Hello Lady Fraktor , I purchased my SM 60 more than 20 tears ago and have used it daily for about 17 of those years and it has served me well and continues to do so . This scope has always been double stacked so has always had an external etalon and did nothing to prevent blocking filter failure .The B/F.'s have 2 filter elements , one is called the ITF , this is the one that "rusts" . I have 3 Coronado solar scopes about the same age and 2 of the 3 B/F"s failed . The 2 I sent to Coronado for repair were B-15 (15mm) so the largest of the 3 options . Cost to repair was about 250 each CAD . I did find a source for the ITF element and bought 2 for $40 each .Lady Fraktor wrote: ↑Thu Mar 16, 2023 6:30 pm Do you know if they were of the new III series?
I thought the external filters were supposed to fix that issue.
I do not consider this a weak point , I was using my scopes literally every the day the sun was out through hot summers and below freezing winters . My scopes were never in the house but climatized at the ready in the garage year round .