I'll chime in with a few points about polar alignment.
As GWAR says, it can be tedious yet it gains importance as your exposure stacks cover more time. Polar alignment and tracking become critical when you are taking pictures that would cause star trails and start to show errors.
When you are using camera lenses (assumption is that you have a focal length less than 300mm) and are imaging targets that are near the celestial equator up to 40 degrees declanation on either side, you have some flexibility. If you keep your sub exposures under 5 minutes, you just need your polar alignment to be within 1/2 degree of true north.
This sounds like a lot. Note that on the image scale that GWAR posted of the view of his polar scope, there's a distance of 4 degrees from Polaris to Delta Ursa Minor. The distance from Polaris to the true north is a bit over a half a degree (.6). Now you can start to see some of the precision involved.
Here's how you can make alignment to north easier:
- Use a compass -- and know the local magnetic deflection -- so that you can set up the tripod properly. Some smartphone apps are able to show north with good accuracy via GPS data.
- Sight along the mount axis to ensure that you're close. This can usually get you within a degree or two with practice. If you have a polar scope in the mount, don't remove it to do this step.
- Use your polar scope, taking care to note the positions of the constellations and stars visible to account for date and time. Assumption here is that you've taken the time to ensure that the polar scope and mount are aligned to the same axis. With proper effort, you can get within a half a degree of true north and you may be finished.
- For greater precision, use drift alignment. Assumption here is that you have a way to measure drift over time and can reposition the mount with accuracy over large swings on the sky.
Here are some further details about the above:
Alignment of the polar scope with the mount axis.
This is a tedious step and is dependent on two activities - once during the day and once during the evening.
For the daytime, you need to make sure that the polar scope's axis is aligned with the axis of the mount. There may be some wobble as the two spin about the sky. Measuring this deflection is done this way:
- Sight through the polar scope at some distant object (top of a tree, building, etc) -- you may need to adjust the altitude of the scope to see such a target.
- Swing the mount back and forth in Right Ascention (east to west) to see if the target remains centered. Don't use the marks visible on the reticle - at this point we don't know if these marks are centered! Pay attention to how the view is in the field as a whole - look along the edge if you are confused.
- If it stays centered, great - you're done. More than likely, it will go in a small circle. The goal here is to nudge the polar scope in the right direction so that this circle becomes smaller.
- Depending on the way the polar scope attaches to the mount, this may be as simple as wrapping the scope body with some tape to shim up any wiggle room (which is what I did) or by adjusting washer positions where the scope screws.
- Sometimes you can't make this adjustment perfect for the whole circle but can only get it great for part of the circle. In that case, make it good enough for the time of year that you have now and then plan to readjust as the seasons change.
All these steps have done is make sure that the scope body is pointed in the right way. Now the trick will be to adjust the reticle. The assumption here is that you can adjust the reticle. Many times there are small setscrews around the body that will allow you to nudge the glass insert.
You can do some of this during the daytime and the last step will be to do drift alignment and then adjusting the reticle to match the sky.
Two methods of adjusting the reticle:
- Do it out of the mount - place the polar scope in a cradle and spin it while sighting through it. If the reticle wobbles, then it's not centered. Adjust accordingly. Some people do this step before the shimming exercise above.
- Do it through the mount - once the shimming exercise is completed above, swing the mount back and forth in RA and adjust the reticle to keep the lines on the target properly.
All this should get your polar scope lined up with the mount very well.
Doing drift alignment.
This is something that is done under the stars every time you set up. It can take a while to do properly but it gets faster with practice and technology.
Three major steps with drift aligment:
Get as level as possible.
Check the stars at the meridian & equator - adjust mount in azimuth
Check the stars at the east or west & equator - adjust mount in altitude
A friend of mine uses his camera to do this checking.
He points the camera at the desired place in the sky, exposes for a few extra seconds, then while still having the shutter open, he slews the mount east or west for about 30 seconds, then goes back the other way for the same time. Then he looks at the image. If the line of the stars is anything other than a perfect line (and not a wedge) then he adjusts in azimuth or altitude. You'll get quick results for making the wrong adjustment so it's pretty fast.
Assumption here is that you can slew in RA. I'm not sure the Astrotrack can do this. Alternately, you could let it run with tracking off for a minute, then turn tracking on. That would give a hockey stick with any deflection being seen as a "leg." The goal of adjustment would be to reduce the size of this leg.
Hope this helps!
