SharpCap will guide you through this process with on-screen instructions, including a live update of how far you still need to move the mount in each direction to get perfect alignment.
Since SharpCap has worked out exactly what RA&Dec the telescope was pointing at in each image, it knows where in the image (or perhaps how far off the image) the celestial pole is. It also knows the point about which the stars seem to rotate - that's where your RA axis is currently pointing. If those two points are the same then your polar alignment is perfect. If they are not the same, all you need to do is adjust the Altitude and Azimuth adjusters of your mount until they are the same point and you have completed your polar alignment.
2. The center of rotation about which the stars seem to rotate when going from the first to the second image.
1. The exact area of sky represented in each image - this process is called Plate Solving. SharpCap has a built in plate solving algorithm that doesn't need an internet connection or any other program or database to be installed. SharpCap's plate solving only works within 5 degrees of the pole though (N or S).
Polar Alignment is a new feature in SharpCap 2.9. The idea was inspired by the PhotoPolarAlign application created by Themos Tsikas. Themos has been kind enough to help with testing and suggestions during the development of the polar alignment feature in SharpCap.The polar alignment works by analyzing two pictures taken of the area near the pole. You take one picture, let SharpCap analyze it, rotate your mount by about 90 degrees about the RA axis and take the second picture. By recognizing the stars in each of the pictures, SharpCap can work out two things:
You can start the polar alignment procedure from the 'Tools' menu.
When you first select the Polar Alignment tool, SharpCap will try to plate solve each frame coming from the camera. If enough stars are detected and the field-of-view is the right size and close enough to the pole you should see something like this
The stars SharpCap is using to perform the plate solving are highlighted in yellow, other stars are highlighted in red. The North (or South) celestial pole is shown and circles of different radii are shown around it. Note that the pole may be out of view - don't worry if it is, carry on to the next stage.
If the plate solving fails then there are three possible causes - not enough stars detected, too far from pole, field of view too large or too small. The last two require you to make physical changes to the setup, but the first might be fixable by adjusting the star detection parameters at the bottom of the screen or by adjusting the exposure or gain of the camera you are using. If the stars are too faint, try turning up the exposure, gain or digital gain. If hot pixels or noise are being picked up as stars, try turning up the noise reduction control.
Once you have the first frame solved, press the 'NEXT' button to move to the next stage.
After pressing 'NEXT', rotate the RA axis through about 90 degrees. You can do this either by unlocking the RA clutch or by using the mount's GOTO system if it has one.
SharpCap will continue attempting to plate solve each frame - once it manages to solve a frame that has rotated far enough it will offer the option to move on to the adjustment phase - looking a bit like this.
At this point, SharpCap has calculated the position in the image that the RA axis is pointing at - this is the point around which the image appeared to rotate. The RA axis point might be out of view, so don't worry if it doesn't show up, as long as the 'NEXT' button becomes enabled.
If you can't get the 'NEXT' button to enable then try different amounts of rotation (or rotating in the opposite direction). If that still fails then it's likely that you aren't picking up enough stars in the rotated position - the best way to fix this is to leave the mount in the rotated position and press the 'Restart' button to go back to the start of the alignment process. This leaves you looking at the rotated position with the star detection controls available to play with until you can get a plate solve in that orientation.
DO NOT ADJUST THE ALTITUDE OR AZIMUTH UNTIL YOU HAVE PRESSED NEXT TO MOVE TO THE FINAL STAGE!
Once you've pressed the button to move to the adjustment stage, one of the brighter stars on screen will be highlighted with an arrow pointing to a target, like this.
All you need to do to get good polar alignment is to move the indicated star into the target - if you do this you will also have lined up the NCP with the RA axis and you'll be done. At the bottom of the screen, below the Polar Align Error figure are some guidelines indicating which direction you need to move your mount. These are calculated based on your estimated longitude which is calculated from the offset between your local time and GMT, so they are not exact.
During the adjustment phase the highlighted star might switch to a different one - no need to worry, just keep adjusting. As you get closer the arrow and target changes to a pair of parallel lines that need to be brought together to finalise the alignment process.
If you find that the length of the arrow isn't updating, or is only updating now and then, it's likely that the plate solving isn't working for every frame - you can either ignore the problem and just get the star into the target or alternatively you can try further tweaks to the gain or exposure to get the star detection and plate solving working more reliably.