Using a refractor telescope 5" aperture, 900mm focal length for terrestrial viewing Can I use a refractor telescope 5&quot; aperture, 900mm focal length on an equatorial mount for terrestrial viewing.
Yes, potentially.  There are a few limitations:

It will be easier to use in alt azimuth mode, which in practice means setting the polar axis to 90 (ie straight up).  Not all equatorial mounts can do this.

With the standard astronomical diagonal, images will be inverted left to right.  You can get diagonals for terrestrial viewing that will fix this.  If you don't use a diagonal, the image will be upside down.

The field of view will be quite restricted if you use a typical eyepiece.  For example a 20mm plossl would give a magnification of 45 and a field of view of about 1 degree.  To get more useful field of view, you would need longer focal length eyepieces, with with a wide angle design.  These are more expensive than ordinary eyepieces.

Unless your scope is an apo, you may see quite noticeable chromatic aberration, particularly at the edges of objects with a bright background (eg birds).