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Drift align with help of PHD2

Semi automatically drift align

  1. My choice of equipment and software to do drift align
  2. How to use PHD2 for drift align, azimuth
  3. How to use PHD2 for drift align, altitude
  4. Final result
  5. Update tips on polar error

1: My choice of equipment and software to do drift align

If you have done a drift align to get your mount polar aligned you know it's very precise but also very time consuming. There are ways to do this very easy and fast but still very precise. You need help by a guide camera and some kind of controlling software. Here is what I have and how I do it.

The hardware you need to do it the same way as I do it. I have a EQ6 Synscan controlled by EQMOD, but I believe most computer controlled goto mount will do. And you also need a guide camera, I have the QHY5 and it's connected to the main telescope through an off-axis adapter.

Here is a list of what software I use and where you can download them, all of them are free.

There are other software that can do this too.

I assumes you are already familiar how to use a guide camera that is computer controlled before continue with drift align. And you must know how to setup PHD2 for auto guiding, otherwise it will be too complicated. See the PHD2 manual, PHD2 is under development so function will be added and changed from time to time.

In its simplest form you only need the guide camera and PHD2, but then you loose some functions.

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2: How to use PHD2 for drift align, azimuth

PHD2 (Push Here Dummy) has a really nice support implemented for drift align. Before starting the drift align enter the guide telescope's focal length and guide cameras pixelsize (you find it under the brain symbol). Then do a calibration of PHD2. Now you are ready to do a drift align.

PHD2 drift align azimuth (Click on the image and you get a full resolution image in a new window)

If you have meridian flip enabled. Decide if you want to do the adjustment to the West or East horizon, depends most on your local place which is best for you, choose the direction where you have a free horizon.

If you choice the West, let it pointing a few degrees to the West of meridian to avoid the automatically meridian flip of the telescope when it passes the meridian line. If you choice the East, let it pointing a few degrees to the East of meridian.

When choosing the utility of drift align (under tools) in PHD2 a window will pop up showing where the telescope is pointing (if it's connected to the mount) and the offset from the meridian line and declination. The first step is to adjust the azimuth orientation of the mount. In CdC, select a star along the meridian (south) and declination 0 degrees with a slight offset to the meridian as mentioned above. Approximately 30 degrees above the horizon here at my place, latitude +59 degree in Stockholm, Sweden.

Directly after pressing adjust button PHD2 find a guiding star automatically (if not, you can search for a bright nearby star in CdC to center the gudie camera on) and a graph (you must have opened it under view in PHD2 first) start showing the trend and after about 1 minute (in my setup) it has stabilized itself (but still drift if not polar aligned yet).

Now, look at the trend for Dec, usually the red line, if it drift upwards or downwards, push adjust, adjust your mounts azimuth, you have to try first time to find direction. Write something in the note window, example, which direction to turn the adjusting knobs, then you know to the next time. Then push drift and look if the drift is more stable, trend line. When you come close enough a circle will show up that tells how far away the correct directions is. I will write more about this circle later.

When you feel you are close enough you switch over to the altitude adjustment.

You have to repeat this Azimuth adjustment process at least one time after have done the Altitude adjustment below.

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3: How to use PHD2 for drift align, altitude

Push the altitude button in PHD2 drift align window.

PHD2 drift align azimuth (Click on the image and you get a full resolution image in a new window)

Ideally we find a star on the horizon at the meridian in the West or East. In actual practice, we must have it about 20 degrees above the horizon, but still on the declination 0 degrees. In my location (latitude +59 degree) it show up here as +66 degree offset from meridian in PHD2 drift align window.

Repeat this altitude process as you did for the azimuth, but now adjust the mounts altitude, up or down. Make a note in note window how to perform, up/down etc.

NOTE! Only look at the trend of declination, in my case the red one, the red trend line for both azimuth and altitude adjustments, the trend for RA doesn't matter!

After a few times adjusting I got a deviation of 3 arc minutes from the Earth's axis.

You must go back and readjust the azimuth at least one time and after that at least do a check of altitude again. Have you done this drift align a few times, then it's possibly to do this in less of ten minutes and get a good precision.

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4: Final result

The result after this drift align, as you can see the guiding hold it in less than one arc second. But this has more to do with the precision of the mount. What's more important is that the rotation of field when doing long exposures is minimized.

Drift precision after drift align

Could it be easier?

Thanks to the people from the Cloudy Nights forum to refine my instructions!

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5: Update, a tips how to see that there is a polar align error

Something that has been a mystery for me. When I use CdC to lock on an object, I lock and center CdC to M13 in this example. CdC is setup to report back coordinates from EQMOD. After a while the telescope pointing starts to drift away. Patrick who develop CdC (Skychart) give me an explanation why it behave like this. It's because of bad polar align. PHD guide the telescope to keep it centered on the object but because of bad polar align it doesn't follow the RA and DEC coordinates and that's the coordinates it reports to CdC. We get an drift as in the image below.

M13 polar error

This is how it could look, after about one hour and an error of polar align of about 10' or more. The inner circle is telescope pointing, and as you see the M13 is not anymore centered in the telescope (but it's centered in the camera because of auto guiding). That's an indication that you maybe have to improve your polar align.

Thanks Patrick for this explanation! Maybe obvious for others but not to me until now.

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