Daytime Polar Alignment
Posted 27 April 2013 - 11:35 AM
In particular, the AP manual describes a process for polar aligning the mount during the daytime using a bubble level, slews to a number of reference positions, and a final goto to an object that you can see during the day. Since I have a PST, I have been polar aligning on the Sun. I have found the daytime polar alignment to be surprisingly accurate. I have, for example, been able to get reasonably accurate goto performance based only on this. I've also been imaging with 10 minute subs at 2300mm focal length with no sign of field rotation.
I got to thinking about this, and I don't see any reason why the procedure could not work on any mount capable of doing a goto to ground referenced position.
My other GEMs are Celestron CG-5s and a CGE. These mounts have a hand controller command to goto an alt-az position, so it seems like I could adapt the procedure easily enough. Unfortunately, it's cloudy here today, so I cannot field test it, but I just ran through it in my basement and it seems like it will work great.
You may ask why I would want to do this. Well, I am finding that the procedure with my AP mount is at least as accurate as the All-Star Polar Alignment feature on the Celestron mounts. The cool thing is that, by doing this during the day right after I set up, the mount is ready to go well before dark.
I will be going to a dark sky site next week and taking at least one of each type of mount. I will have a chance to actually test and evaluate the effectiveness on the CG-5 and CGE.
If anyone else is interested, I've listed the steps below based on the NexStar hand controller. The instructions are intended for the Northern Hemisphere. I presume that they could be adapted for Southern Hemisphere as well, but that does not apply to me.
1. Power on the mount as normal (either align the index marks, or let the homing switches do their thing). Enter time and site information as required.
2. Select Quick Alignment.
3. Turn off tracking.
4. In the Utilities menu, find Goto Axis Position. Enter 90 00 00 for azimuth and 180 00 00 for altitude and press enter. The mount should slew such that the OTA is pointing to the western horizon and is level.
5. While in this position, loosen the declination clutch and use a bubble level to make the OTA perfectly horizontal. Tighten the declination clutch. This step registers the OTA's declination to the mount's declination.
6. Go back to the Utilities menu and find Goto Axis Position again. This time, go to 180 00 00 for the azimuth. For the altitude, you will need to know your latitude very accurately. You will want to enter an altitude that is 180 minus your latitude. Hit enter. This time, the mount will slew so that the OTA is on the west side of the mount and level pointed north.
7. Use a bubble level again to ensure that the OTA is perfectly horizontal. This time, though, do not release any clutches and do not hit any buttons. Use the mount's altitude adjustment to do this. The mount is now correctly polar aligned in altitude.
8. Turn on tracking.
9. Use goto to pick any object that you can see during the day, like the Moon or the Sun or Venus if you are sharp-eyed. Since I have a PST, I use the Sun (with all appropriate warnings about looking at the Sun).
10. When the mount is done slewing, use the mount's azimuth adjustment to center the object. Do not release any clutches and do not hit any buttons. At this point, the mount is polar aligned in both altitude and azimuth.
When it gets dark enough, you can now power cycle the mount, do your normal 2+4 alignment routine and start using the mount.
The steps may seem complicated at first, but once you've done it a couple of times, it's really easy and quick to do.
Posted 27 April 2013 - 12:10 PM
Posted 27 April 2013 - 12:50 PM
I find there is almost always a reasonable period of time where I can see enough stars (inc Polaris) to polar align with a polar scope, but it is still too bright to observe or image. Now if the view of Polaris was blocked, or if this technique provided a more accurate alignment, or if I wanted to leave the scope to start an automated run, then I can see a real advantage. One advantage I can see is it does insure the scope is synced.
Still, always good to have another alignment trick up the sleeve.
Posted 27 April 2013 - 12:56 PM
Posted 27 April 2013 - 01:01 PM
Of course, this is just one sample, and it was with the AP mount. And I was very careful to do the daytime routine as accurately as possible.
Posted 27 April 2013 - 01:17 PM
I may need to account for this. The way to do it would be to insert the following steps directly after turning off tracking, between steps 3 and 4:
Slew to azimuth 180, altitude (180 - latitude) to put the mount with the scope on the west side with the OTA pointed north. Release the RA clutch and use a bubble level to make the counterweight shaft perfectly horizontal. Tighten the RA clutch.
Then, resume at step 4.
Posted 27 April 2013 - 02:58 PM
I do solar outreach quite often and here's what I do:
--> Setup mount and eyeball the north leg somewhere close to north.
--> Using a compass (with a cover & line scribed within), step back from the mount ~ 3 meters and align the north leg with true north. You must know the magnetic declination of your site and adjust the compass accordingly.
--> Level the tripod in the (3) planes containng the tripod legs using, of all things, a LEVEL!! I have yet to see a mount that has a built-in level that really is level.
--> If you are using a GEM, ensure that the polar axis is set to (90 - latidude) degrees; I use an actual inclinometer for this.
If you take your time, you will have an alignment that requires minimal correction. In my case, I can routinely achieve an adjustment only twice an hour to keep the Sun centered.
Posted 27 April 2013 - 03:29 PM
What you describe is pretty much what I do for a quick visual session, or solar viewing. The procedure I describe above is much more accurate. For me, it has been sufficient even for imaging at long focal lengths. Oh, and it does not require that the tripod be level.
Posted 27 April 2013 - 09:19 PM
For visual use, the procedure Fish describes works great. You can also use a bubble level to insure the mount starts in an accurate CWD position with dec at 90 deg. This (and the proper location and time) should give you goto's that are close enough and no discernible drift for visual.
But the quest remains - sub arc min polar alignment without wasting dark time.
Posted 28 April 2013 - 09:51 AM
Obviously, have a permanent setup.
But the quest remains - sub arc min polar alignment without wasting dark time.
Doing a sub arc minute polar alignment must need a way of setting the scope position to better than an arc minute and AIUI that's beyond what a normal spirit level will deliver.
Better levels, such an engineer's level, will - at a cost - I saw a level that has a precision of 20 arc seconds for about $140.
Now you are getting into the realms of worrying about the alignment of the mechanical and optical axes of the OTA and using the refracted pole - doesn't this depend on the altitude of the object you are trying to track?
You can align using the most precise things you have available - the stars - as soon as it's dark enough to see them and long before it's dark enough to image.
Posted 28 April 2013 - 02:51 PM
I'm doing this on a CGE PRO and I'm located near Houston, TX.
Posted 28 April 2013 - 07:36 PM
I live live near the 47th parallel, but in my basement test, I found that I needed 43 degrees offset. What I forgot is that the last time I used this mount, I was at a dark sky site near the 43rd parallel.
You are, of course, right that the correct calculation is not 180 minus latitude. It is 90 plus latitude.
Once again, I appreciate the correction.
After I have a chance to run all of my mounts through this to see how they do, I will post a concise set of steps.
Posted 28 April 2013 - 08:14 PM