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| This is your defective grade F mirror with several zones and defects, holes and rough spots. After testing with a judiciously placed swath of measurements, the mirror appears to become grade A at 97. You cannot trust the paper documents that come with the mirror. |
1) The mirror will incur scratches during the polishing and the finished aluminized mirror will show these defects, which will lower image contrast. The larger the mirror, the more scratches.
2) Fast polishing means pits on the mirror limiting contrast etc.
3) The telescope is only good for deep sky objects. At higher mag, the planets will fall apart and the image may not have good contrast, sharp focus, or will have other defects. The fast mirror is good at lower power with smaller images which are more forgiving. A focal reducer could also improve the view.
4) Also some fudging may take place when holding the mirror analysis as detailed in the above rough sketch.
What is the solution? You get what you pay for? What happens when a quality mirror is no longer affordable? Do you settle for a so so mirror and simply bypass the passion of owning a superb mirror or give up the hobby? Sometimes life is a compromise. Maybe you already have an excellent Celestron C9.25 or C14 for planetary and a good fast big Dob is just what you need!
https://zambutomirrors.com/mirrors.html
http://hubbleoptics.com/mirrors.html
http://www.kennedy-optics.com/telescopes.htm
https://www.obsessiontelescopes.com/
http://www.jimsmobile.com/buy_mirrors.htm
https://www.telescope.com/Orion/2160.home
https://explorescientificusa.com/products/explore-scientific-generation-ii-20-inch-truss-tube-dobsonian-telescope-dob2036-00
https://optcorp.com/products/sky-watcher-20-inch-stargate-truss-tube-dobsonian
https://astromart.com/classifieds/astromart-classifieds/telescope-making/show/finished-mirrors-and-mirror-cells-in-stock
