Switching mirrors need help

I have a new stock Orion xt8 f5.9 8" dob. I picked up an older discovery 8" dob with a nice Pyrex f6 mirror. The condition of the Discovery tube is soso.
I would like to put the mirror from the Discovery into the xt8 tube. I was told the focal length difference would be about 3/4 of an inch. I measured from the face of the primary to the center of the secondary on both, and sure enough, the difference was 3/4 of an inch. The other issue is the Discovery is a 1.5 inch mirror and the Orion is about 3/4 of an inch. So now I have 1.25" difference.
Now, here's the question. Would a simple focuser extension do the trick for the extra length needed or would I be better trying to use the discovery tube which is a little on the rough side?

Victor: Is there good reason to believe the Discovery Mirror is superior to the Orion mirror?

Jon

First, have both mirrors tested (or use both of them for a bit) to make sure that one mirror is in fact better than the other.

Then, it is just a question of the mechanics of the thing. As I understand it, you need to move the focuser and the mirror about 1.25 inches further apart.

Before you make any modifications, you may want to check things out. When you use your eyepieces, how racked out is your focuser? It could be that you have enough out-travel to accomodate your eyepiece collection without any modification, even if you change the mirror.

If you still want to change your mirror, and you need to make other modifications, you can get that distance several ways.

The easiest is to do what you suggest--use an extender at the focuser. Just an additional 1.25 inch (long) tube stuck in the focuser tube, and from that point on, just fit your eyepieces to that.

Another way to do it is to raise your focuser up off the tube by inserting a couple of spacers (or something).

With both of the above solutions, you may experience some vignetting--although I doubt it would be noticeable. Some loss of stability may happen, also, but on a good old 8 inch F6--you'll be fine.

A third way to do the trick is to cut a new hole for the focuser 1.25 inches further forward. This can get messy quick, involving a change of the secondary spider, etc.

Another easy way to do it--if your tube and how it swings through the base will accommodate it, is to move the mirror cell back a bit toward the end of the tube. Be careful to re-balance when you do this, perhaps moving the bearings.

And if it turns oout you just need a bit of movement, you may get this with just changing where your collimation screws are set.

(I'd either get some focus tube extenders or put the focuser on stilts.)

Alex