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

Echelle-Mode Suggested Exposure Times

Echellette Sensitivities

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

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.

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.

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.

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