SkyWatcher Explorer 300PDS

Wanting to be able to easily check and correct collimation of my Celestron C11 XLT EdgeHD I built myself a collimator. Inspired by a very similar design at the Sterrenwacht Almere, the search was on for an approximately 300mm and at least f/5 parabolic mirror. Initially I found these TS-optics mirrors, but that I found a tad too expensive for this just-for-fun project.

A few months later a SkyWatcher Explorer 300PDS came onto the second hand market. The previous owner had tried it on a GEM to find out that he needed a ladder to be able to look through the eyepiece. With an overall length of 1.43 metres the OTA is indeed quite large, while it weighs about 25kg!

For my purpose, however, this was the ideal instrument and I acquired it for a very decent price. Initially I wanted to remove the secondary mirror, as that is how I had seen it being built in Almere. Having a larger central obstructing than this Newton, my EdgeHD would not see the secondary of the Newton, so I decided to first try it using the original focuser and secondary.

Now I needed an artificial star to be placed at the centre of focus. A friend of mine made me a circuitry diagram for a controller that allows to switch between three LEDs (on of which is a RGB-LED) and to dim them (see figure 2 and figure 4). Each LED is mounted in a connector for a 9μm glass-fiber cable. Once the glass-fiber cable is connected to the LED, the other end will produce a 9μm artificial star that needs to be placed at the Newton's centre of focus.

A simple piece of aluminium rod was used to create a 1.25" adapter that fits the focuser of the Newton, while having a fibre-optic cable connector at the other end (see figure 5). Using the original focuser, the artificial star is set to infinity. In order to accomplish that a reference scope (can be any telescope) is aimed at the heavens and focused, after which it is placed in front of the collimator-Newton. Then the focuser of the collimator is adjusted until a sharp image of the artificial is can be seen in the eyepiece of the reference-scope. Once accomplished the collimator focuser is marked and the scope that requires collimation placed in front of the collimator.

First light was produced on 30 December 2019 while collimating my friend's RC10 (see figure 1 and figure 3). At the time I used FireCapture and a ZWO ASI174MM camera to analyse the collimation. Later on I wrote a dedicated piece of software called CollimatorGrabber (see figure 6), that allows me to cut the image in halves or even quarters and project the parts in a mirrored way next to each other. By doing so any deviation from perfect is more easily detected.

I automatically detects the donut and grabs a square area around it. Depending on user settings it is then directly displayed or mirrored in two or four sections.

Once the donut looks symmetrical, the collimator allows to scrutinise the collimation with the scope in focus, showing, if all was done well, a very nice diffraction pattern (see figure 7).

For a more detailed explanation and the underlying physics, see my article on Starry-Night (in Dutch, but most browsers will translate it).