loose particles from the lens surface. Don’t blow the dust off with your mouth, as droplets of moisture can get on the lens, causing spots. If the eyepiece has particles stuck to it that can’t be blown off, moisten (do not soak) a tissue with window cleaning solution and gently blot the surface, without rubbing. On smaller lenses, use a cotton swab or fold a tissue into a steep triangle, moistened with window cleaning solution. Use the puffer again to blow off any more dust.

Second, moisten (do not soak) a cotton swab or tissue with window cleaning solution and gently wipe from the center out to the edge, using a circular motion. Move the cotton swab or tissue slowly enough so that the cleaning fluid appears to “follow” the cotton swab or tissue around and is re-absorbed. If you move it too quickly, some of the liquid will “break away” and dry separately, leaving spots.

You’ll probably use several tissues or cotton swabs to thoroughly clean a lens. Use a new tissue or cotton swab after each swipe; this will prevent any contaminants from getting back onto the lens or scratching it. Do not touch the lens with your fingers, as the grease on your hands and fingers will cause smudges. Also, be careful the grease from your hands and fingers does not get on the cotton swabs or tissues; it will smudge the lens. If it does, throw it away and use a clean one.
Cleaning the edge of a lens is the most difficult part, especially where the cotton swab or tissue is lifted from the surface. Don’t use too much liquid or put the liquid directly onto the surface being cleaned, because capillary action could draw the liquid inside. Try folding a tissue into a sharp point, moistening it slightly and use it to wipe around the edge.

If, after cleaning, any spots remain, try “fogging” the lens surface with your breath, then wiping it with a cotton swab or tissue moistened with window cleaning solution.

Your eyepieces should be stored, dry, and in as clean condition as possible, optically and mechanically. This will make them ready for use on demand. Keeping your eyepieces as clean as possible during use upholds their condition and performance, and will make preparation for storage quite easy. After inspecting the condition of your eyepieces and making sure they are clean and dry, place on the lens covers and store them indoors, protected in a suitable case.

By making sure eyepieces are clean, the chance of transmitting debris to the scope optics is reduced or eliminated. Properly maintained, Sky-Watcher eyepieces will provide long-lasting, consistent, and dependable performance.

Classicals and Modern Eyepieces

Technical Insights Posted on Fri, May 23, 2014 17:35:49

When speaking about eyepiece designs most astronomers classify the eyepieces in “old but good” designs like the Plössl eyepiece (field 45°) and the “new and large” designs with large apparent field of view, from 60° (a “small” large) to… 120° (definitively a “large” large).

I was surprised, and I bet most of the readers will also be, that a very large eyepiece design existed in WWII for submarine periscopes with an apparent field of view of 120°! The design of this eyepiece (called Koehler eyepiece) was available after WWII and look like this:
Eleven lenses are used inside and the apparent field of view is 120°. The happy customers who found one in WWII surplus and tested it on their telescope are all enthusiasts.

But the Koehler design is still a classical design. Before computers existed and was available to a large public, opticians did calculations by hand. It was a consuming task so the opticians calculated one “formula” for an eyepiece (let’s say 20mm) and other eyepieces were enlargement (40mm) or reduction (10mm) of the “master” 20mm eyepiece.

A classical eyepiece design is a design were all eyepiece share exactly the same design, scaled to the intended focal. By example a 20mm Plössl is exactly the double than a 10mm Plössl, except for the apparent field of view that stays constant. If the 20mm Plössl has an eye relief of 16mm and internal lenses of 18mm; the 10mm will have an eye relief of 8mm and internal lenses of 9mm.
Classical designs have one important drawback: for short focal eyepieces the eye relief is so short that observations are difficult: you can not keep eyeglasses and staying in position so close to the view lens is a tremendous and rapidity tiring task impacting quality of observations.

A modern eyepiece design
involves computer assisted design. Each eyepiece focal length uses an adjusted design, different from all other eyepieces in the same group. Calculating and adjusting the design for each focal length will be a time-life task without a computer.

Eyepieces based on a modern design share the same field of view -but unlike the classical design- keep the eye relief constant. A group of eyepieces based on a modern design will be about the same diameter, with the shorter focal eyepieces being the longer.

As eyepieces based on modern design share the same eye relief short focal eyepieces are very comfortable to use (user can keep its eyeglasses) at the expense of a large and quite heavy construction.