Seeing here is usually so bad, I often resort to using an artificial star for collimation and sometimes for a star test; though I know the latter is a slippery slope.
This is the formula, as I understand it, to determine how far an artificial star should be from the scope for star testing...
Determining SA for Near Focus Artificial Star:
N (ft) = 28 ( D/F )²
N is the distance where 1/4λ spherical aberration in the form of under-correction is added due to proximity.
D is the Objective diameter in inches
F is the focal ratio
So, as an example, lets say you have a 16" Newtonian, f/4.
By the formula, if the artificial star is 448 ft from the scope, you will get 1/4λ spherical aberration of under-correction added to whatever the mirror already has. To reduce that to 1/16 λ spherical aberration of added under-correction, so it wouldn't affect a star test as much, the artificial star must be a whopping 1792 ft distant. That's a third of a mile, or nearly 6 football fields!
This approach supposedly works for a Newtonian reflector. For some reason, the distance for a refractor is much more forgiving, and the artificial star can be closer without adding nearly as much proximity under-correction. I don't know if there is a different formula for a refractor, however.
My question is for an SCT, specifically my vintage C-8.
If, and this is what I don't know, the formula applies to an SCT the same way it would for a reflector, the result will give you a distance of about 18 ft with the added 1/4λ spherical aberration. And further, if you wanted to reduce that added error to 1/16λ, The artificial star would need to be a scant 72 ft distant!
In fact, if you can get the artificial star 144 ft away from the C-8, the added under-correction would only be 1/32λ. And that distance is doable in my back yard!
I'll add that I know the light source must be small enough as to be seen as a point source, not an extended object. I use a small (1/8") ball bearing with the sun, or a small but bright, single bulb LED flashlight several feet away at night.
So, after all this rambling, my question is... will this arrangement work as a star test for the C-8 if said light source is 144ft or more distant?
Believe it or not, the image is affected by seeing even at that distance on the ground over grass. But not nearly as much as it would be through my NJ atmosphere.
Thanks very much!