by C.Y. Tan 2015
Most telescopes used for astrophotography are equatorially mounted. However, the more popular computer controlled telescopes are alt-az mounted and are usually deemed unsuitable for astrophotography. The reason is that alt-az mounted telescopes have one more degree of freedom that is not corrected because it does not affect visual observations of celestial objects. Unfortunately, by leaving it uncorrected, alt-az telescopes cannot be used for long exposure astrophotography. In order to overcome this flaw in alt-az mounted telescopes, I have designed and built a field derotator that removes this degree of freedom and thus allows alt-az telescopes to become astrophotography platforms.
I have built a fully functional field derotator that consists of both a mechanical derotator and an Arduino based controller for the LX200. All hardware, drawings, schematics, software and documentation are open source with the respective licenses described below.
I hope that this device will be useful for astrophotographers who find polar mounts a real pain in the rear to use, and prefer alt-az mounts instead.
Field derotation cannot work without an off-axis guider. If you don't plan on investing in an off-axis guider, don't bother building this device because field derotation will not work without it. See field_derotator_formula.pdf for the explanation.
A compiled app bundle for the Mac can be downloaded from the releases repository.
All the software have been successfully compiled on Mac OS X 10.10.5 (Yosemite).
I don't do Windows or Linux so, I cannot answer questions for these two platforms.
This is unsupported software and hardware. Build at your own peril! :)
You can submit questions or bug reports using the issues tab on the right and then click on NEW.
Have fun!
- arduino The C++ code for the arduino MEGA2560.
- arduino_case The case design for the field derotator controller.
- bom Bill of materials for 1 field derotator. I also show the cost for building one.
- eagle Eagle schematic and board files.
- emachineshop eMachineshop files for the derotator aluminium shell.
- field_derotator_formula derivation of the derotation formula, analysis of how off-axis guiding is improved with derotation and limitation of derotation.
- frontend C++ code for the GUI frontend for the user to control the derotator.
- howto Guides for how to assemble the derotator and the controller.
More information can be found in he README.md files in each directory and the wiki of this project on github.
This project has been published in Circuit Cellar Ink Issues 309 (April 2016) and 310 (May 2016), titled "A Field DeRotator for Astrophotography by C.Y. Tan" Parts 1 and 2. Details of the motivation for building the field derotator can be found in Issue 309 and the description of the electronic circuits, and gear ratio calculations can be found in Issue 310.
All the software, documentation, hardware that I have written is copyright 2015 C.Y. Tan.
All software is released under GPLv3
All documentation is released under Creative Commons Attribution-ShareAlike 3.0 Unported License or GNU Free Documentation License, Version 1.3
All hardware is released under CERN Hardware Open License v1.2