Reposition mount with good pointing model?

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This is a transfered topic from the ASA Yahoo Group.

 

 

Posted By: astrowhwiii Wed Apr 17, 2013 5:37 am

hiya
i have a good pointing model, but have had to move my DDM60.
i recall reading about a way to easily reposition the mount without building a
new pointing model. does anyone have a link to the technique?

thanks

-bill w
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Posted By: bernd_eppinger Wed Apr 17, 2013 9:15 am

Hi, bill,

Maybe you mean the Super Fast Polar Alignment method by Philipp Keller. See here:

http://forum.astrosysteme.com/index.php?/topic/42-very-simple-and-fast-polar-align-method/

The method uses a previously made pointing model to get the polar alignment when the mount is repositioned. This is done by centering a star with the polar alignment screws. It is then assumed that if the polar alignment is the same, then the pointing model can be reused.

However, in my opinion, the method in this simple form has a theoretical flaw and can therefore only be considered as an approximation: The orientation of the mount has 3 degrees of freedom. The alignment to a star resolves only 2 of them. One degree of freedom therefore remains. For example, a star can be centered with the same RA- and DEC-motor position with either a horizontal or a slanted base-plate. Unless this extra degree of freedom is controlled by other means, the polar alignment will therefore not be reproduced by the method.

Note: I try to control the extra degree of freedom by using a self-made Alt-Az-style polar alignment and controlling the orientation of the mount plate with a precision spirit level. But the reproducibility is still poor, probably because I can only poorly reproduce the orientation of the telescope on the mount's dovetail plate. Replacing this by a more reproducible mechanics will be one of the next things I have to do.

The following may work for you, but only if your old mount position had a good polar alignment: The first step is to try to get a very good polar alignment at the new location, too. The SuperFastPA method may give you a start value, but you will need additional iterations. Once you achieved good polar alignment, you still have the rotation of the mount around the RA-axis as an unresolved degree of freedom. This can (hopefully) be resolved by loading your previous "good" pointing model, then syncing to a star, and then saving the resulting pointing model under a new name and using it in future.

As a better alternative to the last step, you may use the "3 star Polar Alignment with same Declination, optimizing Polar Alignment only" option. See:

Autoslew5100.pdf

Best regards,

Bernd

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Posted By: portaball2001 Wed Apr 17, 2013 10:56 am


Hi Bill,

As Bernd already pointed out it is possible to realign a repositioned
mount quite easily within a few minutes. I have to do it every time when
I setup my mount ( I have no permanent pier).

I do following steps:

- Setup mount and try to polar align as good as possible ( a built in
laser is a big help)

- Start Autoslew with your old pointing file and Home position the mount

- Slew to a brighter star close to the meridian in the south (alltitude
20deg - 40deg)

- Center the star in the field of view with the adjustment screws of the
mount

- Generate a three star autopointing file with Sequence ( there is a
special option there to do this)

- Load this autopointing file in Autoslew and do a 'Calculate
configuration' with the 'Three star' option ( the first one in the
pop-up window), which will just update the currently pointing file in
use

- Save the updated pointing file with the 'Use now' or 'Use now and load
next time' option.

This procedure usually brings me within five arc-minutes of the pole,
which is good enough for ten minutes exposures ( and probably more) with
my equipment (f=900mm).

Best regards,

Gerhard