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Thread: LRGB Filters

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    Default LRGB Filters



    Last night I ordered a ZWO ASI 1600 (mono) to replace my Sony DSLR for astrophotography. I opted to not purchase the filter wheel and filter package with it since I've read some bad stuff about their filters and want to get something of better quality. I'll probably end up getting a ZWO filter wheel but don't get all of the various sizes, 1.25", 31mm, etc. I'll be using an Orion 115mm EON refractor for my imaging, with the 1600 camera, so what filter size should I desire and why? Also it seems filters are mounted and unmounted, which type works in a filter wheel?


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    Default Re: LRGB Filters

    The 1.25" filters are mounted which means the filter glass is fitted into a threaded cell much like a DSLR filter. The cell screws into the filter wheel or even onto any standard 1.25" fitting. The exposed part of the filter is less than 1.25" - more like 28mm. The 31mm and 36mm filters are just the glass, sometimes with an unthreaded cell to protect the edges. They are held in place in the filter wheel by small screws around the edge.
    The filter size you need depends on the size of your sensor and your focal ratio. With a large sensor and low focal ratio a larger filter is needed to avoid vignetting. That is, where the filter is not big enough for the sensor to be fully illuminated.
    The ASI1600 and ZWO filter wheel combination places the sensor close enough to the filter that even with 1.25" mounted filters it can image down to f/5 or so without vignetting - or so little that it calibrates out easily with flats. This is an advantage because larger filters cost more and mounted filters are easier to handle. Your Orion 115 at f/7 should have no problems. And even if you reduce 0.75 you should still be ok.
    The ZWO 8 position filter wheel can take 1.25" mounted filters and 31mm unmounted filters. For 36mm filters they have a 7 position filter wheel.
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    Default Re: LRGB Filters

    For my filter wheel (Atik EFW2), I chose the 36mm unmounted filters. I figured that 1.25" would be too small and would vignette in my f/4 scope. I could have gone with 2" filters, but that is probably overkill for avoiding vignetting, and I don't need the threaded mounts, since I would not be using any of my AP filters with an eyepiece.

    If I recall, I think I did the math to check that 36mm would be okay with my scope and camera. And the mounting wheel for the 36mm size allowed for seven filters. That allows one clear (luminance) filter, three RGB filters, and three narrowband filters, all I am ever likely to need.
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    Newtonian Astrograph; ATIK 383L+; EFW2 filter wheel; Astrodon Ha,LRGB filters; KWIQ/QHY5 guide
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