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Aperture mask
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The front dust covers on most telescopes have a secondary light reducing aperture. The aperture mask is used to stop down the telescope's aperture to a smaller diameter for looking at the moon, bright objects or daylight viewing. A neutral density filter is commonly used instead of using an aperture mask.
With reflectors, a circular hole in the aperture mask is positioned off to one side, (not directly in the center), turning it into a unobstructed reflector with increased contrast.
The aperture mask may also decrease off-axis aberrations in reflectors. With fast achromatic refractors using a reducing aperture will effectively increase the focal length, and consequently decrease the achromatic aberration.
An aperture mask that increases the focal length to f8 or above will remove most of the achromatic aberration in a fast refractor.

Strangely, subtle details on an object that are lost due to 'colour fringing' can be regained by decreasing the aperture. Decreasing the aperture will improve the sharpness of the view.
However, the main drawback of reducing the precious aperture, is that the telescopes resolution is lowered (ie losing some detail, and effectively reversing the gains of reducing the achromatic aberration). So, it is worth experimenting with different sizes of DIY masks.
A circular piece of glued cardboard, plastic, wood or metal with a suitable sized hole can be made to fit over the front cell of your telescope. 

A small tip for smaller sized telescopes is that once an ideal aperture mask size is found, a suitably sized clear UV photographic filter can be permanently fitted over the aperture. Other colour filters can be stacked onto the UV filter.



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