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I thought some folks here may find this helpful. This is the response to a person's request for help with his PHD Brain settings because his graph was all crazy and his stars misshapen after making some changes and not being clear on what he was changing.
On May 20, 2011, at 1:56 AM, Neil Heacock wrote:
1) RA aggressiveness at 120 means that you are going to apply 120% of the correction to the RA movement. This is probably too much. Anywhere from 80 to 100 should likely be what you want. But not over 100.
2) RA Hysteresis is applying a portion of the previous correction(s) to the current correction assuming that there is some error due to lag or seeing or some other such thing and the current correction is exactly.. umm.. correct. The higher the number, the more of the previous correction is applied to the current correction.
3) Min Motion (pixels) - this is the minimal amount that the star is allowed to move *without* sending a correction. If that number were .25, then the star would be allowed to "float around" a quarter pixel without PHD sending corrections to the mount. This is partly to counteract seeing and partly to counteract normal gear error. A setting of 0.05 means that at a 20th of a pixel fire off a correction. This is sure to introduce oscillation and loads of over correction. I'd raise this to nothing lower then .15 or you are sure to send a correction for every guide frame - which is *not* what you want if you can help it.
4) Max [RA/Dec] Duration is how large of a *maximum correction* PHD is allowed to make (in milliseconds). 360/350 (respectively) are probably adequate unless things are considerably bad like massive periodic error or significant polar mis-alignment. You can raise it to, say, 800, but If PHD needs to send an 800ms pulse (for example) then you've most likely got serious polar alignment problems that you're asking the guider to accommodate for that are probably unreasonable. I'd recommend rather then increasing that number too high (500 should be more then enough for any mount), learn how to better polar align. There are lots of tutorials and options online. I've personally been using PHD to aid in drift aligning and it works great.
5) Calibration step - this is the length of a pulse in milliseconds that PHD will send to your mount during the calibration process. If 500ms move the star a pixel or so during calibration, then leave it there. But if it barely moves and you need to make like 50 steps in the various directions and calibration is taking 3 or 4 minutes, then it's way too low. To know where to put that number, play with it until you see the star moving about a pixel for each step and PHD is only taking about 15 to 20 steps in any direction and fully calibrates in about a 90 seconds or so. For me, my 50mm finder/guider is set to 2500 and my 400mm ZS66 guider is set to 1000.
6) Star Mass tolerance.. I haven't figured this one out yet... but in Craig's documentation he says if we set it to 1.0 then it will basically turn it off (as of 1.12.2 I think). I have mine set to 1.0 because until I understand what it does, I want it to not affect things. I have no idea if your 0.3 is the right number for the typical guide stars you get. My recommendation, it sounds like you have enough on your hands to deal with... turn it off by setting it to 1.0.
7) Regarding your guide exposures of 10 seconds in one link and 1 second in the other link, it's important to understand that too short of guide subs will possibly correct too frequently causing the problem of "chasing the seeing" (which isn't too bad if you lower your aggressiveness to 75-80% or so), while too long of guide subs - coupled with bad polar alignment - will cause lots of Dec corrections and never RA corrections because PHD can only send one correction at a time (unless you're pulse guiding) and if every 10 seconds it needs to correct for Dec because of bad polar alignment, then it will never correct for RA. So, my recommendation is to find a sub exposure that is somewhere between error and seeing. Probably 1.5, 2, 2.5 or maybe 3, but based on what you are showing us, I'd be surprised if 3 is good for you. Try 2 second exposures along with the other things I've mentioned here and see if you can get things to settle down.
I hope this helps you and possibly some other folks who read this.
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