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Hi Pete,
The Frosty Leo Nebula (Designator IRAS 09371+1212; RA 09 39 54.0, DEC +11 58 53) is a protoplanetary nebula. I don't think a filter will help you much with this target as protoplanetaries behave more like reflection nebula, than true planetaries, and the filter will usually just knock em out. Much like a filter does on the Homunculus.
I have only observed it twice in my 18" scope, under dark still conditions on both occasions. I observed it at 100X and it appears just off stellar. At 200X it is well elongated and is clearly a non stellar object. At 300X and 405X it appears as a white elongated smudge about 10" X 7" in apparent size. It is fairly small but reasonably bright at about mag 11.5. Keep in mind its relative size is small, so it needs plenty of power to blow it out and increase the contrast. For reference the Homunculus is about 18" x 10".
Whilst I haven't tried it in my 10" or 14" scopes I have no doubt, whilst it won't be easy, it is visible in a good telescope over about 10" or 12" aperture, under good conditions, by a skilled observer.
Cheers,
John B
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