With my above post in mind, here is some information that you might find useful.
The "Optiamal" back focus of Celestron SCTs other than the C14 appears to be about 96mm. If the scope were perfectly made, this would give a focal lenght of about 2032 (f/10) and assuming again that the spacing were perfect, this would give perfect SA correction.
Now, let's get down to reality.. The reality is that it is rare to have an SCT that has perfect spherical aberration correction. Now just because time is money, the sooner the worker gets the mirror off of the grinder, the sooner they can start production on another mirror, and for this reason, the predominant spherical aberration correction in most telescopes (if there is any) is undercorrection.
Now the interesting thing about the SCT design is that it is actually possible to improve SA correction by increasing back focus.
And this gets to your question. While the optimal back focus by design is 96mm, most SCTs will not have perfect correction at this amount of back focus.
To know the perfect back focus for your scope, it would be necessary to see how much spherical aberration is present when the scope is set in the configuration you normally use.
The easiest way to do this is to use the star test with 4.4mm of inward focus (as measured at the eyepiece) and 4.4mm of outward focus. If you do this test and the secondary shadow ratio (size of shadow vs the diameter of the rings where perfect would be exactly 1/3rd of that pattern size if the obstrution was 33%) then the SA correction is perfect.
If it were not perfect, in theory, you could shorten or lengthen the light path to change the corrector/primary spacing to make the SA perfect. The amount you would have to move it is given in the post above (25mm to achieve a 1/23rd change in SA correction). So, if you determined that you had 1/8th wave of undercorrection, you could move the back focus about 75mmmm further to the rear (3/23rds or about 1/8th wave) and get the correction perfect.
If though, you actually had an SCT with only 1/8th of a wave of correction, moving the focal plane to the rear by 75mm would not make a change in contrast you could see (though a camera can see contrast change to very tiny levels.) If you had larger amounts, so if you had larger amounts, the physical limits to the configuration might become undesirable, and the baffle design usually makes between 170mm and 230mm the practical limit because past this and the scope may start to loose aperture. And of course if you had an SCT with 1/8th wave of SA, this is about as good as they typically get and since it would be impossible to improve the visual contrast change past this, there would be little reason to do so.
As photoracer has kind of indicated, a small amount of change (50mm) is going to barely make a difference that you can see, so unless your SA was very strong, there would be little merit in attempting to change it because it could take more back space than in piratical to do so and if you SA is very bad, the physical configuration would put top end limit of the amount of back focus you could add.
Bottom line: As long as you stay reasonably close to the nominal back focus (96mm for most SCTs) there is no point in worrying about the change in SA and photoracer's advice is pretty valid.
The primary time this is important is in the specialized cases of attempting to use excessive focal reduction and/or Binoviewers, where SA can push the correction below the diffraction limit. I have tested configurations that have shown as much as 2/3rd wave of SA. It is easy to see the amount of contrast 2/3rds of a wave makes. It is virtually impossible to see the difference 1/8th wave makes.
But putting a 2" diagonal or reverting to the factory supplied components and their 96mm back spacing won't budge the ball enough to see. If it is really bad, you can't really fix it, and if it is not that bad, making small changes is not going to make enough difference to see.
And to re-iterate, unless you test, you won't know. If you do the above test with your focal reducer in place (though you have to reduce the amount of in and out focus to about 3.1mm) and the secondary shadow is roughly the same size, then you are good to go. If you observe a lot of SA, then remove it for planetary observing.
Edited by Eddgie, 21 June 2019 - 12:05 PM.