I think a reducer won't hurt in most situations but there are several things going on, and it isn't simply turning a 10" f/8 guidestar into a 10" f/4 guidestar.
OAG optics are simple in principle but there are subtle things involved that can have a big impact on guidestar quality. So a while ago I made a tool to visualize OAG geometry and the light loss involved. Here is an example:
A key thing to note here is that the cone of light usually doesn't fit into the prism - and all that light is lost. The focal length is the same, but you have effectively lost aperture. So an 8" f/8 scope may end up with a 6" f/10.4 guidestar. You can then go ahead and add a reducer and it will make it faster, but with that same, smaller, aperture - so maybe it is a 6" f/8 guidestar.
But if you had used a slower scope in the first place, the light cone might fit into the prism and you could have an 8" f/10 scope with an 8" f/10 guidestar. Although that may seem "slow" - it is full aperture and there won't be any distortions caused by having the cone clipped on the way to the sensor. So that 8" f/10 guidestar may be perfectly fine - in terms of being bright and a good size - i.e. not too small and not too big on the scale of the pixels.
The key to getting the light cone to fit into the prism is to have the guide sensor as close to the prism as possible. If you don't and the sensor is far away, you may be able to bring the sensor closer to the prism by inserting a reducing lens - but you won't gain any of the light lost entering the prism.
So I doubt that a reducing lens would really be beneficial if the oag is set up right in the first place. And no matter what the oag should have a wide view of the prism - and a good size prism. If the oag restricts light from the pupil, that is a big problem right there. And on the topic of this thread - the last thing I would want to do if I have light loss, is insert a filter that throws away even more photons.
Edited by freestar8n, 24 July 2021 - 06:34 PM.