Specifications

2.3 POSITION ANGLE

The common way to measure position angle in the sky is to start at north at 0 degrees and rotate

clockwise (abbreviated CW) through East (90 degrees PA), South (180 degrees PA), West (270

degrees PA) and back to North for a full 360 degrees. When using a telescope, the apparent

rotation of the camera may be opposite when compared to the rotational direction in the sky. For

this reason, it may be necessary to reverse the default direction. This can be done in the setup

menu of the Pyxis control program or by using the CDnxx command described in Table 4-3.

Refractors and Cassegrain telescopes have the same apparent direction of field rotation.

Newtonians, on the other hand, are opposite. Depending on the number of reflections in the

telescope system, the default or reverse direction may have to be selected in order for the

rotational direction to match the sky. Trial and error may be necessary.

Once the proper default direction for rotation is obtained, the desired position angle can then be

achieved. For position angle, the Pyxis will only accept integer entries from 0 to 359 degrees.

Since there are 14.6 steps (128 steps for the Pyxis 3-inch) for each degree, there is a small round

off error of 0.5 steps maximum for each degree interval. This translates to a maximum error for

any position angle of 2 (0.6 for the Pyxis 3-inch) arc minutes. This value does not take into

account any gear backlash, which may be on the same order in magnitude.

The new position angle can be entered in either the Pyxis control program or by using the

CPAxxx command described in Table 4-6. At the default pulse delay of 8, it takes approximately

120 milliseconds for the Pyxis to rotate 1 degree. The default pulse delay for the Pyxis 3-inch is

6, which results in a rotational rate of 785 milliseconds per degree. During the rotational time, the

Pyxis cannot accept any commands until the move is completed. If the camera were tugging at

cables or mechanically obstructed for some other reason, it would be necessary to cut power in

order to abort the move. As mentioned before, the Pyxis will not be damaged if the rotational

movement is blocked. However, the user should use care and common sense with attached

cameras and cables to ensure they will not be damaged. When used in position angle mode the

rotator will never pass PA=180 degrees to avoid wrapping or twisting any attached cables.

2.4 ALT-AZIMUTH COMPENSATION

Alt-Azimuth mounts from some makers may have the ability to track celestial objects with

sufficient accuracy for imaging. The Pyxis has the ability to correct for field rotation that is the

undesired consequence of using an alt-azimuth mount with a camera. Normally, field rotation will

be a problem for any exposure over a minute in length. This depends greatly on the users latitude

and position of the object in the sky. Because the correcting rotation rate increase very

dramatically near the zenith, the Pyxis control program will not accept values greater than 80

degrees to zenith.