Some background before we jump in. A couple years ago, I bought a new Skywatcher 350P 14" dob. After noting a few minor issues with it, I had a 1/2 hour session (in poor conditions) to make sure nothing was horribly wrong optically with it. A short time later, circumstances led me to take a break from observing, which lasted some 16 months or so.
One of the things I did during that time off was remove the secondary mirror and test it using a contact flat. See the photo in the next post. It shows a secondary with the typical hill in the center of the long axis, about 1/4 wave surface. Not a great result; definitely room for improvement there.
Last week I ordered and received a new Antares 3.1" 1/15 wave elliptical to replace the stock 82mm one. It arrived with good-looking data, but was thicker than spec'ed, 0.88" vs 0.70". This means I would definitely have to cut down the 350P's metal mounting stalk to use it. Not being well-set-up for such metalwork, I decided to take a closer look at the stock secondary before proceeding.
I had read posts from other SW collapsible dob owners who had removed their 2ndary, saying it was siliconed to the stalk around its perimeter. That made me wonder, how much of the mirror's error is actually on its surface, and how much is caused by the way it is attached to the stalk?
So I decided to remove the 2ndary from the stalk, test it again, and compare the results. The first pic shows the assembly in question.
A long serrated knife is usually a good way to break a silicone joint. So I dug in with one. Little white sticky chunks came out, but it wasn't silicone; it was the typical 2-sided foam tape so often used to secure secondaries to holders. And the tape was starting to lift up on the end opposite to where the most weight hung. See 2nd photo.
At this point, my enthusiasm for this project soared. Twice before I have remounted secondaries that had been attached with 2-sided foam tape, and the results were fantastic. So I sensed this would be no different.
I put away the knife, heated up the metal stalk with a hot air gun, and gently separated the stalk from the mirror. This exercise alone will convince you that 2-sided tape is not a good way to mount a large secondary. It didn't take that much heat to cleanly separate the stalk from the mirror. See 3rd pic.
Next step is to clean it up and test it. I gave the mirror some 6 hours to return to ambient temps before proceeding.
Edited by precaud, 21 April 2019 - 03:12 PM.