Hi all,
Thought I'd mention to folk a rather interesting development on the Yahoo PHD forums. Craig has finally advised us on the 12th Dec that due to overwhelming demand he is plannng to add polar alignment capabilities to PHD:
OK, by far the #1 requested feature for PHD Guiding is some tool to help users polar align their scopes. I've had 3 requests just this week for it! So far, my hopes that this will go away or that someone will do it in OpenPHD haven't come true, so ... let's talk here as a group.
Now, there are several ways in which a computer can help you align your mount:
1) It slews around to a bunch of stars, centers them (or has you help here), figures out (or already knows) exactly what they are, figures out your flex and other misalignment issues. This is the kind of thing that T-point does. In the limit it will tell you "Turn your Az knob 1/8 turn clockwise." Uber-cool, but it needs to know *a lot* about your mount, scope, camera, needs to slew, may need to plate solve, and nee ds more time than I have to code. So, we're not going to be that cool!
2) It could track your error as you're guiding and tell you something about your current error. To know this, it will need to know where in the sky you are pointing. Folks with ASCOM mounts will have this info available, but anyone (like me) guiding with an ST-4 port won't. Now, you could tell PHD where in the sky you are imaging and, with some math I'd need to think about a bit, it should be able to say things like "I think you're aimed a bit high" . While this sounds pretty cool, it's also an after-the-fact kind of thing. It won't be the most efficient way to get you polar aligned.
3) It could have you go through a drift alignment procedure, giving you the info you need to adjust the mount. Drift alignment needs two stars -- one near the meridian to adjust your azimuth and one near the horizon to adjust your altitude (
http://www.astropix.com/HTML/I_ASTROP/TRACKED/POLAR.HTM). Once calibrated PHD knows N, S, E, and W of course and if it knew you were on the meridian star it could say "Hey, it's going north -- you should move your mount to the east".
Now, #3 already exists to some degree. If you fire up the graph, turn off guide outputs, and watch the Dec curve, it will tell you if the star is moving N or S. If you know you're on the meridian star, you can use this to adjust your azimuth. Of course, if you're on the horizon star, you can use this to adjust your altitude. What's not there is the "cheat sheet" to help you remember these bits or walk you through them. This could be as easy as a detailed bit in the Help file or it could be some dialog that pops up (has you select meridian vs. horizon star and E or W star for the horizon, shows the current N-S error, and what this means to adjust.
Collective thoughts?
Craig
Everyone seems to like Option 3. I've pointed the man to this site and these algorithms if it helps!
Allen Gilchrist wrote and excellent calculator in 2002 from memory – else try http://canburytech.net/DriftAlign/Spreadsheet.html or download the 1.5MB calculator plus doc and data
http://canburytech.net/DriftAlign/do...rift_align.zip
Calculations required documented on the same site for all http://canburytech.net/DriftAlign/Equations.html