In this tutorial I will try to explain the meaning of the exact alignment of the Polar Scope to RA axis of the mount and will cover the steps on how to collimate it again if it got out of alignment.
Polar Scope is the optical finder (small refractor) that is built in inside of the RA axis of the mount and helps us to perform Polar Alignment of the mount before night of observation or imaging. As you probably already know, the Earth's rotational axis does not point exactly to the north star Polaris, it is pointing 0.7° far from it. So the Polaris doesn't "stand" in place, it's actually also rotates around the celestial Earth's axis.
To have perfect Polar Alignment we have to align our mount's RA axis to the Earth's rotational axis and it's done with the help of the Polar Scope. To compensate for the distance of Earth's axis from Polaris, inside Polar Scope there is printed drawings of central cross, large and small circles that represent the position of Polaris in relation to the Earth's axis.
Unfortunately, the Polar Scope might come out of alignment, especially if we transport it a lot and will cause us to have bad polar alignment, which in return might cause an effect of Field Rotation, bad tracking or guiding and bad performances of GOTO commands of the mount. In my case, I transport my mount a lot and have to set up each night, so I often check the alignment of my Polar Scope and adjust if needed..
In this tutorial I will mention 3 axises :
1. The Earth's axis
2. RA axis of the mount
3. Polar Scope axis
Our target is to bring all of the three axises into position when they exactly parallel. I represent this as arcs of each axis on the left. While objects rise on East and set on West, they move in Celestial Arc and when our mount's axis will be exactly parallel to the Earth's axis , the telescope that is pointing to an object will also follow the same arc. Meaning that it will allow us to track the objects for a long period of time when observing and will ease tremendously on guiding system when imaging.
So what happens when Polar Scope's axis is not aligned?
Let's pretend that our Polar Scope is out of collimation and we arrive at the field and do our usual Polar Alignment. What will actually happen, is that we align only two axeses of three, which is the Polar Scope's axis and Earth's axis.
The missaligned Polar Scope will actually cause our mount's RA axis to be misaligned and it will have different celestial arc of movement, which I'm showing as a drawing to the left.
Now you can see that while our telescope is pointing to an object in the sky, it will move in different celestial arc and eventually will loose the object out of the field.
So what do we need? The advantage is that we don't have to do it at night, doing so at the daylight is even easier. Some distant antenna, electric pole, something that is stationary and doesn't move with the wind.
I'm personally like to do it with the end of the antenna, because they tend to have 90 degrees corners which can also help in the alignment. And if I do so at evening, I can use these red warning lights that placed on the high building, works just as well.
After placing my mount in the direction of the object, I like to use small chair and do so while I'm sitting. Depending on the missalignment of the Polar Scope, it can take some time and if you not sit comfortably it will only hard on the whole process.
Now we ready to continue. First thing I do is pulling out the counterweight shaft as it obscuring the Polar Scop view inside the mount. Then I release the RA clutch and turn the RA full 360° to make sure that the counterweight shaft doesn't hit one of the legs or anything else close by.
Of course not to forget to remove the cap.
And turning DEC axis 90° so that the opening in the DEC shaft will allow us to see thru it.
Take a peek inside to make sure all is good.
Now taking a look thru the actual Polar Scope I can see that I'm almost on the target. And my target for this proceedure would be the end of the antenna.
The first step of this proceedure would be locking the RA axis with the counterweight shaft pointing straight down and using the Altitude and Azimuth bolts to bring the target to the center of a cross drawing of the Polar Scope.
Like so.
Next step would be rotating the RA axis 180°.
Like this.
Now looking thru Polar Scope again we can clearly see that the cross center had shifted from the target and that is clear sign of our polar Scope axis being not aligned with the RA axis of the mount. Using it in this condition for Polar Alignment will definitely cause an error.
Next step for correcting this, would be adjusting the three small grub screws, that push the peace of glass with the cross drawings that installed inside.
Make sure you doing a very small adjustments and release opposite screws to allow a movement. Although the peace of glass is installed in the dovetailed metal ring, pushing it too hard may cause it to crack, so be careful.
Now, it's really important, that you don't correct the full error back and bring the cross again on the target. You should fix only half of it.
I demonstrate this on the image to the left. Before correction and After, but just half of it.
That is for preventing the error shifting to the other side when we rotate the mount in the next steps.
After fixing just half of the error, we need to use Alt/Az bolts again to bring the cross on the target. This is very important step that should not being forget.
Now the cross once again centered on the target.
Next step we going to rotate RA 180° back to it's original position with counterweight shaft pointing to the ground.
Looking thru the Polar Scope again we can notice, that this time the error still there, but it's much smaller.
And once again, carefully adjusting the small grub screws we need to fix only half of the present error.
I demonstrate this again on the images to the left. Before the correction and After. The correction might not be clearly visible, but it's there. If it's hard to fix half of, it's OK to fix just a bit of it, as long as you don't bring the cross back to the center.
Adjust each time small amounts is fine.
Once again, the important step would be using Alt/Az bolts to bring the cross back exactly on the target.
Like this.
And as you might have already guessed, we turn the RA axis once again for full 180° and repeat the previous steps for correcting the error.
And just to sum up the steps:
1. Put cross over the target
2. Rotate RA axis 180°
3. Fix half of the error with screws
4. Bring cross back on to the target using the Alt/Az bolts
Repeat until you done.
What we aim to achieve, is that while looking thru the Polar Scope and rotating RA axis for full 360°, the cross should stay exactly on the target. Managing that and we can be sure that our Polar Scope axis is perfectly aligned with the mount's RA axis and will get us to very good Polar Alignment using the Polar Scope.
Another way to make sure you have achieved the goal, is to place the target on the large circle, as for this example I'm using the warning red lights and turning the RA axis for full 360°. The target should run on that circle and not slip out of it.
That would be end of, hope you find it useful.
