Interestingly in a dicussing with another observer we conlcuded that the seperation between the pair can be measured to far better accuracy than the diffration limit of the scope itself. (1.8" for 2.4-inch). If the airy disks are magnified enough (high power eyepiece) so they are large enough one only need to measure the speration between the centres of the disks with a micrometer. It theoretically should then be possible to measure the sepration of Porrima to fraction of an acrsecond with a 2.4 inch refractor!
Hello Marty0750, I was surprised when I did this observation with the small Vixen FL55SS, specially because it has only 300mm focal length, I usually use it as a wide field telescope.
The first thing I noticed was that it is less affected by seeing, and the Airy disks and first ring were very stable. I haven't done other double star observations with it, but I must try some more.
The theoretical limit for this telescope is 2.11" Dawes, and 2.51" Rayleigh. So Porrima at 2.81" is feasible.
I don't know what you mean by a micrometer, but I do have an astrometric eyepiece that I haven't used in quite a while, mainly because I think it's accuracy depends on magnification, meaning that greater magnifications will give better results.
Here is a sketch done a few years ago using the astrometric eyepiece:
The star is barely visible on the sketched scale, but it's there
The SD number is obtained after running a star several times through the central scale, and averaging the time of the runs. It gives you the true size of the scale for the telescope in use.
From what we can see here a 2.8" separation needs a lot more magnification and a larger aperture to be more accurately measured.