Pulse guiding is a method that can change, subtly or dramatically, the tracking rate of your mount. Roland Christen suggested this technique, which consists of precisely timed commands that emulate move the mount at the guide rate speed. The changes are so small and happen so quickly that they appear to smoothly change the tracking rate.
This page is divided into three main sections. In the Pulse Guiding section you enter the rate that you wish to set and turn pulse guiding on and off. In the Realtime Status section information about pulses widths and counts is displayed. The Measure Rate Tool section allows you to measure the motion of a star, asteroid, comet, moon, etc. then immediately use the rate.
To use the pulse guiding feature you must enter a known rate or directly measure it. Unfortunately, even with a known rate of, say, an asteroid, it is unlikely that just entering that rate will allow perfect tracking because the mount itself is not tracking celestial objects precisely because of the effects of refraction and slight polar misalignment.
So
to get accurate tracking you need to measure the “tracking imperfection”
an object for a period of time. You can do this in a couple of ways. One
way is to center a star visually with a crosshair eyepiece or 
with a CCD camera. When
ready press the Start button in the Measure Rate section. It
is best to wait for an exact multiple of the worm gear period (628 seconds)
but you can wait however long you like. . The Time field will increment
each second and you can see the total change in RA and Dec in the dRA
and dDec fields.
If you are doing the centering manually you can press the movement buttons. The orientation and operation of these buttons is the same as defined in the Guide/Slew Page. If you pressed one of the Guide/Center rate buttons then the rate will display in the center of the buttons.
Alternatively, you could use an auto-guider to measure the rate. But you might ask why would you even care to pulse guide if you have an auto-guider? Well, one example is if you are doing imaging with a single CCD camera and you do not have an auto-guider. You could pulse guide to enable you to guide without a guider. The same applies if you have an SBIG camera and are doing Hydrogen Alpha imaging and you cannot find a bright enough guide star through the Hydrogen Alpha filter.
If you are auto-guiding with a CCD camera you just need to press Stop. If not you must manually re-center the star visually or with your CCD camera. In either case you will see the following dialog when you press the Stop button:
If you don’t want to use the rates just press Cancel.
Use Both Rates: Automatically fills the RA Arcsec/sec and Dec ArcSec/Sec fields with the displayed RA and Dec rates, respectively.
Use RA Rate: Automatically fills the RA Arcsec/sec field with the displayed RA rate.
Use RA Rate: Automatically fills the Dec ArcSec/Sec field with the displayed Dec rate.
Cancel: Neither rate is used.
When the correct rate has been set pulse guiding will allow you to do unguided CCD exposures for many minutes, even when polar alignment and/or refraction effects the rate that the stars move relative to the motion of the mount.
You activate pulse guiding by pressing the On button. Immediately PulseGuide will command the mount at precise times with commands sent on the serial port. The real-time rate that PulseGuide calculates is displayed just to the left of the On and Off buttons.
The pictures below are cropped 5-minute ST-10XE images (1.2 arc-sec/pixel) taken with a 1200GTO mount and AP155 refractor at 1085 mm focal length. The mount’s polar axis was purposely misaligned to show the capabilities of PulseGuide. The image on the left shows the extent of the mistracking. Turning pulse guiding on results in a good exposure:
Press the Off button to turn off the pulse guider. While pulse guiding is enabled certain features like PEM record are disabled.
Note: While a pulse guiding is on you cannot disconnect from the mount or exit PulseGuide.
When pulse guiding is active there is a variety of real-time
information available. The numbers circled in the picture to the above
right are the calculated rates in RA/Dec based on the duration of the
movement commands sent to the mount. These
are theoretical rates.
The Measure Rate Tool has this additional information.
Time: Active duration of this pulse guiding session.
dRA: This is the actual measured change in Right Ascension divided by the time active.
dDec: This is the actual measured change in Declination divided by the time active.
Even though 6 decimal places are displayed for dRA and dDec the accuracy of these measurements is typically no more than 1 or 2 decimal places.
Because software timers do not always execute exactly when requested the rates most likely will not exactly match but there is a close-loop algorithm in the software that tries to home in on the desired rate.
The Realtime Status section shows the latest periods of time that the mount moves are active and the total number of commands (pulses) sent to the mount:
If you are having trouble getting good results:
1. Make sure you have trained PEM on a steady night and have turned PEM on.
2. Try measuring the rate for a longer period (the longer the better). If you measure the rate for less than 7-8 minutes the mount will not go through a complete periodic error cycle.
3. Try changing the Max Pulses per second. If you have a slower CPU or have lots of tasks active try lowering it to 3, then 2, then 1.
4. If the Dec rate in the Pulse Guiding area is reversed in sign to the value in dDec then you need to swap the N/S buttons. This is shown in the picture below:
