Background

Observers sometimes want to acquire ESI spectra of targets which are not visible on the guider. This document describes two procedures for doing so.

Method A: Offset from known star

Sometimes pre-imaging of the desired object will be available from a camera which has been photometrically calibrated. In this case, the RA/Dec offset between the faint object and a brighter (R<20) star visible to the ESI guider can be measured. This works best when the offset star is within about 1 arcmin of the target, because in this case the offset can be completed without losing the guide star and is thus more accurate. A finder chart showing this "offset star" is essential in this case, and the observer should produce one prior to the observing run.

Prerequisites

Offsets from observable star.
Finder chart showing locations of offset star and target, plus cardinal directions (north and east) and pixel scale.

Procedure

With the finder chart and offsets in hand, here are the steps to follow:

Have OA send target to REF pointing origin. It should be positioned at about pixel (512,256) on the guider display.
Determine whether the target is visible on the guider. Note that you can use various guider filters (B,V,R,I) to improve contrast (but note that you must change filters using the TV_B, etc., setup files in order to get the guider refocused properly!).
If the target is visible, then offsetting is not needed and you can simple have the OA put the target onto the slit.
If the target is invisible, then execute the inverse offset to bring the offset star to the current location of the target (512,256).
Identify the offset star and give the coordinates to the OA.
Have OA center offset star on REF, send star to slit, and center star on slit.
Select a guide star which will provide sufficient S/N for guiding and, preferably, will not move off the slit area when the offset is made. Begin guiding on this star, and tweak pointing to re-center offset star on slit as needed.
Have OA mark base and execute the offset move. If the guide star remains on the slit then begin exposure. If the guide star was lost, then undo the offset by returning to base, select a different offset star, and re-try the offset.

Method B: Imaging of target and slit

When no offset star has been identified but a finder chart is available, then the best way to put a faint target on the slit is by imaging the target, imaging the slit, and moving the target onto the slit.

Prerequisites

Target coordinates accurate to ±3 arcsec.
Finder chart showing location of target, plus cardinal directions (north and east) and pixel scale.

Procedure

Follow these steps:

Configure ESI for imaging using the appropriate broadband filter to enhance contrast with the background. Note: you must use the pre-defined setup files for performing the configuration in order to ensure that the instrument is properly focused for imaging!
Set slit to 6 arcsec wide slit.
Have OA check pointing near the target coordinates, then send target to REF (near pixel 512,256 on guider).
Confirm via finder chart that the field is correct.
Send target to SLIT pointing origin and begin guiding.
Acquire a direct image through the slit of suitable exposure length (20-200 sec) to reveal the target.
Measure the pixel position of the center of the target (x_t,y_t) in FIGDISP, either by:
- pressing the "s" key to measure the center;
- using "stats box" mode to defining a box around the target and measure the center; or,
- visually estimating the center by reading the approximate (x,y) pixel position off the FIGDISP title bar.
Change the slit to the one you want to observe through.
Acquire a direct image through the slit using an exposure time just long enough to show significant sky flux through the slit (20-60 sec).
Measure the center of the slit (x_s,y_s) by using one of the methods described above for measuring the target center.
In a kanaha window, use the mov command to move the target onto the slit:
```
	mov x_t y_t x_s y_s
```
The target should now be on the slit.
If desired, acquire another direct image to verify that the object is now observed in the slit. Note that you may need to manually adjust the scaling limits in FIGDISP to make the target visible.
Use the appropriate named setup file to reconfigure ESI for spectroscopy with your chosen slit. Since the slit itself should already be in place, the top wheel should not move and guiding should not be interrupted.
Acquire spectra.