Some posts back you were setting up an optical bench but you already have everything to hand to assess the DX-8 even if there is no flat (?). Why not use the Tinsley 6'' in a single pass collimated test? Not perfect but it'll give you a very good map of the sct's wavefront.
Layout is simple; torch/diffuser, slit at focus of DX-8 then in reverse; cassegrain facing the Dynamax and knife edge at it's focus. Or, instead of knife edge, Ronchi grating in and around focus. The knife edge can be quantitative. The two optical axes should be parallel but can be, should be off centre.
you just went above his paygrade with those instructions
Not at all! But I should have taken some more time.
Perhaps you have done a knife edge test at the ROC of a spherical mirror? This is just the same. What does a perfect sphere show? Why the even greying? What if there are zones or a turned edge or it's smooth but not spherical. Why are some parts then lighter and others darker? How can you measure what you see in terms of waves of error?
The slit at the focus of the first telescope acts like a point source of light.. A pinhole could be used but the slit is easier. Make it fine. Although either scope can be used as the collimator it's maybe better to use the Tinsley rather than the DX-8 here. Then alternative scopes can be tested.
Light travels in reverse through the scope and exits as a collimated beam of parallel light 6inches across. This beam's wavefront is as perfectly plane as the errors of the Tinsley allows. Now, this 'perfect' collimated 'starlight' enters the front of the telescope to be tested and is brought to a focus where it can be tested by a Ronchi grating, a knife edge or viewed with an eyepiece. As the light passes through the faulty optics of the scope under test, parts or zones of the wavefront, which now should become uniformly spherical, are advanced or retarded by whatever errors they've met along the way and will then focus longer or shorter. This is what's shown by the knife edge or Ronchi. It's good to have the two alternative tests. For example the Ronchi may show a diffraction curling back effect around the edge which might be mistaken for a turned down edge. The knife edge can be used to check on this.
A couple of practical tips: make the slit 1/4inch long, tape the blades onto a flat eg glass surface. On the glass side away from the scope and facing the torch tape something eg tissue paper to diffuse the light.
At the other scope's focus ensure the testing knife edge or Ronchi is aligned parallel with the slit which will be obvious in practise.
Try it on a Zeiss and see what perfection looks like. Compare that with the DX-8 and maybe think Wow! the errors are set out in such detail and so precisely. Swap for the Ronchi and compare. Which tells me more. Swap for the eyepiece and check/confirm astigmatism and general standard of imagery.
Many contributing to and following this thread will be practical, especially BB. I see long hours of careful, skillful work spent on restoring classic scopes. So much for pay grades.
Most in this forum will have a collection of scopes including some with excellent optics, better than 1/8 wave and maybe better than many flats.
You can set up these tests with a Ronchi or knife edge, with a flat or auxiliary scope. Pros and cons. Versions of the same thing. I find the knife edge test quick, easy, informative and fun. It takes maybe half an hour to set up. Why not try it and see for yourself?
Yes, as Greyday says, pictures would explain things best. I must work on that.