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Another method is to use a "de-rotator". This is a mechanical device mounted between the telescope and the camera. It "reverse rotates" the camera at just the right ratę to compensate for field rotation. Large professional telescopes that use Alt-Az mountings use this technigue.

De-Rotator Notice: pdate •* May 15,2020

When I wrote this article a long time ago. a mechanical and programmable de-rotator seemed an expensive option to solve the rotation problem for amateur Alt-Az telescopes. So a wedge was the simplest solution for some. But not all Alt-Az telescopes can be mounted on a wedge.

On May 15. 2020 I received an e-mail from Tomasz Kruk in the UK. saying he had done just that! So I watched his YouTube video on how he designed it. 3D-Printed a lot of'PLAfilament' (a kind of thermoplastic materiał) parts. programmed it, and got it working on his (motorized auto-tracking) Dobsonian Alt-Az telescope. This was an amazing project and he thought of everything!

I was totally impressed with how he used current 3D printing. microprocessor technology and a robust lightweight construction to produce a truły remarkable de-rotator - AND. it worked very well. Congratulations Tomasz!!

Tomasz Kruks completed de-rotator attached

telescope. Photo Credit: Tomasz Kruk video.

Can it be removed after the fact?

If a series of shorter exposure photographs (a few seconds) are taken, then some post-processing image editing software can "re-align" the series of photos to remove most of the effects of field rotation. It does so by "reverse rotating" all photos after the first one to align the stars. However, with each photo rotated, the overall usable portion of the "stack" of pictures changes. Only the portion of the image that is common to all rotations can be used (see the description of this nearthe end ofthis web page).