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What is this second mounting screw for on a CG-5?

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#1 nexguy

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Posted 06 November 2013 - 11:00 PM

The dovetail slot has two screws. What is the smaller silver one for?
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#2 orlyandico

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Posted 06 November 2013 - 11:07 PM

safety screw. Provide additional securing of your dovetail.

#3 core

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Posted 06 November 2013 - 11:21 PM

... and there's actually a procedure to tighten them - when mounting scope, large knob first, then the silver one; you don't actually have to turn it down tight and put a blister on your fingers (the silver knob). Removing scope, loosen the silver one first, then the large knob. You'll notice that if you loosen the large knob first when removing scope, it will actually get harder to loosen the silver knob.

There's usually just a tiny bit of 'compression'/movement of the dovetail bar into the saddle when you first tighten the large knob. Tightening the silver knob slightly thereafter sets that silver knob in place. If you were loosen the large knob first, the dovetail/saddle 'springs' back/moves into the pre-stressed silver knob, and it'll dig just a bit more, making it harder to turn. Try it out for yourself.

See, we can even talk about proper techniques in mounting screws on a cloudynight ;)

#4 orlyandico

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Posted 06 November 2013 - 11:25 PM

I didn't know about that procedure...

That said I think the practice of using one (or two..) screws to mount a dovetail is Bad Practice. We should all be using clamps.

Even the stock saddle on the CGEM uses a clamp..

#5 jrbarnett

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Posted 06 November 2013 - 11:31 PM

That's a safety set screw designed for the type of dovetail almost no one uses anymore. One pioneered by Vixen, copied by Synta, that has detents carved in the edges, and has largely been replaced with straight-sided Vixen style dovetails.

Example:

http://www.myastrosh.../vixen-2661.jpg

Regards,

Jim

#6 jimbo728

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Posted 08 November 2013 - 01:17 AM

I still use that old Vixen plate because it holds the rings nicely on my old NightHawk and fits. The little set screw fits into a cut out. I tighten them both when using a regular Vixen rail for no other reason than they are there. Just a small measure of safety, I guess.

#7 Eddgie

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Posted 08 November 2013 - 08:33 AM

That's a safety set screw designed for the type of dovetail almost no one uses anymore.


Yes. Exactly.

#8 jrbarnett

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Posted 08 November 2013 - 11:19 AM

I think it's a nice, secure dovetail design. Nothing wrong with it at all. The reason for its replacement with straight sided iterations is more likely cheapness. Casting or machining the more complex shape with the detents likely costs more than doing a simple straight-edged version. The straight edged versions, similarly, may be more versatile to accommodate a wider range of rings and other OTA fitments.

For a refractor of appropriate length for the dovetail's apcing, using rings, there's probably not a better V-series dovetail solution.

- Jim

#9 jrcrilly

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Posted 08 November 2013 - 11:27 AM

I think it's a nice, secure dovetail design. Nothing wrong with it at all.


It is fine for use with tube rings, as the tube can be shifted within the rings to achieve balance. The smooth profile plates are required when the plate is directly attached to an OTA (commonly done with an SCT) because in that configuration the plate itself must move within the saddle to achieve balance.






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