What is the overall procedure for polar aligning my wedge-mounted Fork Arm Alt/Azm mount?
November 12, 2008
Your Forkarm scope can accurately track objects when used in its Alt-Az position. However, you will need to polar align and track in an equatorial mode for long-exposure astroimaging and to eliminate field rotation during a night’s observing.
To do this, you need to buy or build a wedge that fits on the Mount tripod. The wedge allows the scope to be tilted back in altitude at an angle equal to your latitude to be polar aligned. Celestron’s Heavy-Duty Wedge features fine adjustments for altitude (latitude) and azimuth for refining polar alignment of the wedge.
Once you have the wedge, the Fork arm scope’s polar alignment consists of 4 parts: Rough Alignment, EQ North Align, Wedge Align and repeating EQ North Align. (In the Southern Hemisphere, substitute EQ South Align.)
1. Incline the wedge to your latitude using its latitude scale.
2. Set up the telescope on the wedge and roughly position the mount in azimuth and altitude towards Polaris. The hinge or pivot will be towards the north.
3. Move the tube and scope until the tube is parallel to the fork arms, the fork is pointing north, tube handle down and star diagonal upright.
4. Lock the scope’s clamps and move the entire tripod-wedge-scope assembly so that the tube points close to Polaris. Get it close enough so that it is within the limit of subsequent adjustments that can be made using the altitude-azimuth adjustment screws on your wedge (if it has them).
5. Turn your wedge’s altitude adjustment screw(s) to fine-tune the altitude. Likewise, use the azimuth adjustment screw(s) to fine-tune azimuth to properly position the mount for polar alignment.
6. Align the Fork Arm to the sky by selecting EQ North Align, either allowing the GPS to deliver the time-site data or enter it yourself by turning the GPS off, and then choosing either EQ AutoAlign or EQ Two-Star Align for the most accurate polar alignment. EQ AutoAlign requires alignment of the scope’s altitude to the altitude index marks and also alignment of the scope’s east-west orientation until the optical tube is pointing at the meridian. Use the hand control direction buttons to move the scope for both alignments. AutoAlign then chooses two alignment stars for you. EQ Two-Star Align doesn’t require the altitude and meridian alignments, but you have to locate two stars for the alignment procedure.
(The above is for the Fork arm mounts with the version 4.0 or higher flashable hand control firmware. You will also have this feature if you have the newer Nexstar+ hand control. The older Alt/Azm telescopes (those with version 2.2 or older hand controls and those without versions for the hand control) have the North and Level method programmed in the firmware. These scopes will align to magnetic north and level when initializing during the Alt-Az alignment process. use older instruments on a wedge using GPS data, orient the tube so that the Celestron logo on the base is to the north at bottom and the tube is pointed downwards into the base of the fork. When EQ North Align is chosen and Enter pressed, the GPS will move to a position pointing approximately at the celestial equator on the meridian. When the GPS is off with these older telescopes, use the same procedure for the GPS with version 4.0 or later hand controls.)
7. After either Two-Star alignment, select Utilities Wedge Align from the menu and press Enter. The GPS mount will then slew to where it thinks Polaris is located, based on the positioning of the mount and the Two-Star alignment performed earlier. Now use only the altitude-azimuth adjustment screws to center Polaris in the eyepiece. (Do not use the hand control!) Always move the scope up when centering on Polaris to take up any backlash in the adjustment screws. Once it is centered in the eyepiece, press Enter. Now the polar axis is pointing towards the north celestial pole. (The function takes into account the offset of Polaris from the true pole.)
Repeat the EQ AutoAlign or EQ Two-Star Align, so your NexStar GPS scope is again aligned to the sky.