Thanks again all for your comments. Some good points about the dovetail, and I did consider mounting the dovetail rotated 90 degrees so as to fit better with the central brace. I also think the mount would work just fine that way too. But in this case I would not be able to adjust the balance on the declination axis. I much prefer to be able to quickly move the OTA up or down a little, if required when using different equipment, rather than having to add additional counterweights here and there to balance out things. It would also add to the load the mount has to carry.
Fixing the dovetail to the trusses for further stability is not possible as it would directly interfere with the very purpose of the design. The point of the original Serrurier truss design is that sagging/flexure is
unavoidable in any OTA, but the two ends of the scope can be made to sag equally when they are both balanced and mounted on respective trusses from a central brace. This then precisely maintains the optical axis of the instrument.
Note that this is different from the common truss tube design of large dobs, which are often incorrectly referred to as Serrurier trusses. The wikipedia page has some additional details:
http://en.wikipedia.org/wiki/Serrurier_truss
I'm aware that if I had to move the OTA because of an introduced imbalance, then it would also mean that the properties of the Serrurier truss design are no longer optimal for the particular equipment that causes an imbalance. But I think this effect would in fact be rather small and I much prefer this trade-off over fastening anything to the trusses for the reasons stated above.
In fact I built the OTA purposely with a slight imbalance so that it will be perfectly balanced with the CCD cam I plan to purchase (QSI583wsg). This means that imbalance will occur only when I'm using the OTA for visual work, with just an eyepiece, and here the exact collimation would matter less so some differential sagging could be tolerated. Again this may be rather theoretical for just a 10" sized OTA, but at least it is accounted for.
In any case the current attachment of the dovetail is actually very sturdy, more so than one might think. It also helps that the OTA is quite light. I could always rotate the dovetail later if I notice any problems. Again, thanks for your thoughts everyone, they are all good valid points that need to be considered when undertaking these things.
