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| I was reading somewhere that a UV/IR filter is not required for a pure reflector system, could someone shed some light on this for me? Thankyou. |
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| Refractors are designed to focus the visible rgb light. CCD/CMOS sensors detect into UV and IR. See your camera's QE curve. UV and IR light in refractive telescope will be slightly out of focus if you don't filter them out. In a purely reflective system you don't get this prism effect refractors have. So you can use a clear filters instead of an L filter. However I am not sure if you can have a truly pure reflective system. There is usually glass somewhere. |
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Tolga: My Ritchey-Chretien is 2 hyperbolic mirrors and a camera sensor. By definition of truly pure reflective system, that is it. |
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| Wouldn't an RC with a focal reducer/flattener change that? |
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Dale A Chamberlain: yes because at this point you would have introduced glass into the imaging train |
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Dale A Chamberlain: Sure, but I'm not using one. |
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In that case, the only glass I can think of would be the sensor protective window, which in case of your ASI 294 is an AR-coated thin parallel piece of glass and thus it should not cause any major abberations. You should definitely give it a try! Especially in some dusty regions like the cygnus wall area, loads of stars and some little nebulosities unseen in visible light start to pop out in IR. I tried it once with my APO but of course the stars were fluffy as hell ;) But on a purely reflective system like yours they should be nice and crisp anyway! Another consideration is what color the additional IR light will have on a OSC camera like your ASI 294. Since the spectral response of the IMX294-chip looks something like this:  (From MallinCam SkyRaider DS10C TEC, same sensor as ASI294) I would guess everything might get a bit reddish. One more thing: main mirror reflectivity tends to twindle out over 700 nm, which might act like an IR-filter in itself. So I guess the bottom line is like always: you have to try to find out. But isn't that the fun part of the hobby ;) |
