There is a very interesting fine-tuning feature offered with the CGEM (and CGEM DX) mount from Celestron. This is the procedure Polar Align. This function, if executed correctly, will allow exposure times of 10 minutes or more per photo with autoguiding in parallel.
One does not need to see the Celestial North Pole (PNC) from its observation site. We then choose a bright visible star, preferably near the South meridian and the celestial equator. The mount software will synchronize to this star and ask to move the mount to the location où it should be located thus allowing a precise alignment on the axis of rotation of the earth or the PNC.
Here is the detailed procedure to achieve this precise polar alignment:
Preparation for station setup
- Follow steps 1 to 14 of the section Setting up the telescope.
- Start the mount and follow the initialization instructions.
Alignment on 3 stars and more - Two Star Align and one or more additional stars
This step will allow a score Goto precise. It will not improve the precise stationing on the PNC. By choosing 3 stars or more, the mount software will accurately note the misalignment on the PNC, thus allowing an accurate GoTo pointing. Note that he will use this information for thePolar alignment which we will see in the next step.
An illuminated reticulated eyepiece must be used to properly center the stars and thus maximize alignment accuracy. If we use a CCD camera (or webcam) to center the stars, we can use the object Target (Bullseye) of the PHD Guiding software as in the following visual example:
- Choose your favorite Two Stars Align.
- Select the first star suggested by the frame software by pressing Enter. If the star is not visible due to an obstruction, press the key undo and the frame will offer another star. To select the star of your choice, press the keys Up down to select it from among the 200 stars in the database. To change the selection of West / East stars, press the key. Menu.
- Center the star in the finder and press Enter.
- Center the star in the eyepiece or screen and press the key Align.
- Choose the second star and repeat the same process as the first star.
- Add Calib Star? To push on Enter to choose a third star and follow the same procedure (I recommend doing the alignment on 5 stars to maximize the frame's score).
- To push on UNDO to complete the aiming accuracy of the mount.
- Watch the controller display to see Alignment successful.
Polar Align
It is important to note that everything we have done so far has been used only for the precise pointing of objects (GoTo). No improvement in the stationing on the polar axis of rotation has been achieved. This step will be used to increase the alignment accuracy on the PNC.
- Return to the main screen (UNDO to come back). With the help of a planetarium, choose a bright star near the South Meridian at around 40o above the horizon. Preferably choose a star on the east side of the meridian. Write down the name of the star.
- Push the button List.
- Select Named Stars.
- Select the star from the list (for example Algenib).
- To push on Enter.
- The mount moves to the chosen star.
- Check that the star appears in the viewfinder.
- Return to the main screen (UNDO to come back).
- Push the button Align.
- Choose your favorite Polar Align and press Enter.
- Select Align Mount.
- The mount moves to center the chosen star.
- Using the joystick, center the star in the finder scope and press Enter.
- Center the star in the eyepiece or screen and press the key Align.
- The mount syncs to the chosen Star.
- To push on Enter. The mount moves to where the star should be with precise alignment to the PNC.
- The mount asks to center the star using its Latitude and Azimuth adjustments (do not use the joystick).
- After moving the frame, press the button Enter.
- From this point on, the mount is aligned precisely with the PNC.
- Display align : To push on Enter to see the alignment error. Take note of your misalignment. If this procedure is performed correctly, the alignment error will be less than 1 ′ of arc. If this is not the case, turn off the mount and return it to its starting position (point towards the celestial North Pole using the guides on the mount). Then restart the mount and start from the beginning. In order not to start over, it is important to use an illuminated reticulated eyepiece or the Target Object of PHD Guiding to center the stars as mentioned above. Also, make sure that the frame is perfectly level.
Realignment of the mount following setting up on the cabin crew
During precise alignment with the PNC, the mount was moved using its Latitude and Azimuth adjustments. It is therefore necessary to restore the pointing accuracy (GoTo) of the mount following this movement.
- Return to the main screen (UNDO to come back).
- Push the button Align.
- Select Undo Sync (to cancel the current punch) and press Enter.
- The message Complete appears on the screen.
To re-align the frame
- Return to the main screen (UNDO to come back)
- Push the button Align.
- Select Alignment Stars.
- Select the original star.
- Center the star in the finder and press Enter.
- Center the star in the eyepiece or screen and press the key Align.
- The display shows Align Successfull.
Note: as the mount is aligned precisely on the PNC, it is not necessary to perform an alignment (GoTo pointing) on 3 stars and more.
Richard Beauregard
Sky Astro - CCD
Revised 2021/01/16