Tip-Tilt Autoguider Commands

The f/70 Tip-Tilt system is an alternative to the Shepherd Autoguider. As described earlier, the Mirella software running on SAM is used to control the r-theta offset guider mechanism. A PDP-11 computer (MINC) runs the software that controls the guiding for the Tip-Tilt Autoguider. The MINC terminal is located to the left of the Spectrocam 10 workstation, usually near the SAM terminal.

From Tom Murphy's PIFS manual:

If using the f/70 tip-tilt guider controller (the MINC) rather than the Palomar facility guider, the guiding workload is higher, but with potentially sharper images / spectra. This section is by no means complete, and may be the only documentation ever attempted for this system. Only Keith Matthews, who built the f/70 tip-tilt secondary and wrote the control software, knows this system cold.

Basic Setup and Operation

If you know nothing else about the guider, at least know the GO command. This activates guiding. How is guiding stopped? By hitting <RETURN>. The GO command has three bits to toggle. GO7 runs the guider with all three options turned on: (1) Basic guiding (called auto-guiding), (2) Tip-tilt guiding, and (4) the graphical stripchart. Simply multiply the numbers in parentheses above by 1 (on) or 0 (off) and add for functionality. Thus, GO5 runs the guider without the tip-tilt. GO4 turns the cursor on and activates the stripchart without guiding or tip-tilt (useful for viewing the guide cursor without moving anything).

Aside from the GO command, there are a number of parameters to set. This is not done in stack-style, but in a more conventional command argument syntax. Entering the command with no argument returns the current value, with no change made; a convenient way of checking the current parameter values.

Be aware that this system was created without concern for robustness or user-friendliness. Therefore, if the wrong thing is typed, "The whole world may blow up and you'll be left with nothing." --- K. Matthews

The Commands

GOi i is a bitmask, with 1=guiding, 2=tip-tilt, and 4=stripchart. See the explanation above if confused. To stop guiding, hit <RETURN>.

Examples:

GO1     Guide without stripchart and without tip-tilt.

GO5     Guide with stripchart, but without tip-tilt.

GO7     Guide with tip-tilt and stripchart.

GO If no number is provided, the last value is used.

HEAD [] Angle of guide head. Must match actual angle of guide head within 10 degrees for proper guiding. To get correct value, type og in MIRELLA to find out.

F70A [] Angle of the current secondary chop direction. Should match the actual angle within 10 degrees for proper guiding. Look at numeric LED display near bottom of f/70 rack for appropriate angle. Remember to change this parameter if the secondary is moved.

BOX [n] Set guide box to n pixels across. Usually n = 4 is good, though bad seeing calls for a bigger box (n = 5) and good seeing will benefit from a smaller box (n = 3). I have seen conditions good enough to use a 2 pixel box, but that's very rare. BE AWARE that the smaller the box, the faster the computation, therefore the faster the guiding feedback. The rate-controllers below (or "gains") are therefore affected by the box size, and are not invariant in their feedback rate.

TVLL [x] TV Low Level control. Default is 0.1. Keith likes 0.03 - 0.07. This sets the sky background level above which to look for the guide star. If the guider is not working, perhaps you need to lower this value in order to "see" the star.

AGG [i] Auto Guiding Gain. This sets how fast the system issues telescope move commands in response to excursions from the box center. High number is fast, low number is slow. Good conditions are suited by a value for i (an integer) of 5 - 10. If the star is faint, lower this number to the 1 - 4 range. Keep in mind that box size also affects the response time.

F70G [i] The f/70 Gain. This sets how fast the secondary tries to track image wander in response to excursions. For faint stars or poor seeing, use a low value for i (an integer), from 1 - 4. For decent conditions with a reasonable guide star, set this to 5 - 10. For the best of conditions (ie. 9th magnitude guide star, 0.5" seeing) this value may go up to 15 or so. Keep in mind that box size also affects the response time. When changing F70G, adjust F70D accordingly.

F70D [i] The f/70 Damping. For low values of F70G (< 5), this parameter doesn't do much. For these cases, use a value of i = 0. For higher values of F70G, use a value for i that is maybe 70% of the F70G value. Or set i to be smaller than F70G by about 3 - 4. Only Keith will get it right.

HELP Display a list of all of the commands used at the MINC.

Normal Operation

Normally, when going to a new guide star, do the following things in roughly the suggested order. Remember that to check the values of parameters, simply type in the parameter name with no argument.

Retrieve HEAD angle for guide head (from MIRELLA og command) and set.
Check that F70A is properly set. Only changes when secondary chop angle is changed.
Verify (visually on guider TV) that box size is reasonable for the current conditions.
Check the values of AGG, F70G, and F70D for reasonable settings.

Now type GO, or some variant thereof, to start guiding.

Be aware that if issuing a large mtg command, the guide star's position angle relative to the new field may significantly differ from the old position angle. This means that the HEAD angle has changed. Be sure to check the head angle after large moves, and reset if necessary.

Note that when you stop guiding with a <RETURN>, the tip-tilt mirror gets flattened (X and Y = 0). If for some reason you want to pause guiding without flattening the tip-tilt mirror, issue an agoff command in the Mirella terminal (or Xterm). To restart, issue an agon command in the same terminal. The tip-tilt range is defined as +/- 2.5" * box size / 5.

That Wacky Stripchart

The graphical stripchart displayed on the guide computer screen is actually a helpful tool. To first order it signals whether or not guiding is going on. If points are being added to the right, and the dots are scrolling by, the guider is doing its thing (at least it thinks it is).

The shape of the stripchart can indicate at a glance the stability of the atmospheric conditions. A level stripchart chugging along indicates a stable sky; quite possibly photometric. There are two points plotted for each time step. The top point is the total flux inside the box, with some weighting unbeknownst to me. The lower point is the sum of the flux in the central four pixels. So under photometric conditions with variable seeing, the spacing between these two points will reflect the seeing fluctuations. Under less than ideal conditions, or with a faint guide star, the chart may be all over the place to the point that distinguishing two separate stripcharts is impossible.

Abnormal Operation

If the guide star is pushed out of the box, or appears to orbit the box, the angle information is wrong (by ~ 180° in the first case, and ~ 90° in the second case). Check the HEAD and F70Avalues.

If you accidentaly hit the BREAK key on the MINC keyboard, as I have done plenty of times, the familiar $ prompt will disappear, and a scary @ prompt appears. Hit the P key, the <RETURN> to get back to the $ prompt (of course!).

If the system goes totally haywire, go over to the MINC computer (far left), and follow the instructions taped to the top of the box. The few times I've had to do this, it has worked well:

Shut off the power to the PDP-11 computer.
Close both RX02 doors on the PDP-11 computer.
Turn on the power to the PDP-11 computer.
Once the computer has booted up, you should see the following prompts:

28
START?

Type:

Y <RETURN>
Type:

$ RMACF CU
Continue with:

$ /CU (this sets the cursor to the proper place).
Open both RX02 doors. The MINC should be ready for the checkout procedure, and if all is well, it should be ready for Tip-Tilt Autoguiding.

Moving the Cursor

First and foremost, Keith Matthews has warned that nobody should ever move the cursor. In practice, if you have setup the offset guider correctly, you will not likely have to do this. However, there may exist an unknown offset between the Caltech IR instruments and Spectrocam 10, in which case you might have to tweak the guide cursor slightly.

BE AWARE that you can lose the guide cursor easily if you type in the wrong thing. Be careful.

CUX This will return the current X position of the guide cursor. Note this value before you try to move anything.

CUY This will return the current Y position of the guide cursor. Note this value before you try to move anything.

CUX [i] This will move the guide cursor i pixels in the X direction. BE CAREFUL; move in small steps (like 1 pixel at a time!). Larger values move the box left, smaller values move the box to the right.

CUY [i] This will move the guide cursor i pixels in the Y direction. BE CAREFUL; move in small steps (like 1 pixel at a time!). Larger values move the box down, smaller values move the box up.

/CU This will reset the cursor to the default values, which should center the cursor in the middle of the hexagon guider field. Use this to get your guider cursor back if you accidentally lose it. You should restart the MINC (see above) if you are hopelessly lost.

GOi	i is a bitmask, with 1=guiding, 2=tip-tilt, and 4=stripchart. See the explanation above if confused. To stop guiding, hit <RETURN>. Examples: GO1 Guide without stripchart and without tip-tilt. GO5 Guide with stripchart, but without tip-tilt. GO7 Guide with tip-tilt and stripchart.
GO	If no number is provided, the last value is used.
HEAD []	Angle of guide head. Must match actual angle of guide head within 10 degrees for proper guiding. To get correct value, type og in MIRELLA to find out.
F70A []	Angle of the current secondary chop direction. Should match the actual angle within 10 degrees for proper guiding. Look at numeric LED display near bottom of f/70 rack for appropriate angle. Remember to change this parameter if the secondary is moved.
BOX [n]	Set guide box to n pixels across. Usually n = 4 is good, though bad seeing calls for a bigger box (n = 5) and good seeing will benefit from a smaller box (n = 3). I have seen conditions good enough to use a 2 pixel box, but that's very rare. BE AWARE that the smaller the box, the faster the computation, therefore the faster the guiding feedback. The rate-controllers below (or "gains") are therefore affected by the box size, and are not invariant in their feedback rate.
TVLL [x]	TV Low Level control. Default is 0.1. Keith likes 0.03 - 0.07. This sets the sky background level above which to look for the guide star. If the guider is not working, perhaps you need to lower this value in order to "see" the star.
AGG [i]	Auto Guiding Gain. This sets how fast the system issues telescope move commands in response to excursions from the box center. High number is fast, low number is slow. Good conditions are suited by a value for i (an integer) of 5 - 10. If the star is faint, lower this number to the 1 - 4 range. Keep in mind that box size also affects the response time.
F70G [i]	The f/70 Gain. This sets how fast the secondary tries to track image wander in response to excursions. For faint stars or poor seeing, use a low value for i (an integer), from 1 - 4. For decent conditions with a reasonable guide star, set this to 5 - 10. For the best of conditions (ie. 9th magnitude guide star, 0.5" seeing) this value may go up to 15 or so. Keep in mind that box size also affects the response time. When changing F70G, adjust F70D accordingly.
F70D [i]	The f/70 Damping. For low values of F70G (< 5), this parameter doesn't do much. For these cases, use a value of i = 0. For higher values of F70G, use a value for i that is maybe 70% of the F70G value. Or set i to be smaller than F70G by about 3 - 4. Only Keith will get it right.
HELP	Display a list of all of the commands used at the MINC.

CUX	This will return the current X position of the guide cursor. Note this value before you try to move anything.
CUY	This will return the current Y position of the guide cursor. Note this value before you try to move anything.
CUX [i]	This will move the guide cursor i pixels in the X direction. BE CAREFUL; move in small steps (like 1 pixel at a time!). Larger values move the box left, smaller values move the box to the right.
CUY [i]	This will move the guide cursor i pixels in the Y direction. BE CAREFUL; move in small steps (like 1 pixel at a time!). Larger values move the box down, smaller values move the box up.
/CU	This will reset the cursor to the default values, which should center the cursor in the middle of the hexagon guider field. Use this to get your guider cursor back if you accidentally lose it. You should restart the MINC (see above) if you are hopelessly lost.