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7/5/2010

If you  are into astrophotography, as I am, having the GEM mount properly aligned to  the North Celestial Pole (NCP) is a MUST. There are numerous ways to accomplish  this. Some of the processes vary with  the make  of mount you  have.

There are a number of polar alignment processes on the LINKS page at the left.

Using Polar Scope on the mount:

If your mount is equipped with  a Polar Alignment Scope (PAS) it can be used to set the NCP of your mount. The technique varies with  the scope manufacturer and you  should consult the manual or get help from members of various tech groups on the web. One thing that  is  very important is that  the PAS optical axis must be aligned with  the R.A. axis of the mount This process also varies with the mount manufacturer.

Drift alignment:

      With drift alignment you  position the telescope  close to  the Meridian and at  0 elevation. You  then watch to see if the star is drifting North or South and adjust  the AZ knobs to  stop the drift.

While watching declination (DEC) drift only, if the guide star moves UP, rotate the mount's AZIMUTH so that the guide star moves to the RIGHT in your eyepiece. If the guide star moves DOWN, rotate the azimuth so that the star moves LEFT. The reason you make these moves is described in the TUTORIAL. DO NOT use the slew buttons on the hand controller to do this. Use the AZIMUTH adjustment screws on the telescope mount.
The faster the guide star drifts up or down, the more azimuth corrections you must make. If the guide star drifts quickly, your mount is much further away from the celestial pole, and will need to be moved a lot further.. so far in fact that the guide star will move right out of the field of view! No worry.. you can always grab another star to finish the alignment process.

An excellent simulator that  allows you  to  practice this technique can be found at Polar alignment (How to do Drift)

      TO properly do the drift alignment you  should have an eyepiece with  an illuminated reticule. If that's not available you  can find programs on the web that  will allow you  to  put a cross hair on your screen. One such is STARTARG  and can be found at Andy's Shot Glass for a nominal fee.

 

Drift alignment do's and don'ts:

  • DO level the equatorial head before you start. This isn't essential but it means that azimuth adjustments won't affect elevation adjustments (or vice-versa).

  • DO make sure that the optical train is not flexing. Mirror flop, bendy focusers etc will ruin your attempts at polar alignment.

  • DO make the azimuth adjustment first. It isn't significantly affected by refraction since stars are moving parallel to the horizon as they cross the meridian, so you can take time to get this as accurate as you wish.

  • DO use the polar drift method at least for the elevation adjustment if you have a view of the pole. This is quicker and more accurate than the standard declination drift method above the horizon.

  • DO use a star in the eastern half of the sky for the elevation adjustment if you can, when using the standard method. The effects of refraction will decrease as it rises higher in the sky. Avoid anything much below about 30 degrees altitude where the effect of refraction becomes more severe.

  • DO use a star in the same region of sky each time for the elevation adjustment when using the standard method, if you are iterating between azimuth and elevation adjustments. Otherwise you will find a different elevation adjustment each time you do it.

  • DO approach the pole from below for the elevation adjustment - you are raising the polar axis against gravity which may give fewer problems with backlash.

  • DON'T expect perfection - there will always be drift in hour angle and declination as a consequence of atmospheric refraction - and we need the atmosphere to breathe!

  • DON'T fiddle on all night - an important part of drift aligning is knowing when to stop. Remember to take some images before the clouds roll in!

Excepts from http://canburytech.net/DriftAlign/DriftAlign_3.html

 

CCD Drift Method:

Another method that  I have used is the CCD drift method. Here you  use you  imaging camera to  get a picture of the drift angle. Basically you  set up  just  as in the Drift method above and take a 125 second image of a star in the FOV. Hold that  position for the first 5 seconds to  get a reference point then move the scope with the controller West for one minute. Immediately after move the scope EAST for the remaining minute. IF you scope is aligned properly the trace created by the East movement should overlap the trace created by the WEST movement. If it's out of alignment the image will create a  horizontal "V" where the amount of correction is shown by the  angle created by the intersecting lines. More details on this process can be found at CCD Drift  alignment . 

Drift Alignment Using PHD Guiding Software:

Just recently an acquaintance ( Ted Rafferty) put me on to  doing the drift alignment using the popular auto guiding program (PHD) available for free at Stark-Labs.

I would recommend that  you  align your camera to the RA/DEC axis of the telescope. I usually shoot images with the DEC axis being up and down on the CCD chip. This also helps me orient the image later as I prefer to show them with North being up.

The process begins as all the other drift alignments. To check the RA alignment choose a star at 0 elevation close to  the meridian. To set the DEC alignment choose a star as close to  the E or W horizon as possible.

  1. Turn on the guiding program (PHD) , select the mount and camera and run thru your  calibration a usual.
  2. To Check the RA select a star near the Meridian at 0 elevation.
  3. Start Guiding on the selected star.
  4. Turn on the GRAPH and select DX/DY instead of RA/DEC.
  5. Turn off the DEC guiding
  6. If your mount is perfectly aligned the DY ( red line) should track across the graph near the center line. You  may see some wiggle above and below the center line.  If it drift up  or down you  need to  make a very small adjustment to your AZIMUTH screws to compensate for that. You  will see the change immediately. No need to  wait 5 or 10 minutes for the visual drift.
  7. Now set up  the elevation.
  8. Stop the guiding and look for a star near the western or eastern horizon at whatever elevation allows you to clear any obstruction. The lower the better.
  9. Recalibrate the guiding on the new star.
  10. As in step 3,4 & 5 above start guiding and make sure that  the DEC guiding is off.
  11. Once again if your alignment is on then the DY trace should not drift from the horizontal graph line. If it does, carefully make small adjusts to the ELEVATION screws to compensate. Again you should see an immediate change in the graph.

This process is simple enough to allow you to check you  polar alignment whenever you  feel it necessary. The only requirement is that  you  get a copy of PHD and have a camera that  is supported by the program ( there are many).

3/24/14

As an update, the new PHD2 version 2.2.2 has a wizard that facilitates the above process by turning off the guiding and prompting you what  to do next. I haven't used it yet but it sounds like a winner if you  use this type of alignment. You can view the help file info for the wizard here. PHD2 is available free HERE.

Here is an excellent YouTube Video on how to use the PHD2 polar alignment process.

9/20/14

An update to the above. I tried the Drift alignment tool in PHD2 and found it to be very easy to use and resulted in very good polar alignment with little difficulty. Definitely something to keep in your bag of tricks.

Using SBIG CCDops:

I've just  run across another method that  uses the CCDops available from SBIG. The details written up by Mike Malik can be found on the HERE. The Polar alignment instructions can be found in Appendix P.  It's a bit more time consuming but should also yield excellent polar alignment.   

(This Page was last changed 09/24/2019)

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