Maciek_Cz

@jetstream Here are the results for the TEC160ED f/8 (spherical aberration only): green 1/12 - 0.97 red 1/10 - 0.96 blue 1/5 - 0.87 I am not talking about other brands here. For some, it's probably similar to what SV does. We are talking about few tested specimens of SV telescopes. We have SVX180T and one SVX127D that have this problem and on the other hand we have another SVX127D and SVR102T one excellent and one very good. Now, as for the distortion of the lens of the camera taking the photo of the ronchigram, of course, a weak lens can deform the ronchigram, but then the whole would be stretched, not just the bands. From what I've read in the DPAC threads on CN, they all use good lenses and cameras. In general, all ronchigrams are as if stretched in a plane perpendicular to the lines of the Ronchi screen. But this stretching is not caused by the optics of the camera but by the Talbot effect (diffraction on the Ronchi screen). This does not cause distortion of the bands, but only their duplication, which does not interfere with the assessment of the curvature.

The orientation effect is probably there, but with a properly constructed cell and even pressure of the lens blocking ring, it is insignificant. In this case, the manufacturer is definitely the culprit of this state of affairs. Of course, SV lenses meet the SV specification. So a high Strehl for the red color. Unfortunately, the lack of control for the wider spectrum leaves the other colors with a rather random spherical aberration correction. This is clearly shown by the DPAC tests. The curvature of the Ronchi bands cannot be explained by anything other than the presence of excess SA. The DPAC for red shows a great SA correction for this line. Which immediately suggests that the tests were carried out very carefully, in thermal equilibrium conditions.

@Jim L, Of course you're right, variables such as ambient temperature and thermal stability are essential for this type of measurement. A colleague Jeff from CN showed in one of his threads DPAC measurements performed during the optics' coming to a state of thermal equilibrium with the environment. SA correction changes for all colors over time were significant! Scott, who studied the first of the two SVX127D samples, says nothing about thermal stabilization time. Moshen also makes no mention of it. However, if it was about the lack of a fixed temperature, the correction in red would also be worse, which is not the case. The position of a lens of this size doesn't matter at all. If this were the case, we would see astigmatism or coma depending on the height above the horizon of the observed object. And that's not what happens. This would be a significant design flaw of the lens cell. Now I will refer to the internal standards and the ZYGO test. As I wrote earlier, perfect tuning for the red light line does not guarantee the same for the other lines (green and blue). With red only, they were able to eliminate SA for that color (red only) and coma, astigmatism, zones, and turned edges for all colors (red, green, and blue). To sum up, the authors of the DPAC tests should be asked about the thermal stabilization time of the tested telescopes. Each measurement of this type requires diligence, honesty, certain knowledge and significant time, above all, in the interpretation of the results. I also don't like how excessive optimism and excitement about the topic affect the interpretation of the results. I've often noticed this in some threads where AOS was used. However, anyone who has ronchigrams available on the forum can repeat the analysis himself and check whether the author of the results has not suffered too much fantasy. I do just that.

I will only add that this analysis takes into account only spherical aberration. Please tell me if the focal ratio is correct because I found information about the TEC 160 f/7 but also f/8. The given focal length significantly influences the result. If it's f/8 then we're dealing with an even better result around ~0.89.

@jetstream ready:-) This photo was taken by Jeff. Its grid is 5,236 lines per mm. The lens is f/7. Below is the result.

Ok, I can do the analysis. Just give me the parameters of the Ronchi screen (lines per mm) and the focal length of the lens.

Hi Jim, I won't say anything about anti-SV group because I don't know anything about it. However, I can comment on the tested few SV telescopes. As for the model in the title of this thread, I analyzed both DPAC tests with the AOS program of my authorship. It is precisely about estimating the correction of spherical aberration. The recently tested copy is indeed perfect, this is beyond dispute. However, the one tested earlier is simply good. I wonder what the differences are. And indeed the only thing that comes to my mind is the lack of quality control for wavelengths other than the one used in the ZYGO interferometer used by SV. We have two telescopes, the same models, from the same manufacturer with very similar certificates provided by the manufacturer. However, the DPAC tests show that they differ significantly, one has an SA correction of 1/10 lambda or better over a wide range of the light spectrum, the other has such a good correction only for red light, green is at the level of 1/6-1 /7 lambda, and blue 1/4 lambda. Clearly, the manufacturer may not have had any idea about the situation for green and blue. With the help of control on ZYGO, he minimized astigmatism, coma, zones and edge. However, spherochromatism cannot be predicted by testing only one of the wavelengths. There is a risk that not all of these telescopes are as finely corrected as the last one studied. However, there is nothing to complain about, this earlier copy is good. It all depends on what the buyer expects. I suspect most amateur astronomers wouldn't be able to tell the difference between the two telescopes when observing the sky.

I think your assessment is very unfair. Many of the ATM specialists have experience in the field of optics and quality analysis with the help of interferometry, the Foucault/Ronchi test and others comparable or maybe even the same as SV employees. The "DPAC" is such a simple test that you don't really need any knowledge to perform it correctly. It seems to me that it is the opposite of what you wrote, that people who complain about these tests have no clue how they are carried out and how they work.

My SW102 f/9.8 with a Baader Q-Turret and Plossl eyepieces.

Hi, I have a similar setup, AR90s on AZ Nano mount from Bresser.