Many new mounts have polar scopes and of course drift align is a popular and necessary technique. However, when it's time to align the DS16 GOTO mount I'm building, this issue will not be trivial. The mount is extremely massive and lacks fine motion polar controls or a polar scope. This seems to indicate hours of frustration spending time discerning drift, followed by attempts to delicately adjust a hundred pound EQ head with a mallet or something.
So I decided to try out the feasibility of using a GLP mounted on the polar axis and a second scope to see where the laser and thus the polar axis is. This way I can delicately whack while directly observing the results. I'm posting this here for anyone who has a need to simplify polar alignment for large heavy mounts with no polar scope.
Day 1)
So I took my fresh from China 5mW GLP, wired it for 3V regulated DC (no batts!), placed it in my ghetto GLP mount, and plugged that into the polar scope hole in the dec axis as shown below.
Then I set up my ST90 where I could look through it as I rotated the LXD75 by hand, also shown in the gallery.
When I kicked on the GLP and started rotating the LXD 180 degrees while watching the laser, it looked pretty good. The laser made a big circle as I spun it, just as I wanted. For coarse adjust, i manhandled the entire laser housing a bit and made the circling laser as small a circle as possible.
Then I put my 20mm Erfle in the ST90 and went to fine tuning. It was confusing at first, but not nearly as bad as I expected. In short order I had the laser error down to about 1/4 of the FOV using the fine adjust screws on the GLP mount.
At this point I popped in my DSI-C and repeated the process. It actually became simpler at this point as the view on the laptop is easier to work with in realtime than reaching to do fine adjusts to the GLP screws while trying to look in an eyepiece.
Also I realized that while a dead centered, stationary beam end would be nice, a small enough circle is also workable.
Within a half hour, I had the beam nailed down to a tiny fraction of the imager field in terms of error circle. I'd guess a couple minutes of arc. Not bad for a first try. At this point I set up for 15 sec subs and did a run of about 50 frames, clicking the laser on and off every other frame or so. This made a pretty nice set to work from, and an sweet AVI of the pole rotating while the laser flicks to either side of it's error circle.
So I stacked a few frames of the laser at either end of it's arc, and compared the result to Stellarium. See the gallery below
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So I'm thinking this trial run was a pretty solid win. Fine tuning the laser was way easier than I thought, and if you mess with the imager settings you can make the tip of the beam fairly small. Plus the error in my system is clearly visible and immediately so without drifting by comparison of the beam tip to a decent chart.
Now my thinking is I can go ahead and go for a tighter field now that I'm comfortable it will work. I think tonight I'll use my Z12 instead of the ST90, beginning with my 30mm 80 degree eyepiece and then moving to the imager again. The saving grace of all this is for once...the target stays put!
Then once I've convinced myself it's actually working right, I'll go ahead and risk tweaking in my LXD. It's obviously off by a fair amount and I may as well try this for real.
Day 2)
Worked on it a bit more last night under thickening clouds with a few sucker holes. I'd left the laser in place so I was able to get right to it. I went for it and adjusted the LXD since it was obviously off a bit.
The laser was quite enlightening and explained why I had always battled the mount more than I thought made sense when doing drift. Having the polar laser on while adjusting things let me see how well the adjusters work. The az push pulls on the LXD work fine. No slop, smooth, little to no shift when snugging up the opposing bolt when you have it where you want it.
Altitude...different story. It's smooth enough when one push pull is loose and you're adjusting with the other, but when you go to snug the opposing screw, the alt you just set shifts by what can be 2-4 minutes (roughly calculated). By seeing this I was able to prepare for it by stopping a bit short and then using the shift during the tightening to to take it the final distance to where I wanted it.
Over all, I think I cut my polar error by half in short order. With the clouds coming and going I didn't have time to really nail it down as I stuck with the 30' field of view using the ST90 instead of hauling out the big dob to zoom in with 3x the focal length. Didn't have time to do a three star and see what Autostar reported or attempt a verification using drift, as the clouds finally closed in.
I'm now pretty sure this method will work and work well when I'm ready to finesse the DS16 into polar alignment with my hammer.
Day 3)
Was good and clear tonight, did some further testing.
The Z12 is great for centering and way too tight for imaging with 1500mm of FL on a dob mount with a tiny chip. A larger chip or DSLR would have more field and be fine. As it was my window on the sky was so small I had a heck of a time and bagged that. I dragged out the ST90 again and put the DSI back in it, with a barlow. This was better than all previous nights for accuracy, and more workable than the Z12's tight field.
This said, using the dob for centering was awesome. I had bumped the laser and had to recenter it. With the faint stars from the light bucket and rotating the LXD, you can pretty much see where it's orbiting. Then I just manhandled the entire laser housing in smaller increments towards that spot, and wound up with my smallest error circle yet in 5 minutes without ever touching the three align screws.
I'm uploading some video of 37 min of polar rotation while I occasionally move the LXD axis 180 degrees from it's previous location and then back, while turning on the laser for a few seconds at each spot. This defines both sides of the laser's error circle. The LXD's pole must lie at the midpoint of the line between the two laser endpoints.
And cool enough...the adjustments to the LXD a couple nights ago appears to have worked very well. I'm hard pressed to see how the spot between the two endpoints of the laser isn't danged near right on top of the point the stars orbit, and this with twice the image scale of prior nights.
37 minutes of polar laser and pole rotation, 20 fps
http://www.flickr.com/photos/35709891@N07/4480784145 same, 10 fps
http://www.flickr.com/photos/35709891@N07/4480752269/in/photostream/
