Sky and Telescope - December 2017 - 67

t GUIDING
ALGORITHMS
The Algorithms tab in
the Advanced Setup
window lets you change
the mode that PHD2
uses, though the default
settings work well. Here
is where you can adjust
the aggressiveness
values when imaging
under unstable seeing
conditions.

t CALIBRATION
CALCULATOR
This advanced
tool computes
how much PHD2
moves your
mount in both
axes to make
corrections,
training the program to get the
best results from
your telescope
mount.

the Guide Stats window. A readout of 1 means the program is
changing directions after every exposure, so reduce the aggressiveness. A readout below 0.3 means you're not making enough
corrections, requiring you to increase the aggressiveness.
Choose a guide star with the correct brightness. One that's
too bright might saturate, leading to less accurate centroid
calculations. Too dim, and it might disappear if conditions
deteriorate. Select View > Display Star Proﬁle from the pulldown menu. If the top of the peak is ﬂat, then the star is being
overexposed. Correctly set the calibration step size. During
calibration, PHD2 will issue a move command in milliseconds
in right ascension and declination to see how far the star
moves. It will do this a number of times to get a good average.
If you have the calibration step size set too low, it will take a
long time to calibrate. Set it too large and it might not make
enough steps to get accurate calibration data. Aim for about 10
to 15 steps. If you correctly input your guidescope focal length
and autoguider camera pixel size, PHD2 will automatically
calculate a recommended step size with which to start.
Change the minimum move parameters. The minimum move
(MnMo) setting tells PHD2 how far a star is allowed to move
before a correction is issued. If this is set too small, it will try
to chase the seeing. Too large a value, and the software might
not make needed corrections. Remember, the MnMo readout
is for the guidescope camera but needs to be conﬁgured in
relation to the resolution of your imaging scope and camera.
If you're shooting with a longer focal length than your guidescope's, the MnMo needs to be set to a smaller number.
Recalibrate only if necessary. If your mount reports the
declination, you don't need to recalibrate when you move to
a new target. If your setup is permanent, or the same every
time out, and you orient the guide camera reasonably close to
the last time you calibrated, you can save the calibration and
use it again - you don't need to calibrate initially or recalibrate for a new target or after a meridian ﬂip.
Accurately input your camera and guidescope parameters. Be
sure of the focal length and guide-camera pixel size. This will
give you a tremendous amount of feedback as to how well
your setup is guiding. Note that you can set the History graph

to measure corrections in either pixels or arcseconds. Setting
this to arcseconds makes the graph readout appear much worse
than if set to pixels, even though the guiding is exactly the
same. Set it to arcseconds so you can see how you are really
doing in relation to your imaging rig's resolution.
Align your autoguiding camera. While PHD2 doesn't need the
autoguider camera aligned with right ascension and declination axes to do its job, it does help you when diagnosing
problems after a night of imaging. If stars are trailed east-west,
there is a problem in right ascension. If stars are elongated
north-south, the issue is in your declination guiding. Aligning your guide camera makes it easy to determine where the
problem resides.
Interpreting the readouts. RMS (root mean square) is a mathematical average of the guiding accuracy that gives you an indication of how well the guiding is performing. You can judge it
in relation to the seeing. For high-resolution work under excellent seeing with top-end equipment, your RMS should be about
¼ of the FWHM (full-width, half-maximum) you hope to
achieve. When imaging with the shorter focal lengths found
on camera lenses, guiding requirements are much more lax.

In Closing
Autoguiding can signiﬁcantly improve your astrophotos by
allowing you to shoot longer untrailed images - and more of
them - to achieve a much better signal-to-noise ratio. Autoguiding with PHD2 frees you from the tedious chore of having
to do it manually.
Remember, however, that the map is not the territory. The
ultimate judge of your guiding should be the size and shape
of the stars in your images. It doesn't matter if you have 0.25
arcsecond of total RMS guiding error if your stars aren't round
in your light frames. Likewise, if your stars are tight and round
in your lights, and the history chart looks like a foreboding
mountain range, don't obsess about this. Simply tell your scope
to push here, dummy!
¢ Contributing Editor JERRY LODRIGUSS has been photographing the night sky for more than four decades.
s k y a n d t e l e s c o p e . c o m * D E C E M B E R 2 0 17

http://www.skyandtelescope.com

Sky and Telescope - December 2017

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