Astrophotography is an expensive hobby. When you assemble even a basic setup including a telescope, camera, guidance equipment and mount, you can easily get equipment worth thousands of dollars. To reduce a bit the monetary sting, [td0g] has developed an innovative homebrew mounting and guidance solution that can be assembled by almost any enthusiast, with parts costing around $ 100. The precision required to obtain high quality astrophotography is very demanding. We are therefore impressed by what he has achieved with a limited budget.
The inspiration for this design comes from an incredibly simple star tracking device, known as Barn Door Tracking, or Mount Haig. Invented by George Haig in the 1970s, this mount is nothing more than a hinge aligned with the axis of rotation of the Earth. A threaded rod or screw, rotated at a constant speed, is used to slowly open the hinge so that a mounted camera tracks the apparent movement of the sky. Therefore, long exposures can show accurate images of stars and precise details of deep sky objects, instead of curved star trails. [td0g] adapted this technique to a more traditional telescope mount, using drive screws resembling a barn door on the right ascension and declination axes. A pair of NEMA 17 stepper motors drive Acme threaded rods at 4 mm pitch through 3D toothed pulleys from PETG.
Speaking of 3D printed parts, this version is a good example of the judicious use of technology: where metal parts are guaranteed, metal parts are used and the printed plastic is relegated to places where it can do its job properly. [td0g] placed the STL files of the printed parts on Thingiverse in case you want to replicate the drive.
The non-linear relationship between the rotation of the threaded rod and the right ascension training rate generally limits the exposure time you can reasonably achieve with a trap tracker. To adjust for that, [td0g] created a lookup table in the firmware to compensate for the drive and allow longer exposures. He mentions that the reader will operate for three hours before reaching the end of the screw stroke and must be reset. However, if he manages to manage exhibitions of three hours, his sky must be much darker than ours!
M31, Andromeda Galaxy captured with mounting tracking
If you are wondering what is the accuracy of the follow-up, [td0g] Recalls the precision required to obtain blur-free images from a tracking mount: each pixel in the camera / telescope combination equates to only 1.5 arc seconds, or 0.0004 degrees of rotation for Frame. Of course, maintaining such a close follow-up over a significant period of time for longer exposures quickly meets the limitations of mechanical precision, especially in the case of a DIY fixture. To correct the inevitable mistakes, [td0g] included a guide camera and a telescope in the construction. A modified web camera and a 100mm digital SLR lens are superimposed on the mount to automatically correct errors and track the mount through the rotation of the Earth. The images from the webcam are transmitted to the open source PHD2 automatic guidance software, which detects bright stars in the webcam image and sends corrections to the stepper motor drivers to lock them in place. This apparently works very well, as shown by the image of M31, the Andromeda galaxy.
If you want to introduce yourself to astrophotography and that a simpler tracker is faster, take a look at those we've touched on in the past: from the traditional crank-type to the single-engined, through a Printed version in 3D.