![]() ![]() The first stage is to center the mirror holder relatively to the central axis of the OTA. In my case a Cheshire wouldn't allow it and I had to use a collimation cap. The tool must allow us to see the edges of the secondary and the whole primary mirror reflection (as seen below). For this task we need a collimating tool such as a Chesire eyepiece or a collimation cap. This is an example of a basic procedure of secondary mirror collimation mostly intended for a visual use. The only way to be safe is to use mirror clamps, or at least (if you own a really expensive primary mirror), add a safety string. Note that in this article I've only given an example of how I glued my mirror, and I can't guarantee that it will work for others. The last stage is to insert the secondary mirror assembly back into the OTA and to collimate the telescope. If you are uncomfortable doing it - you can find a different method - for example to firmly hold the mirror by its edges. I simply pushed it against the mirror while it's still lying on the tissue paper (this left no marks or scratches). The next part was to firmly press the secondary mirror against the holder. Note that it's easier first to align the mirror on a square sheet of a tissue paper and then to align the spider relatively to the paper. Then I've aligned the mirror holder assembly against the new secondary mirror and carefully attached it between the markings. First I cleaned the mirror back and the holder surfaces with an alcohol and firmly attached an exposed 25x25mm tape pad to the holder. I've attached my mirror using "Scotch" indoor mounting tape. Methods which involve covering a large surface of the mirror with adhesive (or tape) or using non-elastic adhesives (such as epoxy) should be avoided, since they may introduce mechanical stress which can bend the mirror and cause an astigmatism. This will prevent the mirror from falling in case the adhesive fails. A safety feature can be added by attaching a short, strong string between the mirror and its holder using a strong epoxy. The tape patch shouldn't be too small in order to provide enough grip but also not too large so it won't introduce stress. Special care should be taken when choosing the tape - if it's too soft it won't be strong enough and if it's too hard it won't be elastic enough. It is faster and used by many manufacturers (Skywatcher, GSO), though it's arguably weaker and often avoided when attaching heavy mirrors for large telescopes. In this case the distance between the mirror and the holder should be controlled with matchsticks and the adhesive should be allowed enough time to set.Īnother method is to use a double-sided foam adhesive tape. There are various methods of attaching the mirror to a holder without clips: One is to use an aquarium silicone sealant which is applied to 3 evenly spaced spots or to a large single central spot. In my case I had to replace the stock 58mm diagonal of my Skywatcher 10" Newtonian with a larger 63mm one because it was too small. The solution can be either to move the primary mirror further away from the secondary (often not applicable) or to increase the secondary mirror diameter.Ģ) The telescope is used for deep sky astrophotography and a larger secondary mirror is required to achieve a wide fully illuminated field of view.ģ) The stock mirror is attached in a such way that it's under mechanical stress, which causes astigmatism and image degradation, as described here.Ĥ) The offset of the secondary mirror isn't set correctly, which results in unevenly illuminated field of view and complicates the collimation process.ĥ) The stock mirror is of inferior quality and, usually, causes astigmatism.Ħ) The mirror is too old and needs to be re-coated or replaced. Or calculate if the fully illuminated field of view is less then zero. This issue can be diagnosed by looking through an empty focuser exactly from the focal plane - if you can't see the whole primary mirror reflection (including its clamps as seen in this image) - then the diagonal is too small. This happens because some manufacturers are trying to achieve mutually exclusive features: A short, fast OTA with a long back focus and a small central obstruction. And calculation of the fully illuminated field of viewġ) The stock mirror is often too small and causes vignetting (darkening) even at the center of field of view.
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