Summary

This document describes the operation of the IDL-based IFU Centerup tool for centering a target in the ESI IFU. Given a dome or sky flat acquired with the IFU, and an IFU spectrum of the target, this procedure will determine and print the telescope move required to bring the target to the center of the IFU. The observer then needs to execute the given move command to complete the centering.

Requirements

Dome flat acquired with the ESI IFU.
IFU spectrum of target.

Invocation

From the window manager background menu, select
```
	ESI Utilities > IFU Centering Tool
```
to launch the IDL widget.

Procedure

Follow these steps to center your target on the IFU.

Configure ESI. Set ESI up for echellette-mode spectroscopy, being sure to set the CCD integration time to an appropriate length to produce a good spectrum.
Acquire target at IFU. Have the OA acquire the target, send it to the IFU pointing origin, and begin guiding.
Take spectrum. On the centerup tool, clock the Acquire button to Acquire a spectrum of sufficient depth to see continuum (or prominent spectral lines).
Identify region of interest. Note the location of a spectral region which has:
- strong continuum or emission, and
- spectrum running as close to the column direction as possible.
Note the first and last rows and columns in this region.
Select images. On the centerup tool, enter the name of the Target Image (which you just acquired) and the Flatfield image (which you took in the afternoon). You can use the Browse... buttons to select images from the data directory, or click the Last button to select the most recent spectrum.
Define analysis region. Enter the coordinates for the region of interest in the corresponding boxes labeled X start, X end, Y start, and Y end.
Analyze images. Click the Analyze button. The tool will then:
- Read in the ESI images.
- Extract the region of interest and sum the rows.
- Remove cosmic rays in the region of interest.
- Analyze the flatfield image to determine the slitlet endpoints.
- Measure relative flux among slitlets to determine decentering in the direction across the slits (U axis).
- Measure sum of the flux profiles within all slits to measure decentering in the direction along the slits (V axis).
- Derive and display the telescope move required to center the target.
Move telescope. Click the Move button to offset the telescope.
Iterate as required. Repeat preceding steps until convergence is achieved (i.e., recommended moves are less than 0.2 arcsec or so).

Notes

The region of interest should be chosen to include a decent amount of signal, either continuum light from the source or a strong emission feature.
Since no correction is made for curvature of the orders, the region of interest should be chosen so that the spectral direction is well aligned with CCD columns; otherwise, the signal will be “smeared”.

Screenshots

Main Window

ATV window

The ATV window displays the target spectrum and overlays the slitlet regions within the region of interest as red rectangles.

Row Sum Flat Plot

This plot shows a row sum of the region of interest in the flatfield spectrum and is used to determine the slitlet endpoints. Elements:

green line: threshold value for determining slitlet endpoints
yellow line: row sum of flatfield after thresholding; regions below threshold set to 1
grey line: row sum of flatfield prior to thresholding
red lines: slitlet extents

Row Sums Plot

This plot shows the row sum of the region of interest target spectrum and is used to measure the decentering of the target. Elements:

yellow line: row sum of target spectrum
red lines: slitlet extents

If the profiles within each slitlet are decentered from their respective boxes, that indicates decentering in the direction along the slitlets (V axis). The relative distribution of light among the slitlets indicates the centering in the direction perpendicular to the slits (U axis).

U Center Plot

This plot shows the decentering in the axis perpendicular to the slit direction. Elements of note:

yellow line: relative amounts of light in slitlets
blue vertical line: measured center
green vertical line: desired center

V Center Plot

This plot shows the decentering in the axis parallel to the slit direction. The mean value of the profile is subtracted so that only the values above the mean are included in centroiding. Elements of note:

yellow line: sumed profile across all slitlets
blue vertical line: measured center
green vertical line: desired center
green horizontal line: mean profile level