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Motorised Flip Mirror

Installing a flip mirror on the optical train of your telescope gives a new dimension to your astronomical equipment. In my case, as I use the telescope for photometric and spectrometric measurements, the flip mirror allows you to use the telescope with two instruments without having to put on and take off the one you want every time, which always requires adjustments.

In one position of the flip mirror you can make the light captured by the telescope pass through the flip mirror without deflection (we will call this the open position) and in the other position it is deflected laterally (we will call this the diagonal position). In my case, in the open position of the flip mirror, I operate a spectrograph and in the diagonal position a camera for photometry or astrometry. The case of astrometry is an interesting application, since the auxiliary camera of the spectrograph usually has a small sensor and it is difficult to resolve the plate (plate solve) while with the flip mirror in diagonal position with the right camera we will obtain a good field to resolve the plate, which will help a lot to make the correct centring of the star in the slit of the spectrograph.

The problem with the flip mirror is that during the course of an observing session, we will need to change the position at different times. Doing this manually is impractical because it does not allow us to leave the session programmed and requires us to be attentive and close to the telescope to do it manually. The ideal is to be able to change the position automatically, in a similar way to changing the filter with a motorised filter wheel.

To realise this automation function we will use a commercial flip mirror, the one from Baader, which is already prepared for the integration of a motor and has an excellent mechanical construction. The manufacturer mentions this possibility, but does not market it, at least at the time of writing, and does not even give any clues as to how the automation can be done. In this article we are going to propose a simple and effective solution, which I have implemented in my observatory.

Actually, the only thing that enables Baader to say that his flip mirror is motorisable is that the knob used to manoeuvre the flip is toothed. It is the same knob that is used for manual, but it has teeth so that it can be driven with a toothed belt. The manufacturer does not indicate in their specifications what type of toothed belt, and the only way to find out is to look at a video on their website in which a synchronous belt is shown for a fraction of a second a synchronous belt with HTD 3M screen-printed on it, which indicates the type of HTD tooth and the pitch of 3mm. 

Once we know the pitch and type of belt, we can now detail the elements we need for the mechanical automation.
we need for the mechanical automation:

  • Stepper motor, I have used the 28BYJ-48 model.

  • HTD synchronous belt of 6mm width and 3mm pitch. The length depends on the implementation, in this proposal around 170mm.

  • Motor pulley with the same HTD 3mm pitch.

  • Hall effect sensor for position detection and small magnet.

  • Plate to attach the motor to the flip mirror. The motor is fixed with spacers and the design allows to adjust the distance from the motor shaft to the wheel and therefore the belt tension.

An image with the elements is shown below:

In the image above, the engine is underneath and is not visible, but it can be seen in the following video:

The plate that fixes the motor to the flip mirror is made with a 3D printer and has a very simple design. After manufacturing it, I noticed that the knob is off-centre on the Baader flip mirror, so the right side of the mounting plate channel needs to be filed down a little to get the belt to fit correctly. The plate is fixed to the Baader flip mirror using two screws that the manufacturer indicates are for fixing external elements and that are indicated in the image.

The automation of the stepper motor can be done with any controller, and in my case I have used an Arduino Uno board and the ULN2003 driver. One input on the Arduino board is used for the Hall effect sensor, so that the position of the mirror can be known at any time thanks to a properly placed magnet and when the sensor detects it, the mirror is in a certain position.


The software on the Arduino board must move the motor the appropriate number of steps (1800 according to my implementation) in one direction or another depending on whether the Hall effect sensor detects that the mirror is in the diagonal or open position. The Arduino board is commanded via USB so that the system can be integrated into an ASCOM driver and the flip mirror can be operated as a filter.

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