WHT Autoguider version 4.1.29 Release note

Autoguider release 4.1.29 ( UES centroid improvements )

Title and Release number

UES slit centroid compatibility mode version.
Release 4.1.29

Reason for release

The previous releases have had the centroid algorithm improved such that the accuracy of centroid and thus gain in the position to error signal transformation has increased. This has lead to over guiding on the UES slit window. Version 4.1.28 incorporated a modification to allow comparisons between an old algorithm and the latest one. The older algorithm was found to suit the slit window best, and was requested to be incorporated as a permanent feature of the system. This release provides that functionality.

Release History

Release note summary
4.1.26: Latest centroid algorithm with local sky estimation. UES slit has simple centroid in the Y direction, without star extraction.
4.1.27: System beeps when going into suspended mode.
4.1.28: old v4.0.2d centroid added for experiment

Changes in this release

The centroid algorithm used with the AF2 version used an algorithm which extracted a small box from the readout window and extracted the star round the peak in that window, before calculating the centroid. This was found not to work effectively with the UES slit, following the A&G upgrade. Version 4.1.28 added the centroid algorithm from V4.0.2d as a temporary addition to the code.

This version (4.1.29) includes this old algorithm, coded to be compatible with the rest of the system methodology. This allows both algorithms to be available to the operator, so that the best one can be chosen for the current conditions.

The old algorithm simply takes the whole readout window, thresholds it with a threshold of 20% of peak pixel value and centroids those samples above this level. The algorithm takes no account of local sky level or system noise in the centroid calculation. Ramps in the stellar profile can be removed simply by taking the two end samples and using a straight line between then as the base line for the centroid calculation. This ramp removal feature can be enabled or disabled. It is disabled by default.

The old version is affected by unusual sky profiles, especially at low signal to noise levels. The most prominent defect is that the local sky level is not used as the base line for the centroid calculation. The effect of this is to dilute the area of the stellar profile under the star with the area under the noise floor over the whole width of the window. The observed effect is that the centroid position is always biased towards the centre of the readout window. This leads to a loss of loop gain in the autoguider-TCS-telescope control system loop.

The loop gain affects the overall system operation and the later algorithms provide more accurate centroids with more effective gain, thus the problem of over guiding can be a problem with the later systems. for a given TCS loop gain, this old centroid algorithm was found to match the system better than the new algorithm. This is primarily why it was requested to be included.

Operation Instructions

The new centroid mode is enabled with the SEL-CEN-TYPE command, available when the ENGINEER vocabulary is enabled.The SEL-CEN-TYPE command gives the operator a menu from which the appropriate option can be chosen. The current options are: 0 Version 4.1.28 (latest) algorithm 1 old centroid algorithm - raw centre of gravity. 2 old centroid algorithm - 20% peak threshold then centre of gravity. 3 old centroid algorithm - 20% threshold,ramp removal & centre of gravity.

The threshold point can be adjusted with the SEL-CEN-THR command which prompts for the % of peak value to be used.

Bugs fixed

None.

Known problems and restrictions

The old algorithms are NOT enabled by default. The old algorithms cause the centroid to be biased towards the centre of the window.

Installation instructions

Backing up the previous version

Copy the running version to the released place on disk with QUANTUM BULK QUANTUM
source start 1000
dest start 4000
destination 11000

Release Via FTP

Take a blank PC 3.5" floppy disk. Copy the release file from the directory on the workstation to a single PC file on the disk with
cd ~/ag/releases
mcopy rel4129.dat /a: .
Put the disk in the floptical drive.
run AG-RESTORE with option 7.

Release via Floptical

Put the disk into the AG system floptical drive. Copy the floptical onto the hard disk with
AG-RESTORE option 6.


16/12/94 A B Gentles

abg@ast.cam.ac.uk