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Information about Drift Alignment

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#1 Zi Hao

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Posted 08 November 2018 - 08:20 PM

Hi all,

I have been doing manual drift alignment using DSLR D5300 before the weather in Malaysia turns awful, but the results were not the best. I am using a skywatcher reflector 150/750,mount eq3 pro,maximum getting 20s of subs (unguiding, only with drift alignment and sidereal rate, sometimes with accurate GOTO but 20s of subs also), therefore I would like to ask is 20 seconds exposure adequate , or is it the best my scope can do, if with guiding, what is the maximum exposure time it can do?


Can anyone here share their drift alignment star image(I mean the one with angle of deviation, and please indicate the directions in the image as well, so I can have a better reference), thanks.


And one more thing, do I have to turn on the sidereal tracking when I am doing the drift alignment?
Any advice given would be greatly appreciated. Zi Hao

#2 Alex McConahay

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Posted 08 November 2018 - 08:45 PM

20 seconds will get you something, but it is not very productive. Do what you can to get longer. 


As for how to polar align, I found it easier to quit worrying about what the directions say (If the star drifts up, move the azimuth  wherever...….." maybe because I use different scopes. What I find more useful is to center the star and let it drift for one minute, and note how far it drifted in that time, and which direction. Then adjust one way or another (does not matter which, although the smart people have it worked out). Then recenter the star and let it drift the same one minute. If it drifts less, I adjust again the same direction, whatever it was, recenter, and so forth. If it drifts more, I adjust THE OTHER WAY (plus a little to make up for my first misadjustment. And I watch how it drifts again for one minute. Hopefully this time is drifts less than it did on the first try. If it does, but it still drifts, I adjust a bit more that direction. And try again. If it ever starts to drift the other way, I know I have gone too far. After a while I may bump the magnification up to speed things up. 


Point is......don't get too fancy with angles and charts and all that. You don't need an image. All you need to know is if the star is moving more or less than it was in the last iteration. 



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#3 SteveInNZ


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Posted 08 November 2018 - 09:03 PM

Yes, you want sidereal tracking on when drift aligning.


If you have a look at your images, you can do some self-diagnosis. If the star elongation is in the Dec direction, your most likely source of error is polar alignment. If it's in the RA direction, it's most likely periodic error.

I would expect with that mount and 750mm focal length, you are seeing mostly periodic error. The solution to that is guiding but if you are going to try imaging without guiding, 20-30 seconds is about the practical exposure limit per sub. You can go longer and throw out more but that seems pointless to me.

Since your mount is dictating the maximum exposure length and the focal ratio is fixed, crank up your ISO to get the histogram off the left axis and take lots of subs. You can fix noise by stacking but you can't fix bad/wobbly stars (easily).



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#4 fewayne


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Posted 09 November 2018 - 01:40 PM

I always thought the slew-during-exposure method to be pretty reasonable, though since I started using the Ekos polar-alignment tool I confess I haven't used it.

  1. Put a bright star close the meridian near one edge of your image.
  2. Start a two-minute exposure.
  3. After 30 seconds, slew in RA for 45 seconds (in the direction that makes it traverse the sensor instead of sliding off the edge), at a speed that mostly keeps it within the frame (first image); if it goes out of frame a bit, that's OK (second image).
  4. Immediately slew back the other way in RA for 45 seconds.
  5. Examine your image. If you have a straight line, good; if you have a V, adjust azimuth. The bright dot at one end of the V indicates your starting point, so you know which way to adjust (but frankly I like Alex's simple algorithm!).
  6. Repeat until the V closes into a line with a bright dot at the end.
  7. Pick a star near the horizon, and repeat for altitude adjustment.

Attached Thumbnails

  • align1.png
  • align2.png

#5 pedxing



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Posted 09 November 2018 - 02:55 PM

Something I've found to be useful is to use Alex's method first to get into the ballpark before using DARV (or something else) to fine tune.


The fast way to do the rough alignment is to slew to the equator/meridian intersection, start sidereal tracking, start looping in PHD, and turn on the fine grid view.


Then, watch a star against the fine grid to see the drift in declination (really helps if your guide camera is aligned to N/S). The star will also move back and forth in RA, but you ignore that, just watch for motion in DEC.


Adjust azimuth and observe the effect on the DEC drift. Iterate until the star appears to stay steady for 60 seconds in DEC.


Slew to a point about 30 degrees above either the east or west horizon and do the same thing but making adjustments to altitude.


If your mount is level to start with, azimuth and altitude adjustments shouldn't affect each other. With practice, this initial alignment shouldn't take much more than about 5 minutes.


Then you can switch to DARV (or one of the drift alignment tools) and fine-tune the alignment.


There are a couple of reasons why I recommend this initial method if you're going to drift align. First, if you are going to use one of the PHD drift align tools, you have to calibrate before you can use the tool, but if your PA is off too far, you don't get a very good calibration. If you do the rough drift first, then your calibration is much better and the drift align tools will work better. Second, the drift align tools or DARV can suck up a lot of time, so a little time getting roughly aligned first makes the more time-intensive fine-tuning operations go faster.

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