I have been looking at reflection curves for a mirror coatings suitable for UBV photometry with an Opted SSP-3. I was little surprised at the sharp fall off for protected Al and Enhanced Al coatings below 400 nm. I believe from my readings in Astronomical Photometry by A. Henden and R. Kaitchuck that proper Data Reduction can transform observation from almost any system into reliable data. But I still wished to know if this observing community has a preferred coating, maybe 'Beral' just based on published curve; for the optics of a 10" f/8 newtonian and a 16" f/11 classic cassegrain. Note: Both OTA do NOT have a corrector plate. I see some published UV enhanced coating curves that do NOT have sharp fallout below 400 nm, but again I question whether this is wrong-headed thinking for photometric observing at 1,250 feet above sea level in semi-arid central Kansas.
Tue, 05/06/2014 - 22:33
Jeff:
Be aware of the following caution in the Optec SSP3 manual:
Blocking the red leak of the glass used in the U filter results in loss of UV transmission from 300 to 350 nm. The filter is still useful for comparison purposes even though it does not transform into the standard Johnson system precisely. It is interesting to note that the U filter is the most difficult one to transform accurately for all observers including those with photomultiplier systems. UV transmission through the atmosphere varies considerably and any optical glass in the telescope (corrector lens) or photometer will absorb light shorter than 350 nm making accurate U magnitude deter- minations difficult at best.
I am planning to experiment with a U filter made by another company but I haven't started. Given all of the cautionary advice I was given about U band, I am only attempting this because I can use a 24" telescope, and then I only expect to reach 7th magnutude, at best.
Tom