Tired of moving and aligning the scope every single night, I eventually decided to install a fixed pier.
First
problem was the location: my terrace is made of plates, which slightly
move and vibrate. In fact, there is an empty space of about 20 cms.
bellow the plates, so it's a kind of "false floor", very useful to have
cables and other stuff below, but unsuitable to fix a telescope pier.
So, the first task was to setup a convenient floor for it.
That was made of concrete, removing some (4) of the plates and filling the empy space below with the concrete.
About 120 Kgs of concrete was set up there, yielding to an approximate 1 square-meter surface, with a thickness of about 20 cms.
Second task was to build the pier. That was made by my telescope provider, who assembled a metal pier, very nice and solid.
Some improvements were made then to the rough pier:
-
as the Meade bolts, to fix the equatorial wedge to the pier, have a
slight, lateral movement, which difficults a lot the alignment process,
a central bolt and nut was built in the per, so the wedge could be
fixed to the center of the pier, avoiding unwanted movements.
-
the same mechanism which Meade uses to allow for fine azimuth aligment
in the tripod was adapted to the pier. That was really nice.
- a
very simple, but effective, method for avoiding too much friction
between the wedge and the pier was installed. This mechanism consists
of three small nuts were attached on top of the pier, so the surface
which is in contact with the pier is greately reduced. The movements
while aligning the azimuth are then very smooth.
Next
thing was to fix the pier to the floor. Here, the critical thing was to
do it so that the pier remained fairly oriented to the North. Although
the bolts which are used to attach the wedge to the pier allow for some
rotation east-west, I computed that this movement allowed for about +/-
6 degrees in each direction. Ok, that is quite nice, but
nevertheless I wanted to be sure that I fixed the pier in between those
theresholds (of course, after setting the pier one must align it
precisely, and further east-west movement may be needed. For that
reason it is important that the pier is centerd to the North as
precisely as possible).
For this porpouse, I was decided to
use the method of the shadow projection. Giving the longitude of my
observing site, the Sun crossed the meridian at about 13.57 in the
afternoon. At that time, the shadow, of course, gives you the North
direction.
So, I designed a home-made (very simple, but I effective!) of being able to mark the North line:
In
the pictures you can see the prehistoric mechanism which I used, but it
worked, after some weeks of clouds and rain! In the second picture, the
shadow casted by the stick is indicating the Noth direction, at exactly
13.57. The pier was manually moved until the shadow felt over the
drawed "middle" point of the pier. The rest was straighforward: I
attached the aligned pier to the floor using metallic bolts, specially
designed to be used with concrete. And this is the final result, waiting to be polar-aligned, which I will do the first clear night I have!
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This was my nightly routine... until I got my permanent pier in place! It
is often told, in books and documentation, that a good polar alignment
is good for general purpouses, but essencial for astrophotography.
This
is absolutely true. A correct polar alignment will allow you to image
with longer exposures, without being much affected by effects as field
rotation. And, it will allow almost perfects GOTOs movements with your
mount.
Just a few words about the GOTOs capacity. Imaging with a
so small chip as the one in the Meade DSI Pro CCD is almost impossible
with a very accurate GOTOs movements. Except for very large and bright
targets, tinny nebulae and galaxies are almost impossible to find in
the reduced field of the CCD just by chasing the target around. A
perfect GOTO will simply put the target in the field of the CCD. And,
once again, an accurate polar aligment is mandatory for this.
Of
course, there are many different techniques, very well documented in
internet and books, about polar alignment. And here I will describe
which method works for me.
Bur before I begin, let me simply
list three common methods, wide used, from which I've picked up parts
for developing my approach:
- The polar iterative method,
consisting of a "trial-and-error" process, going from a reference star
to Polaris, adjusting azimuth and altitude, and repeating the whole
process until the GOTOs from the reference star to Polaris, and back,
work fine.
- The drift method, in which the drift or shift of two
reference stars, one in the meridian and close to the celestial
equator, and the other near the equator and just above horizon (east or
west), is used to detect misalignment in azimuth and/or altitude.
-
The "Kochab Clock" method, a very smart one, that uses the fact that
star Kochab in Ursa Minor, Polaris and the true celestial pole are
almost aligned by chance. So looking at the sky, how Kochab and Polaris
are aligned in a given date, and knowing the approximate position of
the true celestial pole on this imaginary line, it is possible to point
the telescope in ist "home" position to the celestial pole, adjusting
this way azimuth and altitude.
The method I will be describing
is really a combination of these techniques and other ideas I've been
collecting from a lot of different sources. It is of course feasible to
combine different ideas, parts of methods, etc., to create your own
routine, with which you feel comfortable.
I will go step by
step. Important note: I do everything with the telescope powered off,
so there is no authomatic movement generated from the telescope. During
the process, when I say "move in declination" or "move in RA", these
movements are achieved manually, either loosening the knobs, or by
using the fine control adjustments if any, but NOT using the keypad
control of the telescope.
STEP 1: alignment of your finderscope.You
can use a terrestial target if you want. The final porpouse of this
step is being sure that the finderscope and the OTA are parallel. Be
careful and tighten the screws of your finderscope once aligned, so it
cannot move. We will use the finderscope widely throughout the
alignment process.
STEP 2: setting your mount and leveling it.Yes,
it is important that your mount is leveled. First, set up your mount.
Doing so, try to orient the telescope as close as you can to the north.
If you can see Polaris, it should be fairly easy to do so. I normally
set the OTA in a 90-degree Dec position (pointing to the
not-already-aligned celestial Noth pole) and look through the
finderscope, or just simply trying to visually point everything as
close North as I can.
After setting the mount that way, use
levels and move the legs of your tripod (if using one) until you get a
good level. Just think on this: a small error in leveling is then
amplified with movements of the telescope while observing, so keep
errors at this stage as low as possible. For this, I simply rely on the
bubble level already mounted in the Meade tripod for the LX200.
Another
tip: if using (like me) a Meade equatorial wedge, or any other wedge
which has a built-in, fine azimuth adjustment mechanism, be sure that
this mechanism is, in this stage, at its "medium" position, so it has
plenty of available movement to both sides. It can be frustrating to
discover that this fine control gets to its limit, either east or west,
during the following steps, and that no further fine azimuth adjustment
is feasible without moving the whole mount (and beginning everything
again!).
STEP 3: finding the 90-degree dec position in your mount.This step is much more difficult that may seem from its title.
First,
the declination circle, in most scopes (at that is also true in the
LX200) cannot provide accuracy enough for our purpouses (it may get
about 0.5 degrees accurate, but not more).
Second, we need a starting reference point. The "zero" position must be callibrated.
The
objective of this step is getting the DEC axis pointing, as precisely
as possible, to the mount polar point. Remember, that polar point, at
this stage, is a false polar point, because the whole mount is not yet
polar aligned! But it doesn't matter so far, as all we need for now is
the telescope pointing to what the mount "believes" its the true pole
(that is, finding the 90-degree position in declination).
STEP 3A: setting a reference point.First
thing to do is finding a first reference. I do this by moving the
telescope in declination (remember that at this stage the telescope is
powered off, so I'm moving it manually, loosening the knobs) until it
point exactly to the zenith (vertical position).
And here is the
trick: I protect the telescope optics, covering it, and I use this
cover tap as a flat surface to put a level on. So, with a level,
oriented north-south, I move the telescope, now with the manual fine
knob, until the cover tap is level. Now, I orient the level east-west,
keeping it on the tap, and move the RA axis manually, until it reads
level. I now read again the level in a noth-south direction, to be sure
that I've kept the level position. I firmly tighten the declination and
RA knobs, so the telescope cannot now move accidentaly 
What
we have now is the telescope pointing to the zenith (in a vertical
position), and perpendicular with the RA axis. This IS a reference
point, because, in that position, the Declination of the telescope is
EQUAL to our latitude (by definition)! So, we can now see what the
declination circle reads, and we can carefully adjust the circle so it
marks our current latitude (as I said before, we have here an accuracy
of about half degree, but it is enough so far, and we will refine it in
our second movement in this step).
This declination circle
setting depends upon the telescope model. In my LX, you can loosen and
move the graduated circle, until it reads what you want. My latitude is
about 41 degrees, and this is the reading which I want to read in the
circle. If not reading so, I adjust the circle until it shows 41
degrees. Once done one time, you'll usually be able to have a good
reading in future nights, but you'll need anyway to check it each time,
and repeat every night the process of pointing to the zenith to be sure
that you have a reliable reference point in the Dec circle.
STEP 3B: moving towards false celestial pole.Ok,
now that we have set a reference point, it's time to move to the
"false" celestial pole! We do so manually, loosening the declination
and RA knobs, and moving the whole telescope until the declination
circle reads 90 degrees (now you see why we did the previous
calibration).
As told before, as the accuracy of the
declination circle is not better than half a degree, we need a
refinement to that movement. So, when we are a 90 degrees declination,
we tighten the knobs for a moment so that the telescope cannot move.
We
will now use the finderscope, and afterwards an eyepiece. Looking
through the finderscope we should be able to see Polaris in the field
(that is the thing which we assured with step 2). Now, forget Polaris
for a while, and loose the RA knob of your telescope while looking
through the finderscope, and make the telescope spin in RA. Observe the
movement of the stars in the field. You'll see that they are
moving in circles (or in arcs, because you'll probably won't be able to
make the telescope spin a whole 360-degree turn in RA). The stars will
be moving around an imaginary point, which is precisely our false polar
point (remember: it's false because we have,'t aligned the telescope
yet!).
This imaginary point might not even be in the field.
Doesn't matter. What is really important is that you figure out where
this point roughly is just by looking the arcs pictured by the stars as
you move the RA of the telescope.
Now, use the manual
declination knob in your telescope and move it a bit, trying to chase
this imaginary point. Move declination a bit, fix it, and again move in
RA to see the new arcs, and to figure out where the "central" point. If
you have moved the declination in the right direction, now the
imaginary central point should be clearer to you. The game here is
trying to put this point in the middle of your finderscope (crosshairs).
So
keep adjusting declination step by step, and moving RA while looking
through the finderscope, until you center the point, and everything in
the field appears as rotation around this point, in the center of the
finderscope.
It takes a while getting used to this iterative method, but you'll see that it's very easy once you get the trick.
Now
use an eyepiece and do the same. Ideally it should be an eyepiece with
illuminated crosshairs, but I've been using this method with a normal
eyepiece as well, for a not-so-perfect polar alignment.
You will
not probably have a perfect match with that point being in the middle
of the crosshairs, and this will be due to flexures in your mount and
OTA. The amount of flexure will put a limit on the accuracy of our
method.
So, try to center this point as much as you can. If,
after all our method, we still need a more accurate polar aligment, we
will need to refine everything with the drift method. But, for now,
let's go forward.
OK! We have already found our false celestial
pole! This is real 90 degrees declination for our mount. We have been
able to bypass the lack of accuracy of the declination circle. STEP 4: The REAL alignment.We
have'nt yet adjusted azimuth or altitude of our mount, and now it's
time for us to do so and to convert our false celestial pole into the
real one.
So, tighten the declination and RA knobs, as we will
NOT move them anymore during the rest of our method, and be ready to
move now the fine adjustment controls for azimuth and altitude.
Our
job: moving in altitude so our RA axis is parallel to the Earth
rotation axis in our latitude. And moving in azimuth, so as the RA axis
points exactly North.
There are several mechanisms to do so, but I will describe my favorite one, which is pretty smart and simple.
Looking
through the finderscope, you'll recognize a clear pattern with some
stars, one of them being Polaris. The pattern is like this: 
Of course, the exact orientation of this pattern will change depending on the hour of observation and the date.
So what really matters here is being able to identify the pattern (that triangle, with one vertix being Polaris).
The
TRUE celestial pole is just where the crosshairs in the picture point
to. It's roughly about 40% of the way between Polaris and the opposite
vertix, and a bit above the imaginary line linking these two vertices.
Now, ALL we have to do is moving the mount in azimuth and altitude until we CENTER in the crosshair that point! Remember
that the pattern will be rotated from that one shown here, depending on
the hour and date. But the true celestial pole will always be in that
relative position, taking as a reference the triangle.
Move
carefully the adjustment controls for azimuth (to move east or west the
mount), and altitude (to move up and down the mount) until you roughly
have this point in the middle.
And that's it. We made it! Tighten altitude and azimuth, so as they don't move anymore during the night!
With
that mechanism we will get very close to the true pole, and it will be
more than enough for our observing sessions, alike our imaging ones. Of
course, if we need more accuracy, we will need to refine our job with
the dirft method. But even if that's the case, we will have got a very
good starting point for the final and accurate refinement of the drift
method.
STEP 5: Sync on a star.At this point,
I finally turn on the telescope. I manually move it first, in
declination and RA so as it points roughly South and with approximate
declination 0, as this is the position in which a Meade is supposed to
be swicthed on.
Whe switched on, my Meade does his internal
things. I have programmed AutoStar in my scope so as it gets GPS data
as soon as it begins operation. Now, my telescope knows latitude, date
and time.
I SKIP the alignment routines suggested by the
software, as we have already aligned it, and I then instruct my
telescope to GOTO to a known bright star in the sky. If
everything is OK, the GOTO operation will get very close to that star.
I then center the star in the crosshairs using the control pad of the
telescope, moving in declination and RA. When centered, I simply SYNC
on it.
Now the telescope is ready for operation. It is polar
aligned, it knows latitude, date and time, and it knows exactly where
it is pointing at (i.e. it konws declination and RA of the current
position of the OTA, so it can now move taking as a reference this
"starting" point).
References:
- Clay's
Kochab Clock method: http://www.arksky.org/kochab.htm, from where I
borrowed and adapted the pictures of the OTA pointing to the zenith and
to the pole. - Polar Aligment, at
http://geocities.com/jmmahony/LX10/polaralign.html?20076, from where I
borrowed the picture containing the star pattern. - Drift method,
at http://www.darkskyimages.com/gpolar.html, for a very good
description of this method, which we can use to refine out alignment.
More accuracy?
Ok,
I must check this stuff by myself yet, but it may be possible to get
more accuracy with this method (before going to tha drift method, or
even avoiding it!) by using more accurate charts and, while close to
the pole, using a normal view through an ayepiece. So, here are two
charts which may be useful
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It
is well know, and described and documented in many books and articles,
that the signal-to-noise ratio dramatically improves when stacking
frames.
Stacking frames consists of, through different
algorithms, "add" frames to obtain a new, improved image. As said, we
have two benefits of doing so.
In first place, the signal (or,
simple said, the light which is the target of our images) adds at a
more fast pace than noise, simply because, by definition, noise has a
somehow random nature, and this "accumulation" tends to cancell out
most of it.
Secondly, and this is a great advantage of the digital world, we accumulate light, simulation, this way, much longer exposures.
So,
why not, directly, taking very long exposures? Because there ara a lot
of things which can go wrong in a very long exposure. Here are some:
- long exposures need a perfect polar aligment, to avoid nasty things like field rotation
-
a plane or satellite track can ruin a picture. If taking short
exposures, we can afterwards discard the frames which have been ruined,
but saving the rest. But imagine what is loosing a costly exposure of,
lets say, 1 hour!
- wind, vibration, etc., can also ruin a long exposure image
And, let's take short exposures! It's not that easy.
A short exposure doesn't receive so much light, so tinny objects are not caught, and noise is king in this kind of exposures.
So,
the solution is taking as long exposures as possible and acceptable,
and stacking them afterwards, discarding the bad frames.
With a
"typical" polar aligmnent I can take unguided exposures of about 1
minute with my LX200 8" at F3.3. So, I take many images, and then stack
them to "simulate" much longer exposures.
Let's test the effects of stacking: This
is an image of the famous Horsehead Nebula, near the Flame Nebula, in
Orion. There has not been any histogram adjustment to the images, so
they are "rough".
The first one, on the left, is a SINGLE frame:
an exposure of 21 seconds. See the noise. It shows as grains
everywhere. This "grainny" look is very typical of images with a lot of
noise.
The second one, in the middle, is an image composed of 3
stacked frames, each one of 21-second exposure. We can clearly see how
the noise has reduced a lot, and the image is clearer and with better
contrast. Look specally the nebulosity around the Flame, bottom right,
to notice the improvement.
The third image is a composition of
80 stacked frames, each one of 21-second exposure. The improvent is
remarkable. There is no evidence of noise, and the transitions between
dark and bright zones is smooth. Again, around the Flame we can even
see now the shape of the nebula, and notice faint details of it. go to the top... |
