AutoGuider Image Scale:
It is important that your image scale is very accurate. The best way to determine your image scale
is to take an image with the *autoguider* (un-binned) and do a plate solve to resolve the actual image scale. If this is impossible,
I would download Ron Wodaski's CCD Calculator
to determine image scale.
Peak to Peak Maximum Allowable Error:
Here we try to determine when we want your mount to actually make a correction. If your guider error is only .05" arc-seconds, there is no need to make
an correction. If your min move setting is set to low, then you guider relays will move the mount every guider cycle. This can result in mount oscillations or
chasing atmospheric turbulence. My best suggestion in setting this value is about 75% the image scale in which you are imaging. For example, if you are imaging at
1.2" asp, then set this value to 0.9" asp. This way a guider correction is only sent when the centroid of the guide star is 0.45" asp off in either the
X or Y axis from the selected guide star position.
Maximum Allowable Movement:
This value is not required to achieve great autoguiding, but it does protect us from some extreme situations that could ruin a sub-exposure.
For instance, if a cosmic ray hit is sensed on the autoguider CCD, this could cause your autoguiding software to think that the centroid of the
star has moved many, many pixels from center. Therefore it is going to attempt to correct the mount for this error. If you have no max move setting,
this will result in a very large correction and a ruined exposure. If you have a limit to the size of the correction, i.e. max move, then this
effect can be mitigated by not allowing a large correction. Setting this value too low may result in under correction, so its best to set this value about
2X - 4X larger
than the above 'Peak to Peak Maximum Allowable Error' setting.
Additional Comments on Setting your Maximum Allowable Error:
Setting your maximum allowable error depends on many factors; seeing
conditions, wind speed, the accuracy of your mount, the image scale
of your imaging setup, etc. Finding the right balance of your
systems capabilities and when you should issue a correction takes
some experimentation. I have provided these illustrations to help
with this process.
In this common example, we see a situation where the seeing and
mount accuracy of the imaging system are ~1.00 arc-seconds. The
minimum movement is set very low at 0.30" arc-seconds. The guide
star centroid is staying well within the systems capabilities, yet
the autoguider relays are making unnecessary corrections leading to
mount oscillations, over-corrections, unpredictable movements
because of backlash, etc. In this example, the stellar profiles of
the stars in the image are going to be distorted unnaturally because
of poor autoguiding settings.
By increasing our maximum allowable peak to peak error to 1
arc-second we only make corrections when the guide star centroid
falls out of our systems capabilities i.e. when a correction is
truly needed. As you can see we have reduced unnecessary guider
corrections which will result in better stellar profiles and rounder
Here we have an example of the maximum allowable error being set to
high. This could be the same imaging system on a night of excellent
seeing or a different imaging system with a more accurate mount. As
you can see, guide star centroids outside of our capabilities are
not being corrected.
By reducing the maximum allowable error to 0.60" arc-seconds, we are
now correcting for the movements that fall out of our systems
Richard A. Bennion
(c) 2006 CCDWare Publishing