Abstract:
A temperature compensating focuser and method for use, with a telescope includes providing a temperature sensor adapted to measuring temperature at the telescope body, a lens support for one of the focus optics and a control. The control includes a motor adjusting the support to modify a distance between the focus optics of the telescope in response to changes in temperature sensed by the temperature sensor. The control may further include a learning mode for establishing a temperature coefficient of the telescope. The control may further include a memory for storing a relative position of the lens support and a temperature value when power is removed from the control. The control adjusts the lens support according to the relative position and temperature values in memory and temperature sensed by the temperature sensor when power is restored to the control.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to a focuser for controlling the focus of a telescope and, more particularly, to a temperature compensating focuser compensating for changes in ambient temperature. 
     Particular telescope configurations, such as the Cassegrain configuration, which include primary and secondary mirrors, are susceptible to temperature drift effect on the back focuser. It is not uncommon to experience up to a 0.010-inch temperature drift for each 0° C. in temperature change. Because telescopes are used during nighttime hours when temperature changes are often extensive, it is possible for the telescope to experience significant focus drift especially during long exposure periods. It is not unusual during an observation session, for the ambient temperature to change by at least 10° C. within the time span of a few hours. Applications which require exact focus, such as CCD imaging or film astrophotography, typically also frequently require long exposure periods. By way of example, a typical RGB exposure sequence can last one hour. The change in focus due to the temperature is a serious problem for these telescopes resulting in frequent focusing before exposures and bloated stars during long exposures. 
     Although Cassegrain-type telescopes are exceptionally sensitive to temperature drift, other telescopes would exhibit such drift to a greater or lesser extent. 
     SUMMARY OF THE INVENTION 
     The present invention provides a temperature compensating focuser and method for use with a telescope having a focus which changes with ambient temperature. In particular, the invention is useful with a telescope having a body supporting a primary focus optic and a secondary focus optic which define a focus and wherein the body changes relative spacing of the focus optics with changes in ambient temperature. 
     A temperature compensating focuser, according to an aspect of the invention, includes a lens support establishing a focus of the telescope and a temperature sensor. The focuser further includes a control that is responsive to the temperature sensor for adjusting the lens support in response to changes in temperature sensed by the temperature sensor. 
     A temperature compensating focuser and method, according to another aspect of the invention, includes a temperature sensor adapted to measuring temperature at the telescope body and a support for one of the focus optics. A motor is provided for adjusting the support to modify a distance between the focus optics. A control is provided which receives input from the temperature sensor and produces an output to the motor to maintain the distance between the focus optics, notwithstanding changes in temperature at the telescope body. 
     A temperature compensating focuser, according to another aspect of the invention, includes a temperature sensor adapted to measuring the temperature at the telescope body and a support for one of the focus optics. A motor is provided for adjusting the support to modify the distance between the focus optics. A control is provided which receives an input from the temperature sensor and produces an output to the motor. The control has a learning mode to establish a temperature coefficient of the telescope. The control further includes a control mode to adjust the motor in response to the temperature sensor and a temperature coefficient determined during the learning mode. 
     According to yet another aspect of the invention, a temperature compensating focuser includes a temperature sensor measuring temperature at the telescope body and a support for one of the focus optics which is moveable with respect to a base. A motor adjusts the support to modify the distance between the focus optics. A control is provided which receives an input from the temperature sensor and produces an output to the motor. The control establishes a relative position of the support with respect to the base and includes a memory for storing a relative position of the support and a temperature value sensed by the temperature sensor when power is removed from the control. The controls adjust the support according to relative position and temperature values and memory and temperature sensed by the temperature sensor when power is restored to the controller. 
     A method of compensating the focus of a telescope includes providing a lens support establishing a focus of the telescope, sensing ambient temperature and adjusting the lens support in response to change in sensed ambient temperature. 
     A method of compensating the focus of a telescope, according to another aspect of the invention, includes measuring temperature at the telescope body, adjustably supporting one of the focus optics in a manner that a distance between the focus optics can be adjusted. The method further includes maintaining the distance between the focus optics notwithstanding changes in temperature at the telescope body. 
     A method of compensating the focus of a telescope, according to another aspect of the invention, includes measuring temperature at the telescope body and adjustably supporting one of the focus optics in a manner that a distance between the focus optics can be adjusted. The method further includes establishing a temperature coefficient of the telescope in a first mode and adjusting the distance between the focus optics during a second mode. According to this aspect of the invention, the adjusting is in response to the temperature at the telescope body and a temperature coefficient determined during the first mode. 
     A method of compensating the focuser of a telescope, according to yet another aspect of the invention, includes measuring temperature at the telescope body and adjustably supporting one of the focus objects in a manner that a distance between the focus objects can be adjusted. The method further includes establishing a distance value between the focus objects that corresponds to a temperature value at the telescope body. The method further includes storing the distance value and the corresponding temperature value and adjusting the distance between the focus objects according to the stored distance value and temperature and a current temperature at the telescope body. 
     Although the present invention is illustrated in use with a Cassegrain-type telescope, it should be understood that its principles may be applied to other telescope configurations that exhibit temperature drift. The present invention may be built into the overall structure of the telescope or, advantageously, provided as a field installable add-on kit to existing telescopes. 
     These and other objects, advantages and features of this invention will become apparent upon review of the following specification in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a Cassegrain configuration telescope including a temperature compensating focuser, according to the invention; 
     FIG. 2 is a perspective view of a temperature compensating focuser, according to the invention; 
     FIG. 3 is a sectional view taken along the lines III—III in FIG. 2; 
     FIG. 4 is a sectional view taken along the lines IV—IV in FIG. 3; 
     FIG. 5 is a block diagram of an electronic control; 
     FIGS. 6 a  and  6   b  is a schematic diagram of the electronic control in FIG. 5; and 
     FIG. 7 is a flowchart of a learn-mode subroutine. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now specifically to the drawings, and the illustrative embodiments depicted therein, a telescope assembly  10  includes a telescope  12  and a temperature compensating focuser  14  (FIG.  1 ). Telescope  12  is a conventional telescope, such as a Cassegrain configuration, having a body which supports a primary optic (not shown). Telescope  12  may be of any configuration. Temperature compensating focuser  14  supports a secondary optic  16  which, in combination with the primary optic, produces an image that may be focused, for example, on a CCD camera. Temperature compensating focus  14  can be used with other imaging devices, such as film and astrophotography, as well as for manual observation. As is conventional, telescope  12  includes a computer control  18  for controlling the position of the telescope. 
     Temperature compensating focuser  14  includes a lens support  20 , an electronic control  22  and a temperature sensor  24  (FIGS.  2  and  5 ). In the illustrated embodiment, temperature sensor  24  is a digital temperature sensor of the type produced by Analog Devices which produce a modulated waveform. However, an analog temperature sensor may, alternatively, be utilized. In the illustrated embodiment, temperature sensor  24  is mounted to a temperature buffer  26  which prevents rapid temperature fluctuations from affecting temperature sensor  24 . In the illustrated embodiment, temperature buffer  26  is a foam pad with adhesive backing which adheres to the body  28  of the telescope. Preferably, the temperature buffer is a ¼-inch foam pad. Alternatively, temperature sensor  24  could be mounted in a hole drilled in body  28 , especially if the temperature compensating focuser is permanently mounted to the telescope. 
     Lens support  20  includes a base  30  and a draw tube  32  that is guided in and out with respect to base  30  on a set of roller bearings  34  and a shaft  36  which is frictionally coupled to draw tube  32 . Lens support  20 , as previously described, utilizes many of the principles of a Crayford focuser, which is conventionally used to adjust the position of the secondary optic with respect to the primary optic to focus a telescope. Shaft  36  is driven by a motor  38  which, in the illustrated embodiment, is a stepper motor. However, other types of motors, such as synchronous motors, motors with encoders, and the like, may be utilized. Motor  38  drives shaft  36  through a gear set  40  which has a gear reduction of 50:1. In the illustrated embodiment, each step from motor  28  moves draw tube  32  0.000085 inches. The motor and drive gears are protected in a motor housing  42 . Draw tube  32  moves a total distance of 0.6 inches with respect to base  30 . Draw tube  32  has a home position, illustrated in FIG. 4, at which motor  38  will stall when the draw tube reaches that position. When draw tube  32  reaches the home position, the control  22  sets the zero, or home, position at that point. 
     Electronic control  22  includes a microcontroller  44  which supplies outputs to a stepper motor controller  46  which, in turn, supplies signals to drive stepper motor  38 . Microcontroller  44  also supplies outputs to a display  48  and receives inputs from a mode switch  50 , a learn switch  52  and a learn button  54 . Microcontroller  44  additionally receives inputs from an IN button  56  and an OUT button  58 . Microcontroller  44  also supplies outputs to an IN indicator  60  and an OUT indicator  62 . Microcontroller  44  additionally exchanges data with an erasable programmable memory (EEPROM)  64 . Microcontroller  44  additionally receives inputs from an external PC input  66 . In the illustrated embodiment, microcontroller  44  is a microcomputer-based processor marketed by PIC Microchip under Model PIC16C63. A computer program for the microcontroller is written in the Basic language and compiled to the code for that microcontroller. 
     When mode switch  50  is set to the Manual mode, depressing of button  56  will cause controller  44  to issue commands to move the draw tube  32  in one direction while actuating of OUT button  58  will cause the controller to issue commands to move the draw tube in the opposite direction. This provides manual focusing of the telescope lens by operating buttons  56 ,  58 , while the operator views the image observed by the telescope until the image is perceived to be in focus. Alternatively, manual focus may be determined by taking multiple exposures with a camera as would be apparent to the skilled artisan. Display  48  displays the position of draw tube  32  with respect to its home position. In the illustrated embodiment, each step displayed by display  48  represents relative position of draw tube  32  in increments of 0.000085 of an inch. 
     Temperature compensating focuser  14  includes the ability to determine a temperature coefficient for the telescope  12  that the focuser is combined with. This is accomplished by placing learn switch  52  in the learn mode and placing the mode switch in the mode A or B that it is desired to calculate a temperature coefficient for. After the switches are placed to the appropriate mode, the telescope is focused and a significant change in temperature is awaited. In the illustrated embodiment, a temperature change of at least approximately 5° C. is chosen, although other temperature differences may be utilized. The greater the change in temperature, the more accurate the calculation of the temperature coefficient. After the temperature change, the telescope is then, again, focused utilizing buttons  56 ,  58 . The learn button  54  is then actuated. Microcontroller  44  calculates a temperature coefficient as the linear slope between the two positions of the draw tube and the two measured temperatures corresponding to those points. The linear slope establishes the change in length of the separation between the primary and secondary optics resulting from expansion of body  28  for each degree C. change of temperature at body  28 . Advantageously, temperature compensating focus  14  allows two such temperature coefficients to be determined. This allows the device to be moved from one telescope to the other or to be used with different setups for the same telescope. However, the invention comprehends that only one temperature coefficient could be determined or more than two temperature coefficients could be determined according to the particular application. 
     Once a temperature coefficient is determined, the user can set up the telescope and establish a focus as previously described. The learn switch  52  is then placed to the run position and the mode switch is placed to the auto-A or auto-B position depending upon the location at which the corresponding temperature coefficient is stored. Temperature compensating focuser  14  will then adjust the position of draw tube  32  according to the changes in ambient temperature as measured by temperature sensor  24  and utilizing the previously determined temperature coefficient. 
     A routine  70  for determining a temperature coefficient begins at  72  by determining that the learn-step parameter is set to 0 and at  72  that the digital readout unit is actuated at  73 . The learn-step parameter is set to 1 at  74  and it is determined at  76  whether the learn-step parameter is set to 1 and that the learn button  54  is actuated. If so, then the learn-step parameter is set to 2 at  77  and the position-start parameter is set to the current position of the draw tube at  78 . The value of the current temperature is retrieved at  80  and the temperature-start parameter is set to the current temperature at  82 . 
     After it is confirmed at  84  that the learn-step parameter is set to 2, the routine determines at  86  the difference between the current temperature and the temperature-start parameter. The temperature difference is displayed at  88  on the digital readout display  48 . 
     It is then determined at  90  whether the learn button  54  is actuated. If so, then the difference between the current position of the draw tube and the start position of the draw tube is determined at  92 . The temperature difference between the current temperature and the start-temperature is determined at  94 . A temperature coefficient is determined at  96  at the difference in position divided by the difference in temperature. 
     If it is determined at  98  that mode switch  50  is set to auto-A, then the calculated temperature coefficient is stored at  100  in the A memory location. If it is determined at  102  that the mode switch  50  is in the auto-B position, then the calculated temperature coefficient is stored at  104  in the memory B location. The temperature coefficient is ready for use in adjusting draw tube  32  in response to changes in the temperature at body  28  as sensed by temperature sensor  24 . 
     When a power switch  49  for control  22  is placed in the OFF position, there is a momentary delay as the microcontroller  44  writes the current position and temperature of temperature compensating focuser  14  to memory  64 . When power switch  49  is turned to the ON position, the microcontroller drives the draw tube  32  to the home position and retrieves the last position and last temperature before it is shut down from the memory  64 . The focuser will then go to that position taking into account the difference between the last temperature and the current temperature. This feature allows the temperature compensating focuser to be powered down when not in use and powered up, without the necessity to refocus the telescope, and taking into account the temperature of the telescope when the unit is powered up. 
     External computer input  66  provides an interface with a computer such as an IBM-compatible personal computer. Input  66  includes four lines: an IN line, an OUT line, a PC line and a COM line. When the IN line is actuated, the effect is the same as operating push button  56 . When the OUT line is actuated, the effect is the same as operating push button  58 . This allows a computer  68  to focus the lens support by supplying pulses to electronic control  22  that are the equivalent of operation of the IN or OUT push buttons  56 ,  58 . Alternatively, the computer may operate the electronic control by supplying a pulse between the PC input and the COM input. Microcontroller  44  will respond to the width of the pulse by making adjustments to the position of the draw tube that are proportional to the width of the pulse. The PC would, concurrently, actuate either the IN or OUT inputs in order to indicate to the microcontroller the direction that the signal on the PC input is intended to effect. This second mode offers greater versatility and control of the temperature compensating focuser. 
     Operation of the external PC input is best understood by considering an IBM PC-compatible computer  68 , such as one having a Pentium processor, or better, connected with external PC input  66 . The personal computer is preferably programmed with imaging software  69 , such as the MAXIM DL 2.0 CCD imaging software from Cyanogen Productions, Inc. Such imaging software has the capability to determine whether the captured image is in focus by evaluating the centroid of the image. Utilizing such imaging software, the personal computer can issue appropriate commands to external PC input  66  in order to adjust the position of the draw tube in order to thereby move the secondary mirror with respect to the primary mirror. When the imaging software obtains a best fit focus, adjustments of draw tube  32  to compensate for changes in ambient temperature are carried out in a manner previously described. 
     The present invention provides the ability to compensate for changes to the focus of a telescope, or other instrument, based upon changes in ambient temperature without the necessity for complex algorithms to calculate the focus of the telescope. This is accomplished because the temperature compensating focuser only determines the change in focus created by the change in temperature and does not require knowledge of the actual focus per se. The present invention provides a convenient and rugged technique for compensating for temperature changes to the focus of telescopes of a wide range of designs including homemade telescopes, amateur telescopes and high-quality professional telescopes. Furthermore, the present invention may be provided as a field-installed unit and thereby be capable of movement from one telescope to another telescope. This aspect is enhanced by the ability to calculate and store multiple temperature coefficients. Furthermore, the present invention provides a unique shutdown mode wherein, upon power-up, the focus of the telescope is re-established without the necessity for intervention and taking into account the ambient temperature at the time of power-up. 
     Changes and modifications in the specifically described embodiments can be carried out without departing from the principles of the invention which is intended to be limited only by the scope of the appended claims, as interpreted according to the principles of patent law including the doctrine of equivalents.