Balancing a telescope

[szymon]

I have a Celestron Nexstar 80GTL.  This is a pretty heavy 80mm f/11 (900mm long) refractor on a standard Celestron GT mount.  When using it, it feels like the scope is already pretty much at the limit of the mount.

Today I tried using a heavier eyepiece (a BST Explorer ED), and the mount seemed to struggle.  When powered, it was having difficulty moving the front of the scope down, it sounded under strain and it kind of felt like the gears were slipping.  When moving it around unpowered, the eyepiece was clearly unbalancing the telescope -- when I let go, it would slew up of its own accord.

My solution was to loosen the OTA in the mount and move it forward a bit - this seems to have improved things by fixing the balance problem.  My question is, is this a reasonable solution?  I've read about people putting counter-weights on the front to balance the extra weight at the eyepiece, but it seems to me that this would just add more overall weight and while it might solve the balance issues, it would put even more stress on an already overworked mount.

One issue I have with my solution is that my 25mm eyepiece is very light, and when the balance is set 'back' for the heavy eyepiece, the 25mm is too light.  I know I know, I should really just have two eyepieces of the same weight, but I wasn't clever enough to order a wider BST among the eyepieces I have for testing at the moment (I got a 5mm, an 8mm and a 12mm -- I'm really regretting not getting the 25mm sent too!).  Moving the OTA in the mount after the goto has been set up and calibrated, or even during the goto setup, just to get the bigger eyepiece on, is probably not sensible.

Any thoughts?

-simon

[ronin]

Looking at the scope and mount I would half guess that the scope is not balanced when you receive it. I would suspect that it is positioned about half way for packaging.

So I would say that you would have to determine and set the balance as you have.

I would not consider a BST a heavy eyepiece, there are lot bigger ones running around out there. There is one around that is something like 1.5Kg, but a little bigger then 1.25" fitting.

I would probably determine the CoG of the OTA with the diagonal on, put this position at the centre of the mount arm then slide it back a small amount, say 5-10mm - although the eyepiece would cause the same effect. It is sometimes best that the scope has a small bias one way to take up any slack/free play in the gears.

[lensman57]

Looking at the scope and mount I would half guess that the scope is not balanced when you receive it. I would suspect that it is positioned about half way for packaging.

So I would say that you would have to determine and set the balance as you have.

I would not consider a BST a heavy eyepiece, there are lot bigger ones running around out there. There is one around that is something like 1.5Kg, but a little bigger then 1.25" fitting.

I would probably determine the CoG of the OTA with the diagonal on, put this position at the centre of the mount arm then slide it back a small amount, say 5-10mm - although the eyepiece would cause the same effect. It is sometimes best that the scope has a small bias one way to take up any slack/free play in the gears.

Take the scope off the mount, I think that yours is a GoTo AltAz one,  and put the diaognal and your usual heavy EP in . Lay the scope on a flat table, kitchen table will do, slide a round piece of tubing under the scope and adjust the position of the tube till u feel that the scope is roughly balanced. Mark the position of the tube relative to the scope and once mounted make sure that this position is where the mount's locking screw connects with  the dovetail, with a bit of luck you should be balanced.

A.G

[Stu]

Szymon, balancing the scope is something you should do every time you use it, and the method you used is correct.

You need to balance in both axes though.

The method should be to carefully unlock the the DEC clamp whilst holding the OTA to make sure it doesn't swing on its own. Then, as you have been doing, slide the OTA forward or backwards until it is balanced. This can be done by shifting the scope within its rings or the Dovetail within the clamp.

Once done, lock the DEC clamp and carefully release the RA clamp, holding the counterweight bar to make sure it doesn't swing. This time, you adjust the position of the weight on the CW bar until balanced, then re lock the clamp.

Once this is done, everything should be balanced and the motors and gears will not have to over work to slew the scope.

EDIT The point about eyepieces is valid, as is focus position. It's probably best to put a middling weight eyepiece in, and have the focuser roughly where it normally is, as this affects balance too (I know specifically from using manual giro mounts where balance is more critical)

Hope that helps.

Stu

[szymon]

There is only one clamp, it's a simple alt/az goto mount.  Basically a ring around the telescope which you tighten with a thumbscrew (there is no dovetail).  There is a line marked on the OTA which is where you're "supposed" to put the clamp, but I guess that's with the very very light eyepieces that come with it.

Balancing the scope by itself on a table with a rubber tube on it is a good idea, I'll try that and see if I can make a second mark for use with the heavier eyepiece.

Thanks!

[Stu]

My apologies, I must have looked up the wrong mount. My advice applied to an EQ mount!

I think the advice you have received should help you. I would reiterate my point about focus position though because with heavier eyepieces this can make quite a difference to the balance point. As you say, you may need multiple marks to align to.

Stu

[szymon]

No need to apologise, I guessed you were talking about a GEM. I've been learning the differences in theory at least ;-)

Yes, I'll make sure to get the focus "correct" with the heavier eyepiece first and then balance out, and will play with multiple lines on the scope. The BSTs are parfocal, I guess this is one reason why people buy "sets" of matching eyepieces.

That said, it's looking like I'd really want three eyepieces (wide, medium, high), the Hyperion Zoom is looking ever more attractive in price and quality (and not having to change the eyepieces or rebalance the scope).

[trazor]

You will find that it is not possible to balance the scope in every orientation, due to uneven weight distribution.

I aim for best balance when the scope is at about 60 degrees, somewhere near your average viewing angle.

This video and its follow ups are very good for explaining balance, pick what is of interest.

[szymon]

Great video.  Only downside is it's made me want to buy a better mount!! :-)

[PeterCPC]

I have a completely different mount but still need to balance. What I have done is to balance the scope on it's mount with different configurations of equipment added and measure where the dovetail needs to be and apply self adhesive labels on the OTA for them. This acts as a good starting point when checking the balance before use.

Peter

[szymon]

Yes, I'm currently doing exactly that with masking tape (I didn't want to take a permanent marker to the tube!).

[Davey-T]

Isn't that the one that has the posItion for the clamp marked on the tube like this, only a guide for the supplied eyepieces though

Did manage to balance it with a DSLR on it to image the Moon which worked OK, note the felt tip marks for balancing other stuff.

Also found it needed really high AH rechargeable batteries, fully charged to work properly.

Also found a half decent diagonal transformed the views

Dave

 

[szymon]

That's the one :-)

No problems with power, I'm using an 8.6Ah motorcycle battery (which also adds some stability to the tripod when it sits on the tray - it's the perfect size for the mount).  I've been wondering about the diagonal, which one did you use and what exactly is half-decent?

-simon

[Davey-T]

That's the one :-)

No problems with power, I'm using an 8.6Ah motorcycle battery (which also adds some stability to the tripod when it sits on the tray - it's the perfect size for the mount).  I've been wondering about the diagonal, which one did you use and what exactly is half-decent?

-simon

I used a Meade one that came with a scope that I changed to 2" so the 1.25 was redundant, something like the William Optic one, around £60.00 or something similar.(nearly as much as the scope cost no doubt) but a keeper so worth it.

The objective seemed pretty good , helped by the long focal length no doubt, once it has decent diagonal and eyepieces it's a nice scope for planetary and lunar, as I said even managed a few decent pic's of the Moon.

Dave

[szymon]

Coincidentally I was looking at an advert for a used William Optics diagonal today; I've put in an offer :-)