And for those that do not know how to judge the severity of a turned edge, here is the way it works.
Many think it is the severity of the angle that is the major attribute of a turned edge that affects the image but in reality, it is the width of the turned edge that is more significant.
The physics here is just based on simple geometry.
Suppose you had a 1mm wide band running around the diameter of the mirror or lens 50% from the center of the mirror that had a little ridge ( called an S zone). Because the diameter of that band is smaller than it would be at the edge of the mirror, there is less surface area (light reflecting area) that is exposed to the incoming light, and that means that this small amount of energy and due to the shallow angle, that energy is broadcast over the entire field does little damage.
If though, you made that small band 3mm wide, you are tripling the amount of energy involved (and this energy is also taking away from the total energy at the center of the field.)
Now because the diameter of a 1mm band out by the edge of a mirror would be far larger, the total surface area affected would be many times greater than the 1mm band closer to the center of the mirror so this is why a zonal error is more serious if it is further from the center of the mirror. It is the simple geometry that the further from the center of the mirror the band is, for a give width, there is more surface area involved and this means more energy that is going somewhere rather than where it is supposed to go.
Now looking at the TEC vs the C5, the total amount of area that is involved in each is considerably different. In the TEC, it is this tiny tiny little sliver right at the very edge of the mirror and while there is more area encompassed in this band than if the error was near the center of the mirror, it is still a very very tiny amount of surface.
In the C5 though, it is clear that the turned edge (though I believe it to be a S zone, but a turned edge is essentially an S zone that has the outer half of the ring sliced off) extends far deeper into the diameter of the mirror. The C5 S zone or turned edge is covering far more total area than the zone or turned edge in the TEC does, and appears far sharper. But look at the total area of the mirror involved in the C5. If the mirror was unobstructed, a 5mm wide zone at the outside of the mirror would cover 1.3 square inches of the mirrors total area, but in the C5, we only get to use 90% of the mirrors area, so of the total area avialable (lets say 15 square inches) the zonal area would constitute almost 10% of the total energy that was falling on to its surface. (I am not trying to estimate the area involved in the C5 above, only using a figure of convenience.)
I see a lot of tests with a very narrow but sharp turned edge and because of the surface area involved, it is hard to believe that this small amount of area would be of any consequence (I mean it all has a consequence, but I am speaking of a consequence that would have a meaningful affect on performance). I do see tests though where there is either an S zone or turned edge that is wide enough to involve a significant amount of the mirror's or lenses total area, and these are the ones that are going to have an effect on the image.
Sometimes we see zones in premium refractors (sub aperture polishing?) but these zones are often closer to the center (less area) and mild, so once again, since there is less area, the zone would do far less damage than if a zone of that same width and height was moved out to the edge of the mirror, were far more surface was affected.
An S zone near the center of the optic will have the tendency to broadcast the energy over the entire field as a veiling glare, but since the total area is low and the energy is diffused over the entire field, the zone would have to be quite severe to have much consequence. The consequence would be that the background glow across the whole field would rise very very very slightly. The global conrtrast transfer (the contrast of all size features) would be lowered perhaps a fraction of a percent or in other words, far below the amount that even the most skillful observer in the world would be able to see.
When it is at the edge of the field, the energy tends to be directed and concentrated towards the center of the field, and because more area is involved, and that energy is now being concentrated, a strong zone or wide turned edge will have a meaningful affect on planetary observing.
I am sure many readers already have read Suiter's book on star testing and know how each kind of error manifests itself in terms of contrast transfer function, but for those that don't this post was intended to explain what is going on with turned edges (a kind of zonal error) and how to know when and when not to panic. A razor thin and mild zone at the edge of the lens or mirror is not fun to see, but if it does not show in the star test, it is safe to ignore it. If on the other hand, the zone is wide and steep, and can easily be seen in the star test or DPAC or even single pass Ronchi test, the effect can be so serious that for best planetary work, one needs to make the decision whether it is better to mask off the zonal error and reduce the aperture, or live with the zone. This can be calculated and modeled, but the easiest thing to do would be to make a mask that covered the zone and compare with and without the mask in place.
With the C5, I see some zonal errors that I take to be meaningful. With the TEC, while I do see a slight mild turned edge, I doubt that it has any meaningful affect on Strehl or contrast transfer. These are super super super sensitive tests. That is why people that want to assess optical quality use them. They disclose even the tiniest and most inconsequential error.
One of these instruments is near perfect, one is further away. How far away each is, I can only guess. My guess is that the C5 is perhaps a bit below average in terms of mass produced quality standards, but as I have said twice before, I do not know how to quantify DPAC tests and am just correlating these images to tests I have seen elsewhere that also included interferometer tests. Since I don't consider even an excellent C5 to be a good choice for serious planetary observing, it gets a pass from me because it is an inexpensive, mass produced instrument, and I hold no great expectation that such an instrument will have high quality optics. For the money, I would have been shocked if it looked close to the TEC. Not that this never happens, but I just would not expect it to happen and people buying MCTs and SCTs at this price point would be well served to adopt the same expectation.
Edited by Eddgie, 13 February 2020 - 10:42 AM.