You have the idea Paul. Straight into bed rock another way that this is done with multi story buildings is drilling down into the soil until they hit bed rock and upon pulling the auger out the back fill with a cementious slurry then connect them with a head of concrete. Also your fins are an attempt to increase the thickness of the section which is the way to go but like you have built it they come right up the section any moment that is taken by the fins takes a path down to the footing which takes the moment and then distributes it into the soil/bedrock. This also decreases the effective length of the un-braced section, if you where to do the same from the top end, this will be in effective because the flex will originate at the end of the fins and there is no where for the moment at the top to be resisted. I would have to draw the pier in 3 space in a program such as auto cad and import it to multi frame or space gass to give you exact figures of what is happening, but as you said you have hit it and at a longish focal length there was no "noticeable" movement. Have you tried this theory while guiding? that will give you a quantitative value of movement not just qualitative.
Although i agree with you that 32/40 MPa is achievable by your average DIY warrior, i doubt that without knowing what they are doing that the concrete will get anywhere near it and once again it comes down to she will be right chuck another 2 or 3 bags in the mix. Im also a tad confused by your 40Mpa (slump at pour) they are two different things ones the characteristic strength 40Mpa and slump is a measure of workability nominally you would get a 80 slump unless you specify differently in which the concrete company will adjust the mix with either plasticisers to give better W/C ratio. How do I know about this stuff? I worked as a concrete batcher for Hanson's for 3 1/2 years, backed with a year and a half of university about concrete as a material, and designing with concrete to Aus standard 3600 (that code is a nightmare!). Its not a dig at you mate not in the slightest I know you have a lot of years in the building industry, I just want this thread for your average fella that most of this has gone over their head to have the right information so they can research it till they understand what it is they are doing and to make it not just another Chinese whisper thread.
Also robin you are right the concrete will try to equalise with ambient temps that means when you drop 15 deg in the night it will release stored energy it almost goes to peter ward's post about big optics suffering local conditions more. The Mylar just traps the heat. In a lot of peoples systems the heat haze will not be noticed as the system is not producing the resolution due to focusing, tracking, flexure and a whole raft of other things. If you really want to see what's happening drop your scope onto a reasonably bright star say Mag 2, defocus until the star is a good 1/4 - 3/4's of your eyepiece and then look critically, you will see heat trapped in the tube if its not at equilibrium, seeing from the atmosphere, different parts of your scope releasing its energy very interesting to see. if you have a DSLR with live view this is even better to see. move your hand under the the tubes aperture you will see it massively swirling away
