PolarAlign from COAA

PolarAlign is a utility that helps you to set up an equatorial telescope mount.

For perfect tracking of the night sky, the polar axis of an equatorial telescope must be accurately aligned with the axis of rotation of the Earth. PolarAlign helps you to do that quickly and accurately.

How does it work?

The program determines the current axis of the mount by analysing three images that you capture using the mount, to determine (by plate solving) the centre of each image in celestial coordinates. From those three points, it then determines the current axis of rotation of the mount in celestial coordinates. Having established that, and taking into account the date and time of the exposures and the latitude and longitude of the observer, it then calculates the correction required to bring the axis into alignment with the exact pole and reports the adjustments necessary in azimuth and elevation to make it quick and easy to bring the mount into perfect alignment.

How do I use it?

Firstly, the program needs to know where you are and what are the characteristics of the camera, the lens and the mount. Click on the Obs button on the toolbar and a dialogue opens that allows you to enter these details.

Location

The observatory location is required in degrees and decimals of a degree. It does not accept degrees and minutes nor degrees, minutes and seconds. North latitudes are positive. South latitudes require a minus sign before the degrees. East longitudes are positive. West longitudes require a minus sign before the degrees.

Camera

The camera pixel size is required in microns. For many popular DSLR cameras, the program already contains the required data. Click on the button marked “Look up camera parameters” and scroll down to the camera type that you are using. Be careful not to confuse “Canon 30D” with “Canon “300”, which are quite different. Note that there is an implicit assumption that the captured image is at the native resolution and there has been no binning or scaling of the image between capture and processing.

Lens

The lens focal length is required in millimetres. We recommend using a 200mm focal length lens as this gives a useful area of sky in the image ensuring that PolarAlign will be able to identify the starts and perform the plate solving. Be particularly careful if you capture images in some format other than JPG and then convert the images to JPG format for PolarAlign. Some conversion utilities rescale the image which invalidates the calibration.

Polar axis adjustment

By default, the program reports the adjustment required in the two axes (azimuth and elevation) in degrees. However, if you provide the details of the mount, PolarAlign will convert those adjustments into turns of the adjusting screws in the two axes so that the correction that you make, will be exact the first time.

To convert degrees of adjustment into turns of the adjusting screws, the program needs the adjusting screw thread pitch and the action radius in both elevation and azimuth. For some popular mounts, the program already contains the required data. Click on the button marked “”Look up mount adjustment parameters” and scroll down to the type of mount that you are using, highlight that entry and click OK to proceed. If your mount is not listed, then you can measure the relevant values using a thread gauge for the screw pitch and a normal measuring scale to estimate the action radius. If you measure those values yourself, we would appreciate you letting us know the figures so that we can add them to the table within the program.

The details that you have entered are saved by the program for subsequent use so you only need to enter the data once.

Calibration Images

To perform the polar alignment calculation, the program requires three images of the polar region captured using the mount. No telescope is required for this; a DSLR with a 200m telephoto lens is perfect. The images should be in JPG format. For typical focal ratios and sky conditions, an exposure of 5 seconds or so should be sufficient for each of the images. The pointing of the camera is not critical but the pole star (Polaris, Alpha Ursae Majoris) should be somewhere in or near each image so that the plate solving algorithm will find all the stars. To that end, there is an implicit assumption that the mount is already within a few degrees of the pole.

Between each of the three images the mount should be rotated in Right Ascension through a substantial angle. A rotation of 90° between successive images would be ideal but this is often difficult for practical reasons. A rotation of around 60° between each image give perfectly good results. Note that the mount must not be moved in Declination between the images; it must remain locked in Declination and it is important that the camera is firmly secured to the mount so that there is no possibility that it might droop between the three images, which would invalidate the calculation of the axis.

These three pictures illustrate typical mount positions for the three reference images. They would , of course, be captured at night not in the daytime as shown.

The program reads JPG format images so you should set the camera to record that format before capturing the three images. Once the three images are captured, each one is entered into PolarAlign. The order of entering the images is not important. Press the “Img” button to input each image.

The time tag of the JPG file is assumed to be the time that the image was captured in UTC. For each image, a dialogue appears asking you to confirm that the time is indeed correct in UTC. If the time is wrong in UTC, either because of a time zone difference or because the file was transferred between servers and re-timed, correct it and ensure that it is in UTC before you click OK. This is important to allow PolarAlign to calculate the elevation and azimuth correction from the measured offset in celestial coordinates.

After each image is loaded, PolarAlign will display the image (in monochrome) with the reference stars highlighted by blue circles and the image centre marked with an asterisk. The position of the pole is marked with an “N”.

If the blue circles are not coincident (or almost coincident) with stars in the image, then there has been a failure of the plate solving. Check that the input image shows stars within about 5° of the pole, that the camera parameters (pixel size, lens focal length) are correct, and than the signal to noise ratio in the image is reasonable. Do not proceed unless there is a good match between the stars in the image and the catalogue stars (blue circles).

When the third and final image has been entered, the program analyses the three image centres to find the current polar axis and finally reports the correction required. The main window shows the third reference image, together with symbols to mark the centres of each of the three reference images (“Cen-1”, etc.) and the calculated position of the current polar axis (marked “Axis”).

Finally, a new window opens showing an image of a typical mount displaying the adjustment direction and amount in the two axes. The red arrows show the direction in which the screws are to be adjusted, as shown below.

If you entered the mount alignment adjustment parameters at the beginning of the process, the corrections to make, will be reported in turns of the two pairs of adjusting screws.

If you left the parameters at the default, the corrections will be reported in degrees, which you will have to interpret in the adjustments that you make.

Once you have made the recommend adjustments to the mount alignment, you can repeat the process once more to check that the alignment is now correct. Select File..Delete image data to start the analysis process again. All being well, the next cycle will result in the alignment errors being reported as zero in both axes as shown below.

You can now use the mount, confident that the tracking will be as close to perfect as possible.