This may not be an issue in the present case. But I would note just for general forum interest that my experience is the same as Gus reported - dramatic loss of contrast/detail for my Lunt below roughly 8-10C. I do go through a maximum in contrast as I increase pressure, at about the usual setting, the contrast is just very low. Using a heater does correct the issue.GUS wrote: ↑Wed Apr 07, 2021 5:03 am Terry, I lose detail in both my lunt BF's below 10 degrees celcius, a dew heater brings them back on band. I see no detail on the disc or proms before heating it, but plenty once warmed up. It could be an O ring issue, it could also be a BF issue. I also borrowed another BF from the local astro store where I purchased the scope from and it had the same issue below about 10 deg C. Using a Coronado BF filter with the Lunt scopes I don't have that issue, they work down to at least -5 deg C ( the lowest temps I get in my area) without any issue. Might not be the problem, but putting it out there that this is a known issue below 10 deg C.
Kevin