How to guide a mount with PHD?

2011-08-11

PHD is a free and popular application for mount guiding. Applications checks position of a star on an image from a guide camera, and if it detects any shift in the position of the star - it sends corrections to the mount. This allows taking very long exposures of deep space objects. PHD is easy to use and can be used with variety of mounts and guide cameras.

Download PHD and install it on your PC or Mac (there is also OpenPHD for Linux, but it's bit different and limited by low Linux support rate among vendors). It is also very likely you will need platformy ASCOM platform and Celestron Unified for SkyWatcher SynScan and Celestron mounts (if you can't guide them via ST-4 port).

When you start the application you will see something like that:

On the bottom you will find all the menu options of PHD. The photo camera icon lets you choose a guide cam type. Telescope icon allows you to choose a mount type. The arrow icon starts frame capture, while the brain icon holds guide settings. In general when you select guide camera and mount connection you start the frame capture, select a star and then hit the "PHD" icon to start calibration and guiding. In the top menu there is also "Mount" menu that allow you to choose a platform you want to use (ASCOM, ST4, relay boxes..). The trick is that there are many ways of connecting to a mount and many types of guide cameras.

Selecting guide camera in PHD

When you click on the camera icon you will see a list of cameras or cameras groups. From that list you have to select the guide camera you want to use:

Webcams and USB planetary cameras like DMK/DFK/DBK and also for example Basler (GigE) are hidden under "Windows WDM-style webcam camera". Dedicated guide cameras will provide they own drivers and will show up directly on the list, or if they have ASCOM driver - will be listed under "ASCOM v5 Camera":

For webcam-like cameras (Basler and others) PHD may not be able to set exposure time or other setting. In such cases start the app you use for planetary imaging and set exposure time and other settings if needed, quit and launch PHD. Set the same exposure time on the select and it should cooperate nicely.

When the camera is selected you can choose the exposure time and start frame capture. You need to find a star that can be used for guiding. Note that even small defocus may blur stars to non-detectable level - so focus is important. Select semi-bright star that isn't on the edge of the field and start calibration by hitting the "PHD" icon. The application will calibrate the guide camera and if it succeeds - will start guiding... but we also have to connect to the mount.

Connecting mount to telescope

There are two (or even three) options to do this:

ST-4 guide camera and ST-4 capable mount connected together (skip this section)
Guide camera without ST-4 port - you need to connect the mount to the computer and use ASCOM mount drivers (with RS-232 cable).
Guide camera without ST-4 port but Shoestring GPUSB ST-4 relaybox used to connect to the mount ST-4 port from a computer (skip this section)

For mounts like EQ6/HEQ5 SynScan you will need a RS-232 cable (available in some astro or electronic shops) and COM/RS-232 to USB adapter (electronic stores):

Connect RS-232 cable to the SynScan control panel using the available slot on the bottom. Then connect the COM to the COM/USB adapter, and that to the computer. To control the mount you need to use the ASCOM drivers (Celestron Unified for SkyWatcher and Celestron mounts).

NOTE: do NOT set the control to "PC Direct Mode". This is only if you want to use EQmod platform.

RS-232/USB adapter should be provided with a software to select a virtual COM port for the adapter to use. You need to select one, reboot and then use that COM port (like COM1) in the PHD mount settings.

Syntrek mount need a EQDir adapter to connect them to a PC. To control those mount you will need EQMod platform and software.

Mounts like Syntrek, SynScan and other with ST4 port may use a ST4 relay box like Shoestring GPUSB. You won't need fancy cable or adapters. Just USB to the computer and ST4 to the mount, and in PHD this device should be either on the ASCOM mount list (if it has ASCOM drivers) or selectable from the top mount platform menu (GPUSB type).

Selecting a mount in PHD

For ST-4 guider select in the top mount menu "On Camera". For SynScan (and Celestron mounts) use the Celestron drivers:

Under Properties you can set the correct COM port and other things:

When you click "Ok" it should connect to the mount. If you don't get any error-popups then it succeed and we can continue. If you got "ASCOM error" then something is not right (COM/USB adapter or the connection to the mount)...

PHD Settings

Under the brain icon we have a lot of guide parameters:

When you start the calibration PHD will star moving the scope observing how the star moves on the guide camera FOV. It calculates camera axis orientation, as well as effects of correction pulses, slacks in the mount... If you change the guide camera position or PHD guide settings you have to force a new calibration. (select check box on the bottom of the settings window).

On default settings PHD should do just fine - unless you have quite big or low focal length on the guide camera (OAG, guide scope - whatever). For small focal length you have to increase "calibration step", and decrease it for big focal lengths (SCTs and alike). If the drift is high (from for example big resolution) increase "Max Dec duration" slightly if needed.

If PHD will show a message "Star did not move enough" then the calibration failed as the "calibration step" was to low and the star didn't moved as much as needed. Open settings, increase the value, force calibration and start over again (check also DEC guide mode - if it's off set it to auto).

Corrections aggressiveness may also depend on guide speed settings in mount controls. Try to use as low as possible. To strong corrections and the guide star will bounce back and forth resulting in elongated stars on the main camera.

The Guiding

When the calibration finished without any errors and targeting lines in PHD turned green - guiding have started. Check how it's working. If the guide star is bounced back and forth then the corrections are to strong. If it drifts away from the guide point then corrections may be to weak or calibration wasn't as good as it should be (wrong calculations due to fast drift, wind buffet) - recalibration needed.

If it's guiding as required you may stop it, correct the FOV on the imaging camera and start guiding on other guide star. No recalibration is needed.

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