TVGuider astronomical telescope autoguider

Transcription

1 TVGuider astronomical telescope autoguider Components Sturdy aluminium case containing the electronics Sensitive black and white camera All connection cables Basic function A star image is captured with a highly sensitive black and white video camera and displayed on the LCD screen. The autoguider checks the position of a guidestar in real time and any deviation from the cross hair position results in a correction being sent to the telescope mount. The LCD monitor can be turned off to save power after the star has been centered and focused. Figure 1. The figure depicts the user-friendly menu layout of the LCD monitor. Individual menu items are set by pressing the up and down buttons, which then mark the active menu item with a > symbol. Their values are changed by pressing left and right buttons. By pressing the centre button the values are stored and remain in non-volatile memory even if the instrument is turned off. There are two ways of communicating with telescope mount: 1) MOUNT A.G. output standard autoguider output is compatible with most modern mounts. Connector has 4 outputs for direction RA+/- and DE+/-, which uses the standard ST4 type communication with a common earth. The maximum allowed current is 180mA. Output utilizes an optocoupler to protect the mount. This type of connection was tested with the following mounts:- Meade LX200, CG5 GT, Losmandy G11, EQ6 Skyscan and should work with other types which span positive current (log 1) against common earth and active levels (when providing correction) in making contact log 0. If there are any doubts about compatibility with other mount, contact the author. 2) COM Output The current version of the firmware communicates with the mount using the LX200 protocol. The serial cable sends the following LX200 protocol commands: Activating correction in the direction of East, West, North, South: Me# :Mw# :Mn# :Ms# Closing correction in the direction of East, West, North, South: Qe# :Qw# :Qn# :Qs# Switching off all corrections: Q# Communication takes place at 9.6kbps, 8 data bits, 1 stopbit without parity, at +/-12V (standard RS232).

2 Therefore the autoguider should be suitable for use with all mounts which support the basic LX200 protocol. To make a cable for connecting a specific mount please feel free to consult with me. There is also a possibility to write firmware for specific mounts which behave differently; again please contact the author. Screen menu description: RAT (RATIO, 1-10) This setting determines the ratio between deviation of the guidestar and length of correcting impulse. The higher the RAT value the longer the pulse with which the autoguider reacts to the deviation. The best ratio is set experimentally in conjunction with the guide-speed of your mount. One option is to set the parameters to NO, in which case the system behaves like simple positional feedback. If the deviation of the guide-star is greater than TOL (Tolerance of position) then the correction (in the correct direction) is sent and continues while star is outside the tolerance limit. This mode (with RAT parameter set to NO) is suitable only if reaction time of TVGuider is high (i.e. if the AVG parameter set up to 6-8 frames in maximum). If you want use higher number of averaged frames (on good mount with precise tracking, or for eliminating bad seeing), you should use RAT parameter set to suitable value for your mount. If the ratio is set too high and the correction overshoots the correct position, then decrease this setting. AVG number of frames to average (1-50). When star visibility or seeing is poor, it helps you to get stable star coordinates. Increasing the AVG value will lead to a stabilizing of the star s position. This also makes it possible to reach sub pixel accuracy of the star position. It is recommended to use an average of at least 5 frames which will result in a resolution of around 0.2 pixels. This will not greatly affect the speed of the correction response (calculation and response is 5x/sec in this case). If you need use higher value than 8, do not forget that you must use a properly set RAT parameter (see above). Note: Averaging is done in TVGuider memory only. It does not affect visual appearance of star image on TV monitor. TOL area of non responsiveness (0.1-10pixels). A correction signal is sent only if the guidestar moves away from the centre of the crosshairs by more pixels than the set TOL parameter. With the WAT902 H3 camera, the 5.6um pixel size of the CCD will effectively result in a star occupying a 1 single pixel at the focal plane of the guidescope. A star may be larger than one pixel without affecting the accuracy as the instrument as the software determines the theoretical center of the star (circle/disc) and displays the value in the STAR function (this function is also suitable for assisting with focusing). A general setting for the guidestar selection is around 2-6 pixels. A TOL parameter of pixel is recommended when using a guidescope of around 1 meter focal length, depending on atmospheric conditions. If conditions are good or you use very short guidescope, TOL value can be tightened down to pixel. You will have to established the optimal value for your setup and the prevailing conditions. LVL % sets the brightness of star which the TVGuider automatically detects; stars with lower value are ignored. This parameter is useful when there are number of stars in the vicinity of the cross or if there is interference (snowing) due to camera noise. By pressing the LEVEL CHECK button it is possible to determine which stars have brightness greater than the actual LVL value, or if the instrument is suffering from hot pixels or camera noise. Areas

3 of higher values are colored white upon pressing the button. If you see snowing in the LCD image, increase the LVL value until the interference disappears. BOX sets the frame size area around the crosshairs to contain the guidestar. This parameter can be varied from pixels. Stars outside this area are ignored. If there is more than one star in the guide star box, the instrument determines the average position of all the stars in the area. In this situation, the autoguider works by picking a point between the stars. It is possible to move the crosshair on screen to avoid bright (or hot) pixels which are sometimes visible with a very sensitive camera. To move the cross press and hold CROSS CONTROL followed by the relevant movement buttons. It is possible to synchronize the actual star position by pressing the CENTER button while simultaneously hold the CROSS CONTROL (a useful feature when taking photos with a small CCD chip camera, where movement of the telescope scope instead of moving the crosshair will ruin image composition). LCD sets the brightness of LCD monitor. The section marked STAR displays the size of star (or the NO symbol if there is no detectable star present in crosshair box) as well as its deviation from the centre of the crosshair. The GUIDING section is used to set the following: Mode of pointing: STOPPED stopped, corrections are not transmitted RA+DE corrections for both axes sent simultaneously to the mount RA only corrections for RA only, NO corrections in DEC RA/DE corrections for RA and DE are sent sequentially (some mounts can not move in both directions at once) RA: RA dir forward, RA rev. reverse corrections in RA DE: DE dir forward, DE rev. reverse corrections in Dec If the star moves away from the cross instead of being centered, set that particular direction to REVERSE. BP on beeps when star is lost, BP off does not beep when guide star is lost. OTHER control buttons: POWER Switches power ON or OFF to the whole unit LCD ON/OFF Short press turns screen text on/off if you wish to use the LCD as TV monitor. Long press (1sec) turns the monitor off to save power. Autoguiding continues to function and issues commands as evident by red LED blinking in the direction of correction/movement. A short press turns the monitor back on.

4 CROSS CONTROL while pressing this button, you can use crosshair push-buttons to re-position the crosshair displayed on the LCD screen onto the selected guide star, see above. LEVEL CHECK assists with setting-up or selecting appropriate guide star. Pressing down this push-button will result in all stars with a brightness greater than the set level being displayed in full WHITE. Setting the appropriate parameter LVL will assist in the ensuring that the guide star is bright enough and reliably detected. Description of connections: AUTOGUIDER OUTPUT TO MOUNT PIN FUNCTION 1 Not connected (can be 5V or 12V connection on request) 2 GND - Common earth 3 RA WEST 4 DE SOUTH 5 DE NORTH 6 RA EAST (galvanically isolated outputs, 180mA max.) RS232 COM PORT PIN FUNCTION 1 Not connected (can be 12V connection on request) 2 GND - earth 3 Not connected 4 TxD (connect to RxD PC or mount controlled by LX200 protocol) 5 RxD (connect to TxD PC or mount controlled by LX200 protocol) 6 Not connected

5 CAMERA POWER AND VIDEO INPUT PIN FUNCTION 1 Not connected 2 12V 3 TXD (for future RS232 control of camera, not used yet) 4 GND - earth 5 Video signal from camera 6 Not connected CINCH CONNECTORS - VIDEO IN AND OUT VIDEO IN is connected in parallel to pin 5 on RJ camera connector (see table above) You can use it to connect your own camera, which is powered separately. (GND is on shielding, video signal on center pin) VIDEO OUT can be used for an external TV monitor. View is identical with that shown on the LCD monitor. 12V POWER CONNECTORS (2x) These sockets are used to power the TVGuider unit. They are connected to parallel, so you can use second socket for powering of anything else. NOTE! Be careful about accidental interchange of MOUNT A.G. output and COM output. There are +/-12V levels on COM output. If you incorrectly connect to this output/input on your mount, you can damage electronic of mount and/or TVGguider. If unsure ask the author for correct settings for your mount and guider/camera. Tips & tricks for easy use: 1. Device is very accurate (on good-class mount of course). It is possible to guide a 1m focal length scope with 0.5 to 1m guidescope with sharp image and low aberrations (eg. ED refractor), which will also maximize the magnitude of stars detected. Use at least a 50-60mm diameter guidescope, but mm is better. The guidescope should be movable with respect to imaging scope using either adjustable guide rings, or X-Y shifting in the camera focal plane. 2. Finding a guide star insert an eyepiece into guidescope and roughly center a suitable star. Switch on TVGuider and insert the camera instead of eyepiece and focus the star image. Parfocalizing the camera with an eyepiece will make finding, framing and focusing much easier. Move the guidestar into crosshair area (or crosshair to star position) and set-up LVL value on TVGuider so that the star is reliably detected (star size about 2-6pixels) and green LED shines permanently. Rotate camera angle in the eyepiece holder where RA movements appear horizontal and DEC movements are vertical on screen (+/-10 o is an acceptable tolerance). Set the AVG parameter to 5-8 frames. 3. Set guide rate of your mount to about x sidereal rate for a guidescope with approximately 1m focal length. Start GUIDING by setting up one of RA+DE, RA/DE, RA options in the guiding section of the menu. If the star moves away from the crosshair instead of being centered, set that particular direction to REVERSE.

6 4. Now you should have TVGuider controlling your scope. The aim is to optimize guiding corrections to maximum precision. At first, try to set an optimal guiding speed. Look at the issued corrections, if there is overshooting of the desired position, decrease the guiding speed or RAT parameter (if it is used). Then try to optimize tolerance parameter. Its optimal value depends on actual seeing state and focal length. On a night with good seeing, set a TOL value, which does not cause chaotic correction in all directions. When this value is optimized the TVGuider sends corrections in RA direction with a frequency dependent on mount quality (Periodic error). This is usually in one direction for any period, and in reversed direction for rest of worm period. Correction in DEC should be less frequent than in RA, if the mount has been properly polar aligned. DEC corrections should predominate in one direction direction of remaining DE drift. 5. If star coordinates displayed in the menu vary chaotically, there is something wrong. Check LVL parameter. If a low value is set then the TVGuider can detect electronic noise (check this by holding LEVEL CHECK button, only star must brighten, and no snowing must be present in image). In this case, increase LVL parameter. If detection of star becomes unstable (small star size or NO displayed instead of size and green LED is blinking) try to find brighter star. If all is properly set, coordinates are relatively stable, changing within tenths of pixel to slightly over one pixel in worse conditions. 6. You can store final set-up into flash memory of TVGuider by pressing the center button in button cross on right hand side of the case. All parameters are stored permanently even if you switch the device off. Power requirements Device is designed for 12V DC power supply, the same as for most video TV cameras. Current consumption depends on device mode and camera type. TVGuider with LCD switched ON draws about 390mA, with LCD OFF 140mA only. Camera draws about mA, depends on type (about 150mA with WAT902-H3). Use a stabilized power supply or 12V accumulator (in case of field operation). Be aware of cheap non-stabilized adapters as they can have about 18V without load and can destroy TVGuider device and camera. When powered from car (or separate) battery together with the mount drive, the video camera can be interfered by pulsed voltage drop on the leads to the mount. Use a cable with sufficient diameter or separate cabling for TVGuider in this case. Do not use diverging or dual power adaptor at the end of common cable if the mount is driven by large stepper motors. Some people have noticed CCD camera interference with stepper motor driven through a ground loop (ground of mount connected through metal scope tube to metal flange of camera). In this case I can solve this interference by the use of a plastic C-mount camera barrel (I can supply it with camera), or electrically isolate the scope from the mount. Firmware upgrade: Device is designed so firmware can be updated via COM port and special PC application. In case of new firmware, I ll inform you with instructions on how connect the TVGuider device to a PC and how perform the upgrade. More information available on request.

7 Device reset: In some cases, values stored in flash set-up can be damaged and wrong data displayed on the LCD screen. In this case you can perform a reset to factory-default settings as follows. With unit switched off, press center button of cross and turn power unit on. Default parameters will be re-loaded. You can change parameters to values you have previously determined and store them by pressing the center button. Technical support: In case of problems or questions you can contact me (In English) by mail: martin.myslivec@tiscali.cz Thank you for purchase of TVGuider. Have a clear skies and perfect images. Martin Myslivec Milady Horakove Hradec Kralove Czech Republic