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Vixen Equatorial Mount Polar Alignment   
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 Vixen Equatorial Mount and NCP / Polar Alignment
The 200 mm diameter Schmidt-Newtonian telescope and Vixen GP equatorial mount together with the telescope specification.

The stability and polar alignment of the equatorial mount are critical to obtaining good images. The Vixen GP mount is set up on well-bedded concrete slabs. The slabs provide stability but are a source of thermal currents. Repetitive positioning with the polar axis scope pointing due North is ensured by locating the 3 height adjusting leg screws into snugly accommodating 5 mm deep holes drilled in the slabs. In initial set up for North Celestial Pole (NCP) alignment, the central pedestal was first set truly vertical using spirit levels attached in North-South and East-West planes. The polar axis was then accurately established for altitude (see below), and locked hard into position. Everyday use therefore generally only requires minor adjustment of the 3 leg screws for NCP alignment, since the polar axis for altitude is truly located and locked. The whole alignment is checked now and again when the Meridian marker on the polar axis scope is updated.

The Drift Method is a commonly accepted procedure to check and correct for polar alignment. The effective use of periodic error correction (PEC) on telescopes, which have such facilities, has also been discussed (Effective Use of PEC Programming

The Vixen Instruction Manual clearly indicates how to check if the optical axis of the polar axis scope is parallel to the rotational axis of the mount, and how to make it so. A check of my mount showed that I would achieve no better by tampering with the settings. This check is vital, and any required corrections made.

Alignment of the polar axis of the mount is based on observing Polaris through the polar axis scope and aligning it, by moving altitude and azimuth knobs on the mount,  to a marked reticle position for the specific date and time, which are set on the Hour (R.A.) and Date graduation rings around the polar axis of the mount. The reticle is offset East or West of a central Meridian position according to the longitude difference between my location (determined from a Ordnance Survey map) from my standard time meridian (Greenwich). However, on purchase in 1998, the Meridian was set for 1:00 am 10 October 1990 corresponding to an Upper Culmination of Polaris. Hence, for present dates, due to precession, alignment on the reticle was along a subjective path, illustrated in the Instruction Manual, away from the marked reticule altitude positions.

The Meridian position should be changed to a more recent date to allow the Polaris alignment to occur on the marked altitude scale on the reticle. My Meridian is presently set to the Upper Culmination of Polaris at 16:11 hours on 28 February 2000. Sky Map Pro indicating this to be the transit time at my location with Polaris at Declinaton of 89 deg 16 min. Hence, I require no judgement offset of the Meridian for my location being West of Greenwich, and I merely need to align Polaris at a given time and date to 44 minutes on the marked reticle, to correspond to it's position from the North Celestial Pole.This works well as evidenced by long imaging times, of generally 4 minutes or more, with no star trails.
Tracking problems have been evidenced, however. These are generally realised as "double" images at exposure times of 4 minutes or longer, as exemplified in the adjacent 8 minute exposed image. The RA drive gear wheel has 144 teeth, hence the screw (worm) gear driving this does one complete rotation in 10 minutes. It appears, therefore, that the problem is due to backlash in the gear train. Meshing the gears closer has helped but not fully relieved the problem. Loading the gearing by adjusting the counterweight position can help, such that 8 or 10 minute or more exposure times can be obtained, but more by luck than judgement. A reliable 4 minute exposure time, however, is often more than adequate.
Motors fitted to both the RA and Dec. drives have specifications of errors in siderial rates of less than 0.005% i.e. 0.045 arcsec per minute or 0.18 arcsec in 4 minutes. At prime focus, the MX5 CCD field of view for the 9.8 by 12.6 micron pixels is 2.53 by 3.25 arcsec per pixel. Hence, the motor drive errors in themselves constitute no serious threat in a 4 minute exposure. The greater threat is in the gear train, as noted above, lack of good NCP alignment, and lack of rigidity and stability in the optical paths through the telescope and fittings to the CCD camera. The latter cannot be over emphasised, and particularly the screws on the T rings.

Cables to the CCD camera must not be dragged along the ground during viewing, or place a transient weight on the camera. The cables need to be supported and attached to the telescope cradle by Velcro tape or similar. Likewise, any movement around the mount during imaging is usually detrimental, and this can sometimes be due to movements by the wind.

Vibration in the system can originate from many sources and the general considerations have been discussed elsewhere (Vibrations in Telescopes-General Considerations) together with motor drives and control systems.
 

 

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