I am on a time-sensitive search to determine how much the inherent figuring of a craftsman-style Newtonian mirror influences the occurrence of pincushion distortion. I am considering buying an OTA that has a 10" mirror made by a craftsman that has no "certificate" or independent data to "certify" the mirror quality. The seller provided a terrestrial (cellphone) image taken with through a 13mm fl - 82° FOV lens that showed a nearly 3% pincushion distortion (by my crude estimation) at the outer 20% zone (estimated from the approximated center). Forgive my lack of technical knowledge on the subject of mirror figuring and measurement Also, bear in mind that I estimated the percent distortion by reviewing a limited supply of web pages on the subject
I don't know enough to tell if this distortion is influenced by the mirror figuring or if its from the eyepiece. My readings on the subject suggests the eyepiece is the dominant source but I cannot afford to buy a mirror that contributes to such effects that I can avoid by getting a mirror yanked out of a used "store-bought" OTA.
My intent for the OTA is for use in spectroscopy and the quality of the mirror is my prime interest, but the scope will be used to a lesser degree in casual observation and, of course, photography will only be employed to document spectra, etc. The scope OTA is currently unmounted so I think some additional deep sky images won't be available in the time I need to make a decision. Also, if I commit to buying its a done deal and I cannot (and will not) back out as the seller is traveling a distance (a coincidental excursion) with his equipment mingled among his luggage.
Anyone have any insights as to how I may get some clarity (pun unintended) on the quality of the mirror from a terrestrial photo - taken with, maybe, a different eyepiece or of different subject?
All the best!