PoleAlignMax (PAM) is a computer program written by Larry Weber and Steve Brady. See the link at the bottom of this page for a link to their site and to
PoleAlignMax (and FocusMax, another of their very useful programs).
PAM helps you quiclkly and accurately align your mount to point to the Celestial North Pole (CNP) and, in addition, tells you the accuracy of your alignment.
That is key. If you don't know how close you are and if you don't know if you are closer or further after the last tweak you are blundering around in the dark.
The discussion here is in three parts: A few brief remarks on setting up PAM, a slightly non-standard way to use PAM, and a workaround slightly more
accurate alignment.
Initial Setup:
You can use PAM with either CCDSoft + TheSky or with Maxim + PinPoint. The LE version of PinPoint included with Maxim will suffice. Either works but I have
found the plate solves to sometimes fail with CCDSoft and TheSky. I would recommend using Maxim with PAM if you have it.
It’s essential to get the image scale and binning correct. I use the “Change Scale & Binning” button on the Plate Solve tab and then enter my scope’s Focal
Length, Pixel Size and binning and let PAM calculate the image scale.
I have an STL11000 camera which has a long (40 second) download time for "unbinned" (binned 1x1) images so I always use 3x3 binning. This has no
noticeable effect on accuracy.
For the plate solve to succeed you need enough stars in the image. If you have a wide field of view this should be no problem. With the large chip of the STL
used with scopes of up to 1-1.8 meters in focal length, plate solves rarely fail. If you have a narrow field of view (for example an f10 Schmidt Cassegrain
and/or a smaller chip camera) you may need a longer exposure to get enough stars for a successful Plate Solve. Before running PAM you may want to take a
test exposure and do a manual plate solve. If Plate Solve fails or goes into a prolonged search try a longer exposure. Some times trying a slightly different part
of the sky helps (you may be in a region with few stars). Remember, however, the scope will slew so the second and third exposures will be in different regions
though. Try to avoid the full moon and trees along the path of your three exposures :-)
I generally use a 2 second exposure which seems to even out the seeing a little better than a shorter exposure. I generally use a 15 degree slew to have a
longer baseline but remember that the longer the baseline the lower the Alt for the third exposure.
The major strength of PAM is its speed (see below) so don’t use 1x1 binning if that causes a very slow download and don’t use a 20 second exposure if 1 or 2
second will do.
Nightly Use:
Where should you point the mount for the start of the run? You want to start pretty close to the Meridian so that the third slew will not be at a very low Alt and
subject to marked atmospheric refraction and poor seeing. Starting west of Meridian and slewing west has the advantage that your starting point won't cross
the meridian if you wind up making multiple runs. If you don’t have a clear view to the West you can use the East. Just pick a point far enough east of the
Meridian so it won’t cross before you are done. Figure the whole procedure may take you 15 to 30 min (see below). A starting point 10 degrees or so east of
the Meridian will work. If your starting point nears the meridian in later runs you can always choose a new starting point a little further east for the later
runs.
I.e. each run is independent and you don't have to complete the alignment from the same starting point where you began.
As to choice of Dec, you don’t want to do runs near the pole because the plate solves and geometry will be less accurate. Ideally you want to be near the
Celestial equator, but if you live far north or south of the equator your images, particularly the second and third images after slewing, will be low in Alt. I live at
37 North and use a point a little above (closer to the pole and to Zenith) than the celestial equator. Dec +10 works well at my location.
PoleAlignMax makes a very precise measurement of the arc the mount traces as it rotates in RA. Anything that disturbs that measurement will throw the result
off.
Do not walk around the mount while it is exposing and slewing. If you have to be right by the mount, sit or stand motionless. My mount sits on a concrete slab
and I get off that slab while the mount is doing a run. I don’t want to be on that slab while it is taking astro-images either, so I have my laptop set up about 20
feet away (on a different slab of concrete). I sit still there for the few seconds the run takes. This seems to help reproducibility of measurements.
A Method of faster accurate alignment:
PAM has two separate procedures: First it makes three exposures, measures your polar alignment error then reports that error graphically and with a
numerical value. Second, PAM provides a way, by centering a star in crosshairs, to correct that error. I use the first part, but I actually don’t use the second
(the “Center Star on the Crosshairs”) part. Here is why:
PAM’s strength is its speed. I can do a run with exposures and calculations in less than a minute. The Centering procedure takes much longer. I have “learned
my mount” and I know, approximately, how far the mount moves in Alt and Az as I turn the adjustment knobs. You don’t have to know this to 3 significant
figures. For my mount it takes about "one knurl" of the Alt Knob to move the mount about an arc minute. “About an Arc minute” or “about 2 and a half arc
minutes” is as accurately as you need to know this movement.
If, after measuring, PAM tells me the error is +5 arc min Alt and 21 arc min West in Az, I go out to the mount and turn the knobs to roughly make that
correction. Then I do another run. I can do 3 or more runs in that fashion in the time it would take to do one run followed by centering with the crosshairs.
To go to the next run without using the PAM centering module do not hit the "Next" button. Hit "Stop" and then "Run" to begin another run with the adjusted
centering.
I do not try to completely correct the error. If I do I will wind up oscillating on either side of the CNP. If you do use the Crosshairs module I would still recommend
doing only a partial correction for the same reason (and then do another run after your partial correction). I try to correct about 2/3 or 3/4 of the error. You will
find this converges quickly and is much faster than chasing errors to one side of the pole then back to the other.
I usually set up in the same place and I know about where the CNP is (it’s "just to the left of that tree"). So my first run is always within a degree, usually closer.
I don’t use a polar alignment scope. With 4 or 5 runs I am within 1 arc minute which is more accuracy than is needed for imaging. Even if your first
measurement is off by a few degrees it is the nature of converging with multiple approximations that you will get to within an arc min of the CNP in maybe just
one more run or so.
By approximating the correction by counting knurls on the Alt and Az adjustment knobs I can complete one cycle (measurement and correction) in 1 to 1 1/2
minutes. I get 3 approximations in the time it used to take me to do a careful centering with the crosshairs. A lot of approximate corrections will converge much
faster than a few precise corrections.
Finally, note that if you use this "approximate with the marks and knurls on the knobs" method there is no reason to center the first exposure on a star! All you
need is a plate solve. So you are free to pick a starting point at a convenient Dec near the meridian and just start my run.
PAM, JNow and J2000:
As of this writing (Jan 5, 2009) It is my understanding that PAM actually directs you not to the JNow CNP (not to where the CNP is now) but to the J2000 CNP
(to where the CNP was in the year 2000 – that is where the star charts are standardized). Due to precession of the earth on its axis the CNP moves about 20
arc seconds per year. At present (January 2009) the J2000 CNP is ~3 arc min from the current pole. Larry and Steve are aware of this and I gather a fix may
be provided soon.
My first advice is to ignore this. This slight error is not going to cause any noticeable field rotation even with long exposure and you want to spend time
imaging, not fussing with polar alignment all night. My second bit of advice is to follow the workaround correction below only if you are sure you are not using a
future, updated version of PAM which corrects this minor discrepancy.
But if you want to correct for precession of the CNP here is a technique:
In TheSky6 make two User Defined Data entries. The first is “CNP (J2000)”. That is easy. In the User Defined Data menu (Alt-U) enter the J2000 (the default)
coordinates of the CNP:
RA 0 Hrs 0 min 0 Sec Dec North 90 deg 0 min 0 Sec.
To make the second, “CNP (JNow)” you can either
Method A:
Have TheSky6 precess your coordinates:
Enter the same data as above, then hit the Precess button and precess from 2009 to 2000 (ie precess "backwards in time") and TheSky will generate J2000
coordinates for where the CNP (Dec 0:00:00) is now.
Method B:
Zoom in on the region of the CNP and your CNP(J2000) point. Click on the screen and watch the “Equatorial” (not the “Equatorial 2000”) Dec in the Object
Information Window until the Dec is within a smidge of 90:0:0 (the RA doesn’t matter, obviously). Once you are close enough look at the “Equatorial 2000”
values for RA and Dec and enter those as J2000 coordinates in the User Defined Data menu as “CNP (JNow).
At the moment (January 2009) CNP J(Now) is in J2000 values at about RA 00 Hrs 11 min 15s Dec +89 deg 56’ 56” and is about 3 arc min from J2000 CNP.
Note that having created a fixed point "CNP(JNow)" you only have marked where the CNP is "Now" not where the CNP will be "in a bit." The CNP will continue
to move (slowly) so you will have to redo this every so often. If the CNP moves at about 20 arc seconds/year and if PAM has a reproducibility of ~1/3 to ~1/2
arc min resetting the CNP(JNow) every year or two will keep you within the error of the method.
OK, so the distance from the 2000 CNP to the current CNP is about 3 arc min, but in which direction? The CNP(J2000) will rotate about the true CNP during
the night and with the seasons. That’s where you need TheSky.
Look at image below. You can right click and download the image below to zoom it if the lettering is too small to read on your monitor. Note that 1) CNP(J2000)
is not exactly in the same place as CNP(JNow). The difference is small (it is only about 44 arc min from the CNP to Polaris) but there is a small offset 2) Note
that CNP(JNow) is closer to the center of the intersection of the RA lines than CNP(J2000). That will be more obvious in the next image. Note that the direction
from CNP(J200)) (where PAM thinks the pole is) to CNP(JNow) is a little counterclockwise from the direction towards Polaris. TheSky can help you even more,
though.
PoleAlignMax (and FocusMax, another of their very useful programs).
PAM helps you quiclkly and accurately align your mount to point to the Celestial North Pole (CNP) and, in addition, tells you the accuracy of your alignment.
That is key. If you don't know how close you are and if you don't know if you are closer or further after the last tweak you are blundering around in the dark.
The discussion here is in three parts: A few brief remarks on setting up PAM, a slightly non-standard way to use PAM, and a workaround slightly more
accurate alignment.
Initial Setup:
You can use PAM with either CCDSoft + TheSky or with Maxim + PinPoint. The LE version of PinPoint included with Maxim will suffice. Either works but I have
found the plate solves to sometimes fail with CCDSoft and TheSky. I would recommend using Maxim with PAM if you have it.
It’s essential to get the image scale and binning correct. I use the “Change Scale & Binning” button on the Plate Solve tab and then enter my scope’s Focal
Length, Pixel Size and binning and let PAM calculate the image scale.
I have an STL11000 camera which has a long (40 second) download time for "unbinned" (binned 1x1) images so I always use 3x3 binning. This has no
noticeable effect on accuracy.
For the plate solve to succeed you need enough stars in the image. If you have a wide field of view this should be no problem. With the large chip of the STL
used with scopes of up to 1-1.8 meters in focal length, plate solves rarely fail. If you have a narrow field of view (for example an f10 Schmidt Cassegrain
and/or a smaller chip camera) you may need a longer exposure to get enough stars for a successful Plate Solve. Before running PAM you may want to take a
test exposure and do a manual plate solve. If Plate Solve fails or goes into a prolonged search try a longer exposure. Some times trying a slightly different part
of the sky helps (you may be in a region with few stars). Remember, however, the scope will slew so the second and third exposures will be in different regions
though. Try to avoid the full moon and trees along the path of your three exposures :-)
I generally use a 2 second exposure which seems to even out the seeing a little better than a shorter exposure. I generally use a 15 degree slew to have a
longer baseline but remember that the longer the baseline the lower the Alt for the third exposure.
The major strength of PAM is its speed (see below) so don’t use 1x1 binning if that causes a very slow download and don’t use a 20 second exposure if 1 or 2
second will do.
Nightly Use:
Where should you point the mount for the start of the run? You want to start pretty close to the Meridian so that the third slew will not be at a very low Alt and
subject to marked atmospheric refraction and poor seeing. Starting west of Meridian and slewing west has the advantage that your starting point won't cross
the meridian if you wind up making multiple runs. If you don’t have a clear view to the West you can use the East. Just pick a point far enough east of the
Meridian so it won’t cross before you are done. Figure the whole procedure may take you 15 to 30 min (see below). A starting point 10 degrees or so east of
the Meridian will work. If your starting point nears the meridian in later runs you can always choose a new starting point a little further east for the later
runs.
I.e. each run is independent and you don't have to complete the alignment from the same starting point where you began.
As to choice of Dec, you don’t want to do runs near the pole because the plate solves and geometry will be less accurate. Ideally you want to be near the
Celestial equator, but if you live far north or south of the equator your images, particularly the second and third images after slewing, will be low in Alt. I live at
37 North and use a point a little above (closer to the pole and to Zenith) than the celestial equator. Dec +10 works well at my location.
PoleAlignMax makes a very precise measurement of the arc the mount traces as it rotates in RA. Anything that disturbs that measurement will throw the result
off.
Do not walk around the mount while it is exposing and slewing. If you have to be right by the mount, sit or stand motionless. My mount sits on a concrete slab
and I get off that slab while the mount is doing a run. I don’t want to be on that slab while it is taking astro-images either, so I have my laptop set up about 20
feet away (on a different slab of concrete). I sit still there for the few seconds the run takes. This seems to help reproducibility of measurements.
A Method of faster accurate alignment:
PAM has two separate procedures: First it makes three exposures, measures your polar alignment error then reports that error graphically and with a
numerical value. Second, PAM provides a way, by centering a star in crosshairs, to correct that error. I use the first part, but I actually don’t use the second
(the “Center Star on the Crosshairs”) part. Here is why:
PAM’s strength is its speed. I can do a run with exposures and calculations in less than a minute. The Centering procedure takes much longer. I have “learned
my mount” and I know, approximately, how far the mount moves in Alt and Az as I turn the adjustment knobs. You don’t have to know this to 3 significant
figures. For my mount it takes about "one knurl" of the Alt Knob to move the mount about an arc minute. “About an Arc minute” or “about 2 and a half arc
minutes” is as accurately as you need to know this movement.
If, after measuring, PAM tells me the error is +5 arc min Alt and 21 arc min West in Az, I go out to the mount and turn the knobs to roughly make that
correction. Then I do another run. I can do 3 or more runs in that fashion in the time it would take to do one run followed by centering with the crosshairs.
To go to the next run without using the PAM centering module do not hit the "Next" button. Hit "Stop" and then "Run" to begin another run with the adjusted
centering.
I do not try to completely correct the error. If I do I will wind up oscillating on either side of the CNP. If you do use the Crosshairs module I would still recommend
doing only a partial correction for the same reason (and then do another run after your partial correction). I try to correct about 2/3 or 3/4 of the error. You will
find this converges quickly and is much faster than chasing errors to one side of the pole then back to the other.
I usually set up in the same place and I know about where the CNP is (it’s "just to the left of that tree"). So my first run is always within a degree, usually closer.
I don’t use a polar alignment scope. With 4 or 5 runs I am within 1 arc minute which is more accuracy than is needed for imaging. Even if your first
measurement is off by a few degrees it is the nature of converging with multiple approximations that you will get to within an arc min of the CNP in maybe just
one more run or so.
By approximating the correction by counting knurls on the Alt and Az adjustment knobs I can complete one cycle (measurement and correction) in 1 to 1 1/2
minutes. I get 3 approximations in the time it used to take me to do a careful centering with the crosshairs. A lot of approximate corrections will converge much
faster than a few precise corrections.
Finally, note that if you use this "approximate with the marks and knurls on the knobs" method there is no reason to center the first exposure on a star! All you
need is a plate solve. So you are free to pick a starting point at a convenient Dec near the meridian and just start my run.
PAM, JNow and J2000:
As of this writing (Jan 5, 2009) It is my understanding that PAM actually directs you not to the JNow CNP (not to where the CNP is now) but to the J2000 CNP
(to where the CNP was in the year 2000 – that is where the star charts are standardized). Due to precession of the earth on its axis the CNP moves about 20
arc seconds per year. At present (January 2009) the J2000 CNP is ~3 arc min from the current pole. Larry and Steve are aware of this and I gather a fix may
be provided soon.
My first advice is to ignore this. This slight error is not going to cause any noticeable field rotation even with long exposure and you want to spend time
imaging, not fussing with polar alignment all night. My second bit of advice is to follow the workaround correction below only if you are sure you are not using a
future, updated version of PAM which corrects this minor discrepancy.
But if you want to correct for precession of the CNP here is a technique:
In TheSky6 make two User Defined Data entries. The first is “CNP (J2000)”. That is easy. In the User Defined Data menu (Alt-U) enter the J2000 (the default)
coordinates of the CNP:
RA 0 Hrs 0 min 0 Sec Dec North 90 deg 0 min 0 Sec.
To make the second, “CNP (JNow)” you can either
Method A:
Have TheSky6 precess your coordinates:
Enter the same data as above, then hit the Precess button and precess from 2009 to 2000 (ie precess "backwards in time") and TheSky will generate J2000
coordinates for where the CNP (Dec 0:00:00) is now.
Method B:
Zoom in on the region of the CNP and your CNP(J2000) point. Click on the screen and watch the “Equatorial” (not the “Equatorial 2000”) Dec in the Object
Information Window until the Dec is within a smidge of 90:0:0 (the RA doesn’t matter, obviously). Once you are close enough look at the “Equatorial 2000”
values for RA and Dec and enter those as J2000 coordinates in the User Defined Data menu as “CNP (JNow).
At the moment (January 2009) CNP J(Now) is in J2000 values at about RA 00 Hrs 11 min 15s Dec +89 deg 56’ 56” and is about 3 arc min from J2000 CNP.
Note that having created a fixed point "CNP(JNow)" you only have marked where the CNP is "Now" not where the CNP will be "in a bit." The CNP will continue
to move (slowly) so you will have to redo this every so often. If the CNP moves at about 20 arc seconds/year and if PAM has a reproducibility of ~1/3 to ~1/2
arc min resetting the CNP(JNow) every year or two will keep you within the error of the method.
OK, so the distance from the 2000 CNP to the current CNP is about 3 arc min, but in which direction? The CNP(J2000) will rotate about the true CNP during
the night and with the seasons. That’s where you need TheSky.
Look at image below. You can right click and download the image below to zoom it if the lettering is too small to read on your monitor. Note that 1) CNP(J2000)
is not exactly in the same place as CNP(JNow). The difference is small (it is only about 44 arc min from the CNP to Polaris) but there is a small offset 2) Note
that CNP(JNow) is closer to the center of the intersection of the RA lines than CNP(J2000). That will be more obvious in the next image. Note that the direction
from CNP(J200)) (where PAM thinks the pole is) to CNP(JNow) is a little counterclockwise from the direction towards Polaris. TheSky can help you even more,
though.
| PoleAlignMax Tips |
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These images are obtained in TheSky in the correct orientation by first clicking the “N” key to point north then zooming in on the CNP. Do not rotate the
image in TheSky or you will change the orientation of CNP(JNow) to CNP(J2000).
Note that the distance between the two CNP markers is about 3 arc min (listed in the object information window) and that it is now clear that CNP(JNow) is
at the intersection of the RA lines where the CNP(J2000 is slightly offset.
If you are setting up Jan 3 at 7PM the direction from the J2000 CNP (where PAM directs you) to the current (JNow) CNP is in about 10 o’clock direction. So
you should adjust your Alt and Az until PAM says your alignment error ("Dec Error") is about 3 arc min and that your RA axis is pointed to the upper left
quadrant
If you are setting up at 8PM May 3 2009 you can see the true CNP is in the 7 o’clock direction from the J2000 CNP so adjust Alt and Az until PAM says your
error is about 3 arc min down and slightly to the left.
Don’t sweat the small stuff in direction nor magnitude. If you get in the correct quadrant you will be well within the accuracy you need.
The image below is how I set up at 6:24 PM Jan 3, 2009. I have TheSky6 Open to the region right around the CNP (Not centered on where the mount is
pointing to the South at about Dec +10) so I can see where the CNP(JNow) lies relative to the CNP(J2000) at the current time . And I have the Polar Plot
from my last run of PAM showing that the mount is pointing about in the same direction (towards the 11 o'clock direction) with a Dec Error of 2.96' i.e. about
3 arc minutes from CNP(J2000).
I started aligning at 6:12 and the whole thing including about 4 runs to converge and time to record the data with notes and screen captures took 12
minutes. Without the notes and screen captures say maybe 8 minutes. In that time you would not have completed two 5 min drift alignment runs and I am
within 10 arc seconds.
image in TheSky or you will change the orientation of CNP(JNow) to CNP(J2000).
Note that the distance between the two CNP markers is about 3 arc min (listed in the object information window) and that it is now clear that CNP(JNow) is
at the intersection of the RA lines where the CNP(J2000 is slightly offset.
If you are setting up Jan 3 at 7PM the direction from the J2000 CNP (where PAM directs you) to the current (JNow) CNP is in about 10 o’clock direction. So
you should adjust your Alt and Az until PAM says your alignment error ("Dec Error") is about 3 arc min and that your RA axis is pointed to the upper left
quadrant
If you are setting up at 8PM May 3 2009 you can see the true CNP is in the 7 o’clock direction from the J2000 CNP so adjust Alt and Az until PAM says your
error is about 3 arc min down and slightly to the left.
Don’t sweat the small stuff in direction nor magnitude. If you get in the correct quadrant you will be well within the accuracy you need.
The image below is how I set up at 6:24 PM Jan 3, 2009. I have TheSky6 Open to the region right around the CNP (Not centered on where the mount is
pointing to the South at about Dec +10) so I can see where the CNP(JNow) lies relative to the CNP(J2000) at the current time . And I have the Polar Plot
from my last run of PAM showing that the mount is pointing about in the same direction (towards the 11 o'clock direction) with a Dec Error of 2.96' i.e. about
3 arc minutes from CNP(J2000).
I started aligning at 6:12 and the whole thing including about 4 runs to converge and time to record the data with notes and screen captures took 12
minutes. Without the notes and screen captures say maybe 8 minutes. In that time you would not have completed two 5 min drift alignment runs and I am
within 10 arc seconds.
Below are two images, zoomed in closer to the CNP. One is for 7 PM Jan 3 2009 the for 8PM May 3 2009 (the dates and times are indicated in the Time
windows in each image).
windows in each image).
Before deciding how accuratePAM is you have to figure out how reproducible it is. This isn’t a question anything in PAM but of the reproducibility of your
comparing runs on either side of the mount / either side of the meridian. For two successive runs on the same side of the mount 1/4 arc min or less. So once
you are within an arc min (0.5 arc min if you are compulsive) relax and start imaging.
Using all of the above I can get a five minute unguided subexposure with a FWHM of about 3 arc seconds and an aspect ratio of about 10%. That is very
accurate alignment indeed.
Drew Sullivan
