Design Choices

A project log for StarPi

A Raspberry Pi based Astrophotography System

ChrisChris 03/28/2016 at 18:430 Comments

There have been some design decisions made and I still have some to make. I thought I'd go through the thinking behind these choices.

The mounting of the sensors separate to the Pi is for two reasons. The first is to remove the need to align the pi on the lens in relation to the sensors. This allows the Pi to be mounted in the most convenient position and to use the image flipping options of the RPI_Interface to make the image appear in the correct orientation. The second reason is to allow the removal of the Pi from the lens to view the sky with the naked eye without loosing the position. This will allow the GoTo to continue using the position for feedback. Because of these I have designed a mount for the Accelerometer, the Magnetometer and the GPS, although the GPS did not need to be included. I also designed a battery mount to remove the possibility of any wires becoming wrapped around the tripod of the telescope. These may need to be designed for each telescope as they are curved to sit on top of the telescope body and are held in place with Velcro straps.

I currently have the choice of buying another mount, attaching motors to my existing mount or building an entirely new mount.

The existing mount that I have is an Alt-Az mount. This moves relative to the Earth's surface in the Horizontal co-ordinate system. As mentioned in the comments, this has the issue of rotational effects during long exposure shots caused by the Earth's rotation. This could be compensated for by taking a video, splitting it into individual images, rotating these and stacking them together. There is software already available to do this, but this would be more suited to a faster processor than the Raspberry Pi.

The mount does not have motor mounts or manual controls to move the Telescope to track an image. Any addition of motors would require some work to mount along with the counterpart of whatever mechanism is used. I don't want to damage the telescope when adding mounting these parts.

The other type of mount that I'm considering is an equatorial mount. These require some initial alignment before use, but then the axis are relative to the Equatorial co-ordinate system. It is these co-ordinates that are used for star maps as they are not relative to the location of the viewer. The two types of equatorial mounts I want to consider are the German Equatorial Mount and the Barn door mount.

The Barn Door is the simplest of mounts to build as it only has movement is one axis. This means that it can track an object once it has been set-up, but requires re setting to change the object being viewed. Essentially this is a tracking mount, not a GoTo mount.

The GEM mount is a little more complex as it can move in 4 axis and as mentioned in the comments is the best for astrophotography. Two axis are used to align the Telescope with the Earth's rotation. These are then not used, so there is no need to motorise them. The next two move the Telescope in relation to the Right Ascension and Declination Axis. It is these two that can be motorised to give both tracking and GoTo functionality.

I'm not really interested in reinventing telescope mounts or even spending time building one from existing designs, so I am going to put the seed funding towards a GEM mount and put the effort into other parts of the project. If you want to help out hit the like button!