Search the Community
Showing results for tags 'drift polar alignment procedure '.
Hi All, Now that my observatory is finished, I wanted to do a as accurate polar alignment as I humanly could do for my Astro Imaging and so started a adventure. I ended up getting a decent polar alignment after a few tries using different methods and wanted to share my results for anyone out there who needs help in polar aligning their EQ mount with the simplest instructions I can put to words. I want to start by saying that unfortunately it is time consuming and there is NO SHORT CUT if you want accurate PA. The more time you spend on it the more accurate your scope will be aligned to the pole. The below procedure worked for me in the southern hemisphere, but it should work just as well, except change the celestial equator from north to south for the azimuth adjustment. I got a copy of AlignMaster and with the original file, I was very limited to the stars that I was able to use for the alignment, this was due to my house, the observatory hut. Another problem was that the alignment using AlignMaster never coincided with the alignment when I used the Polar scope and/or Celestrons "All Star Polar Alignment feature" (ASPA), not even close. Usually out by a factor of 6-7 DEGREES when checking using celestron polar alignment feature to measure the polar alignment (procedure described below). I tried to run the procedure a number of times, again with limited star selection and the polar alignment compared to ASPA was still way off. I determined that perhaps there are not enough southern stars in the sterne.txt list so I added more stars, carefully made sure that they were J2000 down to arc second detail, all southern biased, all below 15 degrees DEC, and yes I had more stars to chose from in Alignmaster but the result was the same as before, 6-7 degrees OFF!! At this point I spent 2 nights playing with Alignmaster, not getting any closer to my dreamt off perfect polar alignment so I determined that either I'm using it wrong (honestly I doubt it since it is very straight forward) or, most likely, Alignmaster is not as accurate as I thought... either way, I decided that the next few nights I'll revert to drift alignment. I spent the next three night drift aligning the mount using the below described procedure and at the end of it I allowed the mount to drift on a eastern star and than a northern equator star for 30 minutes and both instances the star was within the illuminated line marking in the eyepiece, no drift, so I dare to say that I'm very close to target. ASPA Procedure: Roughly align the mount on the celestial pole using the polar scope. Turn on the mount and do a 2-star alignment followed by 4 calibration stars, preferably with a illuminated reticule eyepiece. Press ALIGN >> POLAR ALIGNMENT >> Display Align, this will display the true error of which the scope is out to the pole. To reduce the error, do an ASPA routine and then turn the mount off and repeat from step 2. NOTE: Steps 2 and 3 have to be performed to get a accurate reading of the error. When I do a ASPA the error os display as 0 00' 00" in both axis, obviously that not right and only after the star alignment in step 2 can the computer in the mount workout the true error with respect to the set position of the ALT and DEC on the mount. DRIFT ALIGNMENT PROCEDURE: Do a rough polar alignment using either ASPA or a polar scope. Align the illuminated reticule so that a star follows along the left/right line, referred to as the RA line when slewing the scope in RA. Find a star and center it in the eyepiece. Facing east choose a star low in the eastern horizon, 30 degrees is a good latitude since it is low and not too much effected by the atmosphere. Allow the mount to track unguided for the chosen amount of time and adjust Altitude (up/down) to bring the drifted star back to the RA line. At first you can overshoot the adjustment until the star starts to drift in the opposite direction or there is no drift. Facing north use a star as close to the celestial equator as possible. Again allow the mount to track unguided for the chosen amount of time and adjust the Azimuth (Left/right) to follow star direction drift back to the RA line. NOTE: The star will move left/right along the RA line when the Azimuth is adjusted and it will need to be chased using the RA buttons on the hand controller as the star moves off the FOV, but as the azimuth knobs are turned the star will move closer to the RA line. DO NOT TOUCH THE DEC BUTTONS SINCE THAT WILL CHANGE THE DRIFT AMOUNT OF THE STAR AND THE NORTH STAR DRIFT WILL HAVE TO BE PERFORMED AGAIN. Repeat steps 2 and 3 with increasing the tracking/drifting time until satisfied with the drift amount. Drifting time will need to be increased to see the drift direction as the polar alignment gets closer to the SCP. Starting with drift time of 5-10 minutes is enough to show drift since the star will drift faster when the mount alignment is further from the pole Once there is no or little drift in 30 minutes than that is very close to PA, MUCH less than an arc minute and good enough for very long guided exposures with no field rotation. Generally I start with 10 minute drift, adjust and do 20 minute drifts than as a final check do a 30 minute drift alignment a few times until ideally the star doesn't move from the RA line at all. Make sure that the drifting time is the same between adjustments, this will allow you to determine if the drift is getting larger or smaller by the distance that the star drifts. I alternate adjustments between east and north (Altitude and Azimuth) alignment as one axis adjustment can slightly alter the other, especially at the start of the drift procedure. Drift alignment is time consuming, for example, for my permanent setup I spent 3 nights drifting to get PA as perfect as I can, ideally i would like to be adjusting the mount until there is no drift for 60 minutes (the 60 minute drift alignment I will do next 2 nights. I figure that while the moon is out it is the perfect time to get PA perfected, so surely 5 night spent on it will be near zero error). Of course the more time that is spent will result in a more accurate alignment but spending this much time on a portable setup is impracticable, so drift aligning can be shortened to accommodate the imagers needs, even a basic ASPA and a 10 minute drifting session a couple of times over will allow 10-15 minute subs when guided, my plan here was to have subs of over 40 minutes, even a hour if need be. AGAIN, this drift alignment procedure was in the SOUTHERN hemisphere, so some aspects might need to be adjusted for the northern hemisphere, such as you can use Polaris, no such option down-under and your celestial equator star will be in the SOUTH. ALSO, If I was drifting on a western horizon star, than, instead of moving the star back to the RA line in the eyepiece, I would have to move it FURTHER away than re-center the star using the hand controller. This is where keeping the drift amount times the same to determine if your drift is improving or getting worse comes in handy, if your star drifts more after a adjustment than you know that you need to turn the AZ/ALT knob the opposite way, SO using a star in the east is simply less troublesome. I hope this info will be useful to someone who need help with polar alignment. Next project... to program the CGEM with PEC using a CCD and PHD to record a few runs of the mounts error and upload the averaged recorded waveform. Clear skies.