Who needs ED and Fluorite Glass in their spotting scope?

 Many  budget spotting scopes and binoculars do not use extra low dispersion (ED) glass which helps to reduce Chromatic Aberration (CA) to a minimum. Intense CA can ruin the view through your glasses as it causes purple and green fringes around the subjects being viewed. It is caused by the objective lens not being able to focus all the wavelengths of light to a single point. CA is especially apparent when viewing very bright and high contrast subjects, such as the Moon, against a dark sky. With a spotting scope it is apparent when observing a bird sat amongst tree branches in very bright and high contrast situations. The tree branches often form a matrix of branches which are sometimes in focus and sometimes not. CA will appear as purple fringing around the branches or even the bird in focus. 

It is possible to reduce or completely eliminate CA  by using triplets, which use three lenses combined to focus blue, green and red to a single focal point, or doublets, which use two lenses combined to focus blue and red light to a single focal point. Triplet and doublet objective lenses, however, have difficulty focusing light to single point when their diameter increases. Doublet lenses usually show some CA when they have an objective lens over 50 mm in size.

Larger triplet and doublet lenses still show some CA when their objectives are made from standard glass. To completely eliminate it they have to use ED glass, but these are expensive; my Viking ED 65mm spotting scope has a list price of about £1,000. Whereas, my Helios 90 mm AR90 scope costs about £180 at full list price, but without ED glass. The Viking scope shows no CA or other aberrations whatsoever at any magnification; it is almost as good as it gets. The Helios 90 mm scope despite its triplet lens does show some CA above 38 magnification under very bright conditions, and when the eye is not focused on the optical axis. The CA gets worse as the magnification increases to 75X.  Otherwise the scope performs exceptionally well at eliminating other forms of optical aberrations, such as astigmatism, coma, field curvature and spherical aberration, and it does this just as well as the Viking.

There is a way of completely eliminating CA without the cost of using ED glass. Here is the trick that astronomers use and it is almost completely effective when using a good scope and it costs almost nothing: it is achieved by stopping down the aperture on the lens.

I have made an aperture mask which stops down the 90 mm lens to 65 mm. I experimented with the size of the mask's aperture, and 65 mm worked best. I made the mask using the black plastic of an A4  paper folder with a compass and white pencil and a craft knife and pair of scissors. I sized the mask to make a 100 mm outer circle which fits perfectly in to the objective case of the scope: it doesn't drop out readily, but it is easy to fit in or remove. The results are amazing; CA is completely eliminated at all magnifications and across the whole field of view. To all intents and purposes it performs as well as the Viking ED scope. I was thinking of upgrading the Helios AR90 to a Viking 85mm with a list price of well over £1,000. The Helios is now a serious competitor and I do not need to upgrade. I can now save up for longer to buy a Swarovski without missing out on much - some hopes!

How does this work? The mask reduces the working aperture of the scope, in my case, I estimate that the aperture is reduced from f4.4 to f6.6. The smaller aperture increases the depth of field, which allows all wavelengths of light to achieve a single focal point somewhere along the optical path. The bird in the trees is no longer surrounded by branches with purple fringes above 40x magnification. There is only one drawback the reduced aperture allows less light in for observing at dusk, but you simply remove the mask to let more light in. Reducing the aperture, however, helps to  dampen down the brightness of the moon if you use the scope for astronomy, so that your eyes do not get so tired. And, you can even do this with your Swarovski or Zeiss spotter to reduce the glare on the Moon and Jupiter.

I also observed this "masking " phenomenon when  using a Starwatcher 80mm achromat (more later) to observe the Moon. The objective lens cap has a  removable 55 mm aperture mask which reduces the glare of the satellite and the CA. But for other types of astronomy you usually want let as much light in as possible.

There you have it: if you are thinking of upgrading but don't have lot's of cash then an otherwise perfectly good scope can be adapted to use an aperture mask that almost completely eliminates CA, if it bothers you. Experiment, and give it a try. If you are short of cash then you might not have to spend wads on an ED scope in the first place. But remember, eyepieces can also introduce CA and other sorts of aberrations which are harder to fix, luckily my Helios eyepiece doesn't do this even though it is a cheap one.