FYI,
B and H Photo has the Lumicon deep-sky filter on sale. This is the one that
threads directly onto the back of a SCT:
"The Cassegrain Deep Sky Filter threads onto your telescope's SCT rear-cell,
and presents male SCT threads for accessory attachment.”
This is the Lumicon LF3020, for $199.95:
https://www.bhphotovideo.com/c/product/457082-REG/Lumicon_LF3020_Deep_Sky_Filter.html
Alex
