Drift aligning using PHD guide
Posted 07 September 2013 - 01:30 AM
Posted 09 September 2013 - 09:20 PM
Posted 12 September 2013 - 01:59 AM
Set your slew rate to 1x sidereal. This should stop the mount's tracking when slewing E. Set the camera for a 70 second exposure. Start the exposure while tracking at sidereal rate and do the following for the indicated time periods.
00-05 track normally
05-35 slew E
35-70 slew W
Look at your image. The tracked portion will generate a star while the slews will create two tails forming a V shape, with the return tail passing through the star. The width of the opening of the V at the star is the drift that occurred during 1 minute of slews while the relationship between the trails is an indication of which way the mount needs to be adjusted. If your first adjustment increases the size of the V, adjust the mount the other way.
When I do this at the meridian and as close to either horizon as I can get (the same points to measure as other drift alignment techniques), I always find some stars in the image. At one minute per measurement, it doesn't take long to drift align. You can double the sensitivity of this test by increasing the exposure time to 130 seconds and slewing for 60 seconds in each direction.
Posted 12 September 2013 - 04:17 AM
Posted 21 September 2013 - 07:46 PM
I did a rough polar alignment first using the standard drift align method then got it spot on using PHD as described below
1)First of all position the scope so it is pointing at a star as near to the Eastern or Western (doesn't matter which) horizon as poss so basically as low as you can get to the horizon
2) Select a star and start the calibration process as normal in PHD and start guiding.
3) While still guiding we now need to turn off the DEC Guiding in PHD so you can either stop guiding and click the 'Brain' and select it at the top right ('Dec Guide Mode' and select 'Off') OR even easier click 'Enable Graph' in Tools (since we need the graph anyway) then select 'off' in the bottom right hand selection box on the graph screen (there is 'Off' 'Auto' 'North' and 'South' in this box we need to select off...this is only referring to the DEC part of guiding)
5) Now on the graph click the 'RA/DEC' button so the display changes to show a blue and red 'dx' and 'dy' (You MUST do this or the red line will not move whatever you adjust once the Dec guiding is turned off)
6) Now look at the red 'dy' line ...if we have got the polar alignment (PA) pretty good the line should only be rising or falling slowly...if it shoots up or down very quickly we are still a way off ...the idea is to get the red line as flat as poss by adjusting the knobs on the Alt axis (remember the scope is pointing at the E or W horizon)
7) After adjusting the knob a very small amount the star will have moved again so click 'Stop' in PHD re centre the star with the scope motors and re click the star then click the 'PHD' button to recommence guiding.....it doesn't go through calibration again as it is the same piece of sky ...just start guiding again and watch the red trace on the graph again (I usually click the 'Clear' box on the graph to start it afresh) ...keep doing this till the line is as flat as you can get it, if it is rising or falling more than before you have turned the knob the wrong way...the line will wobble slightly but we are looking for it to stay as level as poss.....tighten up the ALT adjustment knobs on the mount when done.
8) Now we are ready to fine tune the RA alignment
So using the motors position the scope so it is pointing at a star as near as poss to the Meridian and Celestial Equator.
9) Since we are now in a different piece of sky we will have to recalibrate PHD guiding so after clicking the 'Stop' button in PHD go into the 'Brain' and tick the 'Force Calibration' box at bottom left and on the graph select the DEC guiding box at bottom right to 'Auto' again.
10) Select a star and click the PHD button again and it will start its guiding calibration.
11) Once the guide lines are green indicating guiding has commenced click the bottom right button on the graph again to select DEC guiding to 'Off'
12) As before look at the red 'dy' line but this time adjust the AZ KNOBS on the mount to get a flat line again.
The PA is now pretty much spot on...after adjusting the RA it does affect the DEC a bit so you may want to go back and forth a couple of times refining it
Also the longer the focal length of the scope used the more accurate it will be .......it doesn't matter about it being ABSOLUTELY bang on if you are guiding but if the scope is permanently mounted in an observatory, like mine , it is worth doing to improve gotos etc.
I have attached a screen grabs showing that alignment is still slightly off with the red line going up (the full graph left to right is 10 mins). After aligning this line will stay level.
ignore the 'peaky' blue AZ line ....that was a tracking problem since rectified
I realise this has been a long drawn out explanation but hope it helps as there must be others like me who need their hands holding throughout the whole process
Posted 21 September 2013 - 10:17 PM
PHD2 has some features that make it easier to use as a polar alignment tool
It has a declination trendline graph option that will show the declination line without the need to to stop declination corrections, and if you do stop declination guiding it still graphs the declination line (this eliminates step 3 and 5, as well as 11). For ASCOM mounts, it also changes calibration based on your declination (so no step 9)
PHD2 Beta 2 was just released yesterday at http://www.daddog.com/phd2
Posted 22 September 2013 - 02:02 AM
I'm a bit confused about how you can use dx/dy mode unless your camera x/y axes are pretty precisely aligned to the mount's ra/dec axes.
Displaying the bullseye and doing slow slews is a quick and easy way to align the camera with the mount axes. Just keep a bright star on the lines and you're set.
I agree that PHD2 has some great polar alignment tools that makes PHD1 less desirable for the exercise.
Posted 22 September 2013 - 03:52 AM
I tried it to prove this.... after lining the cam up so it follows inputs on the hand pad going vertically up and down and horizontally left and right I did a calibration in PHD then selected Dec guiding off and watched the red line move up showing PA to be a slight bit off
I then stopped PHD, physically turned the guide cam 45 degrees in the guide scope, re calibrated, turned off Dec guiding and watched the red line again.
It drifted again IN THE SAME DIRECTION AT THE SAME RATE showing that guide cam orientation doesn't matter when doing this procedure as long as you don't alter it's orientation after PHD does it's calibration
After moving it 45 degrees and selecting dx/dy the blue RA graph did of course move downwards but this doesn't matter as we are only interested in the red Dec graph.
I always position the guide cam in line with the axes anyway as its easier to position a star..But it DOESN'T HAVE to be for it to work.
P.S. don't know why I jumped from 3) to 5) above
P.P.S. Interesting about PHD2 ...wasn't aware there was one ...will check it out in that link thanks for that