STATION OF THE TELESCOPE

This section discusses how to set up the telescope (MES) in the equatorial position. As we will see, for long exposure photography (more than 30 seconds), it is important to put the telescope in an equatorial position to compensate for the axis of rotation of the earth.

This method of stationing is valid both for German equatorial mounts and for azimuthal mounts fitted with a equatorial table. Although German equatorial mounts offer the possibility of setting up with a polar finder, the precision is not precise enough for long exposure photography of the deep sky. With the polar finder, we obtain an accuracy of about 5 ′ of arc, at best 2 ′ of arc, which is insufficient. It takes an accuracy of positioning on the North Celestial Pole of 1 ′ of arc and less for long exposure photography (with autoguiding in parallel) which will allow exposure times per photo of 10 minutes and more in autoguiding. The method which will be described will make it possible to achieve this precision. So, do not spend your money to get a polar finder, rather prefer this setting which is more precise.

The North Star is currently at approximately 41 arc minutes (or 0.70o, this measurement varies over time) of the polar axis of rotation. When setting up, aligning the telescope with the Pole Star is not precise enough for long exposure photography, because the Pole Star is far too far from the axis of rotation. As mentioned above, an alignment error of up to 1 arc minute (in right ascension and declination), of the axis of rotation, is considered sufficiently precise for long exposure photography with autoguiding in parallel. .

This method is relatively easy to implement and takes little time (approximately 40 minutes including installation of all equipment). I recommend it for anyone who moves their telescope between their astronomy sessions. 

Setting up the mount

  1. To begin setup, the mount motors must not be operational.
  2. For users of an azimuthal mount, calibrate the telescope (performed once) in declination and right ascension. See below for more details.
  3. Install the telescope stand at a comfortable viewing height.
  4. Make sure the telescope feet are on a solid base. If the ground is not stable, we will lose the MES and there will be a drift of the stars which will produce elongated stars.
  5. Put the top (which will receive the telescope) of the level feet.
  6. Install the German Equatorial Mount or Equatorial Table on top of the foot and ensure that it is oriented approximately towards the Pole Star. To do this, the mount must be in the neutral position (indicate by a mark or 0) of right ascension.
  7. Install the telescope on the mount or equatorial table.
  8. For azimuthal mounts, put the telescope tube in the declination position of 90o so that he points to the polar star.
  9. For German frames, put the frame in the declination position indicated by an arrow or other mark.
  10. Bring the axis of latitude in line with the observation site (Montreal region 45,5o). From this moment, the mount and the telescope points to the polar star.
  11. For the German equatorial mount, install the counterweights.
  12. Install all the instruments used for astrophotography as well as the wiring.
  13. For German equatorial mounts, swing the telescope in right ascension and declination (see details below).
  14. Level everything very precisely. To do this use the spirit level that comes with the German mount or the equatorial table. Be careful, make sure this level is level! On my mount, he was not! I then bought a small spirit level in the store that I put on top of the one that comes with my mount. Pay special attention to leveling, as the telescope's tracking accuracy will be highly dependent on a good leveling (imprecise leveling will cause stars to drift).
  15. To facilitate the search for the polar axis of rotation, install a 40 mm eyepiece preferably.
  16. With the help of the detailed explanations in the next section (Find the polar axis of rotation), precisely align the telescope to the Celestial North Pole (PNC) using the longitude and latitude axis adjustments of the equatorial table or mount.
  17. Start the mount motors for tracking in equatorial mode.
  18. For azimuthal mounts, make sure the mount is in equatorial and non-azimuthal mode.
  19. If you have a computerized telescope (Go To), you can point to 1 or 2 guide star (s) suggested by the software. This will only be used to point the object to make it appear in the eyepiece or on the screen. Of course, it will not improve the alignment accuracy on the PNC.

Details on how to swing a German equatorial mount (point 13 above)

  • Loosen the right ascension screw (AD) of the mount to put the mount in a horizontal position, as shown in the following image:
  • Swing the telescope
  • By moving the counterweights, bring the telescope into perfect balance.
  • For better frame tracking, do the following:
    • To. For an observation towards the west (telescope towards the east), put a slight imbalance on the side of the tube.
    • b. For an eastward observation (telescope west), put a slight imbalance on the counterweight side.
    • So in all situations the imbalance should always be towards the east. This must be very thin. It will prevent "Backlashes" of the frame in autoguiding. Tighten the right ascension screw on the mount securely.

Swing the mount in declination

  • The mount is always in a horizontal position and the right ascension screw is tight.
  • Loosen the Declination (DEC) screw on the mount to bring the telescope (tube) to a horizontal position.
  • Watch which way the tube is out of balance.
  • You have to move the tube forwards or backwards to put it in perfect balance.
  • If it is difficult to move the tube in the horizontal AD position, the AD should be placed vertically (weights down) to facilitate movement of the tube. Then return to the horizontal position of AD to check the balance of the tube in DEC.

Calibration of the azimuth telescope, fitted with an equatorial table, to the declination position of 90o

Before starting this procedure, put the telescope base perfectly level in the azimuthal position. Align the telescope tube perpendicular to the ground. Put a spirit level that rests on the frame (on either side) of the top of the telescope in the direction of the declination. Level the top of the telescope by moving the telescope's declination axis. Set the declination dial to position 90o. This simple procedure, if the telescope is perfectly level in the azimuthal position, will allow the calibration to be precisely 90o.

Calibration of the azimuth telescope, equipped with an equatorial table, at the neutral (0) position of the right ascension

  • This adjustment can be made at home.
  • Carry out steps 1 to 11 (of course, we can skip step 9) of the Preparation of the telescope setup above.
  • Put a spirit level which rests on the 2 forks of the telescope.
  • Loosen the telescope right ascension screw.
  • Rotate the optical tube on the right ascension axis until the 2 forks are level.
  • Tighten the right ascension screw.
  • Set the right ascension dial to the zero position.

Richard Beauregard

Sky Astro - CCD

Revise on 2021/01/14