Tutorial on how to perform Polar Alignment with the "Hour Angle" method.
This tutorial best suited for the EQ/NEQ6/ATLAS mounts. But it also possible to perform with smaller HEQ5 mounts. They are bit different than the larger mounts but the principle is the same. I had performed this method with HEQ5 mount and had no problems, only that followigng this tutorial, only parts of it could be applied.
Important note: You should use this after you have calibrated your finder scope to the RA axis !
I'll go through simple basics that are shown on the image.
It is known that the Earth's North Celestial Pole doesn't point exactly to the north star Polaris. The Polaris also moves on it's orbit around celestial pole. So to help us to perform good polar alignment, manufactures had implemented what's called Polar Scope Finder inside of the mount's RA axis. Which actually is a small refractor with printed drawings of NCP and position of Polaris in relation to it.
The meaning of the Hour Angle method lies in rotation and exact positioning of a small bubble of the Polar Finder according to the actual Hour Angle location of Polaris in the sky for current time and date. Then adjust our mount's altitude and azimuth to bring the Polaris into the small bubble. Doing this properly and we can achieve good Polar Alignment.
You can see that placing Polaris in his bubble while reticle rotated wrongly, can cause large error in Polar Alignment. So it should be done accurately.
Hour Angle, is the time format coordinates for the Polaris. Since Earth's full rotation happens in 24 hours, so does the Polaris. Only countdown goes in counter clockwise direction.
As shown on the Stellarium example image on the left, the Hour Angle of Polaris would read as 2 hours 35 minutes.
When Polaris come to the Meridian and crosses it at the highest point, it's called "Transit". When Polaris in Transit, it means that he completed full rotation around the North Celestial Pole and he's clock is HA 00:00:00 and start new count.
Since our mount's RA axis have hours scale which is also divided to 24 hours, the idea is to calibrate our Polar Finder reticle to the Polaris Transit. So when we on the field, we can use the actual Hour Angle of the Polaris, turn RA axis of the mount to exact position, then using Azimuth and Altitude bolts to bring Polaris to the small bubble on the reticle. Doing so we would be Polar Aligned in very good way.
For calibration purposes we would need some distant object and be able to see it through our mount's polar finder. Couple kilometers from my house there is antenna with the red light on top of it, so I use this.
As for preparations, I set my mount so that I would see my object in the polar finder, pull out the counterweight shaft, remove the cap and turn Dec axis for 90°.
Now using only Altitude and Azimuth bolts of the mount, we need to bring that distant object exactly to the center of Polar Finder reticle.
This is how it would look like. The upside down image is normal, because the Polar Finder is actually a small Refractor and we going to have that in mind in the next steps.
Now, using ONLY Altitude bolts of the mount, we need to bring that distant object down to the large circle. While we use only Altitude bolts we make sure that the object goes straight down. Now in the next step we would bring the small bubble over the object to imitate the Polaris transit.
The object should be brought only down, because the image in a finder is upside down.
The next step is to bring the small bubble exactly over the object. Releasing RA clutch we would slowly turn the RA axis so they would be aligned as shown on the image.
I love to be precise whenever I can, so "just" putting the object in the middle is not enough. I want to put it exactly on the middle and I have found that the width of the small bubble corresponds exactly to one hour on the RA scale.
I have slowly rotated RA axis until my distant object moved to the intersection of the small bubble with the large circle to the left. Then released the RA scale and moved it precisly to the round hour.
Knowing that the small bubble's width corresponds to one hour on the scale, I will rotate it for half an hour on the scale and that would bring the object to the exact center.
After we made sure that the small bubble is located exactly on the bottom, which imitates the position of Polaris's transit, we would release RA scale and set it to 0 hour.
This is how it should look like. Our RA scale set to zero while the small bubble of the Polar Finder exactly on the bottom.
And with this we finished calibrating our mount for performing Polar Alignment with Hour Angle method. So when we get to the field, we would use current Hour Angle of the Polaris, rotate accordingly RA axis and bring the Polaris into the small buble.
For owners of HEQ5:
Your RA scale is a bit different, but you can rotate it and place 0 against indication triangle and fix it with the knob. Since the scale it self doesn't rotate with the RA axis the only option, is to use the white line indicator under the "months" ring and also set it to the zero, as shown on the image.
On HEQ5, as on EQ6 you would use upper half of the scale for North hemisphere and bottom for South hemisphere.
Something to note, when I finished calibration, my mount position were like shown in the image. Depending on the position of polar finder inside RA axis, your mount might be in deifferent position and that's OK.
Now it's time to use this method in the field. As always, I'm leveling the tripod and carefully balance both axises.
Now when I'm imaging, I'm slightly unbalance to the East.
If you in the field and have no Laptop or other devices which would help you to get the Hour Angle of the Polaris, we can get one from the mount it self using the hand controller.
After turning on your mount we should carefully insert all of the parameters it asks. Location coordinates, time zone, date and time, daylight savings.
After going through all of the setting we getting to the screen of Polaris's Hour Angle. You can see on the image it shows HA=06:04.
But before we continue we need to know if this is accurate. In the days that I was writing this, it was known that SynScan software uses J2000 Epoch static coordinates and Hour Angle reported by Handset was outdated. For next years there probably would be some changes in softaware and it might use JNOW coordinates, but if you didn't updated and use old firmware, you should check this out.
To check out for how much my Handset was outdated I used 3 different software to verify the Hour Angle coordinates.
2. Polaris by Gary Gawthrope
3. PolarisFinder v1.5 from Takahashi
I have entered same location coordinates, date and time to each of the three programs and I got very similar results from all three of them and they were agreed that the HA was 02:58 for that time. When I used same data with my handset I got HA of 03:13 , so it was obvious that my Handset reports Hour Angle 15 minutes forward.
So I had noted this and will keep it in mind when I'm next time in the field.
Now, going back two steps, we going to use the reported by Handset Hour Angle position of Polaris and align our mount accordng to it. So the usual next steps would be rotating the DEC axis for 90°, removing the cap and pulling out the counterweight shaft so it let us see through the Polar Finder scope.
Now we release the RA clutch and turning the axis to align the hour scale with the HA we got from Handset. If your handset is updated, or you have precise coordintes, you may use them now, but I had outdated Synscan and as you remember I had it 15 minutes rush. So instead of using 06:04 I will use the corrected HA of 05:49 as shown in the image.
After that, we engage back RA clutch and now will bring the Polaris using Alt Az bolts of the mount to the small bubble of the Polar Finder.
As I have mentioned before, I love to be precise. And after we engage the clutch and try to move our mount with the bolts, Polaris in the sky keep moving while our mount is stationary, which means, that if you fast, then it's ok, but if it takes you so time to bring the Poalris to the bubble, the Poalris have been already moved.
So not to let it escape, right after I engage the RA clutch when aligned on HA, I turn the tracking on to a regular Sidereal rate. Now I can relax and accurately bring the Polaris to he's bubble without being worried for how long it would take me.
Another thing to keep in mind. Probably all of our old (Syntha) Polar Finders are outdated. Since Polaris is not stationary and move one arc-minute closer to NCP every 5 years we should take this in mind. While bringing the Polaris exactly to the center of the bubble was ok in years around 2000, now in years around 2012 you should place Polaris closer to the inner side of the bubble, as sown in the image.
Now we have been Polar Aligned in a vey good way using Hour Angle method.
One thing you should remember, if you did turned ON the tracking while bringing the Polaris to the bubble, you have to park it back before you continue to Star Alignment procedure.
You slect Park Scope from the Menu and after scope being parked (very small movement) you release the RA clutch and bring the mount to the Home Position. That is the right way to do it.
But if you didn't use "Tracking" while Polar Aligning, you can simply put mount back to Home Position and proceed from there.
After we done with Polar Alignment we can proceed to Star Alignment procedure.