ARC 3.5m | ARCES (Echelle)

Last updated: Oct 4, 2013 - AB

Table of Contents

1. Instrument Description
1.1 Spectrograph Format
1.2 Slits
1.3 Calibration channel
2. Instrument Operation
2.1 Software Control
2.2 Configuring the Instrument
2.3 Taking Echelle Exposures
2.4 The Echelle Slitviewer Camera
2.5 Focusing the Telescope
3. Instrument Performance
3.1 Detector Summary
3.2 Detector Layout/Cosmetics
3.3 Detector Linearity/Saturation
3.4 Detector Gain and Readout Noise
3.5 Instrument Sensitivity
3.6 Typical Exposure Times

4. Instrument Calibration
4.1 Calibration Lamps and Control
4.2 Wavelength Calibration
4.3 Flat Fielding
4.4 Dark Frames
4.5 Flux Calibration
5. Instrument Issues
5.1 Spectrograph Focus
5.2 Slitviewer Focus
6. End of Your Run
6.1 Data Storage and Retrieval
7. Data Reduction
Appendicies
A. Instrument Design
B. Instrument History
C. Sample Echelle data
D. Basic low-level commands (for scripting)
E. Other documents

1. Instrument Description

The ARC Echelle Spectrograph (ARCES) is a high resolution, cross-dispersed visible light spectrograph. It captures the entire spectrum between 3200-10000Å (between the atmospheric cutoff and the CCD cutoff) in a single exposure on a 2048x2048 SITe CCD. The spectrograph provides a resolution (2.5 pixels) of R~31,500.

ARCES is permanently mounted on the NA1 port of the 3.5m telescope. This means that it does not require an instrument change to use ARCES with another instrument during one night. ARCES is a single point spectrograph, so the small (!) field of view rotates as an object is tracked across the sky.

1.1. Spectrograph format

The spectra are recorded in ~120 orders that are distributed across the chip. Longer wavelength orders are located at the bottom (low row numbers) of the chip, and shorter wavelength order are located at higher row numbers. Each order is curved significantly, and within each order, wavelength decreases with increasing row number. Compared with some other echelle spectrographs, the orders are relatively narrow and closely spaced, which may require users to take special care when flat fielding and extracting spectra.

The following table gives locations of 107 orders, along with the approximate central wavelength and dispersion of the order. The dispersions are negative, indicating that wavelength decreases as column number increases. Note, as of 07-01-2013, there is a new grating in the spectrograph. This table is still useful, but be aware that some of the lines may fall between orders.

Order ID Row (at chip center) Wavelength (chip center) Dispersion Range (Å / 1600 pixels) Order ID Row (at chip center) Wavelength (chip center) Dispersion Range (Å / 1600 pixels) Order ID Row (at chip center) Wavelength (chip center) Dispersion Range (Å / 1600 pixels)
1
319
10163
-0.1292
207
37
655
6187
-0.0786
126
73
1074
4447
-0.0565
90
2
330
9985
-0.1269
203
38
665
6120
-0.0778
124
74
1087
4413
-0.0560
90
3
340
9813
-0.1247
200
39
675
6055
-0.0769
123
75
1101
4379
-0.0556
89
4
349
9647
-0.1226
196
40
685
5991
-0.0761
122
76
1115
4345
-0.0552
88
5
359
9486
-0.1206
193
41
695
5929
-0.0753
121
77
1130
4312
-0.0548
88
6
368
9330
-0.1186
190
42
706
5868
-0.0745
119
78
1144
4280
-0.0543
87
7
377
9180
-0.1167
187
43
716
5808
-0.0738
118
79
1158
4248
-0.0539
86
8
387
9034
-0.1148
184
44
726
5749
-0.0730
117
80
1173
4216
-0.0535
86
9
396
8893
-0.1130
181
45
737
5692
-0.0723
116
81
1188
4185
-0.0531
85
10
405
8756
-0.1113
178
46
748
5636
-0.0716
115
82
1202
4155
-0.0528
84
11
414
8624
-0.1096
175
47
758
5580
-0.0709
113
83
1216
4125
-0.0524
84
12
424
8495
-0.1080
173
48
769
5526
-0.0702
112
84
1231
4095
-0.0520
83
13
433
8370
-0.1064
170
49
780
5473
-0.0695
111
85
1246
4066
-0.0516
83
14
441
8249
-0.1048
168
50
791
5421
-0.0689
110
86
1261
4037
-0.0513
82
15
450
8131
-0.1033
165
51
802
5370
-0.0682
109
87
1277
4009
-0.0509
81
16
459
8016
-0.1019
163
52
814
5320
-0.0676
108
88
1294
3981
-0.0505
81
17
468
7905
-0.1005
161
53
825
5270
-0.0669
107
89
1309
3953
-0.0502
80
18
477
7797
-0.0991
159
54
836
5222
-0.0663
106
90
1327
3926
-0.0499
80
19
486
7691
-0.0977
156
55
848
5175
-0.0657
105
91
1341
3899
-0.0495
79
20
495
7589
-0.0964
154
56
859
5128
-0.0651
104
92
1358
3872
-0.0492
79
21
504
7489
-0.0952
152
57
871
5082
-0.0645
103
93
1370
3846
-0.0488
78
22
513
7392
-0.0939
150
58
883
5037
-0.0640
102
94
1390
3820
-0.0485
78
23
522
7297
-0.0927
148
59
895
4993
-0.0634
101
95
1407
3795
-0.0482
77
24
531
7205
-0.0915
146
60
907
4950
-0.0629
101
96
1423
3770
-0.0479
77
25
541
7115
-0.0904
145
61
919
4907
-0.0623
100
97
1440
3745
-0.0476
76
26
550
7027
-0.0893
143
62
931
4865
-0.0618
99
98
1456
3721
-0.0472
76
27
559
6941
-0.0882
141
63
944
4824
-0.0613
98
99
1474
3696
-0.0469
75
28
568
6858
-0.0871
139
64
956
4783
-0.0607
97
100
1491
3673
-0.0466
75
29
578
6776
-0.0861
138
65
968
4743
-0.0602
96
101
1509
3649
-0.0463
74
30
587
6696
-0.0851
136
66
982
4704
-0.0597
96
102
1527
3626
-0.0460
74
31
597
6618
-0.0841
135
67
994
4666
-0.0593
95
103
1546
3603
-0.0458
73
32
606
6542
-0.0831
133
68
1007
4628
-0.0588
94
104
1564
3580
-0.0455
73
33
616
6468
-0.0822
131
69
1021
4590
-0.0583
93
105
1583
3558
-0.0452
72
34
624
6395
-0.0813
130
70
1033
4554
-0.0578
93
106
1600
3536
-0.0449
72
35
635
6324
-0.0803
129
71
1047
4518
-0.0574
92
107
1616
3514
-0.0446
71
36
645
6255
-0.0795
127
72
1061
4482
-0.0569
91

1.2. Slits

There is a single slit mounted in the instrument at any given time, but several slits are available. The default slit corresponds to 1.6"x3.2" on the sky. Other slits must be requested in advance and require the observing specialist to physically change the slit in the spectrograph, so it is generally not possible to use more than one slit in a single observing session. The following table gives the available slit sizes.

Slit size in projection on the sky
1.6"x 3.2" ( default )
1.6"x 4.7"
1.6"x 1.6"
0.9"x 0.9" ( pinhole )

For additional measurements of the physical dimensions of each slit made June 2010, please see Appendix A.

1.3. Calibration Channel

The echelle has an internal calibration channel, which provides a ThAr lamp for wavelength calibration, and a quartz lamp for flatfielding. A blue filter can be placed in the calibration beam to produce blue flat field frames (see Flat fielding section below).

There is a controllable mirror that can be moved in and out of the beam to access the calibration channel. Since the echelle calibrations are internal, calibration data can be taken even while other instruments are being used on-sky.

2. Instrument Operation

2.1. Software Control

The Echelle is operated within the Telescope User Interface (TUI) software written and maintained by Russell Owen at the University of Washington. A detailed manual is available here. The main TUI status window looks like this:

2.2. Configuring the Instrument

The instrument is configured using the Echelle window accessible under the Inst menu from the main TUI window. This brings up the Echelle control gui. Here we give a brief overview of the Echelle controls via the Echelle control gui:

This window shows the current configuration (Shutter state, cal. mirror, lamps and cal. filter). The configuration can be changed using the Show Config button. As with other TUI windows, clicking the Apply button will implement any selected changes and all current selections are applied. No changes will be activated until this button is clicked, and all selections will be highlighted pink.

Since the spectral format is fixed with the echelle, the only configurable options relate to the calibration channel: the calibration mirror can be placed in either the sky or calibration position, the two lamps can be turned on and off, and either the blue or no (open) filter can be selected.

2.3. Taking Echelle Exposures

Echelle exposures are taken using the Expose window which is opened using the Expose button in the Echelle configuration window. The Expose gui looks like:

The expose window is in standard TUI format (for detailed description see here ), and, as for other instruments, shows status of the current exposure at the top, and allows you to set the object Type (for the FITS image header), exposure Time, number of Exposures, and root File Name. If you wish to add comments to the file header, place them in the Comments field.

Here are the descriptions of what each gui button in the Expose window.

All data will automatically be stored on a local computer, Newton. Each program has remote access to this computer through a program user name. For example, UW01 can access their data using uwobserver@newton. The password to each user name changes every quarter. Users can get the new password by calling APO directly--we cannot give them out over email or messenger. Once signed in, data can be found under /export/images/<program ID>/UT date/<filename>. However, most users set TUI up to automatically transfer images to their local computer using the Preferences options in the main TUI window menu (see AutoGet and Save To options under Exposures). You can also define a subdirectory within TUI,(TUI will even create it for you) by entering a name such as <subdir1>/<subdir2>/program ID/UT date/<filename> in the File Name line of the Expose gui; note, however, that a separate image number sequence will be started in each subdirectory.

2.4. The Echelle Slitviewer Camera

There is an internal slitviewing camera in ARCES that is used for target acquisition and guiding. The slitviewer uses an e2v (Marconi) 512x512 CCD, with the basic midband AR coating, that is operated using an Apogee camera body and USB controller. The pixel size corresponds to 0.133 arcsec/pixel, giving a field of view of a bit over an arcminute on a side. The readout time for the slitviewer is about 1 second.

Camera Summary:

Control of the slitviewer is obtained via the Echelle Slitviewer window accessible under the Guide selection in the main TUI window. This will open the following GUI:

Acquisition images are obtained by setting an exposure time and using the Expose button. The desired object can be automatically moved to the center of the slit by moving the cursor to the desired object and by doing a CTRL-click to select the guide star (if not already selected with green marks), then click the Center Sel button; the location of the slit center is periodically adjusted by the observing specialists but is located around pixel (290, 233). Always make sure you're looking at a current image before you CTRL-click or start guiding. You must be in default mode (+ cursor selected along top control panel on slitviewer) for CTRL-click to function. CTRL-click has worked if a light blue X and an arrow pointing towards the boresight appear.

The slitviewer can also be used for guiding during the Echelle exposure. A common mode is to guide on the object in the slit, using the small amount of light that leaks out of the slit; this is done using Boresight guiding. It is also possible to find a star located in the slitview image off of the slit, and guide on it via Field Star guiding. Manual guiding does not send guide corrections to the telescope, but just continues to take images.

You can guide on any object which is symmetric (galaxies with strong cores are fine), not too faint, and non-saturated. Optical double stars, late-type galaxies, and bipolar PNs may pose a problem for guiding, as will objects where you desire to observe a position away from the bright center (e.g., SNs near bright galaxies). Any object the guide software thinks is usable will be circled in green. If your favored guide object is not circled, it may be too faint, too bright, or too lumpy. If you really want to use an object that has no circle around it, try dragging a box around it to centroid it. Make sure you are in + cursor mode, then click and drag from the top left to the bottom right of the object. A light blue box will be drawn. If this succeeds (if a circle appears) then you are all set. You can also use this same "drag a box" motion with the right mouse button to zoom in on a specific area.

Use longer guide exposure times to obtain better signal of your star. The best exposure times are 5-30 seconds, but up to 120 seconds is usable if you can be patient. Longer exposure times can also help if you're having data transfer problems. Exposures shorter than about 3s are a waste of image-transfer bandwidth and may cause you to over-guide on seeing fluctuations. See more about guiding in Guiding with TUI User's Guide.

Note on ND filters:

The echelle slitviewer is equipped with five neutral density (ND) filters. These can be used to stop bright objects from saturating in the guide window. For brighter objects, use a higher ND filter.The neutral density filters for the echelle slitviewer are absorptive optical density filters. The amount of fractional transmittance through them at 656.3 nm is equivalent to the negative power of the ND number for that filter. Therefore, ND1 is 0.1 times the transmittance of the clear filter at 656.3 nm. The ND2 filter is 0.01 the transmittance of the clear filter and so forth. The following plots show the response for the guider per wavelength for each of the ND filters when convolved with the QE of the guide camera.

Guider Match Scripts:

On Newton there is a script that can be run from the institutional accounts that will match up the times of the slitviewer to your science images and create a log of each science frame with the nearest guide image and the range of guide frames if a range exists. This script is called: ecam_match. For more details on using this script please see: Guider Match info.

2.5. Focusing the Telescope

The telescope is focused by inspection of images on the Echelle slitviewer. Because the slitviewer focus varies across the field of view, it is important to place an object near the slit for focusing purposes. Generally, focusing is most efficiently accomplished by letting the Observing Specialist focus the telescope. They can run the Echelle focus script to expedite the process and provide a current seeing estimate.

Focus will be unstable at the beginning of the night until the telescope has reached equilibrium. Depending on conditions, focus may change throughout the night.

3. Instrument Performance

3.1 Detector Summary

The echelle detector is a SITe 2048x2048 thinned, backside-illuminated CCD. The following table summarizes some of the chip characteristics (see more detail in subsequent sections):

Device
TK2048E
back illuminated CCD
Serial Number  
Number of rows
2048
Number of columns
2048
Pixel size
24 µm
Gain
3.8e/ADU
Readout noise
7 e
Dark current
0.002 e-/sec/pix
(7.25 e- per hour)
Linearity regime
<35000 DN

The overhead time for each exposure is significant. Before the shutter is opened, the chip is flushed (about 20s). After the exposure is completed, the image takes nearly two minutes to read it out. Experience suggests that the telescope can be safely slewed during readout without any effect on the exposure.

3.2. Detector Layout/Cosmetics

The SITe detector has 2048x2048 active pixels. There are 21 extra inactive pixels on the left side, and we read out an additional 59 pixels horizontally and 20 pixels vertically, for a final image size of 2128x2068. The bias level, normally around 1300 DN, can be determined from the vertical overscan region; as usual, you may wish to avoid the overscan pixels immediately adjacent to the imaging pixels.

(IMPORTANT NOTE: the BIASSEC currently written into the headers is WRONG and should be changed!)

The chip has a number of partially blocked columns that make them unusable, but most of these are located near the top of the chip in an area that is unilluminated (corresponding to very short wavelengths). The following table gives a list of cosmetic defects on the chip. Use this table to create a nominal pixel mask for your pre-data reduction steps.

Column range Row range
788:788 803:2000
1683:1683 664:2000
102:102 220:2068
1285:1285 1793:1836
1356:1356 1475:2068
1603:1603 1419:1783
1383:1383 1906:1944
1417:1417 1927:1975
982:982 1611:1891
491:491 1576:1685
569:569 1711:1723
654:655 1906:1982
854:854 1871:1926

3.3. Detector Linearity/Saturation

The echelle detector is linear up until about 35,000 DN.

3.4. Detector Gain and Readout Noise

The detector gain has been measured to be about 3.8 electrons/DN.

The readout noise is about 7 electrons rms. This readout noise is the primary limiting factor for observing faint targets.

3.5. Instrument Sensitivity

TBD

OLD: The sensitivity of the instrument is 1 count s−1pixel−1Å−1 for a star of mV = 16.

3.6. Typical Exposure Times

This is a link to the NOAO exposure time calculator. Users will need to adjust for the different telescope aperture, from 4m to 3.5m. Users will also need to pick the appropriate gratings and filters to closest match the setup at APO.

http://www.noao.edu/gateway.spectime/kp4mechelle.html

4. Instrument Calibration

Typical echelle calibrations include wavelength calibration, flat fields, bias frames, and, possibly, dark frames. All of these can be achieved using the ARCES internal calibration channel. The echelle slit head is enclosed in a light-tight box also containing calibration lamps, a filter wheel, and a pickoff mirror for changing between "Sky" and "Calibration" mode.

4.1. Calibration Lamps and Control

All of the calibration lamps are controlled in the Echelle configuration window.

4.2. Wavelength Calibration

Wavelength calibration is achieved through exposures with the ThAr lamp. A 30s exposure give high (~150) S/N for most bright lines at the expense of some saturated lines in the reddest orders. Shorter duration exposures can be used to retrieve useful signal in these orders if necessary, but short exposures alone will not result in enough signal for good global dispersion solutions.

The instrument is not perfectly stable, and the ThAr lines will shift in position within nights and/or from night to night. If very accurate wavelength calibration is needed, more frequent ThAr exposures can be taken.

4.3. Flat Fielding

Flat fields taken with ARCES require multiple exposures because the combination of lamp spectrum and instrument sensitivity yields far higher count rates in the red than in the UV/blue. In order to reach sufficient S/N in the blue orders without saturating the red orders, it is necessary to take two sets of flats, one to achieve high S/N in the red, and another to achieve high S/N in the blue. For the latter, a CuSO4 (blue) filter is placed in the beam (done in the Echelle configuration window).

Typical exposure times are 7s for the "red" flats (no filter in beam), and 200s for the "blue" flats (with the CuSO4 filter in the beam).

Flat fields are stable, but small shifts in the location of the orders can sometimes be seen.

The use of flat fields with ARCES can be complicated because of the narrow orders and because the width of object spectra and flat-field spectra are comparable due to the short slit. Because of these, most users do not perform two-dimensional flat fielding, but instead extract object and field field spectra and then perforam 1D flat fielding.

4.4. Dark frames

If long exposures (~1 hour) will be used, you may wish to obtain long dark frames to subtract the dark current. Multiple dark frames will be required to remove cosmic rays.

4.5. Flux Calibration

If you wish to do relative flux calibration for your spectra, you observe some spectrophotometric standards. Information about some standards can be found here: http://www.apo.nmsu.edu/35m_operations/35m_manual/General_Information/Standards/optuvstandards.html

A TUI catalog of the stars described on the standards page can be found here: http://www.apo.nmsu.edu/35m_operations/35m_manual/Instruments/instrumentguides/TUIstandards.txt

5. Instrument Issues

5.1. Spectrograph Focus

Spectrograph focus is stable over five years.

5.2. Slitviewer Focus

The slitviewer focus is manually adjusted by the technical staff. There is a moderately strong variation of focus across the field of view of the slitviewer, so the focus is adjusted to bring the slit most clearly into focus.

6.0. End of Your Run

Be sure to turn off any calibration lamps you may have used, then hand over the instrument by simply quitting out of the Echelle instrument control window in TUI. In some circumstances, you may continue to use the instrument after your shift, e.g., you are the first half observer and the scheduled second-half observer is not using the Echelle. Ask your Observing Specialist for permission to do so if circumstances warrant.

6.1. Data Storage and Retrieval

Your data will remain on newton.apo.nmsu.edu:/export/images/your program code/UTYYMMDD/ for 9 to 12 months before being automatically deleted. Data can be accessed via using scp or sftp to the institutions' observer accounts on newton (you can also use ftp and the images account and password); call APO (505-437-6822) if you are unsure of the correct login.

7. Data Reduction

As of August 2012, the grating has been updated. The table will be updated as soon as possible.

Please use this as a guide. Echelle Data Reduction guide.

Please find additional information for taking and reducing data here.

Appendices

A. Instrument Design

See the Echelle commissioning report.

COLLIMATOR:

GRATING:

CROSS DISPERSION:

PHYSICAL SIZE OF EACH SLIT:

Measured in June 2010 using the Sloan Heidenhain encoder and probe in combination with a mechanical stage on the microscope to measure the physical size of each of the echelle slits, then converted to arcseconds using the quoted telescope platescale 5.86 arcsec/mm.
Slit Short Side (microns) Long Side (microns) Size

Old slit1
(labeled 275 x 825)

294.3 +/-6 832.3 +/-14.5 1.7" x 4.9"
1.6" x 1.6" 291.5 +/-9    
1.6" x 4.7" 283.7 +/-3.2 835.4 +/-3.2 1.7" x 4.9"
1.6" x 3.2" (default) 303.4 +/-4.5 553 +/-4.7 1.8" x 3.2"

100umPinhole
(labeled 0.9" x 0.9")

192.3 +/-2.4 192.0 +/-2.2 1.1" x 1.1"

Associated plots:

B. Instrument History

 

C. Sample Echelle data

Echelle blue flat

Echelle red flat

Echelle ThAr

D. Basic low-level commands (for scripting)

echelleExpose object|flat|dark|bias time=t [name=name] [n=nexp] : takes nexp exposures of specified type, with exposure time t (seconds), with specified root file name (last name used if no name specified)

echelle mirror lamps|sky : send pickoff mirror to lamps or sky position

echelle calfilter Open|Blue : sends filter to Open or Blue(CuSO4) position

echelle lamps 0|1 0|1 0|1 : controls internal calibration lamp (0:off, 1:on). First entry is for flat field lamp, second is for ThAr, third unknown.

TUI can run scripts for all of your calibrations. In TUI, open Scripts > Run_Commands. Your text can be written in the window, or you can upload a text file if you have it written beforehand. For example, here is a script to take a series of echelle calibration data:

echelle mirror lamps
echelle calfilter Open
echelle lamps 0 1 0
echelleExpose object time=30 name=ThAr n=1
echelle lamps 1 0 0
echelleExpose flat time=7 name=Flat n=3
echelle calfilter Blue
echelleExpose flat time=200 name=FlatBlue n=3
echelle calfilter Open
echelle lamps 0 0 0
echelle mirror sky
echelleExpose bias name=Bias n=1

E. Other documents


ThAr line lists:

http://old-www.noao.edu/kpno/specatlas/thar/

https://www.eso.org/sci/facilities/paranal/instruments/uves/tools/tharatlas.html

Observing program help:

http://catserver.ing.iac.es/staralt/

Just be sure to set "Observatory" to "Apache Point Observatory" and have the correct date.