An entertaining and educational cloudy night activity is to go through the interferometry tests of various telescopes. Recently, while trying to decide whether to purchase a Celestron C11 SCT, I found myself looking at C11 interferometry as well as searching out opinions on the internet. EdgeHD versus standard C11? Are post Synta acquisition SCTs optically superior to pre-Synta Celestrons? Is the C11 a meaningful improvement over a C8 in light-gathering and resolution? Will greater sensitivity to seeing largely nullify the optical advantages of the C11 at my location? Opinion differed greatly, and even objective interferometry test results were highly variable. I decided that a summary compilation of test results for C8s and C11s would be helpful.
I simply went through the major interferometry test sites on the internet and found the highest reported Strehl value, regardless of wavelength. Only Strehl values without any correction/removal of detected spherical aberration, coma or astigmatism were used (tilt and defocus may have been removed). Most of these were for green light.
Trying to determine whether an SCT was of Synta origin wasn’t straightforward. I decided that only the Celestron SCTs with Synta style back cell would be classified as Synta (see picture). This means that some older Syntas may have been misclassified, but still permits looking at the often-expressed belief that the more recent the Synta Celestron SCTs have superior optics. Strehls were omitted if there was no picture of the back cell, or other indication of its origin (e.g., Edge).
These Strehl values come from three sources: 1) Wolfgang Rohr/Hassfurt, 🇩🇪 (http://r2.astro-fore...astrofotografie), 2) fidgor.narod/Moscow, 🇷🇺 (http://fidgor.narod.ru/Observers/test.html) or 3) AiryLab/Gréoux les Bains, 🇫🇷 (https://airylab.com/astronomy-test-reports/)
C11 and C8 Strehl ratios: overall averages and standard
deviations as well as for each interferometry test lab.
C11s (Strehl) pre-Synta Synta* EdgeHD
AiryLab average 0.931 0.893 0.853
n 3 4 2
standard deviation 0.012 0.092 0.095
fidgor.narod average 0.802 0.902 0.926
n 4 16 4
standard deviation 0.135 0.049 0.029
Wolfgang Rohr avg 0.949 0.962 0.952
n 4 6 1
standard deviation 0.029 0.015
Overall C11 average 0.890 0.914 0.909
n 11 26 7
standard deviation 0.103 0.054 0.059
C8s (Strehl) pre-Synta Synta* EdgeHD
AiryLab 0.957 0.96 0.96
n 1 1 1
fidgor.narod average 0.698 0.901 0.878
n 4 17 4
standard deviation 0.198 0.044 0.083
Wolfgang Rohr average 0.962 0.965
n 3 1
standard deviation. 0.022
Overall C8 average 0.837 0.904 0.894
n 8 19 5
standard deviation 0.181 0.046 0.065
There is little difference between the pre-Synta and Synta C11s averages (Strehls: .890 vs .914). However, the variability (standard deviation) of the pre-Syntas is roughly twice as much as the Synta Celestron SCTs (.103 vs .054). This suggests that manufacturing consistency and QC has improved. The widely accepted manufactures’ criteria for diffraction-limited telescope performance is a Strehl of .8 or higher. Improved consistency in manufacturing means that there are fewer ‘duds’, but possibly fewer ‘exceptional’ SCTs as well.
The picture for the C8s is a little different. The variability of the pre-Synta C8s is, again, greater than for the Synta Celestrons (standard deviations: .181 vs .046). Furthermore, the average Strehl for the pre-Synta C8s is lower than the Synta Celestron C8s (.837 vs .904). If these differences are real, it suggests that more care was taken in the manufacture of pre-Synta C11s compared to C8s, but that both are made to the same standards in the Synta era.
The Edge SCT test results were included in the Synta category, but were also looked at separately. Edge SCT average Strehls did not appear different from all Syntas for either the C8s (.894 vs .904) or the C11s (.909 vs .914). This is not surprising, since the purpose of the Edge optics is to correct off-axis aberrations inherent in conventional SCTs, but interferometry evaluates optics on-axis. Indeed, if we compare on- and off-axis Strehl ratios for a conventional SCT (figure 9.6, Telescopes, Eyepieces and Astrographs) versus an aplanatic Flat-Field SCT (figure 9.17) that “…differs only slightly from that of Celestron’s EdgeHD” (p. 256), we see that the Edge design has a slightly lower (-.08) Strehl on-axis, but a better and much wider acceptable performance range off-axis
Are these C8s and C11s representative of all Celestron SCTs? The numbers would suggest not, as there were fewer tested C8s (27) than C11s (37). This doesn’t reflect the production numbers, but may result from the willingness of owners/ buyers of the more expensive C11s to pay for interferometry. Also, there were far fewer pre-Synta than Synta ‘scopes being tested. Access to interferometry is a relatively new. Understandably, there seems to be less interest in paying to have an old ‘scope tested.
Differences in the average Strehl values reported by the test sites may be partially explained by differences in test equipment and protocols. In my inexpert opinion, Wolfgang Rohr’s methodology seems to be comparable to others, and, of itself shouldn’t explain the difference between his ratings of the C11 and the other test sites. What accounts for his higher Strehls of the tested C11s? Possibly it is a sampling issue. His test reports suggest that his clients are often demanding amateurs who already possess an excellent instrument that is being tested as part of a repair, or who just want verification that their prized telescope is as good as they believe (or, perhaps, are obsessing over an imagined defect—something that many amateurs can relate to, myself included). On the other hand, the fidgor.narod site often indicates that theirs are new telescopes being tested prior to sale, or used ‘scopes being evaluated as part of a re-sale. Consequently, theirs may be a more representative sample of Celestron SCTs.
G. S. Smith, R. Ceragioli & R. Berry Telescopes, Eyepieces and Astrographs: Design, Analysis and Performance of Modern Astronomical Optics, Willmann-Bell, 2012.