Saw a square mat 600mm x 600mm at Bunnings, was high density foam about 3/4" thick and should be water proof. May cause stability problems if feet are not firmly on the ground. $12.
Plain, thin plastic sheet is the best for a dob. And longer legs.
I've come to see that this is the best solution from experience.
Consider the action of your dob on a hard surface - the action is steady as the mount is working off a solid surface.
Now, put the mount on some spongy surface, lush turf, foam rubber, whatever. First note: the little stumpy feet usually found under the ground board are not long enough to reach through the lush turf to the hard earth. The result is the turf is actually pushing up against the underside of the ground board, just like placing the mount straight onto foam. The result is that the turf/foam introduces its own set of forces and backlash to those normally needed for the dob set on concrete. This effectively more than doubles the force required to get the scope to move, and you introduce a horrible amount of backlash when you release the scope, and the turf/foam pushes back to restore its shape.
Foam rubber of any type is the worst solution. Put the foam onto turf, and the scope on top of that, and you won't be able to get the scope to work at all. No crap.
Thin plastic sheet, garbage bag cut open, builders plastic, plastic cover wrap of a mattress, what ever, will give the least amount of undue influence on the action on the dob mount.
But this too won't overcome the problem of setting down the scope onto turf. The plastic sheet here will also be detrimental to the scope's action. There are only two solutions to this - 1, move the scope to hard dirt or concrete; 2, give the scope longer legs.
Replacing the crappie rubber door stops that are typically found under the ground board for at least 4" long legs is the best thing you can do for your scope. Not only will these legs be able to penetrate more easily through turf, but will also reduce the effects of dew on the underside of the scope.
Have a look at the before and after pics of my 8" dobbie. The new 'leggie' mount has no problems with lush turf, and being made properly, damp turf is also never a problem. For this f/4 dob, the longer legs also raised the position of the eyepiece to a better height, so all I need to use is a humble chair, and no more kneeling on wet turf.
John
I am assuming you are asking about your SDM (#34 which is an 18" I believe?). I have found with my 20" on the ground board I made, which is very similar to an SDM one, at least dimension wise if not quality of workmanship!, that the turf has to be really thick for the it not sit down firmly. But if the ground is damp I like to sometimes sit it on a small tarp just to reduce any moisture getting onto the ground board.
Most days I never bother.
Malcolm's right about the turf needing to be really thick to affect the scope's performance if the instrument is a large one. Thick turf is what most affects my 17.5". But, lay down any type of matting on turf, and this changes things straight away. Mind you, my 17.5" is only 40kg, and an SDM is much heavier again. But a small tarp is also a great idea as it too is thin.
Thank you Alex for your comprehensive reply. It confirms what I suspected may happen with a foam mat. I have used a small tarp but sometimes it gets gathered up as the scope turns in azimuth and then fouls. My concern on bare grass is moisture getting into the feet which even on an SDM only seem to have the merest hint of paint. The electrics are also very close to the wet ground.
Thank you also Malcolm and Allan for your input.
I would have thought that some thin waterproof carpet type material may be best. I will cast around a bit more.
The beauty of a small tarp is that you can peg it out so that it won't gather up!
Having experienced Allan's tarp solution at Bretti last night, I can say it is the best I have seen in application, as it provided a good dry base for the scope plus plenty of space for observers to move around the scope freely.
Rick uses a piece of rubber backed carpet square (marginally bigger all around than the base's footprint) for his 14" DOB and he seems happy with that solution too.
