Quote:
Originally Posted by alistairsam
Thats pretty cool Jen, longest i've been able to go with my home built fork mount was 1 min or so with no guiding.
found some good explanations here
http://www.cloudynights.com/item.php?item_id=2755
in the section "I watch PHD's graph and display and all looks great but my stars are still oblong", one possibility suggested is differential flex.
another thread with changing phd settings
http://www.iceinspace.com.au/forum/s...ad.php?t=48229
From one user "increase "RA agr" to max 100, and reduce "RA hys" to 5 in order to stop the "Osc-index" going further below "0.19" |
Just remember, PHD settings vary based on the focal length of your guide scope. So some tweaking will need to be done for your mini guider.
Personally, i would be looking into your PA first. If alignmaster advises that you are greater than 10" off - then your mount is very far out of alignment.
When i first started i had huge issues getting PA right. Even now i have problems some times.
Just remember - you are after true south, not magnetic south.
The degree off magnetic south varies based on location and time. A calculator is here:
http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp
And since you are using PHD, you can drift align with it.
http://njstargazer.org/PolarAlignment.asp
Basically: Get mount setup as close to true south as possible, set longitude as best as possible. I use a digital inclonometer, and this gets me almost perfect every time! The longitude markings on my HEQ5 are a few degrees out, so i'd recommend not using those unless that is all you have.
Manually move the scope to slightly off true south from park position (i go to the left as facing south) by a few degrees. Keep it as close to the horizon as possible (no more than 30degrees up). Turn on sidreal tracking, tell PHD to calibrate and begin guiding. Once calibrated, stop and go into the settings. Turn DEC guiding off (top right of settings page, should be set to Auto atm). Start guiding again on a star and turn on the PHD graph.
Hit the RA/DEC button which will turn the graph into graphing DX/DY. Look for the red line on the graph (disregard the blue).
If the red line goes up or down (from the middle) then you need to adjust the azimuth screws (move head clockwise/anticlockwise). If you are a long way off, you will actually be able to see the star drift up/down in the main window and the red line will basically shoot off the graph after 1 or 2 guide pulses, and PHD will complain that it lost the guide star. The further off you are, the faster it drifts.
One issue i had was that no matter how much i adjusted the azimuth screws, my line and the speed of drift never changed. I was confused and didn't know why. This occurs if you are a long way out. In my case i was pointing at magnetic south, so greater than 10degrees out. the PHD drift method is very sensitive so your graph will often veer off the screen very fast if you are more than a degree or so out, only when you are close does it stay within the graph screen.
Keep adjusting until the red line is relativly stable near the middle. If you are after perfect polar alignment, then the red line should not go up/down significantly over 10mins or so. If it does, you are slightly out.
Expect a few variations, the graph won't be perfect. This is due to the seeing jumping the star around.
Whilst learning i would suggest just getting it as close as possible, don't wait 10mins and try and get it perfect first time. Getting it close (slight movement (decrease or increase) over 1min) will give you decent images, but not pinpoint stars. Just don't try 10min exposures.
Basically repeat the same thing for logitude (but use the longitude adjust screws) but on another star near the east or west horizon.
If you think you are close, you can always stop guiding and then run alignmaster and it will let you know how far off you are.
Expect to spend nights working on getting this right. It took me two months! but then again, i'm a bit slow with this.
Don't forget to turn DEC guiding back to auto before you start guiding/imaging as well!
Once you know your PA is correct, i'd try imaging again. If you still have elongation, and your PHD is guiding OK, then start looking at flex/mirror slip etc.