PHD Calibration

[AlistairW]

Hello,

Not had to do one of these for a long while. but having changed my set up I do now.

Is it best to be pointing as near to Dec 0 as possible and close to the meridian. If I am out at 9pm today from my SW UK location I can see Altair is at RA/DEC (J2000) 19h50m47s/+8deg52min13s. Does that sound like the right area to be in?

Indeed why does it actually make a difference as to getting a good calibration ?

Thanks

Alistair

[symmetal]

The celestial equator at declination zero is the only altitude where a scope movement of say 1 degree in RA moves the same physical distance as a scope movement of 1 degree in Dec. At any other altitude the scope will move a smaller physical distance for a 1 degree movement in RA.  This just make calibration the most accurate to measure as the distance moved per calibration step should be the same in both axis. If you calibrate at any other altitude the RA steps will be smaller so a multiplication factor dependent on altitude has to be added to RA to compensate.

Anywhere on the celestial equator would do for this but on the meridian the calibration stars are at their highest elevation in the sky so should have better seeing for calibration.

You don't want a bright star for calibration as it will saturate your guide scope. You want calibration exposures around 1 to 2 seconds to average out the seeing. A mid brightness star which gives a nice sharp peak and good SNR figure (100 or more) is ideal and there should be plenty to choose from at Dec 0. I just choose any star close to dec 0 and the meridian on CdC or Stellarium and slew to there., choose a suitable candidate in view and then calibrate. 

Alan

[michael8554]

As Alan said.

Once you've Calibrated at Dec zero, PHD2 will automatically compensate for guiding away from Dec zero, if your scope is connected so that PHD2 knows the RA and Dec position.

So if ST-4 guiding you should calibrate near your target, to get the true RA and Dec movements.

The PHD2 auto select guide star should select a suitable non-saturated star.

Michael