Inspecting Optics with a Light or Flashlight

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How good will Telescope and Eyepiece Optics Look when Scrutinized with a Light, or a Flashlight?  Before we answer this question, we would ask you to look at answers given on an independent user forum. Here the question is raised by an amateur astronomer on a forum called "Cloudy Nights", where you can read a few answers given by those more experienced:
 
https://www.cloudynights.com/topic/371402-beware-of- the-flashlight-test/?p=4753285
 
A misconception about the amateur telescope optics manufacturing is that optics should look cosmetically flawless, even under close up inspection with a light. But the fact of the matter is that when light hits an optical surface a small percentage of it scatters across the highly polished surface, and this can reveal all manner of "phantom" effects that can appear as scratches (called "sleeks"), bright and dark areas, dots, smears, specks of dust, etc. But because of the highly polished surface, the effects that you see are very small indeed. And the brighter the light used for inspection, the more likely it is that you will see artifacts.    The fact of the matter is that such small-scale artifacts have no practical effect on the final image. If it did, then professional telescopes that have had their optics damaged, would show terrible images. Case in point is a story from MacDonald Observatory where many years ago an employee fired a pistol at the large primary mirror of the 107-inch Harlan J. Smith telescope, and hit it with a tool which left significant scratches in the massive optic:
 
http://www.agnew.biz/travel06/2007_02_01_archive.html
 
MacDonald Observatory researchers still continue to this day to retrieve very high-quality images, and the damage done has had very little effect on the overall performance. When we say very little effect, you can't see problems in the star images. But because there are holes in the mirror that were made by the bullets, the light gathering of the instrument was reduced from 107 inches to 106 inches. Indeed, it would be unlikely that anyone could tell the difference between images given by a 107 inch to a 106-inch telescope. But we use this extreme example to illustrate how insignificant an effect would be by tiny artifacts on telescope optics.   So, if you see thin sleeks, specks of dust, etc. during your inspection, you will not see them through the instrument when you are observing. Why is this so? Because these artifacts are not at the point of focus. If you think about it, Newtonian Reflectors, and Schmidt-Cassegrain telescopes have secondary mirrors that block the center of the large mirror. At night when your dilated, you can't see the secondary mirror effect at all. So, if your mirror has some dust or sleeks, or if there are dust particles and sleeks on your refractor (even trapped between the lens elements) they are no cause for concern. However, if you allow a telescope optics to be completely covered with dust, it will reduce contrast, and it should be cleaned to provide you with the best performance. We provide instructions on cleaning optics yourself and we also provide a cleaning service at Explore Scientific should you ever require it.   All optics manufacturers aim towards high-quality optical correction, and optics used for amateur astronomy are held to very tight tolerances. But sometimes amateur telescope optics makers are often held to an even higher standard than professional telescope optics makers in that they are also expected to make cosmetically perfect optics. It is common among optics enthusiasts, to look upon an optic and marvel at its beauty and optical performance, but what allows us all to explore and perhaps see something that we have never seen before is the image performance of the optic.