orthogonal with german equatorial mount

[Domol]

Hi
anyone out there ever added shims to their OTA dovetail ?
I'm looking to add shims to get my C11 perfectly orthogonal. I've found guidance for our northern cousins and am wondering it there any for the southern hemisphere instructions? Here's what I have for Northern hemisphere. Should I reverse it for us?

Once you have Polaris centered, flip the OTA to the west side, and set the RA so that the weight shaft is parallel to the ground.

If your mount is orthogonal, then as you tip the mount in Dec, Polaris should perfectly center in the field.

If it does not, then you have some cone error.

If the miss is such that the star passes to the right side of the field (I would look though a straight though finder to see where it went and avoid diagonal reversal) then the fix is to place shims between the rear ring and dovetail.
If the miss is to the left, shim at the front.

Thanks for your expert advice!:thumbsup:
Regards
Dom

[Steffen]

Cone error works the same on the northern and southern hemisphere :)

You don't need Polaris, or even a polar-aligned mount. Just swing around RA so that the Dec axis is horizontal (with the scope on one side of the mount and the counterweights on the other), go up and down in Dec a bit to find and centre a star (or far away object) in the eyepiece, swing around the RA axis so that scope and weights swap places and see whether your object is still in the centre.

By nudging the end of scope left or right to get the object closer to the centre you can figure out where the shim needs to go.

Cheers
Steffen.

[stevous67]

Hi Dom,

I was thinking about somehow using grub screws to make the adjustments? I could help you with that.

Steve :question:

[Domol]

Thanks for your reply Stefan. I'll give it another try, when the cloud / rain clears! I think I was trying to get too technical, like trying to get the dovetail exactly parallel and at exact right angles to the counter weight bar!!! Cheers Dom

[Domol]

oh, and thanks Steve! (just saw your reply). :thumbsup:
It is very tough taking the dovetail off every time!! talk later
Cheers Dom

[Steffen]

It would be nice (mount manufactures feel free to use this idea without charge) to have some kind of adjustment mechanism on the saddle for eliminating cone error. Shims are such a pain to apply properly…

Cheers
Steffen.

[Peter Ward]

Losmandy made doveplates with optional riser screw-adjustments for a short time.... sadly the market was not appreciative :confused2:

[Barrykgerdes]

The problem of orthogonality won't be fixed by adjusting the dovetail. This needs to be fixed by aligning the declination shaft to be exactly orthogonal with the RA shaft. Do not confuse cone error with orthogonality.

The first problem is to devise a method of measurement with arc second precision.

If you can do this a G11 could be adjusted by shimming the joint between the Declination shaft assembly and the RA shaft assembly or re-machining these surfaces with a greater precision and accuracy. However I don't believe you will be able to improve on the factory machining accuracy.

Barry

[Merlin66]

Barry,
With a GEM, I'm not sure I agree....
The final outcome is to get the optical axis of the OTA parallel with the RA axis.

This can be achieved by shimming the mount joints or by doing the same with the saddle/ rings/ OTA - the SW method of "cone" correction - push pull screws at the ends of the dovetail/ rings should work?

[Barrykgerdes]

No orthogonality is to get the declination rotation axis perfectly orthogonal to the RA axis. Shimming the dovetail will only to get rid of the OTA cone error, it will not correct orthogonality, that can only be done by correctly aligning the declination and RA axes.



This is cone error correction not orthogonality

Barry

[Merlin66]

Barry,
I agree with your terminology, but surely the outcome is the same?

[Barrykgerdes]

For most intents and purposes of tracking, as long as your RA axis is aligned with the earth's axis you won't generally notice and problems with orthogonality. It is when you want to drive the assembly with a set of coordinates to a precise position that orthogonality is actually noticed.

With a good GEM the orthogonality is usually extremely close and can be ignored however the errors in gotos can usually be traced to cone error which is the alignment if the OTA to the declnation plane, assuming you are already correctly polar aligned. This can be corrected by shims if vertical alignment is required.

I have not had much experience with GEM's (My son has my G11 and I have never used it myself). However I have had a lot of experience with fork mounted LX200's and developed methods of measuring as well as correction. The first step is to get the Dec axis orthogonal to the RA axis and when this is correct (In could never get better than about 3 arc minutes) then get the OTA orthogonal to the dec axis to remove the cone error. These corrections I did for many people to get their gotos reasonable (about three arc minutes anywhere).

Barry

[Merlin66]

Barry,
Having lived with various Lx200 fork mounts over the years, I 100% agree with the need to check the forks for orthongonaly. Not an issue.
I think, if you analyse the layout of the GEM, you'll find it presents a different problem/ and hence solution.
Once the optical axis is aligned with the RA axis they can perform GOTO and sky alignment very well.......

[gary]

I see there is a little confusion with nomenclature in this thread. :)

Three axes -
The RA axis, the Dec axis and the Optical axis.

Orthogonal in geometrical terms, of course, simply means perpendicular (i.e. at right angles).

Non-orthogonal, of course, simply means not perpendicular (i.e. not at right angles)

RA axis to Dec axis non-orthogonality, to borrow from TPOINT nomenclature, is known
as Non Perpendicular Axis Error, or NP for short.

Optical axis to Dec axis non-orthogonality, to borrow again from TPOINT nomenclature,
is known as Collimation Error in Hour Angle, or CH for short.

The later is what is referred to in some Chinese telescope instruction manuals as
"cone error". However, I would suggest this term be best avoided as there are
some users on the net who refer to the NP effect as cone error. I would
recommend to best refer to them as NP and CH and to stop using the expression
"cone error" altogether.

Both NP and CH have the side-effect that there is an area around the mount's pole to
which the scope can never point.

It is the "east-west" component of these two forms of non-orthogonality that are
of primary interest. Both have the affect that when the OTA is "flipped"
on a GEM the direction of the error is reversed.

On GEM's, any NP the mount has was pretty much put there the day it was
assembled by the manufacturer. However, CH can result from the user not having
the dovetail plate "square" on the mount or the OTA not "square" on the dovetail,
or by having the telescope out of optical collimation, or by having an eyepiece
or camera tilting in the eyepiece holder or so on.

Since NP tends to be more carefully controlled by the manufacturer, on most
good commercial mounts, it tends to be small. CH, however, can be significant
and is worth correcting.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

[Merlin66]

gary,
Thanks for the input.
How do you actually measure (other than with micrometers etc) the NP error independantly from the CH error?
If the CH error is corrected, relative to the RA axis (not just the Dec axis) does this not also correct the inherent NP error?????

The answer: No!
As the dec axis "wobbles" unfortunately so does the optical axis...
As Gary/ Barry have said - the axes need to be corrected to sit at 90 degrees to each other - first! Then the optical axis aligned with the RA axis.
Apologies for adding to (my) confusion!

[gary]

Hi Ken,

You are most welcome.

We ourselves take a numerical analysis approach to unraveling NP from CH and
for that matter, we simultaneously unravel other errors, such as polar misalignment,
declination axis flexure error, tube flexure, eccentric bearing errors and so on.
We do this by way of a star pointing test where we sample stars across the sky
and tabulate their catalog RA/Dec positions against their RA/Dec positions as
computed by the telescope computer.

With regards the hour angle components of CH and NP, CH is constant for
all values of Dec whereas NP is zero at the equator and reaches a maximum at
the pole. The GIF animations on our web site attempt to illustrate this -
http://www.wildcard-innovations.com.au/geometric_mount_errors.html

Surveyors that used the old optical theodolites had to remove errors caused
by non-orthogonal axes as well.
Here are a couple of pages for interested readers on theodolite error and adjustment -
http://whistleralley.com/surveying/theoerror/
http://whistleralley.com/surveying/theoadjust/

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

[Domol]

Thanks for everyone's input and especially your 101 on the topic Gary.
Being a visual astronomer seeking high precision gotos I don't think there has been enough importance placed on this topic with GEM. The Gemini measures NP error so now I understand this relates to Orthogonally. :thanx: