Barlow Lens

MOON- 3X BARLOW LENS

To ensure the quality of telescope optics, a very sensitive technique known as star testing is performed. Albeit an easy technique, correct interpretations of the results require careful examination and sufficient knowledge. Specifically, optical aberrations (oftentimes caused by imperfect optical systems or bad weather conditions) can be determined (and thus corrected) only if you are careful and knowledgeable when inspecting the patterns of diffraction images. This article lists the common optical aberrations observed by star testing.

Firstly, it should be noted that although it is commonplace for star tests to be carried out in night conditions on bright stars, some people may opt for it to be done using artificial stars since it is nondependent on time of the day. Example of an artificial star is the reflection off a Christmas tree ornament placed at an appropriate distance.

Now, to star test, telescopes with good quality eyepieces (best not to use a Barlow lens as it may introduce its own aberrations) have to be focused at an extremely high magnification (i.e.: at least 100x per mm of aperture). Once set up, you should observe the patterns through the scope for images that are in focus, and ones that are very slightly inside and outside of the focal point.

For focused images that have diffraction, the pattern has a sharp, bright Airy disk, surrounded by increasingly faint concentric circles. Importantly, patterns at similar points both inside and outside of focus are round and identical to each other. An out-of-focus image for a refractor without diffraction is a multitude of alternating light and dark, concentric circles. On the other hand, out-of-focus images with diffraction show a dark central shadow. Therefore, problems are determined by comparing differences in pattern between the inside and outside focus images and from the overall shape of a diffraction pattern. These problems indicate things that can be easily corrected.

Image diffraction may be due to many reasons, such as atmospheric turbulence and/or temperature inequilibrium. During turbulence, it is best to conduct star testing on another night. A telescope's temperature that is in inequilibrium results in tube currents, indicating that you should wait awhile for the temperature to come to equilibrium. Poor collimation in telescopes means you should readjust the mirrors of your telescope. If "pinched optics" patterns exist, it means that you should find your optical system for sources of stress on the optics, such as overly tightened collimation screws or tube-ring clamps that are too tight. Additionally, the existence of astigmatism might be due to poor collimation on the telescopes, or that a secondary/diagonal mirror that is not perfectly flat. Finally, some aberrations result from poor shaping and/or polishing during the mirror-manufactured process may be harder to correct by the user. These include mirror over-correction and under-correction, some causes of astigmatism, edge defects, zonal defects and rough surface. In this case, it is best to leave it to the experts!

You can find out more about telescopes at OZScopes - The Australian Telescope Experts.

If I have a 3x barlow, how to get a 5x barlow lens?

The magnification of the barlow depends on the separation between it and the eyepiece, so introducing an appropriately sized extension tube will give you 5x. Whether it will actually work for you is another matter. There are two potential problems - not having enough focuser travel, and possible vignetting.




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