In the classic "Amateur Astronomer's Handbook", author J.B. Sidgwick states that a dewshield that is only twice the objective's diameter or shorter is too short. Most of today's refractors would not have an acceptably long enough dew shields. Takahashi comes fairly close, but still no cigar, so to speak. This is especially true as the internal lens cell sits fairly far forward in the cell/dewshield arrangement.
You can go the other way too and have a dewshield which is too long. The problem here is vignetting or light cutoff in the view. Does anyone know how to calculate how long, at max, a dew shield can be.
Thanks!
Barry Simon
You can go the other way too and have a dewshield which is too long. The problem here is vignetting or light cutoff in the view. Does anyone know how to calculate how long, at max, a dew shield can be.
Thanks!
Barry Simon
