I have inherited a large Nikon lens. About 500mm in length, this extends to about 650mm. I know I must use it on M outside of that, I am jiggered. Whenever I manage to get the moon in the viewfinder, the merest speck of wind makes it shake so that the moon flits past the vf and bounces back. It is just a mess. If I dare to use the two extensions in addition to the basic lens, the problem is further throttled. I have a decent tripod, I have considered putting a sandbag on the bottom of the centre pole. I have seen them online, but my local photo shop has never heard of one?
Any suggestions?
You don't mention the focal length of your lens, but I'll guess it's not more than 600mm, in which case my experiences will be relevant.
You claim you have a decent tripod but don't mention the model or the head. There are many 'decent' tripods that won't hold a heavy load stable (the same is also true for heads)
On my heavy weight tripod I have held 1000mm & longer lenses/telescopes steady in moderate breezes, but my normal tripod certainly couldn't cope with my MTO1000mm lens a couple of weeks back in strong wind.
FWIW my heavy weight tripod is a Manfrotto 058B which weighs 6.7kg (so never goes more than 1/4 mile from the house/car. The Gitzo G2380 fluid head fitted to this is also heavier than normal.
Shooting the moon at reasonable magnification subjects movement will be significant, I doubt it took more than a minute or two for the moon to completely cross the lens's FOV at 1000mm.
The following photo was taken using my 650mm f/5 telescope prime focus (body directly on the telescope tube) on a 2x crop body in a moderate wind (the lifeboat was rolling heavily) NB. That's a 1300mm FF equivalent FOV
prime focus small by
Mike Kanssen, on Flickr
Using an a-focal set up with both eyepiece & a 200mm lens the optics become about 5x more powerful (I think it was a 40mm eye piece used) This gives significant diffractive softening, difficulty in focus & magnifies any movement further. Not much camera movement even here:
Afocal 3 (eyepiece & 200mm lens) by
Mike Kanssen, on Flickr
I've had reasonable success using a cheap gimbal head on budget legs when using long lenses for evening shooting, If you don't want to take up body building a head like this might be the way foreword.
Most often if using only a lens around 500mm I'll end up handholding, that worked for this (450mm on APSC):
Wind power - old & new by
Mike Kanssen, on Flickr
and when well braced even for this (a 600mm mirror lens via a focal reducer on MFT for a 900mm equivalent FOV):
Supermoon pre-eclipse by
Mike Kanssen, on Flickr