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Is the emphasis here guiding in NIR or using Sharplock without an ONAG?
There are some gains to be had from guiding in NIR with the aid of a bandpass filter (as Lee is doing with the OAG), I've done it using an Orion 70mm guider/finder and an Astronomik ProPlanet 807. Most filters (ProPlanet included) will still transmit varying amounts of light below the NIR, though.
Going down that path means no SharpLock and leaves you at the mercy of incidental flexure, etc. I wouldn't recommend DIY with a cold/hot mirror, a few folks have tried it with off-the-shelf parts and ran into issues with optical aberration.
An OAG + bandpass filter would give you some NIR guiding but finding the right star in the limited FoV is challenging.
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