By Dave Rowe

This DOS program allows you to quickly polar align an equatorial mount to the North Celestial Pole (NCP) using a CCD camera and 35 mm camera lens. The CCD pixel coordinates of two calibration stars near the NCP are recorded, the mount is rotated about the polar axis, and the pixel coordinates of the same two calibration stars are again recorded. This information is entered into ALIGN, which then calculates the current pointing direction of the polar axis. As an alignment aid, the program also calculates the pixel coordinates of the two calibration stars when the mount is perfectly aligned with the NCP. By using this alignment aid, a single iteration is typically sufficient to align a mount to within one or two arcminutes.

The current version of this program is for northern hemisphere users only. If there is sufficient interest, I will add southern hemisphere coverage.

To align by this method you will need a CCD imager with a wide-angle lens. I use an ST-4 autoguider CCD camera and a 35mm focal length Nikon lens, but many other combinations are possible. The imaging system should have approximately 1 arcminute pixel resolution or better, and cover approximately a four-degree field of view (or larger) to make it easy to find the calibration stars. The ST-4 imager combined with a 35mm focal length camera lens is an excellent combination for this application. I have also used a 50mm focal length lens and the ST-4 with good results.

You will also need a way to point the CCD imager at the NCP. I use a ball swivel, but any reasonable arrangement should do. With a bit of practice, I can now point the CCD camera to within a degree of the pole on the first or second try. It’s not necessary to accurately point the CCD imager. It need only show the two calibration stars before and after a small (20 degree) rotation of the polar axis.

I made an adapter for the ST-4 and camera lens from a short (1.6") length of 2" diameter aluminum tubing, a rear lens cap, and a plywood insert to hold the ST-4 nosepiece. JB Weld epoxy was used to glue the rear lens cap to the aluminum tubing. I’m sure that there are many other quick ways to mount a 35mm camera lens to the ST-4, but the one that I came up with in an hour or two does the job nicely.

You will also need a laptop to take the images and to run ALIGN.

ALIGN uses three different coordinate systems, each of which should be well known and obvious to astrophotographers.

The locations of the two calibration stars are entered in celestial coordinates (right ascension, declination). These coordinates can be found from planetarium programs such as The Sky or Megastar. At the moment, ALIGN does not correct the coordinates for precession. The celestial coordinates should be entered in decimal hours and decimal degrees. By default, ALIGN uses lambda-Ursa Minoris and Polaris as the calibration stars. The user can change to other calibration stars by entering their coordinates in the Setup Panel.

The measured locations of the calibration stars on the CCD image are in units of pixels in the x- and y-directions.

The calculated displacement of the polar axis from the NCP is given in degrees East or West of the pole and in degrees Up or Down from the pole.

ALIGN uses the DOS clock on your computer, so make sure that the time and date are correctly set.

There are three panels in ALIGN, accessed using the function keys. Within a panel, move between entries using the cursor keys.

In this panel the user enters information about their location and time as well as the characteristics of the CCD camera and the locations of the two calibration stars. The program will retain, in a file called ALIGN.DAT, the values that you have entered in the Setup Panel. Unless you change your location or CCD camera, you need only worry about daylight savings after entering the correct information in this panel for the first time.

Longitude – Enter your approximate longitude in degrees from Greenwich. West of Greenwich is positive and east of Greenwich is negative. You need not enter your longitude more accurately than about +/- one degree.

Time Zone – Enter your time zone as an integer. Once again, West of Greenwich is positive and East of Greenwich is negative. As an example, the US West Coast is time zone 8.

Daylight Savings – If it’s daylight savings time, enter 1, otherwise enter 0.

CCD Pixel Dimensions – Enter the x- and y-dimensions of the CCD pixels in microns. For the ST-4, the pixels are 13.75 by 16 microns. If the image that is produced by the CCD camera and lens combination is inverted about the y-axis, then the x-dimension should be negative. Similarly, if the image is inverted about the x-axis, then the y-dimension should be negative. The imaging software that comes with the ST-4, called CCD.exe, appears to the user as right-reading. In other words, the sky looks, on the screen, as it would when viewed through a small telescope. However, the pixel locations in the y-direction increase as you move lower on the image. Thus, the image is effectively inverted about the x-axis. Therefore, for the ST-4 and CCD.exe, the correct pixel dimensions are (13.75, -16.0).

Calibration Star Locations – Enter the Right Ascension in decimal hours and Declination in decimal degrees for the selected calibration stars. The program will default on initial use to lambda- Ursa Minoris as the first cal star, and Polaris as the second cal star. If you don’t like these cal stars, pick two that are within a couple of degrees of the NCP if possible.

The first step is to roughly polar align your mount. I just visually align the axis to Polaris. Within a degree or so is good enough.

The next step in polar alignment is to move the CCD camera until the two calibration stars are in the field of view and will stay in the field of view after a polar axis rotation of about 20 degrees. Lock the CCD camera down and don’t move it.

Record the locations, in pixel coordinates, of the two calibration stars. Be careful with the order. If your imaging system has a resolution of about 1 arcminute per pixel or better, it is sufficiently accurate to measure the center of the calibration star to within one pixel. Higher accuracy can be obtained by calculating the centroid of the cal star, but this is usually unnecessary for this application.

Rotate the polar axis of the mount in either direction by at least 20 degrees. The CCD camera should rotate with the mount, but should not move with respect to it. In addition, the declination axis of the mount should be locked down and not moved if the camera is attached to the telescope or mounting plate.

Once again, record the new pixel locations of the two calibration stars.

In the <F2> panel of ALIGN, enter the four sets of coordinates for the calibration stars. When finished press <F3>.

This panel finds the current polar axis pointing direction, as shown in the right screen as the red cross. In addition, the box labeled Alignment Aid tells you how to move the mount to quickly polar align. Adjust the altitude and azimuth of the mount so that one of the calibration stars has the given pixel locations. You are then polar aligned. Obviously, don’t move the camera or rotate the polar axis of the mount if you want to use this feature.

Also shown in this panel is the time, in various measures, and information about the two images, including the effective focal length of each image and the angle of each image with respect to vertical. The rotation of the polar axis between exposures is also shown. To update the Time and Alignment Aid, press <F3> again.

There are three possible errors that can be generated by faulty data:

1. The cal stars are within one pixel of each other. This usually occurs if no data is entered in the <F2> panel.
2. The polar axis is rotated by less than 2.5 degrees between exposures. This usually occurs if you forget to rotate the polar axis at all.
3. The effective focal length differs by more than 20% between exposures. This usually occurs if you have mis-entered the data.

To run an example, leave all settings in the <F1> panel at their default values. In the <F2> panel, enter the following calibration star locations:

Position 1 Position 2

Cal Star 1 (20,30) (30,20)

Cal Star 2 (100,110) (100,110)

This will allow you to see the results in the <F3> panel, bypassing the error messages.

Please test your polar alignment before exposing. I always check alignment by taking another pair of images and using ALIGN to verify that the polar axis is within my tolerance window. If you are a first time user, make sure that you are using the program correctly by checking the results of this program with your normal alignment method.

Download the WinZipped program and support files here:

align.ZIP

It’s hard to tell when software works well or needs improvement. If you try ALIGN, please send a note with your comments. Thanks, DR.