Anyone who owns a conventional achromatic refractor will be aware of the blue halo surrounding any white or bright object at high powers. Most people will know that this is due to the inability of the two objective elements to bring the complete colour spectrum to focus at the same point (chromatic aberration).
There are of course many products on the market to eliminate this; usually colour filters which block out the blue end of the spectrum normally costing about £30. However, as the problem is generated by the outer edges of the lens, which behave more like a prism (particularly at the extremities) splitting up the light into colours, then obviously one solution is not to use the light from the edges.
The procedure of stopping down the aperture is a well known practice, albeit, sometimes frowned upon, as reducing the aperture not only reduces light grasp but has a resultant theoretical loss of resolution. Nevertheless, after a suggestion that i should try this—- I decided to give it a go!
I currently own the Helios StarTravel 120mm (4-3/4″) refractor and made several discs of eighth inch thick black plastic with an external diameter of 120mm of course fitting over the objective lens. The centres of the discs were cut out to various diameters, giving effective apertures of 4″, 3″, and 2″. Not much difference was noticed with the aperture stopped down to 4″——-however with the 3″ aperture the blue—–yellow halo effect was virtually completely eliminated with hardly any trace detectable, and believe it or not, the image quality was better, sharper and less “confused” even though the aperture has been reduced!!!
Therefore stopping the aperture down to about 65% will be very worth while. For people with 150mm refractors an aperture of 100mm should be tried. For those with 100mm refractors an aperture of 65mm should be tried. It is such a noticeable improvement that i urge you to try this yourself, even with a thick piece of cardboard at first; you can make a proper permanent one at a later date!!!
Another tip for users of colour planetary filters which screw into the eyepiece — try screwing them into the star diagonal instead — this will allow you to change eyepieces without having to unscrew the filter!
Walter Martin (EAAS).