BACKGROUND

Since the introduction of affordable and reliable autoguiders, autoguider
use has increased considerably. And why not? Guiding can be a tedious
and a monotonous task. With an autoguider, one can set an alarm and
wake up hours later refreshed when the exposure is complete. Still, it is
not unusual (though perhaps less so in recent years) to see an amateur
using rather inexpensive equipment, minimal accessories and no
autoguider. That amateur may walk away for a minute or so, return,
converse, and casually make a small correction. Contrast this to the
amateur with many accessories, a much better drive, more expensive
scope, and an autoguider. Of course, the former amateur will get worse
results as compared to the later amateur. Perhaps not- here's why:

Conventional wisdom used for film imaging with high quality mounts under
excellent seeing is not simply transferred to inexpensive amateur
mounts used under typical seeing by many amateurs. I have used
inexpensive mounts simultaneously along side high-end amateur mounts
and produced similar results, but not using the same techniques. Clearly,
it is more of a challenge to obtain similar results with an inexpensive mount.
However, if we modify our techniques a bit to compensate for known
anomalies, good results are possible on inexpensive mounts with a
reasonable level of effort. This is important for those that have limited
resources to invest in astrophotograhy.




WHY USE PEC

PEC is, of course, needed less with high quality worm and worm-wheels
at shorter focal lengths. It is easy to dismiss mass produced mounts
as unusable if we are fortunate enough to own or have access to a
better mount. However, as we start moving up in focal length to over
3000 mm coupled with long exposures, PEC again starts to become
useful, even for the high-end amateur mount. Fortunately, PEC is
available on many of these mounts.

If used properly, PEC and an autoguider can have a synergistic effect.
This is especially noticeable with long exposure tricolor CCD imaging at
3000+ mm using a CCD chip where the CCD chip records small errors
much more efficiently than film. Many PEC's are capable of averaging
corrections over periods of 0.50 Hz (2 seconds) or less (LX-200) while
a typical autoguider is operating at 3-5 second correction rates or greater.
If we increase the autoguider correction rate too much (faster), we may
start chasing the seeing resulting in oscillation of the mount, especially
under average to marginal seeing.




THE ROLE OF AUTOGUIDER CORRECTION RATES

We should not confuse the exposure time setting of the autoguider with
the correction rate which is the sum of exposure time, integration time,
any software/hardware overhead, and drive hysteresis. Thus, the
correction rate is always longer than the autoguider exposure time.
This is an important consideration is we try to use an autoguider to
program the PEC. A short exposure setting might not be adequate
to assure correction rates are less than the desirable sampling rate
of 1/2 the record period or about 1Hz (1 second) for the LX-200.
Then, of course, the autoguider is fully capable of programming non-
periodic seeing related corrections (undesirable) easily ignored by
the human when the seeing isn't superb.




CORRECTION RATE LIMITATIONS IN MOUNTS

We should not assume that because our mount appears to oscillate in
response to guiding corrections as a result of seeing, worm errors,
vibrations, alignment errors, and/or PEC programming errors that a given
mount is not capable of fast guiding rates. For example, a slightly
modified LX-200 is capable of responding to simultaneous X-Y guiding
corrections at 2 Hz (0.5 seconds). For the RA axis, this consists of
typical slowing and speeding of the RA drive. In the Dec axis, full worm
reversal slows drift or changes the direction of star movement. Of
course, obtaining smooth correction rates at 2 Hz requires a thorough
understanding of the relationships of various adjustments in the drive
and mount. For most, such fast correction rates are not needed,
however, the reduction in hysteresis required to achieve these rates
is desirable.




THE PEC ADVANTAGE

For a LX-200, if the PEC was calibrated and updated under excellent
seeing and polar alignment, not only are the worm errors smoothed, but
residual RA drift is virtually stopped as clock crystal drift and minute
alignment errors are corrected. Then, subsequent updates are used to
add or subtract pulses (0.3333 arc second movements) to memory
segments which tweak the drive further allowing the drive correction
to track the worm quite close to real time- much closer than is practical
for an autoguider, especially under marginal seeing.

It is important that permanent PEC programming is preceded with
good polar alignment and initial PEC erasure. Why good polar alignment
BEFORE PEC programming? Because poor polar alignment produces
changes in the RA drift as well. With PEC programming, in addition to
clock frequency errors and worm errors (which are repeatable), we
introduce another error (RA drift) which is corrected by the PEC as well.
The next day, we set up and roughly align again and find our RA drive
doesn't track as well. This is likely because we programmed in the RA
drift produced by the previous night's poor polar alignment. The previous
night's rough polar alignment is usually more difficult to replicate than
accurate polar alignment.

Once the PEC is carefully programmed, we can use the autoguider
independently of PEC at whatever correction rate that produces the best
results- perhaps several seconds to even minutes (if desired) for a well
aligned mount. Now, autoguiding is truly optional because manual
guiding corrections may result in one correction every few minutes or
longer as the PEC has corrected residual RA drift and the excellent polar
alignment checks the Dec drift. Those on a budget can save considerable
expense without sacrificing results using their PEC wisely on a well
aligned mount while manually guiding.




CONCLUSION

This is not to say an inexpensive mount will out-perform a high end
mount, however, clever implementation of PEC such as by the LX-200
can minimize the differences in results- which is what is important.
Moreover, high-end mount users can use well implemented PEC to
improve results at longer focal lengths as well as improve manual
guiding results.






--
Michael Hart
Husen Observatory

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