DIY GEM

[John78]

Right I was looking at the EQ5 now this seems relatively straight forward to construct, assuming the degrees of freedom I think it has are right.

I left off rotation of the whole head for polar alignment as it has to do that.

My thought is you could quite easily build one of these - the only thing I see being really critical is getting the 90 degree T correct which is probably do-able with some kind of pillow block bearing.

Are these accurate assumptions?

[brianb]

My thought is you could quite easily build one of these - the only thing I see being really critical is getting the 90 degree T correct

Many amateurs used to self construct equatorial mountings.

Getting the axes square to each other is important but it's easy to design & build an adjustable fixing for the Dec axis housing onto the RA axis. A similar joint can be used between the tube saddle plate and the Dec axis.

Design & construction to keep vibration out is the hard part.

[John78]

What sort of motion quality or dv/v does a good mount deliver? I do motion quality as part of my day job so making things run vibration free and smooth shouldn't be an enormous challenge (likewise scavenging gears and motors).

[brianb]

What sort of motion quality or dv/v does a good mount deliver?

The better mounts maintain accuracy to stellar rate with an accuracy of 0.2 arc secs or better. 0.5 arc secs is about the maximum acceptable error with a medium aperture scope. Slow deviations can be accepted as autoguiders can deal with them, but vibrations with periods significantly shorter than guider time constants (~5 sec) ruin images and need to be minimised - though some are unavoidable due to ground & wind vibration, they should be damped out rapidly.

This is why wood is such a good material for tripod legs.

By making an equatorial mount to the English (yoke) design instead of the German variety, you avoid the need for meridian reversal at the expense of the loss of access to a small area at the pole. The yoke mount needs two piers but no counterweight and can be conveniently constructed from wood (all except the thrust bearing at the south pier) - railway sleepers would support a scope in the 12-15" class easily enough.

[John78]

Yeah I think I was going to start by building a mount for DSLR and then upgrade to a scope later - I think the GEM looks like a good option?