Question about Secondary Mirror

I have an old coulter 13.1 inch dobsonian with a seemingly hugh secondary attached to a thick metal bar. Its original. I'm considering replacing the secondary and holder. I understand somewhat that the size of the secondary is related to the size of the primary, something to do with the lightcone. I'm not sure at all. I thing this original secondary obstructs someting like 30 percent or so. It looks like a monster when defocused. And it will show it if not in dark skies. Does anyone have any suggestions about a good replacement? What is the minimum size I can use. Thanks

As I recall (I once owned one of these) I think this thing uses a 3.1" secondary which is pretty proper for this size & focal ratio scope, but uses an overly-large and oddly-shaped secondary holder. Yes the support bar is thick on those babies.

I would recommend replacing with a thinner more classical 3-vane or 4-vane spider from Novak or similar. You'll have more diffraction spikes but they will be *much* dimmer than the big bright line you see running through stars right now. If you don't like spikes at all you could get a 3-vane curved spider from 1800 Destiny or something similar.