SpUpNIC: Spectrograph Upgrade – Newly Improved Cassegrain

The Cassegrain Spectrograph Upgrade project was completed in October 2015 and the instrument (SpUpNIC) has been in routine operation and well subscribed ever since.  A SpUpNIC paper was presented at the SPIE Astronomical Telescopes and Instrumentation conference in Edinburgh in June 2016.
dome_flats

SpUpNIC still uses the same set of surface-relief diffraction gratings, arc lamps (CuAr and CuNe – now available simultaneously) and order blocking filters (BG38, BG39 and GG495) as before, but the instrument has new camera and collimator optics, as well as a new detector system.  The new optics and CCD slightly reduce the dispersion, but significantly increase the wavelength ranges delivered by the various gratings – see the table below for details.  The original slit mechanism is still in use and offers slit widths ranging from 0.15″ to 4.2″ (in 0.15″ increments).

Grating
Lines

per mm

Blaze

(Å)

Range

(Å)

Dispersion

(Å/pixel)

Arc Map
4 1200 4600 1250 0.625 G4wc
5 1200 6800 1100 0.525 G5wc
6 600 4600 2800 1.36 G6wc
7 300 4600 5550 2.72 G7wc
8 400 7800 4100 2.21 G8wc
9 830 7800 1700 0.835
10 1200 10000 950 0.470 G10wc
11 600 10000 2600 1.25 G11wc
12 300 10000 5600 2.75 G12wc

The spectrograph camera’s new Folded-Schmidt optical design is much more efficient than the old Maksutov-Cassegrain system, and the new CCD provides an additional sensitivity boost.  The introduction of a rear-of-slit viewing camera allows far more accurate positioning of the target on the slit, and being able to view the star going down the slit makes it easier to focus the telescope reliably.  These critical aspects have substantially increased the spectrograph throughput and overall observing efficiency, particularly for faint targets.  The new instrument control software, as well as the quick-look data reduction tool, further streamline the process and provide access to the data which can be extracted and approximately wavelength calibrated as soon as an image is read out.

Sample spectra are shown in the figure below, with the SDSS g magnitudes of the stars listed to the right.  Most were obtained with the low resolution G7, but the shorter spectrum (second from the bottom) was a 900s exposure with a 1.5″ slit using G6.  The G7 spectra employed slit widths between 1.5″ and 2.7″ (depending on the seeing) and the exposure times ranged between 1800 sec for 16-18 mag, 1200 sec for 15-16 mag, 600-900 sec for 13-15 mag and 200 sec for 11th/12th mag standard stars.  The faintest target observed for this campaign was 18th mag and that required a 2400 sec exposure.  The most extreme use of the high resolution blue G4 has been to observe a V~17.5 mag star for 1200 sec and be able to classify the object as a white dwarf.  See the SpUpNIC exposure time information page for more details, including performance comparisons between the old spectrograph and SpUpNIC.

allspecLisa_mag

Spectro-photometric Standards with Grating 4

G4 - EG 21 G4 - Feige 110G4 - LTT 377

Spectro-photometric Standards with Grating 5

G5 - EG 21 G5 - EG 21 2nd order

Spectro-photometric Standard with Grating 6

G6 - EG 21

Spectro-photometric Standards with Grating 7

G7 - EG 21G7 - LTT 377 G7 - LTT 1020 G7 - LTT 3218

The instrument team is completing remaining commissioning tasks and tests, and making refinements to the system – specifically the software, as well as the grating mechanism which proved to be mechanically unstable.  The latter has been significantly improved and ought to be resolved with the modifications currently in progress.  As a result, we do not yet have a good feel for the instrument’s performance in terms of measuring radial velocities.  This information will be added once the grating stability issues have been addressed and the RV capabilities of the instrument can be established.

The SpUpNIC Wiki page provides detailed information for observers at the telescope, start-offs are provided for new users and telephonic support is available throughout the night should problems arise at the telescope.

We welcome feedback from users, please email the instrument PI (Lisa Crause): lisa(at)saao.ac.za.

SpUpNIC