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Stick to the 500mm f/4L, you can use it as a telescope. For planetary imaging, use a teleconverter (e.g. Canon extender 2x) and a 3x Barlow, which makes it f/24.
If you should purchase it new, then an APO refractor is cheaper, but you already have it.
It has an excellent quality and is even lighter than a 125mm APO refractor.
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