On Slit Guiding

Standard On-Slit Guiding Procedure (using the SCV GUI):

1. Click on the center of your target in any of the following boxes: 2, 3, 5, 6, or 7, click “go R” in box 16, and acquire a new slit camera image by clicking “Single” in box 13.

   1. Ensure the exposure time is reasonable in Box 17. We prefer using 1.63 seconds (1 fowler sample) whenever possible.

2. You can repeat step 1 until you are happy with the centering position of the target on the reference pixel (seen as the green circle in boxes 2 and 3 or the green x in box 5).

   1. It is okay if centering is close but not perfect, the autoguiding software will provide adjustments that can move the target closer to the center of the slit after a few frames.

3. Once you are happy with the centering of your target on the reference pixel, click “go A” in box 16 and “Single” in box 13.

   1. This will move the target at the center of the reference pixel to be centered on the A-nod position on the slit (the red box in boxes 2 and 3).

   2. If the target moves to the A position on the slit successfully, continue to next steps, otherwise repeat steps 1-4.

4. Check that the “A-B” option is selected in box 12.

5. Next, adjust the “BoxSize” parameter found to the bottom right of box 11 so the red box shown in boxes 2 and 3 only encompasses your target star with as little background as possible.

   1. If there is a nearby companion star, try to set a box size that only encompasses the target star and not the companion.

   2. Reasonable box sizes span from 32 (on the larger end) to 16 (on the smaller end) and are usually adjusted in increments of 4.

6. Click the “Guide” button in box 14 to begin guiding on your target.

   1. The histogram in box 20 should be actively updating after this.

   2. Continuous slit camera images will be taken which will be reflected in box 19 as well.

7. Check that the “send AG offsets to TCS” box is checked in box 18.

   1. If this box is not checked then the offsets the guiding program calculates will not be sent to the TCS meaning you are not really guiding. So this is super important!

8. Change the value in box 15 to a reasonable number. This value dictates how frequently coordinate offsets are sent to the TCS to adjust telescope pointing to keep the target on slit--so you usually want a small value!

   1. When the PA is rotated away from 90 deg we have found that it can be necessary to change this value to 1 as the drift on-sky is higher in these orientations. The drift will be the worst at PA +- 180 deg.

   2. The value here also dictates how many slit camera images are saved to the IGRINS hard drive throughout the night (in the example shown on the left it is one in every two images).

   3. 2 is the most commonly used value here but values between 1-3 are probably the most reasonable.

9. The histogram in box 20 will update to show the position of the slit (the well that appears in the PSF) on the target PSF in each slit camera image taken. The goal is to keep the slit on the center of the target PSF while spectra are being acquired.

   1. Failing to keep the target near the center of the slit can introduce undesirable affects to the edges of the orders in reduced 1D spectra, so please pay close attention to this while observing!

10. Watch the positioning of the slit in box 20 for a few slit image acquisitions to ensure the guiding is working well before starting spectra data acquisition using the DTP.

11. Monitor the guiding while spectra are acquired.

    1. You can send manual offsets to the TCS by clicking on the histogram in box 20 and clicking “SEND” in box 21.

       1. Manual offsets are for when the guiding software is struggling to keep the target centered on the slit.

       2. You should click within the wings of the PSF on the histogram as to not introduce too big of a manual correction otherwise you risk the target falling entirely off the slit!

       3. Verify that the correction value looks reasonable before hitting “SEND” in box 21.

    2. You should not send manual offsets more than every x (set in box 15) or so slit viewing camera images.

       1. The guiding software uses an average of the peak of the PSF and how far the peak is away from the center of the slit. Adding a manual offset adjusts the average calculated over the x amount of frames (set in box 15) so adding multiple manual offsets too quickly can skew the average drastically and move the target too far off the slit.

    3. Usually you will find that the guiding software likes to keep the slit skewed to one side of the PSF throughout observations (which side varies with the PA of IGRINS).

       1. This is because the sky is moving and the guiding software is playing catch-up! It does not try to use past RA and DEC corrections to predict new offsets so the software will always lag behind the sky position slightly.

If the target falls off of the slit while acquiring spectra:

1. Abort the frame currently being taken using the DTP.

   1. Aborted frames are not saved if they are aborted before the H- and K-band read out is not yet completed.

   2. If a bad frame is saved, be sure to note it in the IGRINS digital log for the night so it is not used during data reduction.

2. Try using a manual offset to return the target to the slit.

3. If 2 does not work:

   1. Click the “Stop guide” button in box 14

   2. Repeat steps 1-6 above to reset guiding.

      1. Ensure that you send the target to the correct nod position on the slit using “go A” or “go B” to reflect the positioning for the aborted frame.

   3. Once guiding is restarted on target, verify the guiding integrity for a minute or so before restarting observing using the DTP.

      1. You will have to add the nod position for the aborted frame in the front of the Sequence box in the DTP to start where you left off.

If the target does not slew to the correct nod position on the slit:

For example: The target is not centered on the cross-hairs for the correct nod position in box 3 and is instead centered on the slit.

This is a sporadic issue that can observe when observing. It is not common, but is something observers should look out for! We have seen this error occur most frequently when PA = +-180 deg and often happens for specifically the final A frame in an ABBA nod sequence. Usually you will see that the target will end up on the center of the slit rather than the center of the A-nod position (red box in box 3).

1. Abort the frame currently being taken using the DTP.

   1. Aborted frames are not saved if they are aborted before the H- and K-band read out is not yet completed.

   2. If a bad frame is saved, be sure to note it in the IGRINS digital log for the night so it is not used during data reduction.

2. Click the “Stop Guide” button in box 14.

3. Click the center of the target on the image in box 3.

4. Click the corresponding “go A/B” button in box 16 for the nod position you would like to move to.

5. Click “Single” in box 13”

6. Verify that the target is now centered on the right nod position in box 3.

   1. The target should be centered on the cross-hairs for the corresponding nod box on the slit.

7. Click “Guide” in box 14 and monitor the guiding in box 20 for a minute.

8. Resume data acquisition using the DTP.

   1. You will have to add the nod position for the aborted frame in the front of the Sequence box in the DTP to start where you left off.