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IR GUIDING -(JTR 05 Dec 2002 )
ON POINT-SOURCE IN SHORT (15") SLIT
ON EXTENDED OBJECT IN SLIT
ON OBJECT IN SLIT-FIELD BUT NOT IN SLIT
Read 'Guiding' document.
Read 'Array parameters and data' document.
The SpeX guider uses a 512x512 Aladdin
2 Insb array with a pixel scale of 0.12 arcsec and a corresponding FOV of
The slits are actually gold-coated CaF2 substrates with
uncoated slots for transmission into the spectrograph section. Guiding is done
on the reflected slit-mirror image, either on spill-over light from the object
in the slit or by a guide star within the slit-mirror FOV. The slit wheel also
contains a mirror (position #1 mirror/blank - the back of the mirror acts as a
blank for the spectrograph) for imaging with the guider without a slit.
The cross-dispersed and low-resolution (prism) spectral modes use an
the OPEN position in the order sorter filter wheel (OS Flt). This means that
guiding can be done in any filter contained in the guider filter wheel (GFlt):
broadband filters J,H,K,L' and M', and narrowband (1.5%) filters FeII,S(1),BrG, CO,
cont-K and 3.454 micron. (Broadband Z and H+K notch are due sometime in 2003.)
The single-order spectral modes require an order sorting filter (OS
Flt): Long4,L5,L6,Short3,S4,S5,S6,S7. This means that guiding can only be done
through the required order sorting filter (usually with OPEN in GFlt), which
makes IR guiding in the long wavelength single-order modes on all but bright
objects very difficult, due to the high sky background. In these cases
observers should reconsider using the short-slit cross-dispersed modes which
permit IR guiding at any wavelength.
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GUIDING ON POINT-SOURCE IN SHORT (15") SLIT
cross-dispersed modes require short slits.
- Select slit and
position (rotator) angle.
- Select guider filter (GFlt). For unreddened objects J is usually the
best choice, and H or K for reddened or cool objects, or a
narrowband filter for bright objects, although IR guiding even works well on saturated objects.
- Take image of field with guider. In BASIC mode select suitable ITIME,
COADDS, Beam.Pattern (A or B beam) etc. For an object brighter than mag J=15 an
exposure of a few seconds will reveal the object.
Using the GDV STATS window
JHK magnitudes (i.e. JHK filters only)
can be measured (this is useful for identifying
objects): With the image in BUFFER A select a STATS window to examine BUFFER
A. Draw a box over the star in the IMAGE window, then move the box to nearby
sky. In the STATS window click on SET SKY. In the IMAGE window move the box
back onto the star; the magnitude (accurate to about 0.5 mag) is indicated
at the bottom of the STATS window.
- Click on AUTO GUIDEBOX SETUP which is located on the right side of the
Guidedog XUI window. This will draw the A (top) and B beam (bottom) guide boxes on the
slit image in GDV (see above image). If the line joining the two guide boxes is not in the
centre of the slit image change the X cord in the A CENXY and B CENXY boxes
to where the centre should be. The drawn boxes are just a guide, the actual
centering and size of the guide boxes are indicated by A CENXY/WH and B
CENXY/WH. Re-clicking AUTO GUIDEBOX SETUP will reload the default values, so
don't do this if you have changed the boxes. The defaults should be good to
about one pixel but you should check.
- Using the TCS OFFSET button,
move the object into box A in the GDV window.
Place the cursor on the object. Using the centre mouse button and
the SHIFT key, draw line from object into guide box A. Click on OFFSET
TELESCOPE button to move the object. Take another image to confirm that the
object is inside the box. The star does not have to be in the centre since the
guider will take over once the object is inside the box.
NOTE. Drawing a line in the IMAGE window is important. By doing this GDV
reads information in the image header such as plate scale and rotator angle
which are required to calculate telescope offsets and beam switch vectors.
Any time the rotator angle has been changed (eg. keeping the slit at the
parallactic angle) a line should be drawn in the updated image so that GDV
gets the updated rotator angle. The ANGLE and PLATE SCALE used by GDV are
indicated in the TCS OFFSET window. These can also be typed in by hand
(PLATE SCALE 0.12 and ANGLE that indicated by the ROTATOR icon in GXUI).
- Set up telescope nodding/beam switching for guiding in the A and B
beams: When using the default guide box positions, clicking on
AUTO GUIDEBOX SETUP automatically sends the correct beamswitch to the
the TCS. Other (non-default) guidebox positions of the slit are entered into
the A CENXY and B CENXY of Guide Dog XUI as explained in 5. However, for
these positions the telescope beamswitch is set by entering the A and B
guidebox coords into the TO (beam A) and FROM (beam B) parameters in the
TCS OFFSET window. Click on
SEND AB BEAM OFFSETS
to send the beamswitch to the TCS for these
non-default positions. Clicking on
SEND AB BEAM OFFSETS is not required
when AUTO GUIDEBOX SETUP is used.
When the TCS is configured correctly setting the beamswitch
requires no action by the telescope operator.
- Start guiding: Change observing mode from
BASIC to SLOW.GD.
Any parameter preceded by an '*' maybe changed on-the-fly (ie. during guiding).
Choose suitable *ITIME and *COADD. For guiding in both the A and B beams
*GUIDE AB should be set to AB.
Select the guiding algorithm from *METHOD,
use either CENTROID+FLT1 or CENTROID+FLT2 (faint objects).
The magnitude limit for autoguiding on spill-over from the
slit depends on slit width and seeing. The typical limit is about mag 15
(ITIME 10sec with sky subtraction - see below). Autoguiding works best under
low background conditions, so use the J and H filters whenever possible. For
manual guiding in a GDV window using the hand-paddle (or software equivalent in the TCS1_STATUS
widget) the limit is about mag 18 (ITIME 15 sec).
To start guiding click GO. To accept guiding
corrections set *OFFSET TO to TCS. To turn-off guiding corrections select
OFF. Guide images can be monitored on the VIEWSONIC screen or in buffer
D of GDV. Guiding
corrections can be fine-tuned by changing *GAIN XY (e.g. a commanded
correction of 0.5 arsec can be changed to 0.25 arcsec by changing the gain
from 1.0 to 0.5, lowering the gain is sometimes useful to reduce
oscillations when the tip-tilt component of the seeing is high).
faint objects it is best to subtract off a sky background image. This also
removes 'hot' pixels which can incorrectly weight the guiding algorithms.
Once a guider itime/coadd has been choosen move the object out of guide box
and TAKESKY with same itime/coadd. Move object back into guidebox click, on
*SUB AB and start guiding.
Hot/noisy pixels are included in a
mask which removes them from the guiding algorithm. They appear in
the GDV screens but are removed from the VIEWSONIC fastupdate guide
display. Additional hot pixels may also be removed by
including them in the bad pixel mask. 'bm.set X Y' will mark a pixel as bad
and the guider will then ignore it. 'bm.clear X Y' will clear an incorrectly
marked pixel. 'bm.clearall' clears the entire mask (restart Guidedog IC to
return to the default mask). These commands are
entered on the command line at the very bottom of the Guidedogxui window.
For example, if a hot pixel appears at 240, 250 in a guidebox, it can be
removed by entering 'bm.set 240 250' on the command line.
guiding in beam B press the beamswitch button to right of VIEWSONIC (or BEAM_B in
the TCS1_STATUS widget). If
object does not appear in beam B you've probably sent the wrong AB beam
offsets to the TCS - stop guiding and redo step 5. If everything is OK move
back to beam A.
- Proceed with spectrograph set up in
BIGDOGXUI. If the guider ITIME is
long change the BEAM DTIME in BIGDOGXUI to about twice guider ITIME so that
the guider has time to centre the guide star following a beamswitch.
Beamswitches are commanded by the spectrograph (BIGDOGXUI screen).
For objects too faint to autoguide on but still bright enough to be
seen when not in the slit (about mag 18 with 15sec ITIME), the above
procedure is the same except that autoguiding is is turned off (*OFFSET TO
set to OFF) and the object tracked in a DV window using corrections from the
hand-paddle or telescope offset buttons in the TCS1_STATUS widget.
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GUIDING ON EXTENDED OBJECT IN SLIT
As for procedure A. except
When objects are too extended to allow nodding up and down
the short (15 arcsec) slit observers have the option of nodding (ie. beam
switching) out of the slit for sky or using the 60 arsec-long slits. However,
in long-slit mode order sorting filters must be used to avoid orders
If objects are too big to autoguide on (eg. Jupiter and
Saturn), use IR guiding but turn autoguiding off (*OFFSET TO set to OFF). Then
visually track the object on the slit in GDV window.
To nod out of the short slit:
- Select desired location of guidebox A. Typically this would be located
halfway along the slit eg. A CENXY=230,260. For extended objects it is best
to increase the width (W) and height (H) of the guidebox; set WH in box to
right of A CENXY.
- Verbally tell the telescope operator the beamswitch position eg. 60
arscec north (to nod out of the slit). Do not use the SEND AB BEAM OFFSETS
in the TCS OFFSET window.
- Move object into guidebox A.
- In SLOW.GD set *GUIDE AB to A (no guiding in B beam) and and start
Nodding along the long slit:
This is the same as
procedure A. except that the A and B guideboxes are set further apart along
the slit (set A CENXY/WH and B CENXY/WH). Type the required positions and sizes
into A CENXY/WH and B CENXY/WH, and set the beam switch using the TO and FROM
parameters in the TCSOFFSET screen as described in B.6.
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GUIDING ON OBJECT IN SLIT-FIELD BUT NOT IN SLIT
The guider FOV
is 60x60 arcsec and guiding can be done on any suitable object within this
field (allowing for nodding which reduces the effective FOV), so long as the
guide rates are the same (ie. sidereal). The only difference in the methods
described above is that with the object placed at the desired location in the
slit, the guidebox XY coords (A CENXY and B CENXY) are offset an amount equal
to the difference in the XY coords of the program object and the guide object.
Example: dX = -30 (program object X - guide object X) dY = -30 (program object
Y - guide object Y) as measured in GDV; with program object positioned in slit
at A CENXY=230,230 and B CENXY=230,293, the guideboxes are changed to: A CENXY
= 230 - 30, 230 - 50 and B CENXY= 230 - 30, 293 - 50. Set the beamswitch
using the TO and FROM parameters in the TCSOFFSET screen as described in B.6.
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GUIDING TIPS (Slow.guide)
In marginal seeing conditions, short
itimes (<1sec) can lead to positive feedback of guider corrections,
resulting in oscillations of the guide star about the slit. The solution is to
smear-out the seeing jumps by increasing the itime (usually by increasing
co-adds to prevent saturation). Reducing the guider gain also helps. (Tracking
the seeing speckles requires fast guide/tip-tilt correction which is available
with the optical tip-tilt system.) Typically, the best slow guide parameters
are itime ~3sec (eg. 1sec x 3 co-adds) and a gain of 0.5 in x and y. When
guiding is good, the measured psf of spectra (y-cut, 0.15arcsec/pix) should be
only slightly worse than the psf of the 10sec guider (0.12arcsec/pix) seeing
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GUIDING WITH SUB-ARRAYS
It is important to set-up the guide
boxes and telescope nod position before switching to sub-arrays. First, take a
full guidedog image and set-up the guide boxes as described above. Once this
is done select SUBARRAY from the Guidedog XUI and enter where you want the
sub-array(s) to be located. You may then go into SLOW.GD and start guiding as
normal. Guiding will not work if sub-arrays are set-up first.
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