I feel there seems to be a general ignorance upon fluid head and what they can do in astronomy.
A professional fluid head usually has 3 settings, namely 2 drag settings and 1 counterbalance setting. Drag is for the adjustment of movement tension, and counterbalance is for compensating the torque equipment may produce. A good counterbalance mechanism works independently, thus wherever the drag settings are at, it will always hold the load within its tilt range.
Some heads have +-90° tilt range, e.g., Ronford Baker, Oconnor, some Cartoni and Vinten, while some have +90°/-75°, e.g., Sachtler. In binoscope secarios, my advice is always looking for the +-90° models.
Some heads have continuously adjustable counterbalance settings, e.g., Ronford Baker, Oconnor, some Cartoni and Vinten, while some have stepped counterbalance settings, e.g, Sachtler and Miller .In binoscope secarios, my advice is always looking for the continuously adjustable models.
Every prof. fluid head should be accompanied with a counterbalance chart, this is of paramount importance when choosing an appropriate head. Take the Cartoni Focus 22 for example:
The horizontal axis is the payload and the vertical is the equipment center of gravity above the head. As you can see, the maximum payload capacity decreases while the center of gravity increases due to greater torque high gravity equipment generates. But as far as your equipment is within the red area, it will perform perfectly well and hold in the whole +-90° tilt range. In my case, I estimate my 123 binoscope's center of gravity at 125mm above the head, that means the head will support from 2.5kg to nearly 20kg.
According to my experience, fluid heads are best for non-tracking binoculars or binoscopes. There are mainly 3 reasons:
1. Compactness. They usually weighs for a few kilos and not much bigger than a 15cm diameter ball. You can always leave them on the tripod and take them everywhere without worrying about bumping into anything.
2. Versatile. You can put anything with a 1/4 or 3/8 threads on it, e.g., video cameras, DSLRs.
3. Fluidness. The adjustable fluid controls are extremely smooth and easy to operate. You can always adjust the tension according to your lens focal length, i.e., the higher the magnification, the stiffer the drag. That mechanism dampens out a lot of vibration at the expense of movement speed.
But good high payload fluid head are expensive, their prices rocket while the maximum payload build up. Finally it's up to everyone to choose what they prioritize.