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Primary Mirror beveling on BT-200 Telescope.
Hi all,
Just a heads up for those that may have a similar problem with Bintels' BT-200 8" f/4 telescopes. Although may be a small batch of GSO scopes. I have one and have not been able to get a clear image out of it. Stars would end up with weird blobs on the sides of them, collimation always seemed like it was out, even though I went and bought the cats eye collimation sticker and borrowed the tools and got it pretty much spot on. Even the Cheshire said it was collimated. Star tests indicated the black blob in the middle was infact in the middle. I was getting a little bit of warping on the primary due to the clamps being tightened too much, but I adjusted these so a piece of paper could just slide between the mirror and the clamps. I even swapped over the cork backing behind the primary mirror. It all looked ok. Unfortunately star tests proved otherwise. No matter what I did, I couldn't get a clear image, the edges always seemed wrong, which made me second think my purchase.
Well one night I had a thought, I wonder if the bevel has anything to do with it. The bevel was quite rough and had over spray from the Alumanising, which did end up reflective in a couple of chips along the edge of the bevel. I had heard of people masking the mirror to effectively reduce turned down edges on mirrors, so I tried to construct a mask, well this was hard because I didn't have the right tools, so I ended up just painting the bevel and some of the edge of the primary in Matt Black paint, right up to the mirror coating.
I am still yet to do a proper star test on it, due to clouds etc.. (nature of this time of year) But initial images out of the scope indicate that the little blobs on the edges of the stars are gone and the stars do appear much rounder, also the images are much clearer visually from the scope. As I said though, I still need to do a few more quick tests to confirm it has worked.
So if your plagued with unsharp images or weird blobs on your stars from these fast newts, might pay to check out the beveling on the edges of the primary. I am not saying it will be that but it's a start.
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