Testing Optics

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2019-04-08

I actually wrote this article a long time ago - probably around the year 2000 - but I think it outlines some basics of testing optics that, at the very least, will help you discern whether you have an optical problem or not with your telescope (and/or eyepieces). Enjoy!

Testing Variables: Seeing should be 7 to 8 on a scale of 10, Transparency should be 3 to 4 on a scale of 5 and Test on objects that are high in the sky. Scope should be free of dew and obstructions and a high quality eyepiece (Plossl or better) should be used if possible.

First Test: Extraneous Light Transmission

• Remove eyepieces and star diagonals, and look through the tube at a patch of dark sky.
• Look for unwanted light bouncing around (illuminating the inside of the tube) that might be coming from the moon or a street light.
• The inside of the tube should be very dark.

Second Test: The Star Test

• Locate a bright star that is high in the sky and examine it through a moderately powered eyepiece (75x - 125x).
• In focus, the star should appear as a bright pinpoint with a hint of circular shape (Airy disk) with a single ring of faint light around it (diffraction ring).
• Go slightly inside and outside of focus - the images should be identical.
• If not, oval patterns that change axis 90° from inside to outside of focus indicate astigmatism (A defect in which the unequal curvature of one or more refractive surfaces of a lens or other optical system component prevent light rays from converging at a single point, resulting in a blurred or imperfect image).
• Asymmetric (different on both sides) patterns indicate misaligned optics.

Third Test: Epsilon Lyrae

• In the map to the right, Vega is labeled. eps Lyr is at the center of the Telrad circles. The Ring Nebula is labeled as a reference point.
• Bring the double-double into view in a lower power eyepiece and get the two sets of stars split in the middle of your field of view.
• Increasing to a high power eyepiece, you should be able to sit each pair of eps Lyr on opposite sides of your field of view and split both doubles cleanly.
• Astigmatic lenses will require slightly different focus for each pair of stars.

Fourth Test: Flatness, Vignetting and Coma

• Find a star field such as the one Sadr (Gamma Cygni) is in and view at low power.
• Stars should be sharp and in focus from center right to edge of field of view - indicating good flatness.
• Stars at the edge that smear or turn into comet like objects with tails pointing away from center indicate coma.
• Vignetting will result in stars becoming dimmer as you move away from center. Caused by poor optical design blocking some light at the edge of FOV. Usually noticeable in only very low power eyepieces.

Fifth Test: Chromatic Aberration

• View a bright star such as Vega. The diffraction rings should be the same soft white colour as the Airy disk. Similarly, a coloured star such as Arcturus should show no colour changes inside, outside or in focus.
• View of the moon should be uniform - with no purple or green halos around it. Poorly colour corrected optics will show ghostly blue/purple halos around extremely bright objects like Venus and Jupiter. The chromatic aberration can degrade the details visible on the surface of Jupiter or Saturn, and can spoil the splitting of close double stars.

Testing Misconceptions

• Aperture has a much greater impact of a system's ability to split close double stars, or resolve individual stars in globular clusters, or to show spiral arms in Galaxies. These are not necessarily a good measure of optics, although excellent optics will enhance all these test results.
• Different optical systems require different levels of adjustment and maintenance. Reflectors and Cassegrains require periodic collimation of the primary and secondary mirror systems. Refractors and Maksutovs require little to no adjustment as their optical components are usually fixed in place. An out of alignment telescope could easily fail the optics tests, yet if it were collimated, it could perform fine.