Many, if not most, tripod 1/4" bolts are not a thread that goes all the way down to the head. Rather most tripod bolt have a smaller stem that connects between the head and the threaded section that reaches through the coupling plate. All of a sudden you do not have the same tensile strength of a full thread 1/4"... This is an important aspect - how the whole device is constructed in its many parts.
The other aspect is the role for which the mount that couples to the tripod head is designed for - not so much the tripod itself. The Houston-Fearless tripod that is pictured above is not the problem. **** fine tripod that one is! The issue is the mount taking the scope. How it is designed and made is dictated by the job it is intended for. Most video & film mounts are not designed to be used for astro purposes when a big scope is put on them.
Mounts designed for video and film will not have a great deal of capability for high altitude/elevation aiming. The main reason being the way the load acts upon the clamp and mount, and the ability of these to be released and be able to SAFELY move and handle the big load of a big scope, such as a C8. Of course there are many big cameras that are heavier than a C8 and these mounts and tripods handle these without blinking, but these big cameras are not being cantilevered off the hub of the tripod. The problem actually compounds the higher the scope is pointed and the further away the scope is set from the centre of the tripod. The way loads are then being applied to the whole mount-tripod system this way are not for what photo tripods are designed for.
And of course it depends on the mechanical strength of the mount if it can handle a scope on its back and pointed up at zenith.
Alt-az mounts for scopes are designed specifically to deal with the way scopes are used and to handle these loads. These are totally different from the way photo and video mounts are designed, and this is where the distinction between mount and tripod needs to be made. The two are separate entities, even though with a photo tripod the mount/coupling platform and tripod are one unit, the two are actually separate components.
Another part in this is the ability of the clamps to deal with the load in a SAFE manner. If the scope/mount system is very top heavy with the higher the scope is aimed the greater the torque you need to apply to the clamp so it does not slip. You will not have an assembly that is safe for you to use nor will you be able to relax with it because you will always have foremost in your mind if the clamp is tightened up sufficiently and then how easy it will be for you to loosen the clamp without having the scope tip back uncontrolled, and certainly not smack you in the face. You cannot be at ease with such a system.
Then there is the tripod...
AND the final piece of the puzzle is the stability of the whole system. Tip a photo/video mount back to or close to zenith with a C8 on its back, and is the whole system (scope, mount & tripod) a secure one? Or is there a propensity for the thing to topple over will little effort? This is why I hate those blasted binocular-to-tripod stalk adapters - these are diabolical contraptions for astro because most people use these on photo tripods and end up with all the problems mentioned above...
If you do consider a photo tripod for astro use, you need to consider the different components of a photo tripod separately, not as one unit. If one component is not up to the job, then the whole system is not up for the job. Weakest link dictates.
A practical solution option - within limits
There are ways to be able to use a seemingly unsuitable photo tripod for astro use, and within limits. Rather than sitting the scope (or binos) on top of the tripod head, slinging the instrument under a platform what sets the instrument in line with the altitude pivot of the tripod is a much more stable solution. It is then possible to position the instrument in such a way that the scope/binos are also balanced across the altitude pivot and the tripod head clamps can be left totally open! A semi isostatic solution. It is only semi isostatic because the instrument is still cantilevered off from the centre of the tripod, limiting how much load you can hang off the platform before the tripod topples over.
The pics below shows the astro platform I made. The scope is an 80mm f/5 achro with a 2" focuser & diagonal and a huge Meade eyepiece - and the clamps are not engaged! The pic is of the proof of concept set up I made - I don't have a pic of the final rig. The bino set up is the one I always use. The binos in the pic are 11X70. It would be impossible to use this modest photo tripod with either one of these instruments if they were placed on top of the tripod head.
Edited by maroubra_boy, 29 October 2020 - 05:42 PM.