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IR GUIDING -(JTR 05 Dec 2002 )



    '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 60x60 arcsec.

    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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    The cross-dispersed modes require short slits.

    1. Select slit and position (rotator) angle.

    2. 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.

    3. 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.

    4. 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.

    5. 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.

      IMPORTANT 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).

    6. 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.

    7. 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).

      For very 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.

      To check 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.

    8. 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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    As for procedure A. except as follows.

    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 overlapping.

    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:
    1. 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.

    2. 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.

    3. Move object into guidebox A.

    4. In SLOW.GD set *GUIDE AB to A (no guiding in B beam) and and start guiding (GO).

    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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    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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    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 measurement.

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    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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