Using the 7" on Arcturus [low in the west] provided a much brighter image than the Vixen had. The images were larger and still visible on the camera's LCD viewing screen after I had taken each 'snap.'
I was delighted to see the bands were straight by any reasonable standards when well within focus. Exactly how accurate these images are in giving an insight into the spherical correction of the iStar lens I have no real idea.
There was much more movement in the atmosphere tonight after a long sunny day when it reached 70F. Perhaps it was just the larger aperture exposing the thermal seeing?
The first two images have been converted to B&W and maximized for the colour which provided the greatest contrast. Usually green seems best.
All the other images are unfiltered and were brightened considerably in Photofiltre and then cropped for far greater image size on the blog. In each case I joggled between Gamma and Contrast to make the most of the visible bands.
These coloured [white light] images exaggerate the colour well beyond anything I was seeing visually. The camera cannot ignore colour as can the elderly human eye.
The bands can be seen to spread apart and grow broader as one moves towards focus from well inside.
Note how the bands change from one snap to the next. I would probably be foolish to read too much into any particular test image.
I am looking forward to using the Solar Continuum green filter to reduce the multitude of rainbow colours and hopefully darken and sharpen the bands.
The bands are growing wider with each new image which should be a better indication of the actual figure on the complete lens.
In some of the images the edge looks rather soft while in others it looks slightly better.
There does seem to be a definite tendency for the bands to bend outwards towards the edge.
This area can be removed by stopping the lens down slightly. This also improves the colour correction since the lens becomes 'slower.'
Its aperture becomes smaller for the same focal length. Changing the focal ratio in this way reduces chromatic aberration and makes the lens less sensitive to spherical error.
It remains a mystery why I saw such curved lines on the first attempt to using the Neumann Ronchi test eyepiece on the iStar lens.
I have been worrying about this for some time despite the very reasonable images I get visually and when imaging. Though admittedly the iStar is not a patch on my 10" f/8 Newtonian reflector.
I will update this post after further overnight thought.
I am a little confused by the concentric ring inside the image.
It is almost as if it is a double exposure at two zoom settings. Further reading suggests the parallel sides of the transparent media on which the Ronchi grating is etched cause this.
The 45 degree Baader diagonal has been eliminated for these images. I used a star diagonal instead.
I think it is fair to say that the bands are curved inwards towards the center. With a stronger curve nearer the edges. If this was a mirror it would be an oblate spheroid with a turned up edge.
It is decades since I did any serious optical testing and that was almost always on my own ATM mirrors.
I shall have to do some reading on how the bands appear on lenses compared with mirrors.