25 replies to this topic

[Hilmi]

OK, so now my G11 is rather well behaved. I am having an issue with automation though. The issue is one of automation. You can't go around making your mount east heavy every time you do a meridian flip. After all, that sort defies the purpose of trying to automate the entire setup. So, how do you handle this?

I generally have no issue guiding, but when I image something to the north so that the scope can keep on imaging right through from east to west, it gets to a stage where it just goes crazy and starts bouncing back forth. This has already ruined my attempt at m81 a couple of weeks back.

Don't know if I am communicating the issue very well.

Any ideas how to handle this for those who understood what I am trying to say?

Thanks

[jjongmans]

I understand your problem. This is one of the reasons I decided to upgrade to a direct drive mount with encoders, meaning no backlash or periodic error.
You can try to balance it without the east heavy offset, that way you'll have the same performance on both sides of the meridian, but I guess that's not what you want.
You can try to track past the meridian, without flipping, but that will limit your tracking time. I don't know what your G11 is capable of, but at a certain point your mount/telescope will collide with the tripod / pier, that's the point where you want to stop tracking of course.

[orlyandico]

you can use the string and weight trick so that your mount is alway east-heavy.

[Hilmi]

you can use the string and weight trick so that your mount is alway east-heavy.

Please elaborate

[Peter in Reno]

I generally have no issue guiding, but when I image something to the north so that the scope can keep on imaging right through from east to west, it gets to a stage where it just goes crazy and starts bouncing back forth. This has already ruined my attempt at m81 a couple of weeks back.

Does that happen when crossing the Meridian when the counterweight shaft is parallel to the ground and rocks back and forth like a see-saw?

I imaged M82 last weekend with my Mach1GTO and I didn't see anything like this.

Peter

[orlyandico]

.. because the Mach1 doesn't need to be biased east-heavy.

the periodic error is slightly worse when it is west heavy, but still very low (5" to 6" uncorrected, sub-1" with PEM, vs 0.5" with PEM when east-heavy)

at least on mine..

(and... i can detect no slack whatsoever in my Mach1. I think AP purposely loads the worms more heavily because the motors are oversize and can power through even with tight meshing)

[Midnight Dan]

you can use the string and weight trick so that your mount is alway east-heavy.

Which is ... ?

-Dan

[Jeff Phinney]

What orlyandico was referring to was what is commonly referred to as a "pre-load". Simply a weight hung on a wire rope that is wrapped around the axis, applying a constant torque on the shafts. Most large telescopes use the idea in both RA and Dec. to keep the driving gears enmeshed in constant contact. Pretty simple idea, but rarely used on smaller telescopes.

[gdd]

http://groups.yahoo....67/pic/33448...

The link points to a photo of the string method in the Yahoo Losmandy_Users group.

Gale

[Tom and Beth]

By any chance, do you have a finder that throws your balance off?

You should be able to balance your scope pointing in almost any angle (*) with the locks off such that the scope doesn't move when left alone. Pitch, Roll and Yaw is as applicable to GEMs as it is to airplanes.

I balance dec with the scope horizontal, then balance RA with the dec shaft horizonal, then balace for rotation with the Dec horizontal and the scope verticle. A finder scope sometimes will cause the scope to rotate in this last position. A Newtonian also will be off balance in this position if nothing counteracts the focuser/EP.

(*) I don't bother trying to balance a scope when then it's UNDER the mount

[tjugo]

Hi this super high tech solution will solve your problem

The original author of this idea is Rainer Ehlert.

Cheers,

Jose

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[Stew57]

I have done the same sort of thing with my GEM. I balance neutral then balance after adding the east heavy weight tide to a string. Gives the same amount of torque to the east no matter where the mount is pointing.

[Jeff Phinney]

As i said earlier, it's standard practice on larger telescopes.

[Hilmi]

Cool. Now I need to go look for one of those retro looking weights. I guess it should be easy to find in an antiques shop

[Hilmi]

By any chance, do you have a finder that throws your balance off?

Not much can be done about that. The finder is in a fixed position, my scope is not mounted on rings where I could just rotate to balance. Also, there is the fact that the camera needs to be rotated to provide a guide star and that will always throw of balance. You can only go so far with balancing before it gets into OCD territory.

But I think you are right, DEC balance along the OTA axis is what is causing the weird dance when I am imaging to the north and the scope is basically close to the counter weight down position.

[orlyandico]

I just re-measured my mount, and confirmed - the periodic error is 6" p-p when west-heavy, but 3.5" p-p when east heavy. So, Peter, the effect when meridian flipping is still there even with a Mach1, but it's small enough that the guider probably wouldn't notice.

[averen]

I did the "string and weight" trick, but made it a little more permanent...plus it acts as a spacer so I can track CONSIDERABLY farther through the meridian.



More photos:
http://www.flickr.co...ith/6167124016/

Jared

[wolfman_4_ever]

noice!

[Hilmi]

Jared,

I like your idea. I was working on getting myself the complete ovision upgrade with spacer. Maybe I can integrate your idea into the ovision spacer.

Does the free hanging weight cause any trouble when slewing?

[neilson]

Hi Hilmi,
I have a laser with laser finderscope bracket mounted on the opposite side of my OTA as the finder scope. I also attached a small fishing weight since the laser wasn't as heavy as my finder scope. It really made a difference having the OTA balanced all ways.

I use an 8 oz. fishing weight for my East heavy weight. I was attaching it to my counterweight bar. But I am going to try that string method.

Neilson

[Starhawk]

The Mach 1's stiff axes prevent this.

However, this problem gives me an interesting idea for an invention.

-Rich

I generally have no issue guiding, but when I image something to the north so that the scope can keep on imaging right through from east to west, it gets to a stage where it just goes crazy and starts bouncing back forth. This has already ruined my attempt at m81 a couple of weeks back.

Does that happen when crossing the Meridian when the counterweight shaft is parallel to the ground and rocks back and forth like a see-saw?

I imaged M82 last weekend with my Mach1GTO and I didn't see anything like this.

Peter

[Starhawk]

That spacer really has me thinking as well. Did you consider a machined aluminum one?

-Rich

[averen]

That spacer really has me thinking as well. Did you consider a machined aluminum one?

-Rich

Are you referring to my spacer?

If so, yes, I did consider a machined aluminum one. But the cost was considerably more than my MDF spacer (less than $1 and about 30 minutes of my time). The MDF spacer has been in service for a while now and has worked just fine. MDF is very dense and stable so I see no harm in using it for this application. I primed and painted it to keep the dew away from the MDF...which is really the only weakness in this application!

Aluminum would have been the "ideal" choice, but given the cost of the two MDF is more than adequate. I could probably make a couple of hundred MDF spacers for what an aluminum version would cost me. And when it's dark no one can tell...here's the spacer painted.



Jared

[ksipp01]

sorry for the double post...I just posted in DIY and stumbled on this pertinent thread(although I had seen this on flickr) Here's another thought for the same problem, different mount (CGEM).

so far its a 1/2 inch PVC cap that w/ gentle twisting and firm inward pressure, will actually thread right into the polar scope threads. the spring is attached to the center and held by another 6-32 screw. the outer collar is a 3" PVC DWV adapter, (#70330) (I'll cut unused half off later) The threaded portion of the PVC adapter was opened a small amount w/ an oscillating sander (about 1/2 the thread height) and it actually screws on snug and is held in place by 3 screws that line up with the semi-circle mounting screw channels. the torsion spring is held to that by a thumb screw to a small aluminum spacer attached to spring w/ set screw. I could only find a 90 degree torsion spring at the hardware store, 360 degree ordered online, will be here soon. I'll update once complete.

I put a couple brief videos on my website http://www.scopefocu...torsion-balance

Comments and suggestions wanted!
Thanks
Kevin

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[orlyandico]

That is a pretty cool approach! (spring in the polar scope bore). The question is.. does it interfere with GoTo slews?

Rich - the Mach1 is not immune to "easy heavy preference," stiff axes or no. I've measured the PE at 6" peak-to-peak when west-heavy, versus 3.5" peak-to-peak when east heavy. So the bias is still there, but most people would have no trouble guiding out 6" (uncorrected).

After PEM, it's 0.4" p-p east-heavy and 0.9" west-heavy.