How To Read PHD2

Autoguiding is an essential aspect of deep-sky photography. A typical mount can't keep the telescope precisely fixed on its target, so PHD2 locks onto a star(s) and takes frequent pictures.

If the star appears to move in a picture, either the mount has moved off target or the atmosphere has caused the starlight to be in a different place, or both. PHD2 then sends a correction to bring the mount back on target.

Remember- none of the numbers below are important if you're happy with the main camera's images. Don't obsess over guiding graphs!

Image Window

This shows the latest picture from the guide camera. The star in the crosshairs is the main guide star. The circled stars are additional guide stars. Multi-star guiding is better than single-star guiding.

Guiding Graph

This shows the errors in the RA (blue) and Dec (red) axis of the mount for recent images. The scale on the left is in arcseconds. The graph will never be completely smooth, but large jumps might indicate a problem. Here the jumps are only about 1" or less, which is ok for this setup.

Star Profile

This shows us a picture of the main guide star and a graph of it's brightness values from left to right. We want the graph to be pointy on top. A flat top indicates the star is over-exposed, and PHD2 may have a hard time tracking it.

Guide Statistics

These are the numbers to pay attention to. A guide star can move up/down or left/right each exposure. The RMS Error is a way of expressing how far it's off, regardless of +/- values.

RA and Dec are the two axis of the mount. Here the errors are 0.62"/pixel in RA and 0.33"/pixel in Dec. We'd like to see these be closer together but they'll work on this wide field setup. A wide gap between them can indicate that stars will be egg-shaped when imaging at longer focal lengths.

Tot is the combined error and the most important stat. In general a total error of 1.00" or less is good. Certain systems may require better (lower error) guiding, and some, like this one, may be able to image with higher errors. It depends on the focal length of your telescope and the size of the imaging camera's pixels.

It's important to set PHD2 to display errors in arcseconds, not pixels. The error numbers displayed are for the number of exposures along the x-axis of the graph, which can be changed in Settings.

Guide Settings

The guide settings adjust the parameters PHD2 will use when guiding. The settings may be slightly different depending on the algorithm chosen. In this example, I'm using the "hysteresis" algorithm in RA and the "ResistSwitch" algorithm in declination.

Aggression determines how much of the computed correction is used. Here it's set to 35 for both axes.
Hys. determines how much the RA correction is tempered by recent history. Here we're using a low setting of 10.
MnMo determines how far the guide star has to move, in pixels, before triggering a correction. Here, the settings are 0.1 pixels for both axes.
Mx RA and Mx DEC allow you to set a limit to each guide pulse, measured in milliseconds. The longer the guide pulse, the bigger the correction. Here, corrections are limited to 800ms. Mounts that require bigger corrections will need higher values.
Auto lets you specify which direction is used for DEC corrections (north or south) or allows the algorithm to decide (Auto).
The exposure drop-down sets the length of each guide exposure (0.5 seconds in this example).
The blue slider adjusts the brightness of the guide image you see on screen, but doesn't affect anything as far as the algorithm is concerned.

These settings are for my ZWO AM3 harmonic drive mount. It requires very short guide exposures and low settings for best performance. My EQ6-R Pro

at home uses a worm drive and requires much higher settings across the board, especially aggression and exposure length.

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