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PHD guiding setup

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I noticed last night that if I used a very high altitude guide star (~61deg Dec) that the PHD calibration failed because the star didn't move enough. I'm using the default settings. When I choose a lower altitude star (~28 deg Dec) the calibration worked OK. Is the calibration obtained with the lower altitude a star valid when I select a high altitude target, or should I change the PHD settings to get a successful calibration at the high altitude?

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I used to redo the calibration every time for a new area of the sky, but am now not sure it is necessary. I dont adjust it any more unless the target is on the opposite side of the meridian, in which case you HAVE to re-calibrate. But I do control what PHD actually send to the mount with pulse guiding in EQMod, so maybe thats why it doesnt matter for me?

There is a guide within PHD somewhere which will give you an idea of the cal step to use in relation to your focal length.

The author of PHD, Craig Stark is usually very helpful, and quick to respond to email enquiries if you get really stuck :D

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I used to redo the calibration every time for a new area of the sky, but am now not sure it is necessary.

PHD Re-calibration is definitely necessary if you change in declination. At higher declinations the observed "star movement" in Right Ascension caused by a fix length guiding pulse is significantly reduced (by a factor of 1/cos(Dec)). As a result RA guiding pulses need to increase in length with declination. An ASCOM based guiding application could read the mounts declination an adjust accordingly but I don't believe PHD currently does this and so recalibration is required. Alternatively, if using EQASCOM and ASCOM pulse guiding , you can use the "gain sliders" on the pulseguide monitor screen to manually correct for over/under corrections in RA caused by a change in declination.

Because of this declination effect it will most likely be necessary to increase PHD's calibration step to generate enough movement for calibration to complete - particularly if imaging near to the pole.


Edited by chrisshillito
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I used ro recal after every "significant" move it doesnt take that long and is better than messing up a few hours worth of subs...

Have to agree with that Peter, last time I got caught out was a couple of months ago when I did a unattended session, the first target was spot on and the subs were beautiful, 2nd target which I set just before work I didn't have time to watch the first subs come in....I ended up with 4 hours of trash :D

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OK, I guess I'm a little confused due to my lack of knowledge. I use EQMOD to align and steer to/track a target with the EQ6 serial input. I use PHD via a USB link to my Lodestar guide cam, which has on camera guide outputs. This I connect direct to the EQ6 guide input. As far as I know PHD is acting independently of EQMOD.

How do you get PHD to interact with EQMOD and utilise pulse guiding? Is this more accurate than letting PHD run independently, as in my current setup?

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