Mirror Cleaning

This was well past due. The manual for my Orion Sky View XT10 Dobsonian telescope says I should not have to clean the primary mirror more than once a year or so. I bought it over three years ago and have yet to wash it even once. As you can see from the pictures, my mirror was filthy. Not only did it have a healthy layer of dust all over it, but it also had flakes of what I assume were black paint from the tube. The largest of the specks I had noticed the day we brought it home. After researching on the Internet and reading the manual, I decided to give my primary a bath. To remove the mirror cell I just had to unscrew three screws (the ones with springs surrounding them if anyone is using this as a walkthrough) and slide the cell out sideways. To remove the mirror from the metal enclosure, I unscrewed the 8 screws clamping it down. My screws were EXTREMELY tight and I eventually resorted to my power drill to finish the job. It goes without saying, do not touch the top of the mirror as the aluminized coating is easily tarnished. Once the mirror was out I openly admit, I hefted it for a good 30 min. It just felt so powerful in my hands. It was thicker and heavier than I expected. The parabolic surface is so slight it is barely noticeable. The slighter the curvature the longer the focal length, and my telescope has a beastly f.l. of 1200mm. Next I prepared the bath.

Left: focused on surface. Right: focused on image – notice the higher contrast where the mirror was shielded from the dust by the clamp.

The manual suggests to fill a sink with room temperature water, a couple drops of dish soap, and a capful of rubbing alcohol. I swirled it around and gently submerged my precious optical instrument. I let is soak a good thirty minutes while I took the dog for a walk. When I returned, I broke out the cotton balls and got down to business. As per the instructions, I very gently stroked the mirror underwater in straight lines with a fresh cotton ball each stroke. The first round lifted much of the grit but left visible “mud lanes” on the surface. The second round left the mirror about 90 percent clean – a few light lanes were

This is an “after” picture showing a strikingly clean reflection :)

still visible but I decided to forego a third round as I imagined I might do more harm than good to the aluminized coating. I swished the mirror around, drained the sink, and rinsed in lukewarm water. The water slides right off of a clean mirror and the few remaining drops were easily absorbed by the corner of a paper towel. After the mirror was dried I snapped some “after” photos to compare with the “before” photos. It pained me to see it but, as I photographed the mirror the cleanliness fell from about 90 to 85 percent just from the dust in the air. It’s unavoidable, but it’s still 85 percent cleaner than it was an hour ago. Cloudy skies prevented me from trying the new optics out. I’m not expecting anything dramatic, maybe a little more contrast if anything, but I’ll post an update the next chance I get to point it skyward.

 

     

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