Cant comment on the vacuum method .
My scopes had lived in bone dry Northern Ca for their entire lives.
Never had visible condensation on the primary , I've dewed a secondary once or twice (ever) between the 2 Obsessions I have owned, a 15"(traded) and the current 20".
The 20" sees use just a few times a month , where as the 13" sees up to a dozen.
It has been posted repeatedly the loss of reflectivity over a period of time. By those numbers the coating should have degraded some measurable amount.
Side by side with a brand new 18" star splitter last year , the 1999 20" showed an obvious improvement over the 18"- brighter and more detail on same objects.
Can I assume the 20" is still performing at /near its original reflectivity? ( not planning have it tested)
The mirror stays clean and void of stains . Only exception is my site at a high forest meadow , is close enough to main road ( 75 yards or so ) to catch the fine silt of the forest floor when the rare hunter , grower or off-roader wheels by.
My remedy is too dry out my mouth and blast a burst of spittle free air across the primary to remove the silt.
The 20" primary has been cleaned roughly every 3-4 years or less.
The 13" is clearly in need of a re-coat (pinholes) however objects like the horse-head , Stephan's Quintet are quite visible as are many 14m galaxies on my observing list from my home Bortle 4 location. Roughly 21.56 SQM based on the 5 year old data on ligthpolution.info
Coatings are as old as that 13" scope. Got to be 30 plus years easily.
Not to argue with the facts ,but there are always exceptions. The 13" gets a cleaning most every year as the most used scope.
My observations are seat of the pants, no hard science ( or meter)
Great contrast , and visual impressions about what I'd seen on Steve G's NGC notes using a 13.1" telescope.
Edited by Stardust Dave, 17 October 2020 - 03:22 PM.