Measurements, yes, that's one caveat I have to add when comparing your image to what I've seen on my NP127is. Although your stars look well-formed, round, and relatively small without something like a FWHM measurement it's kind of hard to make an absolute judgement on image quality. That being, small and round stars are difficult (my criteria for the NP127is) while just being round may not tell you much. On nights with poor seeing I can get very round stars but they are also significantly larger than when I have a night with good seeing (and focus). The differences can be very dramatic, going from a FWHM of five arc seconds on a "bad" night to something under two arc seconds on a good night (or even approaching one arc second in H-alpha).
That said, your stars look very good for roundness and the center of the globular cluster looks well resolved, suggesting pretty small and tight stars. But, I'm looking at a fully processed image which can also introduce its own problems when evaluating image quality. So, if you do take some FWHM and eccentricity measurements remember that those have to be taken on the linear original or master integrations before any form of processing (except calibration and possibly stacking). That is, no histogram adjustments and certainly nothing like sharpening or deconvolution.
In the end, however, it's really how the images appear to your own eyes. Meaning if it looks fine to you then things like FWHM and eccentricity really don't matter much.
In terms of other samples for the NP127is, I haven't been able to find much. Most images you see are fully processed and that can change the image quality in both good and bad ways. Also, it isn't that common to find image samples that are presented at full, original scale and then there is the issue of the pixel size of the camera and the linear size of the sensor. The thread on image quality that I started several years ago shows the results from at least one other NP127 (the older non-IS version), but in that case the owner was using a fairly small sensor.
The only other thread on Nagler-Petzval image quality that I know of here on CN ended when the owner returned his scope with a partial credit (from the vendor) after first allowing Tele Vue to check and adjust the scope back at the factory. He wasn't happy with the full-field coverage given by the NP101is and finally purchased a Takahashi as a replacement (given a 60% credit on the NP101is). Here is that thread:
All this said, I think I've produced some very good images with my NP127is. In fact, if you search here on CN you'll probably find that I've been the principal contributor of images that have been taken with that scope (perhaps even if you count everything from both the NP127 and the NP101). Of course, samples vary, so I can't really say whether my experience represents the best or worst of what Tele Vue has to offer. I doubt, however, that it is either the best or worst.
Edited by james7ca, 28 May 2019 - 09:08 PM.