Help with old equatorial mount.

[rickj]

Hi. I'm new to the forum and astronomy really. I'm in the process of setting up but on a very tight budget. Hopefully my question will make sense, even with my made up terminology!

I currently have an old equatorial mount and tripod from a Prinz Astral 500 that I am hoping to utilise for another scope. Having removed the 60mm clamshell I was wondering what I could use to attach a standard skywatcher dovetail to the mount? If you look on the attached photo you can see that the nut in the centre protrudes so a dovetail wouldn't sit flush, would it? The distance between the inner edges of the two fixing holes is 53mm. The second picture shows the bottom of the removed clamshell.

Hope I've posted this in the right place. Any help gratefully appreciated.

[JamesM]

Depends on how good your DIY skills are! I also take it you are not going to want to use the clamshell?

For me I would first try and remove the bracket that is attached to the clamshell in order that when it is placed back on the mount, its thickness will successfully negate the effect of the protruding nut on the mount head. I would then attach a dovetail bar to the bracket which is now sat on top of the mount and would drill this dovetail with two holes that match the same two holes that were once used to secure the clamshell to the mount (...hope that's clear). Now some bars have holes already drilled but it is unlikely that they will match, perhaps one of them leaving you to drill the other. The scope of course can be balanced on the mount by sliding it backward/forwards through the scope's rings. These rings can be attached to the dovetail directly.

Does that help or have I misunderstood the question - please come back to me with further questions.

James

[rickj]

Hi James, thanks for the reply.

Unfortunately the bracket and the whole of the clamshell are, I assume, cast from a mold and can't be separated. A shame because I think my DIY skills may just about have stretched to your suggestion. I had considered replicating the holes from the attaching part of the clamshell using hardwood as metal working is beyond my abilities. Can you see any problems with using wood?

Of course I could just keep an eye out for a cheap replacement mount!

[JamesM]

You could probably get away with a bit of hardboard, its only a fillet to get the dovetail over that central nut. Wood will split if you tighten against it but hardboard being a composite material is more stable and better at spreading the load. An alternative would be to use washers but I notice that there is a raised lip around the mount plate that might prevent this.

James

[Domis]

You could probably use a piece of steel or aluminum box tubing as an adapter.Just drill a center hole large enough for center nut to recess into and holes to match the dovetail and old mounting plate bolt holes.Keep the tubing fairly short so you can reach inside to start and tighten the bolts.

[david o]

You could always use longer bolts to mount a suitably drilled dove-tail plate and then use some small nuts on them as spacers to "bridge" the central boss.

[JamesF]

I'd be tempted to make up a wooden or metal plate that bolts to the mount through the existing holes, and then fix a dovetail clamp to the plate. That way the scope would be easier to remove than if the dovetail were bolted directly to the mount.

James

[Jack123456]

I would suggest making a spacer out of MDF (maybe 1/2" think), the same shape as the bracket on the clamshell. the dovetail should then lay flat on top of it.

[Astro Imp]

You could always use longer bolts to mount a suitably drilled dove-tail plate and then use some small nuts on them as spacers to "bridge" the central boss.

I would think this is the easiest way to go.

Hope you are ultimately successful whatever you go with.

Good luck.

[rickj]

Wow! Some excellent advice and ideas. The beauty of all of them is that they cost very little... barring accidents! I may give both mdf and washer methods a go and see which seems most stable/secure.

Thanks everyone your input!

Rick