Updated : 05 july 2011

Autoguider

Autoguiding tool allow to automatically guide from a continuous guiding camera flow on a star kept at the same location by adjusting the mount position. This will ensure better sleep or nice breaks during very long exposures!

Guiding camera can be any camera supported by AudeLA: webcam, long exposure webcam, grabber (ex: Watec video camera through USB grabber), Audine, SBIG, ASCOM driven camera…

Your mount can be any mount supported by AudeLA with slow guiding movements: LX200, SkySensor, Gemini, Ite-lente, Mel Bartels, Celestron, Temma, Ouranos, AudeCom, Ascom driver...

Starting

Setting up acquisition parameters

Configuring autoguiding

a) Autoguiding on star
b) Autoguiding on spectrograph slit
c) Learning (optional)
d) Image preprocessing (optional)

Running autoguiding

Corrections during autoguiding

Tips & Tricks

 

Starting

Note: you can use a second camera for autoguiding and open a second Visu (Visu2) to keep Visu1 for the main acquisition. If you select the Autoguiding tool in that second Visu, it will reopen automatically next time you connect your guiding camera.

Setting up acquisition parameters

Acquisition parameters are displayed on the main screen of the autoguiding tool:

Exemple of guiding on double star (Castor).

 

Exp.: exposure time in seconds.

Interval : Waiting time between exposures; use 0.5 to begin with (see Tips & Tricks).

Consign : Target position for the guiding star; this is mainly used in spectroscopy to bring the target on a specific location (slit or optical fiber hole). Could be sub-pixel values is required. The target position is shown as the center of the blue cross. You can also change this location by left clicking on one of the blue axis and move the mouse around.

option "Axes (AD, Déc.)" : display the blue axis whose center indicates the target location for the guiding star.

option "tracking On" : Guiding movements are sent to the mount only when this option is set ON. You can activate or desactivate this option anytime. For safety reasons, this option is set OFF when you maunch the autoguiding tool to avoid unwanted movements. Also, MAKE SURE TO SET TRACKING OFF when performing a GoTo on your next target – this is common mistake and some mount do not like receiving guiding movement while slewing on the sky... It is recommended to set tracking ON after have the autoguiding configured (you can actually changed the parameters while autoguiding is ON).

Deviation : Difference in X & Y position between the guiding star and the target position.

Pulse. : Pulse duration sent to motors RA & Dec. Value displayed is in milliseconds followed by direction (n=north, s=south, e=east, w=west).

Interval : Total duration between two movements (in milliseconds).

 

Configuring autoguiding

a) Autoguiding on star

You can change the autoguiding parameters by clicking on « Setup ». This opens a new window:

Area size (in pixels)
You can change the size of the red square which defines the area where the software look for the guiding star. Increase this area if you have a bad tracking, bad seeing or cloudy skies; reduce this area if you have a good tracking and a double star for exemple (to select the right star and stay on it). This square is redrawn after each image acquisition:

Detection threshold (pixels)
This parameters indicates if a guiding star is detected or not.

The key figure is the number of pixels above minIntensity = dbgmean + 6.0 * dbgsigma 
with dbgmean : average background value in the red square (searching area)
         dbsigma  : standard deviation of the sky background in this searching area

When the searching area includes a number of pixels below minIntensity, the software consideres thay the star is not visible (sky becoming cloudy for exemple) and stop sending guiding movement to the mount until this value get back above the min Intensity. Intensity indicator is displayed in red in the autoguiding pannel and some help caption is displayed if you move your mouse on it:

If the detection threshold is set to 0 (zero), guiding movement are always sent to the mount even is no star is detected.

Speed (in pixels/ms)
Coefficient to convert number of pixels in pulse duration (in millisecond) to send to the mount. Start with 10 on each axis then increase or descrease the value depending on mount reaction to guiding movements.

Threshold (inn pixels)
Below this distance, no movement is sent to the motor (to avoid back & forth movement). A good value is 1 pixel usually.

Angle (in degrees)
Camera inclination from RA axis.

b) Autoguiding on spectrograph slit

Key principles

Algorythm :
- Measure received flux in two areas above & below the slit (defined by the target position and the slit width parameter)
- Calculate the percentage of flux between those tao areas
- Multiply the step percentage by the %/pixel ratio to convert this value in a pixel shift of the guiding star in the image.
- Divide this value by the speed (pixels/sec) to get the pulse duration to send to the Dec motor of the mount
If the star get too far from the slit (> width of the slit edge), the software switchs back to standard star guiding mode (gaussian adjustement).

Setting up slit autoguiding

Select « slit » in the parameter Setup window.

Default values for a C8 + Lhires III + webcam

Area size (in pixels)
Define the total size of the two rectangles limiting the searching areas. Those rectangles are redrawn after each image acquisition:

slit Width (in pixels)
Spacing between the two rectangle, defining the slit dark area. Flux received in this area is not taken into account in the flux measurement.

Ratio (%/pixel)
Ratio used to convert flux percentage into pixel shift.

Angle (in degrés)
« Angle » IS NOT USED in slit guiding mode. You have to align the slit with the X axis of the image. A rotation of few degrees is allowed.

Other parameters are the same as in star autoguiding mode (PSF: Point Spread Function, gaussian model) – see above.

c) Learning (optional)

Learning allows to find the RA & Dec speed as well as the camera angle. Set a delay for the pulse (in seconds). The software will ask you to click on the star after each acquisition. Make sure the backlash of your mount is properly adjusted for best results.

... to be completed...

 

d) Image preprocessing (optional)

Add exposures

This option allows you to add multiple exposures before calculating the guiding movement to make. Useful when you camera doesn't have long exposure mode and for faint targets.

Dark frame substraction

This option will allow to remove a dark frame before calculating the guiding movement to make. This is very useful for some noisy camera to eliminate hot pixels (ex: long exposure webcam, PL1M or QHY5 cameras...).

Running autoguiding

- Run continuous acquisition (GO CCD button on main panel)

- Click & Drag the blue axis to the desired target position

- Double Left Click on the guide star to set the searching area (red square) on the guiding star even if far from the target position; check that the red square follows the star movement (otherwise look at the detection threshold value)

- Activate the Tracking ON

- Monitor the movement of your guiding star and the pulses sent to the mount.

 

Corrections during autoguiding

All those parameters can be modified during autoguiding (no need to stop it to change one of them).
Exemples :

Change the target position during the autoguiding : in spectrography, this allows to move the target position right on the slit; this can be seen when the star almost disapears in the slit and the flux on each side is minimum. Make sure to adjust your camera setting so the guiding star doesn't saturate (a Watec 120N+ video camera is best as you can adjust exposure and gain very precisely). Leave some intensity room if you sky is not too clear.
Direction : if your guiding star moves to the wrong direction, change the direction of the guiding on the corresponding axis. This is the case on german mount when you do a pier side switch.



Tips & Tricks


=> Do not saturate the guiding star

A saturated star won't allow a precise guiding. Shoot for an intensity around 200 if saturation level is 255...

=> Take long exposure to integrate in bad seeing

Ideally 1 to 2 sec exposure time but of course you have to avoid saturation.
If exposure time is too short, best is to cumulate exposures to avoid guiding on turbulences...

=> If your mount is tracking well

Best is to increase the interval between exposures to limit wring guiding movement due to turbulence.

=> Use your mount PEC

After some experiment, we have found the PID integration algorythm was not a good option to keep as it was conflicting with mount PEC. Final result seems better with mount PEC activated.

=> Slightly unbalance your mount (or a small polar misalignment) is best if your mount has some backlash in order to have guiding movement always in the same direction. This is also true in case of manual guiding by the way! Specially in Dec.

=> Note on spectrograph slit guiding

Ratio %/pixel can have a value between 0.5 and 50 ! We haven't found a way to calculate this paramter automatically. It depends on star luminosity, sky background, ratio between those two values and also on the rectangle size...
When this ratio is too low, Dec movements are too slow or even null.
When this ratio is too high, Dec movements are too large and provoques oscillations.