Cleaning Evacuated Tubes

Our 315 litre 30 evacuated tube solar hot water system produces more than enough hot water for efficient but unrestricted use by the five people in our household throughout most of the year, but last winter we had to turn on the booster for a couple of hours on several days. With the cold season approaching, I decided the evacuated tubes might benefit from a clean as they had not had one in the 5 years since installation. Climbing onto the flat corrugated steel roof where they are installed my suspicions were confirmed.  Despite recent intense torrential rain, the tubes were blotched with sooty deposits - possibly originating from the large Meleuca tree in front of the house.
Luckily cleaning the tubes was easy. I didn't want to use any chemicals because the roof is part of our rainwater system collection area.  Instead I just wiped them clean with a micro-fibre cloth labelled 'glass', rinsing it frequently  in a couple of buckets. The closest three tubes in the photo below have been cleaned. The whole operation took about 20 minutes.
No measure was made of how much improvement was gained by cleaning the tubes but after  I finished I watched with satisfaction as the controller showed the system continuously heating the circulating water by about 10 degrees as it passes through the manifold.  Hooray for showering in solar heated rainwater! 
John