Paramount Polar Alignment
At the Winter Star party, I had two interesting encounters
that, more than anything else, have inspired this post. One was a gentleman who
told me that with his brand-x mount, “I only need to sync on two stars, and
then whatever I slew to, the object is always in my eyepiece”. “Why can’t the
Paramount do that, without having to do all those points?”
On another occasion a gentleman nearby, perhaps trying to
goad me, was being very loud and vocal about that fact that it takes forty-one
stars to polar align a friends paramount, then another forty-one just to see if
he had done it right.
Where do these people get their information?
How many stars does it take to polar align a Paramount?
Well, exactly, precisely, and without exception… one. Uno. Eins. Un. Monad.
Quote me on that, track me down at a star party and hold me to it if you like.
It’s just one. Just like any other German equatorial mount in fact.
Actually, it can also be zero. You could use a planet instead.
The confusion is to some degree understandable. What’s
happening is many people are confusing TPoint’s polar alignment feature with other
software-assisted polar alignment techniques that can be applied to all mounts.
They are different tools, they do different things, but at the same time they
can appear similar. Aspirin and a vaccine both have health benefits, but they
are not performing the same function.
We also have
the problem that the visual crowd and the imaging crowd each have very different
ideas of what polar alignment really is, and how close it needs to be, or even
what the term “sync” means. One of the things TPoint can do is refine your
alignment much like a drift alignment procedure. Except TPoint is faster,
unattended, and does a vastly better job… but it’s not how you do the initial
alignment, and it’s not even necessary for visual observing, or even to some
extent if you just want to guide your way out of a sloppy polar alignment. (TPoint
will be the topic of my next post, but you shouldn’t even start using TPoint
until you have a preliminary polar alignment, so that’s where I’m going to
focus for now.)
Again with the disclaimer: this is how I do MY polar alignment. I’m
portable, in a hurry, and I have my quirks. To start with, I don’t like polar ‘scopes.
They are fine, lots of people swear by them, some swear at them, and I am in
the latter group. Besides, I like to do my initial alignment in the daytime
long before Polaris is visible. I’m also assuming the imaging train is mechanically
STRAIGHT and along the axis with the Versa Plate, and not skewed by some number
of DEGREES up or down, left or right. TPoint can actually do a really good job
of modeling this out later, but your initial alignment is not going to be that
great if this is the case. You have been duly warned.
Step 1: Level the mount as best as possible. See my last blog post for how
wonderful the Pyramid Portable Pier is for this task. This step used to be very
frustrating, but was worth the effort. Now it’s both easy and worth the effort!
Step 2: Loosen the head of the tripod and point the mount north. I use a
compass if necessary and just get it as close as I can make it. If you can see
Polaris when doing this, that’s a great guidepost, naturally. Assuming you’re
not using a polar alignment scope, just get the RA axis pointing more or less
at Polaris. If Polaris is behind some trees or a building, don’t fret over
this. You really don’t need to sight on Polaris if I the imaging system’s
mechanical axes are close to orthogonal (90 degrees of each other) and the
optical axis is aligned with the mechanical axis. Also, be sure to carefully
set the altitude of the mount’s polar axis to your current latitude. You should
have done this already, and it’s easy to do on the Paramount since it has nice
tick marks on the altitude adjustment.
Step 3: Plug in your computer, make sure TheSkyX has the correct lat/long for
your location and the current time, and slew to a nice bright star. Not too
close to the meridian, and a northern declination works best. Alcor and Mizar
are my favorites, because it’s easy to know you’re looking at the right star(s)
when you see the pair in the finder or on camera. A well-aligned finder scope is often missing from many an
imager’s scope. For a portable imager this is a mistake because the next thing
you want to do is center this bright star in the finder. You do this by making
adjustments to the mount. It’s best to not even have your hand paddle
plugged in at this point! An alternative I sometimes use is one of those green
lasers and they work fantastically… but are often banned at star parties. If
you don’t have a camera in place, you can then refine this mechanical alignment
with an eyepiece. Start with a wider field, and then drop down to a higher
power view to center up the star. An eyepiece with an illuminated reticule
works great for this. If you’re a DSLR imager (which I frequently am), then
great, you can look right through the viewfinder and center up the star there
right where it’s going to fall on the chip.
Step 4: For visual work, you’re pretty much done (although we can refine this
more later). In TheSkyX, select “Sync” from the telescope menu to sync on the
star. The visual gentleman at the WSP will find that he can now slew to any
target he likes and with a nice low power eyepiece his target will likely be in
view, just like with his other mount. If the Paramount (or any quality German
equatorial mount) is “good and level”, and your scope is on “good and straight”,
you should be able to slew to any visual object anywhere and find it in a low
power or wide field eyepiece. If you’re planning to image, you might as well go
for the gold. Take a picture here, Image Link, and sync on the Image-Linked
Didn’t I say I do
this in the daytime somewhere? Yep. After getting the mount level and more or
less pointing north, I’ll home the mount and slew to the brightest star in the
sky…. the sun. Oh, I should mention that you better sure has heck have your
scope covered when you do this! Let’s not destroy your optics or camera in the
process. If the moon is up, it works just as well, and possibly is a safer
target depending on the type of your scope (those big open truss
thing-a-ma-bobs can be hard to protect adequately from sunlight). Now, adjust
the mount mechanically until the sun or moon is directly in line with
your scope’s optical tube. A finder scope really helps , but the aforementioned
warning about the sun applies, this time of course, please don’t burn your eyes
by looking at the sun through a finder either. For that matter, you should
cover the finder if you slew to the sun; you very well might set your shirt on
fire even if you don’t plan to look through it!
I do not sync on
the sun or the moon however. They are just guideposts. Oh, well actually for
solar observing (with adequate filtering), you actually can center the sun and
sync, and you’re in great shape all day long. You could also if you’re doing
nighttime imaging go ahead and center the moon, but I like to take it a step
Now that I’m within
a couple degrees of the pole, I can slew to a bright star or planet and they
will usually show up in my finder scope. Yes you can see planets and bright
stars during the daytime; you just need to know where to look (and TheSkyX
makes this easy). The sun and moon make great jumping off points. Now, without
using the hand paddle, I tweak the mounts altitude and azimuth to get the
planet/star centered in the finder scope. If possible, I put in an eyepiece,
one with a reticule is even better, and refine my pointing further just like I
would at night.
I could sync at
this point, but I like to wait until sunset. It’s amazing, but depending on the
optics and camera I can get stars in an image as little as ten minutes after
sunset, and almost always by twenty minutes after sunset. I’ll take a picture
with the Camera Add On, Image Link, and then sync on the photo.
qualification, I can say the mount is now aligned with the pole. For imaging,
I’m going to want to refine this considerably, and I usually call this my
“initial alignment”. For visual I could stop here, or I could tighten it up
just a little more.
mounts have both one and two star alignment routines. What they are attempting
to do with two stars is model the fact that your ‘scope may not be mounted
entirely straight relative to the mechanical axis. The mount may not be
perfectly level or pointed North, the mirror may be tilted slightly, etc. All these
“misalignments” add up.
Even with a perfect
mount, the optics, or the mounting of the optics tends to be a frequent point
of failure for perfect alignment. Adding extra pointing samples allows most
mount control programs to do a better job of when you slew to a star on the
other side of the sky for example. Of course when the object is not in the
field of view you still need to hunt around for it, and often you can add a
supplementary observation to help improve pointing… in that part of the sky.
This is where TPoint
comes in, and it can do this much better, and it only requires six points. Six…
not forty-one and you only have to do the procedure once. Six may be more than
two or three that other fork-mounted ‘scope guys advertise, but when you
consider that you have to keep adding more calibration points when you slew
elsewhere… well why not just get the six right off the bat and be done with it?
One point and a well-aligned optical train and you can get away with wide
fields, or perhaps a little “Star Searching” (a feature of TheSkyX that slews
the mount in an outward spiral pattern to quickly locate objects outside the
current field) to find what you want. Six samples, no further mount adjustments
and TPoint, and your object will always be right in your low powered eyepiece….
every time. For visual, the most you’d ever want is twelve points and even at
high magnification and low fields of view, you’ll find the object always dead
near in the center of the field every time. No further adjustments to the mount
needed either by the way.
What about the
needs of the imager? Well, let’s save that for next time when I talk more about
T-Point for visual use, and polar alignment fine tuning.
08-14-2012 4:38 PM