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I repeat:
On the Canon DSLR >400D there are two filters:
A colour correction filter- which is normally removed for improved Ha transmission
and the front anti-alias filter which also acts as a UV-IR cut filter. This is normally retained.
If you remove the colour correction filter with no replacement it works 100% for astro with the scope etc, but slightly compromises the auto focus with standard lenses.
The CC can be replaced with plain glass (1mm thick). Some modders use the more easily available Baader UV-IR replacement ( not for the UV-IR cut but the price!!)
I use the clip-in CL Astronomics filter (only works with the EF lenses) to allow me to get good focus and use the modded camera with std lenses and an objective grating.
Hope this helps.
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