Download TVGuider – astronomical telescope autoguider

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).
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 0.5-1 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 0.2-0.5 pixel. You will have to established the optimal value for your setup
and the prevailing conditions.
LVL - 0-100% 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
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 10-200pixels. 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.
·
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
(galvanically isolated outputs, 180mA
max.)
5
DE – NORTH
6
RA – EAST
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
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 70-100mm 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 (+/-10o is an acceptable tolerance). Set the AVG parameter to 5-8
frames.
3. Set guide rate of your mount to about 0.25-0.5x 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.
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 100400mA, 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.
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:
[email protected] Thank you for purchase of TVGuider. Have a clear skies and
perfect images.
Martin Myslivec
Milady Horakove 1738
500 06 Hradec Kralove
Czech Republic