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ARC 3.5m | ARCES (Echelle)
Last updated: Feb 23, 2016 - AB
Table of Contents
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.
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)
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.
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.
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.
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.
- Start - This button starts the exposure or exposure sequence.
- Pause - This button pauses the exposure, you can start it again later.
- Stop - This button stops the exposure AND saves the current data to disk
- Abort - This button aborts an exposure. It DOES NOT save the data.
All data will automatically be stored on a local computer, arc-gateway. Each program has remote access to this computer through a program user name. For example, UW01 can access their data using uwobserver@arc-gateway. 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:
- CCD is a e2v (Marconi) CCD77-00 back illuminated 512 x 512 CCD
- CCD pixel size = 24 µm
- CCD gain = 4.6 e-/ADU
- CCD read noise = < 2 e- / pixel
- Quantum Effeciency >80% (450-750nm)
- Dark Current: 0.08 e- / second
- Pixel scale = 0.133"/pixel binned 1x1
- Full well depth: 350k e-
- Linear up to 64k, Saturation at 65K
- Installed filters (User selectable): Clear, ND1, ND2, ND3, ND4, ND5
- Field of View: 63.6"
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 arc-gateway 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.
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.
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 CCDSerial 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.
TBD
OLD: The sensitivity of the instrument is 1 count s−1pixel−1Å−1 for a star of mV = 16.
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.
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.
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.
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.
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.
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
Spectrograph focus is stable over five years.
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.
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 arc-gateway.apo.nmsu.edu:/export/images/your program code/UTYYMMDD/ for 9 to 12 months before being automatically deleted. Data can be accessed via scp or sftp to the institutions' observer accounts on arc-gateway (you can also use ftp and the images account and password); call APO (575-437-6822) if you are unsure of the correct login.
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.
A. Instrument Design
See the Echelle commissioning report.
COLLIMATOR:
- off-axis parabola
- diameter = 225 mm, of which 200 mm (beam size) is actually used
- focal length = 2000 mm
- focal ratio = f/10
GRATING:
- 31.6 grooves/mm, blaze angle b = 63.5 deg (nominal), or tan b = 2
- incident angle = 69.5 deg
CROSS DISPERSION:
- two 45 deg prisms of UBK7, used at minimum deviation
- total deviation through both = 52.7 deg
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:
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
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.
