Hi Stephen, Part I - your scope.
You will need a pretty steady mount for this, and one that tracks. On an unguided mount this is very difficult.
1. Choose a bright star near the zenith mag. 1-3 are ideal - altitude at least 70 degrees above the horizon.
2. Align scope on the star.
3. Make sure the optics are collimated to the absolute best you can, this is a subject in itself.
4. Precisely centre the star in the scope, and increase magnification to at least 2X per mm of aperture (changing eyepieces as needed) and centre it if need be.
5. Accurately focus. You should be able to clearly see the Airy disk and a couple of rings around the star at best focus.
Defocus one way (let’s assume inwards) by a small amount until the central Airy disk is gone and you have 2-3 concentric tiny rings. In the centre of these you will still see a tiny dot of light - the Poisson spot - which is caused by diffraction around the central obstruction in a Newtonian or catadioptric scope.
6. Compare with the images at
https://www.telescope-optics.net/aberrations_point.htm
And
https://www.telescope-optics.net/dif...berrations.htm
7. Refocus and then defocus the other way, and repeat.
Note that the Poisson spot is useful it shows where the exact optical axis is.
Lastly at step 5 there are situations where you may nit be able to clearly see the Airy disk and one or two rings:
- poor seeing,
- thermal issues within the telescope causing air currents,
- poor optics.
I have seen scopes that optically will not show the Airt disk - at high power they give a blurry mess. Sometimes the optics are pinched or strained because someone has over tightened something, or a mirror is badly supported. But some are just plain bad.