由 willis » 2011-10-12, 15:00
For some APMs, there are two sets of back foci which result in different spot diagrams. Take 105/650 as an example, the two sets of back foci result in different spot size & vignetting. When the back focus is 80.7mm, the spot sizes are larger, but the vignetting is smaller (3% at 21mm). When the back focus is 75.85mm, the spot size is smaller, but the vignetting is larger (19% at 21mm). (19% is only half of some well know astrographs, which has 40% vignetting at the FF corner). It seems that the spot size & vignetting are completing. Does it mean one can tune according to needs? Anyway, from the figures the design tolerance is quite big. (80.7 - 75.85 = 4.85mm). If it is tested the stars are still round at the extreme corner of FF for the quoted range, 75.85mm to 80.7mm, it would be great!
Tak FS60C, APM-LOMO 80/480, APM-TMB 105/650, SW Black Diamond 120ED, Borg 125SD, SW 12" Dobs , Lunt LS35T, Lunt LS80T
TMB Super Mono 4mm, Pentax XO 2.5mm, Baader Planetarium 8-24mm, Nagler (2-4mm, 3-6mm, 13mm), Ethos (3.7mm, 6mm, 10mm), Explore Scientific 100º (14mm, 20mm)
DBK 21, DMK 21, DMK 41, DMK 51, ASI 178MC
Nikon D810A, Canon 5D II, Fujifilm X-T10