Somewhere between 1000mm and 2000mm for 35mm format.
The mylar image was taken through a Celestron 8" sct, i.e. 2000mm focal length, while the black polymer image was taken through a Celestron 102mm refractor with 1000mm focal length.
Filters can be purchased at Astro Scientific Center at the Singapore Science Center, at least i got mine there back then.