This is the second part of the "testing of the new GSO RC8 telescope". If you're interested, the first part is found here: M56, Testing the new GSO RC8 telescope PART I

...PART I continued:

[...Doing some raytracing simulations, it turned out that there's a big difference in image quality if back focus is changed from 270 to 254. Therefore, I've chosen to change the mirror spacing to achieve the specified back focus.]

Once I did that, I had to recenter all the optics again. With some practice this job takes about 15 minutes. 5 Minutes if one doesn't use the artificial star in the end.

Now to the actual image assessment. A good target is dense star fields. Even with the nearly fully illuminated moon, it's not going to invalidate most of the conclusions one can draw from these images. I've chosen my DSLR to capture the images since it's the camera with the largest sensor in my pool.

I've chosen to capture two targets since M56 went behind some terrestrial objects. The M39 contains much brighter stars and during that night, the transparency was very bad, leading to the halos that are visible in the image. Yet, the field properties behave rather consistently.

For me, the conclusion is that I can use the scope without corrector if I'm shooting with my ASI294. Anything larger might require a corrector, if one is very picky about the stars. Besides that, I am happy with the quality of the optics so far.

Let's hope that weather permits some nice images in the near future.

A final note on my simple "collimation" procedure: it turns out, the backyard adjustment with laser, Cheshire and artificial star leads to an alignment that only needs slightest adjustments on the secondary. Slightest means that the alignment screws needed turns of less than approx. 10 degrees.
It shouldn't surprise too much as the procedure is about rotational symmetry around the optical axis. The distance of the artificial star doesn't matter that much in that case. The only caveat here is that as the artificial star gets closer to the scope, the further the focus travels backwards. Hence, misalignments of the focuser or its sag my amplify and lead to a shift in the optical axis. Therefore the final tweak on the real star to finalize the alignment or at least confirm it.

Feel free to comment.

Björn