PHD2 Drift Align first, or CGEM 2 Star Align first?

[Robert Birkett]

I'm really not sure if this matters or not, no documentation mentions where to start, but almost all GoTo scope mounts request an alignment be done at startup.  So my question is, if I want to use the "gold standard" and do a drift alignment with PHD2, then which option should I select when I turn on my CGEM?  2 Star, One Star, Quick Align, Last Alignment?  Does it matter?  If I'm going to go through a 20 minute alignment with the CGEM hand controller, what's the point of a drift alignment on top of that......

[Andy Galasso]

Hi Robert,

The mount's star alignment routine synchronizes the mount's controller software with the sky so that you can have accurate Go-To's.

A polar drift alignment routine physically aligns the mount's polar axis to the celestial pole in order to reduce declination drift and field rotation for long-exposure imaging.

The two types of alignment are independent -- you can have a poor star alignment and still have an excellent polar alignment, and vice-versa.

For your CGEM, the answer to what type of alignment you need depends on how accurate you need your Go-to's to be.  If you are using an imaging software package that automatically centers targets with plate solving, then you don't really need accurate go-to's and you'd probably want to select the quickest option to get up and running. On the other hand if you are relying on the mount to center targets accurately then you'll want to have a precise star alignment. You can ask in one of the Celestron or CGEM forums to get more info about the pros and cons of the various CGEM star alignment options.

If you have a mobile rig that you setup and tear down each night the procedure would be:

   1. quick star alignment (1-star?) to get the scope roughly oriented to the sky so you can slew to the general areas you need to be at for polar alignment (meridian/celestial equator/horizon)

   2. polar alignment using drift or any of the other polar alignment methods

   3. (optional, depending on your needs) do a precise star-alignment.

There's no point in doing a precise star alignment before the polar alignment because when you move the mount's polar axis you will invalidate the mount's pointing model (star alignment.)

HTH,

Andy

[peter wolsley]

Andy and Robert,

Is there any limit to how far out the Polar Alignment can be before a drift alignment is not possible?  Many users have lots of trouble calibrating PHD2 simply because of poor polar alignment.

I own a CGEM and use the CGEM's pointing model...I don't use plate solving at present.  I always perform the full 2 star alignment + 4 star calibration.  I tend to set-up just before sunset so there is always time to do this before it is dark enough to take astrophotos.  If you perform the Celestron All Star Polar Alignment as well, you can get rid of gross errors in polar alignment which will also make calibrating and drift aligning using PHD2 more reliable. 

Personally, I use a polar alignment routine that I wrote for my own purposes to initially polar align.  The same thing would be possible if you had a Polemaster device for your CGEM.  Then I align and calibrate.  Finally I fire up PHD2...calibrate and guide.  I rarely drift align.

So now you have a few variations to consider.

Peter

[Andy Galasso]

On Wed, Jan 31, 2018 at 3:08 PM, peter wolsley <wols...@gmail.com> wrote:

Is there any limit to how far out the Polar Alignment can be before a drift alignment is not possible?  Many users have lots of trouble calibrating PHD2 simply because of poor polar alignment.

Peter,

That's an interesting question. In my experience with my mobile setup I would use the mount's polar scope to get fairly well polar aligned -- much less than a degree away from the pole. At that point I could do a PHD2 calibration and then a polar drift alignment and be ready to start imaging, easily accomplished well before astronomical twilight.

If I did not have the polar scope to get the initial alignment, I might think about doing another iteration to ensure a valid calibration:

    1. rough polar align using compass north for azimuth and latitude for altitude

    2. calibrate PHD2

    3. quick drift alignment to get closer to the pole

    4. re-run phd2 calibration

    5. longer more accurate drift alignment

(And always nudge the mount north before starting calibration to remove dec backlash and make the calibration faster and more likely to succeed)

Andy