Compensating for Orthogonality

[cwjohn]

Hi

At the moment I am using 2 tak scopes on a custom plate which is connected to a Losmandy D plate. The custom plate is quite solid and the two scopes are perfectly aligned but the connection between the custom plate and the D plate is problematic and generates orthogonality which is hard to adjust out.

My facility is permanent so my preference is to use a Tpoint or Maxpoint model to compensate, but I would think there would be a certain amount of orthogonality that would be beyond these pointing models to compensate.

I would be interested in feedback from anyone who has managed to compensate for orthogonality with a pointing model and the degree of orthogonality of the system in question.


Thanks in advance
Chris

[gregbradley]

I don't know about compensating for flexure between 2 mounted scopes but T-point does a good job of measuring and compensating for go-tos of non orthogonal aspects of your mount and also for various flexes.

If your 2 mounted scopes are flexing differently to one another that is a physical problem and as far as I know not solvable with software.

You would need a more rigid double mount. I think these setups tend to be problematic.

Greg.

[gary]

Hi Chris,

As a rule of thumb with regards Tpoint CH error, which is what you have,
you want to ensure that it is mechanically about under one degree to
be modelled.

Depending on what your pointing goal is, you would be starting to
push your luck if your CH error is greater than about 5 degrees.

During a pointing run, it is important you take samples across
a whole range of zenith distances and on both sides of the meridian.

It will really pop out on the graphs the first time you do a meridian flip.

It goes without saying that the source of the error can't be random.
If those couplings shift wrt each other, all bets are off.

However if the two are rigidly affixed with respect each, you have
an excellent chance of nulling CH out.

[cwjohn]

Greg

As I said my two scopes are perfectly aligned. No problem with differences between the two. The problem is with orthogonality between the D plate and the scope mounting plate and of course the orthogonality of the D plate vs the mount.

Getting the scope plate and the D plate exactly orthogonal is pretty damn hard as I have to take down the scopes to reset the plate. Assuming the D plate is true the scope plate seems orthogonal based on its situation with respect to the D plate holes but this is a pretty rough guide. I do not know if there is any easy way to make two plates exactly orthogonal before mounting the scopes.

Gary

That is good feedback. My rough estimate of the degree of orthogonality is around 2 degrees at the moment yet to be fully confirmed. I just want to put the destination in the FOV of the camera and I would resynch before imaging. At the moment my FOV is 10 arcminutes but later with larger camera it will be larger than this. I would think I would be happy with 10 arcminutes.