What is guiding Guiding is the process of keeping an object being imaged dead centre in the telescope to allow for long exposures. The guidescope and guide camera are fixed to your telescope and the guide camera is connected to your mount and some software on your laptop or remote PC dependant on how you’re connected. The guide camera is fixed on to a star and the software analyses the feed and sends minute “push” signals to your mount to keep this star centred on the screen. Because the star is (closely) moving at the same rate as the object that you’re imaging through your telescope, by keeping the star centred in the guide camera, the object being imaged through the telescope is kept centred. This means that you can take as long an exposure as your equipment will allow. What you will need 1) A guide camera 2) A guidescope 3) A laptop, mini PC, Raspberry Pi, ASIair or similar 4) Guiding software, PHD2 is free and easy to use but a Google search will offer up other options. You will notice two ports on the camera above. One connects the camera to the mount and this is called the ST4 port. The other connects to the USB port to talk to the guiding software on your laptop/miniPC/Raspberry pi etc... The way it works is like this 1) Connect your camera to the mount and laptop 2) Start the guiding software and initialise the camera 3) Select a star 4) The software will now do some calibration by moving the star in the East, West, North and South directions 5) Once the calibration has completed you will now be guiding on the selected star and good to go 6) The camera sends consecutive short exposures to the guiding software, usually around 3 seconds 7) The guiding software analyses each image received from the guide camera and notices any movement of the star 8) The software sends a signal to the mount via the camera and tells it to move in the correct direction to counteract the movement of the star, thus keeping the star (and your DSO target) bang in the centre Congratulations, you’re now guiding!!! If you connect your equipment to either a mini PC / Raspberry Pi / ASIair or something similar then you can lose the camera to mount cable and utilise a setting called “Pulse Guiding”. This setting can only be used if your mount is connected directly to one of the above devices. It sends the correction “pulse” directly to the mount instead of via the camera and this can help with cable control.
© Paul’s Astrophotography 2021
In order to obtain the lengthy exposures needed for imaging deep sky objects (DSO) then you will need to add a process (and more equipment) to your setup. Paired up with an equatorial mount a guide camera and guidescope will enable your setup to greatly extend your exposure times to capture those all important photons. When imaging a DSO you need to let as much light fall on to the sensor as your setup will allow. The longer you can allow the light to hit the sensor, the more light data builds up in the image file. You then take a series of these long exposures that you will then stack to bring out even more detail. The problem that all mounts have (no matter how accurate their motors are) are the humans that operate them. We already Polar align our mounts to enable us to counteract the spin of the Earth which in turn means that we can track an object in a single axis, right ascension (RA). This is ok for visual observing or shorter exposure lengths, and in general a very well polar aligned mount can give between 1 and a half to 2 minute unguided exposures before you start to see trailing on the stars. This is fine when imaging bright objects like the impressive Orion Nebula but when imaging faint targets then you need to extend your exposure times considerably. And if you decide to dabble in narrowband imaging then you could be needing some seriously long exposures dependant on the camera being used, the only way to achieve this is to implement guiding in to your setup.


Guide/Planetary Camera
Camera & Guidescope Setup