R C O P T I C A L S Y S T E M S

Focus Control Software

RC Optical Systems • 4025 E. Huntington Drive, Suite 105 • Flagstaff, Arizona 86004 • Phone 928-526-5380 • Email - info@rcopticalsystems.com

Focus Control Software

Use the controls on the Focus Tab to control the various functions of the secondary servo focuser.

There are three displayed focus positions: The 'Set Point', and 'Actual' positions display the target and actual servo positions respectively, and are expressed in units of focus counts. Each count corresponds to 1/40,000 inch of secondary mirror movement. The third displayed position is a representation of the offset from ideal mirror spacing, and can be displayed in terms of mirror spacing (Secondary Deviation) or offset at the focal plane (Focal Plane Deviation), and is displayed in units of 0.001 inch. To display the offset from ideal mirror spacing, the ideal mirror spacing must first be determined and set for your telescope. This position is unique for each telescope and is specified in the ideal backspace from the telescope's back plate. To set the ideal focus position the focuser must first be in absolute mode by first homing. Using appropriate spacers, determine the focus count that brings a star into focus at the ideal backspace position. Click the 'Set Ideal Pos' button to store this position in the TCC's non-volatile memory. This position need only be set once. Thereafter, whenever the focuser is in absolute mode the offset from ideal focus position will be displayed. For the TCC to accurately display the offset at the focal plane, the focal plane step size for your telescope must be determined and set. Choose menu item Tools->Options and click the focuser tab. Enter the step size for your telescope if known, or use the provided calculator. See focuser options for more details. See understanding the servo focuser for a more complete description servo control systems.

Pressing the up/down buttons will increase/decrease the focus position by the amount selected to the right of the buttons. Keeping the buttons depressed will cause movement commands to be repeated three times per second for five counts then 20 times per second thereafter, thus allowing large positions changes to be made rapidly.

The TCC has the ability to compensate for temperature dependent focus shift. To setup and use temperature compensation, click the 'Temperature Compensation' button to display the following window:

Before use, the Movement Threshold and Temperature Compensation Coefficient must be set. The temperature compensation coefficient corresponds to the number of focus counts the focus point shifts per degrees F or C. This coefficient can be either positive or negative and must be determined experimentally for your individual setup (telescope, accessories, mounting plate, etc). The movement threshold is the minimum counts that the focuser will move during temperature compensation. If the calculated temperature compensation is less than the set threshold, then this amount is added to the result of the next calculation. The use of a movement threshold will prevent the focuser from constantly adjusting to minor changes in ambient temperature.

Temperature compensation can be disabled or set to run in either automatic or manual mode. In Automatic mode, temperature compensation is calculated every 30 seconds, and an adjustment is made if it exceeds the movement threshold. The temperature used in calculating the temperature compensation is a 5 minute running average of the ambient temperature. This average temperature helps to eliminate the effects of short lived variations in temperature, and more closely matches the temperature time constants of components that contract and expand the optical path. In Manual mode, temperature compensation is calculated only in response to clicking the 'Manual Synch' button. This mode is useful to prevent focuser movement during an exposure.

NOTE: In compliance with the ASCOM standard, focusing is disabled when temperature compensation is active. If the focuser is commanded to move either by the hand controller or via computer an error will be displayed.

To move to a specific location, type the desired position in the text box next to the button then press the button. Absolute positioning is only available after the focuser has either been commanded to seek the HOME position or the TCC started with 'Recall Last Know Focus Position' active.

To move to a previously stored position, choose the entry in the drop down list of saved positions and confirm your intent when prompted. If temperature compensation has been set up for your TCC then you will be prompted to temperature compensate the stored position. Each stored position is paired with the ambient temperature at the time of storage. If the temperature at the time of recall is significantly different, and your telescope exhibits temperature dependant focus shift, then the stored position may not be accurate. If desired, the TCC will compensate for temperature shift that results from the temperature difference. For example, if your telescope exhibits a focus shift of 5 counts/degree and a position is stored with a corresponding temperature of 50 F and it is being recalled when the temperature is 70 F, the TCC will compensate by (70F-50F)*5 (counts/F)=100 counts. Absolute positioning is only available after the focuser has either been commanded to seek the HOME position or the TCC started with 'Recall Last Know Focus Position' active.

To store the current position or edit a previously stored position, press the which will launch the Store Position dialog box. Please refer to Storing and Editing Focus Positions for details of this operation.

Pressing the button will command the TCC to slew to its mechanical home position. The home position is a precise, highly repeatable mechanical stop at the focus actuator's fully retracted position. Upon reaching the home position the position counts are set to 0 then the motor is backed away 20 counts to remove any mechanical strain. Using the home position as a reference point allows repeatable positioning between observing sessions, that is, positioning is Absolute. Once in absolute positioning mode, the TCC can be commanded to move to previously stored positions or to any absolute position by use of the button. NOTE: The TCC needs to be commanded to the home position only once after power up. This command can be issued either via software or through the TCC hand controller. The TCC software will detect the positioning mode (Absolute/Relative) upon start up and enable the absolute positioning options accordingly. If the TCC is started with 'Recall Last Known Position at Startup' enabled, then it will be in absolute mode and homing is not necessary.

Check 'Recall Last Known Position at Startup' to have the TCC automatically save its focus position at power down and recall this position at next power up. This function is only available if the TCC is in absolute mode by Homing or being started from a last known position. The TCC will automatically store a position once it hasn't changed for approximately 3 minutes. When the position is stored the text on the RECORD button will change to

. When the position is changed the stored position is invalidated and the RECORD button will change to

. If the TCC is powered down with an invalidated position it will start up with a position of 0 and will require homing to be in absolute mode. To force the current position to be stored prior to the 3 minute timeout press the RECORD button.

Pressing will stop the motor at its current position. This is useful for aborting a slew to home or to a stored position.

The instantaneous motor current is displayed during a slew and during position maintenance. During normal operation the motor current should read '<0.65' after reaching the desired position. In situations of increased mechanical resistance (such as at the end of secondary actuator travel) increased current may be necessary to maintain the desired position. Prolonged over current can cause heating of the servo motor which will have undesirable optical effect. By default the motor will turn off during these situations. Please see focuser options for details on focuser configuration.

Checking will turn the motor off and disable any motion command being issued from the software. This is useful to prevent inadvertent focus change during imaging. NOTE: Checking will not disable focusing by the TCC hand controller or by any other linked ASCOM session such as an autofocus plugin. The Motor Status is displayed beneath this check box and indicate either ON or OFF. The motor can be turned off either manually by checking or it can be set to turn off automatically when the focus position falls within the deadband range of the Set Point. Excessive position error or over current condition will also result in the motor being turned off. See focuser options to learn how to configure the focuser.

Album last updated on 2010/10/21 15:41:01