Last night I took out my ST80. The weather was clear but cold, in the low 30's F. I usually mount it on a Bogen 055XB tripod with either a MicroStar Deluxe or a 501HDV head. But this time I used a Stellervue M2 on Manfrotto 058B. This setup gave me much better stability and balance control with heavy eyepieces. My selection of eyepieces was 41 Pan, 31 Nagler and 21 Ethos. I had the TSFLAT2 screwed onto the barrel of a Baader 2" BBHS diagonal.
M45, the Pleiades, was the main object for testing the TSFLAT2 with these eyepieces. The 41 Pan and 31 Nagler both showed flat fields. There was some astigmatism, but I think this was from my eyes, not the optical system. I could see astigmatism with my progressive eyeglasses on or off, but much worse with them off. My prescription is two years old, so it has probably changed. The 21 Ethos, with the widest apparent field of view, showed the most astigmatism.
I took the three eyepieces back into my house and screwed a Dioptrx onto each one. I looked at the Pleiades again through each eyepiece with Dioptrx - without my eyeglasses, of course. For each, I dialed in the best correction with the Dioptrx. Now the stars were pretty sharp from center to edge in the 41 Pan and the 31 Nagler. However, the 21 Ethos still showed some aberration at edge of field when the telescope was focused at center, and vice versa. It looked to me like a combination of FC and astigmatism.
One take away from this, for my own purposes, is that I will probably leave a Dioptrx on the 41 Pan, 31 Nager and 21 Ethos. In the past I haven't liked using Dioptrx because with Dioptrx I need to flip my eyeglasses on to see the sky naked eye and read charts, and then flip them off again to view through the eyepiece. This makes star hopping a PITA. But at these wide fields and low powers, finding the object shouldn't really be a big problem.
Edited by Sarkikos, 19 November 2020 - 03:03 PM.