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EFFECTIVE USE OF PEC PROGRAMMING AS RELATED TO POLAR ALIGNMENT AND
AUTOGUIDING
BACKGROUND
Programming an LX200 smart drive (PEC) or another drive with permanent periodic error correction (PPEC) should be done on a night of good seeing, no wind, at high power, and on an excellently polar aligned scope. This is because improper polar alignment introduces RA drift that effects the PEC programming used in other parts of the sky. One possible variation might be to program the PEC near the object of interest to compensate a bit for polar alignment errors that could include Dec drive training. I have used 800-1200X on a 12" LX200 on a star quite near meridian close to the celestial equator. I make 2 to 3 small corrections per second to avoid overcorrecting at each 2.4 second recording cycle. This is done in RA only. No Dec corrections are ever made.
The measured results are consistently around 4 to 5 arc seconds periodic
error peak to peak over the 8 minute worm cycle. Once done, the PEC corrects
for small drive frequency variations and virtually stops RA drift for at
least 2 minutes. With a well programmed PEC and accurately polar aligned
scope, one does not need to make frequent guiding corrections at all. In
fact, a human is often better at guiding than an autoguider during poor
seeing, because the human can easily determine seeing errors from alignment/drive
errors. Attempting to autoguide out seeing anomalies is a daunting task,
especially with a computer controlled autoguider approaching a limited
maximum update rate of 1.45 seconds or greater. A discussion about the
operation of the PEC in the LX200 drive can be found at:
R. A. Greiner -- PEC Correction
DRIFT ALIGN BEFORE PEC PROGRAMMING
If your scope drifts consistently in RA, you may need to tweak polar alignment before PEC programming. Three things happen when we are NOT polar aligned: The stars appear to drift in RA. The stars appear to drift in declination. The field rotates. Using the drift method to polar align is recommended highly for high power visual work and precision imaging. As we approach 3000+ mm focal lengths, alignment that was satisfactory for a camera lens or small refractor is wholly inadequate.
The term "declination drift" in itself implies a polar aligned scope
will drift in declination only. In fact, a scope that is not polar
aligned drifts in RA as well, however, we ignore any RA drift during the
process and watch "declination drift". As we approach polar alignment,
RA drift slows as well. The reason for RA drift is easy to visualize.
If our mount is not quite aligned with the celestial poles, the amount
of misalignment formed causes the RA to appear to drift. The greater
the misalignment, the faster the RA drift. Note: Start by erasing
the PEC to remove any programmed RA drift. The idea is to
line up our scopes to turn parallel with the earth's rotation. Those experiencing
unusual RA drift should assure polar alignment is quite accurate.
LEVELING THE POLAR MOUNT- IS THIS REALLY NECESSARY?
There is a good reason to level the mount along the altitude and azimuth
axis. When drift aligning, a mount out of level will cause adjustments
in one axis to effect the other, increasing the number of iterations that
are required to arrive at a polar alignment solution. For example, a correction
that might require an azimuth move will also move a bit in altitude when
the mount is not level in azimuth, the amount depending on the degree of
error from level. Once quite level, we are then ready to start the declination
drift alignment process. The method outlined in the Meade manual is based
on using a diagonal. Straight through users and south celestial pole users
should reverse the correction directions. It may be a good idea to label
the direction moved on the mount azimuth and altitude knobs to prevent
mistakes that would unnecessarily prolong the process.
A SIMPLIFIED METHOD TO DECLINATION DRIFT ALIGNMENT OF A POLAR MOUNT
Now that we know which star to select and have done so, we're not quite ready to drift align. First, we must align our reticule eyepiece with the RA axis. We can consider this the E-W axis. Everything above the E-W (RA) axis is NORTH, everything below is SOUTH. If you are uncertain, merely moving the scope with the E-W keys will identify the E-W (RA) axis in the eyepiece. It is vitally important that we understand that the use of the term north or south as described in drift alignment procedures is not related to your position at the telescope, rather, the direction the star drifts with respect to the RA axis.
Now, we select a star within 5 degrees or so of the celestial equator and within 30 minutes of the meridian. This provides maximum declination drift which readily speeds the alignment process. A moderately bright star often provides better results than a very bright star. Using a 2-3X barlow with extension or star diagonal will produce powers that are quite high, amplifying small drift movements. If the seeing is so poor that your moderately bright star is too dim and/or moves about, postpone any subsequent PEC programming for a day of better seeing.
If the star drifts north, use the azimuth control to move the
scope east. Keep adjusting and re-centering the star until the movement
virtually stops over a 3-5 minute period. Now, locate a star at about 6
hours RA not much less than 15 degrees of the horizon (this avoids refractive
errors) If the star drifts north, use the altitude control to move
the scope down. Keep adjusting re-centering until the star does
not drift 7-10 minutes. Now return to the paragraph immediately above and
repeat, but strive to improve star drifting from 3-5 minutes to 7-10 minutes.
RETURNING THE TELESCOPE TO A POLAR ALIGNED WEDGE
This leads to the subject of reproducing polar alignment when removing the scope from an adjustable wedge or other device that maintains the scope parallel to the earth's rotation (aligned to either celestial pole). Once the wedge is aligned, we can move the scope base up, down, right or left as long as the scope base is not TILTED by a non-flat surface. I have used indexing pins to position the scope base at the exact point it was aligned. However, are they really needed if you have a flat mounting surface (no bumps or warpage)? An even better question is, how non-flat (irregular) must the surface be to cause a significant polar alignment error?
If moving the scope base laterally across the wedge surface (parallel to the earth's rotation) introduces a 12 arc minute error west (due to a 0.020" bump or warp in the surface), a 60 minute exposure could drift around 3 arc minutes in declination. A 0.020" deviation in a short span of less than 0.25" (maximum anticipated variance) is clearly visible on the wedge surface and quite easy to see in the eyepiece during drift alignment. However, the maximum amount of field rotation seen in a typical 60 minute exposure near the guiding point would be fairly small - around 30 arc seconds. If the amount of field rotation produced is so small, should most bother with indexing pins? Probably not. Why then, bother at all?
Three reasons come to mind - First, manual guided exposures are much
easier with an alignment precision that requires almost no Dec corrections
or RA corrections in two minutes or longer. One can relax a bit.
Thus, more precise polar alignment is desirable. Second, two minute
unguided CCD images will move less than 1 pixel, enabling electronic stacking
with minimal loss of field of view and object centering. Third, guiding
errors while using an autoguider are narrowed to flexure, vibrations, wind,
drive train, and seeing. We could use precision milling of the wedge
surface to 0.002", but simple eyeballing the bolt hole centers or simple
indexing pins as a means to assure returning to precise polar alignment
is quite adequate.
RESULTS OF PRECISE POLAR ALIGNMENT AND PEC PROGRAMMING
To see what a well aligned mass produced mount can do, I took two 15
minute shots under 3 arc second seeing, one using 3 second guiding corrections
and the other unguided. These images demonstrate the power of excellent
polar alignment and PEC working in concert.
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THE AUTOGUIDER ADVANTAGE FOR IMAGING
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. The above illustration demonstrates that constant guiding is not necessary on a well aligned mount when the PEC is accurately programmed. 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,
a good result is possible on inexpensive mounts with a reasonable level
of effort. This is important for those that have limited resources
to invest in astrophotography.
WHY USE PEC WITH AN AUTOGUIDER?
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 PECs are capable of
averaging corrections over periods of 0.50 Hz (2 seconds) or less (LX200)
while a typical autoguider is operating at 3 to 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 IN PEC PROGRAMMING
I tested a number of popular amateur autoguiders employed in PEC programming with mixed results. At first these devices would seem ideal because over long exposures they produce results few (if any) can duplicate with the manual guiding. However, for a short duration of a few minutes it is possible for the human to exceed autoguider results for PEC programming. In addition, few autoguiders have sufficiently high correction rate needed. When we speak off correction rate, we should not confuse the exposure time setting of the autoguider with the correction rate that 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. The shortest possible 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 LX200.
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. Considering the aforementioned limitations in
commercial autoguiders, I believe the most dependable results are obtained
by manual PEC programming. Here, if we study the periodic error for
a few cycles of the worm, we can memorize significant deviations, thus
anticipating them in near real time. Thus, it is my belief, the best
PEC programming is done manually.
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 LX200 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.
See the article concerning adjusting a declination drive adjustments
without a major rebuild at: Declination
Drive Adjustment. Those who have retrograde motion
or suspect they may have retrograde motion can find details to control
this anomaly at: Retro Motion Adjustment.
THE PEC ADVANTAGE
For an LX200, 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
I believe the value of accurate polar alignment is often underestimated.
For casual observing, casual polar alignment is quite adequate. However,
for improved PEC programming and good photography results at focal lengths
of 3000+ mm, a well-aligned mount is essential to excellent results.
This is not to say a bit of alignment or tweaking will cause an inexpensive
mount to out-perform a high end mount, however, clever implementation of
PEC such as by the LX200 and others can minimize the differences in results.
Moreover, high end mount users can use well implemented PEC to improve
results at longer focal lengths and any manual guiding endeavors.
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Michael Hart
Husen Observatory
mhart@netexpress.net
For more high resolution images go to the following web site:
http://www.netexpress.net/~mhart/