Balancing an AZ Goto Mount and ST102 OTA

[AdeKing]

Hi all,

I've got an AZ Goto mount which is attached to EQ5 legs via an adaptor machined for me by a local metal fabricator, which has immeasurably improved the stability of the AZ goto Mount over the original flimsy steel tripod that it came with.

However, whilst using the mount with an ST102 and 2" diagonal on Saturday night, I noticed that this setup is very back heavy (presumably due to the 2" diagonal) and kept tilting back every time there was a gust of wind.

I moved the OTA as far forward as I could before the bolts attaching the dovetail to the tube rings got in the way of the saddle, but it won't move far enough forward to counterbalance the weight of the diagonal and eyepiece.

From what I can tell, the OTA shouldn't really be too heavy for the mount, as it is sold as a combo, but with a 1.25" diagonal, so I'm assuming that its the addition of the 2" diagonal that is causing the problem.

Does anyone have a suggestion for how I can modify the setup so that the OTA can move further forward.  I was thinking of getting the 33cm long skywatcher dovetail, but wondered how easy the Skywatcher Dovetail bars are to drill if the pre-drilled holes don't match up the spacing of the tube rings.

Secondly, is it possible to get an allen bolt that will fit into the countersink holes for the tube rings on the dovetail and dispose of the two small allen bolts that are fitted at the observers end of the dovetail, I assume that the small bolts are to prevent the assembly from slipping out of the saddle when viewing towards the zenith.

Thirdly, would it be easier to attach a relatively small counterweight directly to the objective end of the OTA, or by fitting the longer dovetail with a small weight attached to the end furthest from the observer.

As I normally observe with a Dob, or a 130P newtonian on the AZ Goto, neither of which have much of a balancing problem any recommendations regarding ways to balance would be gratefully received.

Regards,

Ade

 

[parallaxerr]

23 minutes ago, AdeKing said:

Does anyone have a suggestion for how I can modify the setup so that the OTA can move further forward.  I was thinking of getting the 33cm long skywatcher dovetail, but wondered how easy the Skywatcher Dovetail bars are to drill if the pre-drilled holes don't match up the spacing of the tube rings.

Secondly, is it possible to get an allen bolt that will fit into the countersink holes for the tube rings on the dovetail and dispose of the two small allen bolts that are fitted at the observers end of the dovetail, I assume that the small bolts are to prevent the assembly from slipping out of the saddle when viewing towards the zenith.

I had this same problem with one of my scopes. A longer dovetail will help, the SW ones are aluminium and very easy to drill. I used one that extended beyond the focuser when mounting a DSLR camera. One problem you may end up with though is bolt heads where you want the dovetail to be in the clamp, which leads on to the next point.

Aftermarket dovetails such as ADM are available that are counterbored so the allen bolt head is flush and can be run through the dovetail clamp.

The two extra bolts in the end of the SW dovetail are for correcting cone error when aligning two scopes on the same mount, so if you don't require that function, they can be safely removed.

I personally don't like adding counterweights where possible, extra weight = extra vibration.

[jambouk]

If the axes are a bit lax, you can gently tighten the nut under the dovetail to tighten the altitude axis, and there is another nut under a bar at the very bottom on the under surface of the mount head to tighten the azimuth azis. They should be snug and prevent you from easily moving the axis by hand, but not so tight that the motors struggle to move the axis. But ideally you want to balance the kit as much as possible. Can you move the scope in the rings, or get a longer dovetail?
James

 

[AdeKing]

On 06/03/2017 at 13:49, parallaxerr said:

I had this same problem with one of my scopes. A longer dovetail will help, the SW ones are aluminium and very easy to drill. I used one that extended beyond the focuser when mounting a DSLR camera. One problem you may end up with though is bolt heads where you want the dovetail to be in the clamp, which leads on to the next point.

Aftermarket dovetails such as ADM are available that are counterbored so the allen bolt head is flush and can be run through the dovetail clamp.

The two extra bolts in the end of the SW dovetail are for correcting cone error when aligning two scopes on the same mount, so if you don't require that function, they can be safely removed.

I personally don't like adding counterweights where possible, extra weight = extra vibration.

I now have a longer Skywatcher Dovetail, and I have fitted it so that it extends towards the focusser, but I can see what you mean about the bolts getting in the way of the Saddle.

I too would like to avoid counterweights, but I did wonder whether the ADM Mini Dovetail Counterweight might do the trick, but its an expensive option and I wasn't sure that the dovetail would fit a standard saddle.

Thanks for the info about the four extra bolts being for correcting cone error, I shall now remove them.

[AdeKing]

On 07/03/2017 at 20:41, jambouk said:

If the axes are a bit lax, you can gently tighten the nut under the dovetail to tighten the altitude axis, and there is another nut under a bar at the very bottom on the under surface of the mount head to tighten the azimuth azis. They should be snug and prevent you from easily moving the axis by hand, but not so tight that the motors struggle to move the axis. But ideally you want to balance the kit as much as possible. Can you move the scope in the rings, or get a longer dovetail?
James

 

Now you mention it, the Azimuth axis is stiff and cannot be accidentally moved, but the Alt axis is very easy to move, so maybe the Alt axis needs tightening up a bit.

I can't move the tube rings more than about 1cm in each direction, but I have got a longer dovetail so I will try fitting that and see how it goes..

[jambouk]

Yes, tighten up the altitude axis; nip it tight, not overly so. And do some tests with a longer dovetail and check it still all feels stable.

Good luck and do keep us posted.

James

 

[AdeKing]

I've fitted the longer dovetail and marked the balance point, which is further forward than I expected it to be.  I've also tightened the Altitude and the problem has gone.

I think it was a combination of the two factors, but I think that a loose Altitude axis was the major factor.

Thanks for your help everyone.

Ade

[jambouk]

Do make sure it is as balanced as well as possible else you'll annoy the altitude motor, and you might not be aware of it being unbalanced now the axis is tight. You don't want to burn the motor out.

glad it's sorted.

james

 

[AdeKing]

Thanks for the advice James, tomorrows job is marking the balance points for each of the eyepieces I use on the dovetail with white paint pen so that I can get it as well balanced as possible.

Ade