6.10.2020 Supporting a focusable filter and camera column.

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My latest mods to the PST etalon and filter stack are proving problematic. The helical focuser stiffens up far too much with such a cantilevered load hanging from it. It takes too much effort to manually focus now without losing the sun from the field of view. There isn't a chance in hell of using the little, remote focusing motor!

Further thoughts on supporting the long etalon and filter stack: No matter how stiff the components they all hang on a narrow bronze band in the remote end of the focuser's drawtube. Not an ideal engineering solution! Particularly since the loads are radial and rotate with the OTA on its equatorial mounting. 

If the whole lot is cantilevered at that point then moment [Mass x Distance] comes into play. I shall endeavour to calculate the moment of the entire filter stack. What you might call a foul weather pastime. It is raining and blowing all day today.

The little ZWO '174 camera weighs only 133g. The centre of the camera sits about 22 cm from the support point at the focuser's compression band. So the camera alone has a moment of 22x133 =~ 2926 cm/g. [Relative to a point just ahead of the etalon.]

I then supported the complete etalon+filter column at this clamping point. Then used my digital scales to measure the downward force at the camera. The load measured was a full 500g thanks to the contribution by the etalon, its housing, the Clicklock clamp, blocking filter and adapters. So the moment is 22cm x 500g = 11,000 cm/g. Nearly four times the moment of the camera alone!

The entire unit shown above weighs 1 kg. Or about 2.2lbs. It doesn't sound much but the cantilevered load applied to the helical focuser, at the telescope backplate, is enough to make it all but unusable due to friction. Hence the need for a "proper" focuser. More on that in a later post.

The moment of a DSLR mounted directly on the focuser is not remotely the same problem as a long stack of extenders filled with active components and a ZWO camera on the far end. By extending the protective filters forwards of the focuser only a rather small degree of counterbalancing is achieved. They don't have much weight and nor do the M48 extensions. 

One wouldn't want to add more mass to the filters, inside the OTA, to try and balance out the cantilevered loads. Because the distance from the clamping point [fulcrum] is relatively small. It could be argued that doubling the moment by full counterbalancing would be counterproductive. It might see-saw at the clamping band when subjected to vibration or external forces like the wind. It also adds to the loads trying to distort the focuser. These might tend to pull the following components off the optical axis of the light path.

Applied mass within the OTA might absorb heat. Then release it slowly as convection currents in the light path. Baffles tend to concentrate thermal currents within the light path. Since they rise and have no other possible path. Some baffles are edge scalloped. To allow convection currents to rise by that route. Rather than spoiling the view.

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