Echellette Mode

Orientation
Exposure times (suggested)
Sensitivity
Spatial scale
Wavelength finder
Wavelength solutions

Imaging Mode

Focus values (suggested)
Dome flat exposures

Low-D Mode

Exposure times (suggested)
Rotation of spectra

Echelle-Mode Orientation

With slits at bottom of guider, moving object up the slit, moves the trace to the left. See ESI Orientation for details.

Echelle-Mode Suggested Exposure Times

The table below presents suggested exposure times for internal lamps and dome flats, for a 1-arcsec wide slit. If you are using a different slit width, divide the exposures for the internal and dome flats only by the slit width in arcsec. (You do NOT need to do this for emission line sources such as lamp spectra.)

Source	Exp. (s)	Source	Exp. (s)
CuAr	300+	Int. flat	10
HgNe	4	Dome flat	300
Xe	4

Echellette Sensitivities

When using the information below to estimate S/N ratios or exposure times, note that the read noise is 2.5 electrons/pixel and the inverse gain is 1.3 electrons/DN. In general, if a target is faint enough that read noise is significant, the night sky emission will be bright enough to dominate the read noise in any case.

Most of the commissioning data were taken during bright moonlight, hence our night sky brightnesses are not useful for estimating dark sky conditions.

The spectrophotometric standard star BD+28° 4211 (V = 10.55) was observed through a 6 arcsec wide slit in subarcsecond seeing.

The figure shows the image as it is displayed on the data-taking screen, with the bluest order (15) at the left and the reddest (order 6) at right. In each order, blue is at the top and red at the bottom. Clicking on an order (except for order 14, for which we are missing data) will bring up the extracted spectrum, to give an idea of the count rate at various wavelengths in each order. Units on these plots are in DN/pixel; multiply by 1.3/60 to get into units of electrons/pixel/sec.

Below are some tabulated numbers which give the count rates at the peaks of the orders. Click on the order number to display the actual extracted spectrum of the standard star.

(Note that order 14 is missing because the automated reduction routine had problems with the bad columns at the bottom of the CCD. In the spectrum, the wavelengths affected by the bad columns are repeated in order 13 at the top, so while S/N at these wavelengths suffers, there is still full wavelength c overage.)

Order No. Wavelength Electrons/pixel/sec Electrons/Å/sec Dispersion
(Å/pix)

15 4200 1343 8950 0.15

13 4800 2058 11,430 0.18

12 5200 2275 11,970 0.19

11 5650 2285 10,880 0.21

10 6200 1841 8000 0.23

9 6850 1473 5665 0.26

8 7800 1083 3730 0.29

7 8750 758 2300 0.33

6 9800 130 333 0.39

Echellette Spatial Scales

Below are approximate pixel scales (arcsec/pixel) for each of the ten echellette orders. These were calculated by measuring the separation of the pinholes in the "MultiHoles" slit in both echelle modes and imaging modes, and scaling by the imaging pixel scale of 0.1542 arcsec/pixel. The latter was determined using a handful of astrometric binaries. (27 Sep. 2002)

Order No.	arcsec/pixel along slit
15	0.120
14	0.127
13	0.134
12	0.137
11	0.144
10	0.149
9	0.153
8	0.158
7	0.163
6	0.168

Echellette Wavelength Solutions

ESI graphical wavelength finder

Below are approximate wavelength solutions to each order. Note that this should not replace each observer's set of calibration lamps!! It is provided more as a means of judging wavelengths in a quick look at an image, or as starting points for a wavelength calibration, or as a means of identifying lamp lines.

The line lists are provided as a separate file. Eventually each order will have a separate line list, edited to remove lines which are blended at ESI's resolution, or otherwise deemed to be unsuitable.

Polynomial cooefficients are used as:

WL = C0 + C1*(p-2048) + C2*(p-2048)^2 + C3*(p-2048)^3

where WL is the wavelength in Angstroms, and p is the pixel number.

Wavelength solution for order 6 is estimated, from an extrapolation of the parameters for the other orders.

Order No.	C0	C1	C2	C3
15	4077.46	0.154482	-1.140e-6	-3.106e-10
14	4366.24	0.165128	-2.0205e-6	5.71e-10
13	4699.50	0.179043	-1.912e-6	-8.44e-11
12	5088.55	0.194456	-2.140e-6	4.00e-11
11	5549.09	0.212052	-2.365e-6	-1.23e-10
10	6101.46	0.233675	-2.593e-6	-1.105e-10
9	6776.99	0.259847	-2.826e-6	-1.90e-10
8	7621.60	0.29266	-3.203e-6	-2.77e-10
7	8707.59	0.334496	-3.6815e-6	-2.58e-10
6 (est.)	10156	0.39	-4.25e-6	0

Suggested Focus/Exposure Values

In all cases, please Load... a Named Setup via the ESI Dashboard to select observing modes such as "Imaging", "Echelle" or "lowD". The collimator focus values for the named setups are updated before each run.

Below are averaged focus values for various filters from 2010-2015, in particular the facility BVRI filters. As other filters come into use we will attempt to tabulate those focus values as well.

A word about methodology:
Before each run we take exposures of a grid of holes placed in the upper filter wheel, using different collimator focus values. The sizes of holes across the field of view was then analyzed and a compromise ``best focus'' position determined.

Also included are estimated exposure times for dome flats. Use the "Imaging" flat field lamps, which are weaker than the spectroscopic lamps. The numbers below should produce between 40,000 and 45,000 DN in 1x1 binning.

Filter	Focus	Focus Offset (R)	Dome flat exposure (sec)
B	98,400	-10,300	50
V	95,250	-13,450	15
R	108,700	0	7
Ellis R	112,900	4,200	7
I	105,700	-3000	5

Low-D Suggested Exposure Times

The table below presents suggested exposure times for and lamps and dome flats, for a 1-arcsec wide slit. If you are using a different slit width, divide the exposures for the dome flats only by the slit width in arcsec. (You do NOT need to do this for emission line sources such as lamp spectra.)

Note that you may want to take a longer exposure of the Xe lamp to get fainter blue lines. You may also want to take longer flats to get enough signal-to-noise in the blue end, although this will saturate the red end of the spectrum.

Source	Exp. (s)	Source	Exp. (s)
CuAr	300+	Int. flat	too bright!
HgNe	4	Dome flat	30
Xe	4

Rotation of Low-D Spectra

In low-D mode the spectra are tilted across the detector by approximately 6.5 degrees. You can get a quick reduction of the spectra by derotating the image and extracting a small region around the spectrum. For example, in IRAF you could type:

rotate esi0001 rot0001 rotation=-6.5 interpolant=spline

This leaves esi0001 unchanged; rot0001 contains the rotated image.