Altaz mounts - how do people solve this problem?

[RobertI]

I recently bought a Giro-WR from a fellow SGL member and I must say it seems to be a quality piece of kit. I haven't had a clear night to test it yet and am still playing, but I am curious how other members have tackled a basic problem I am having. Basically I can happily balance my 72mm frac on it when it's horizontal, but as I point it towards the zenith, the weight of the eyepiece, which points out at 90 degrees, causes it to become slightly unstable and tip. I think the effect is exaggerated because the eyepiece and diagonal is heavy relative to the lightweight scope, and might be less of a problem with heavier scopes. Anyway I have tried three solutions:

1) Tighten the alt axis just enough to remove the problem - I feel this is making the mount too stiff and is defeating the object of a smooth 'push to' mount;

2) Add a 'levered' weight to counteract the eyepiece (i managed to fashion something from two dovetails bolted at right angles) - worked perfectly, but extra hassle and defeats object of a simple mount;

3) Twist the diagonal so the ep is pointing away from the tripod centre - the most effective solution as there is no instability, no additional weights and the axes can be completely free.

Option 3 seems to work but I have never seen people using a scope in that orientation on an altaz mount, so wondering if I have missed something?? 

Thanks

Rob

[Moonshane]

No you have it right Rob.  achieving balance is a challenge with giro mounts. Worth it though.

[DRT]

I have a similar problem with my TV Pronto on my Ercole and Ercole Mini, Rob, and I think it is a function of using a short scope. I don't experience the same problem with longer OTAs.

The only solution I have found it to put your scope as far forward as possible and use the alt axis tension to hold the scope in place.

[DRT]

That said, I know that @Stu has invented something called Stu's Thingymajig-thingy-balancing-thingy so hopefully he will chime in and shed some light on the problem

[John]

I have a Giro II which I use to carry my 100 F/9, 102 F/6.5 and 120 F/7.5 refractors (sometimes 2 at a time !). I use a combination of adjusting the tension of the altitude axis and having reasonably long dovetail bars on the scopes so that I can quickly adjust the balance of the scope by moving it fore and aft, to keep balance. Overall, having a slightly "weight forward" starting point seems to work best.

My eyepieces vary from the Nagler zoom to the 21mm Ethos so quite a range !

It has taken a little practice to get used to how this works but, as Shane says, it's worth the effort !

[DRT]

8 minutes ago, DRT said:

I have a similar problem with my TV Pronto on my Ercole and Ercole Mini, Rob, and I think it is a function of using a short scope. I don't experience the same problem with longer OTAs.

The only solution I have found it to put your scope as far forward as possible and use the alt axis tension to hold the scope in place.

That final sentence only relates to short OTAs.

[Stu]

2 minutes ago, DRT said:

That said, I know that @Stu has invented something called Stu's Thingymajig-thingy-balancing-thingy so hopefully he will chime in and shed some light on the problem

I think I should patent the Thingymajig-thingy-balancing-thingy, it works well! It's actually not possible to achieve balance by shifting the scope forward in the rings or with a long dovetail because, as you say, the weight of the eyepiece is away from the axis of rotation. The only way to achieve it without needing the clutches is with a counter weight on the opposite side at the front of the scope.

I believe the Giro II/III probably has more progressive clutches than the Giro-WR so it is easier to counter the effect with the larger mounts. I also think achieving balance is easier when using two scopes, not sure why, just more variables to play with I guess.

So, I give you Stu's Thingymajig-thingy-balancing-thingy ??. Shown with an Ercole, but works with a Giro-WR too. Clutches were completely loose for these images, perfect balance.

[DRT]

17 minutes ago, Stu said:

I give you Stu's Thingymajig-thingy-balancing-thingy ??.

Do I get commission?

[RobertI]

A very clever solution Stu. I can see why you need a decent sized counterweight, that eyepiece looks heavier than my scope! I shall watch out for the TTBT in the shops. Seriously, though, that's very useful to see how you have solved the problem. 

[mikeDnight]

Here's a possible solution to the problem that will hopefully do away with counterweights and rebalancing issues. 

Remove the altitude locking knob completely, then cut a piece of nylon rod to about 6mm in length and having a diameter fractionally smaller than the thread size of the locking knob. (Knitting needles are ideal for this purpose)! Insert the piece of nylon/plastic into the hole and replace the locking knob. Attach your scope and tighten the locking knob gently until a measure of friction is achieved that allows both movement of the scope while at the same time holds the scope in position. The locking knob now becomes a clutch and you may find you rarely need to lock the scope down as it should be held by gentle friction.

It is important that there is no oil on the area of the altitude shaft onto which the nylon or plastic rod is to make contact!  A cotton bud inserted into the locking knob hole will clean the area on which the nylon or plastic rod rests. You may need to fully rotate the altitude shaft and use several buds to remove oil or grease from the area.

Mike

[Stu]

2 hours ago, RobertI said:

A very clever solution Stu. I can see why you need a decent sized counterweight, that eyepiece looks heavier than my scope! I shall watch out for the TTBT in the shops. Seriously, though, that's very useful to see how you have solved the problem. 

Thanks! ?

The eyepiece is my poor man's 31mm Nagler aka the 30mm ES 82 degree. Very nice for widefield views.

[gls]

I use a gimbal mount so I dont have this problem. A modified version of one of these. I changed the arm that holds the scope so it balances perfectly on both axis, and my scope (a megrex 72) can reach zenith in it.

 

http://www.lensmaster.co.uk/rh2.htm

[RobertI]

3 hours ago, gls said:

I use a gimbal mount so I dont have this problem. A modified version of one of these. I changed the arm that holds the scope so it balances perfectly on both axis, and my scope (a megrex 72) can reach zenith in it.

 

http://www.lensmaster.co.uk/rh2.htm

Thanks for the link, I think I have seen this before and it looks like a fine bit of engineering. I am currently looking a using an L bracket with my Giro-WR so that the scope 'hangs' under the bracket - this kind of arrangement on my previous setup gave me perfect balance at all angles (by some fluke) with my 72mm frac so should hopefully work with this setup. At £20 for the bracket it's worth a try!

By the way the reason I have moved on from my previous setup for g&g was that the tripod head was not brilliant quality, and homing in on an object at higher power meant moving the scope to slightly beyond the object, to allow the head to 'settle' into the right position. A bit tiresome after a while, but I had plenty of use out of it.

[gls]

It is a good bit of kit, very well made.  But it's not made for astronomy so you do need to modify it for best results.  All I did was make a new L bracket and fit a dovetail clamp to it. I also added a long dovetail bar to my megrez.

One thing I like about it is that the centre of gravity is above the top of the mount so I don't need to worry about it tipping over or adding counterweights. 

 

Here's 2 pictures of it. Incase anyone is interested. 

 

 

[Stu]

Looks a nice piece of kit but fundamentally it still has the same issues as the Giro style mounts as the weight of the eyepiece is not compensated. It must only be held steady by the tensioning of the bracket, which is also possible with the Giros.

[gls]

When I modified it I balanced it so it needs minimal tension to hold it steady because the centre of gravity is in the centre of the scope and eyepiece. If you let go it stays there. All the knob really does is stop the L bracket wobbling about because it's too loose.

 

edit - to be honest though I never use that eyepiece with it because this is my grab and go scope and carting my ES 100 degree eyepieces about sort of defeats the purpose of it. I use them in my dobs mainly. I just took the picture with it to show that you can use them, as I have on many occasions, but if you balance them properly it would have no problem imo. All it would need is the hole drilling in a slightly different place to get a perfect balance with them. With the eyepieces I made the mount to work with, it works just like the video in the link I posted, with minimal effort and it stays put.

[iPeace]

Started out with the same issues with my Ercole Mini. As others have said, it's a matter of finding the right balance and the best amount of tension. It feels like a faff until you're used to it, but indeed well worth the effort.