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I have a spatial filter, which I bought about 10 years ago.
I need a new microscope objective for this.
How do I determine, if the microscope objective has metric or english thread?
Or are there no different threads with microscope objectives, and any fits?

I have this one:
http://en.opmount.com.tw/product.php?Lv ... AutoID=236

And second, as I remember I had bought microscope objectives which couldn't be used in a spatial filter,
because the focus was inside the optics of the microscope objective. How can I distinguish, and how can I be sure it is a objective that has the focus outside the optics?

I use a Newport 900 SF (almost sure!) with M-series optical objetives. It uses a standard RMS thread, 0.800-36. It came with M-10x objective (sure..)

Probably, (Taiwan!) it is metric, see the precision numeration screw.