WHT Nasmyth derotators

Four Nasmyth derotators are available at the WHT:

No.  New name      Old name          Notes

1    Optical 1     GHRIL optical     Formerly used by SCAM.
2    Optical 2     UES old           No longer used.  Similar to 1.
3    UV/optical    UES new, or UV    Used by SCAM 7/04.
4    IR            GHRIL IR          Permanently in GRACE.

Derotators 1 and 4 were originally used mainly at the GHRIL Nasmyth focus, while 2 and 3 were used for the Utrech Echelle Spectrograph (UES), formerly mounted at the opposite Nasmyth focus (now hosting the GRACE enclosure). The GHRIL and GRACE mounting flanges are identical, and lie at the same distance along the telescope's optical path. The derotators can only be changed during the day by trained staff (hand-lifting of heavy equipment required). The correct focal-station name must be selected in the TCS when a derotator is changed.

Individual derotators

1. Optical 1 (long-slit) derotator

   The optics are similar to those of optical derotator 2, as described in La Palma Technical Note 9. It comprises a flat mirror and a pair of fused silica prisms (each with lens surfaces) with total internal reflection. The derotator needs to be balanced with specific counter-weights.

   This is the derotator usually used by visiting instruments mounted at the Nasmyth focus, e.g. ESA SCAM/STJ, or Rafael Rebolo's FASTCAM.

   Field-of-view: 2.5 arcmin diameter unvignetted, 50% vignetted at 5 arcmin (7 arcmin unvignetted field without a derotator).

   The throughput was measured by Tibor on 10/7/08, to be 0.50 at 543 nm. After cleaning it, the throughput was 0.55. I can find no records at ING of on-sky measurements of optical-derotator throughputs (i.e. comparison of counts on detector with and without derotator).

2. Optical 2 (long-slit) derotator

   The optics are similar to those of optical derotator 1, as described in La Palma Technical Note 9. It comprises a flat mirror and a pair of fused silica prisms (each with lens surfaces) with total internal reflection. The derotator needs to be balanced with specific counter-weights.

   As of 12/09, Juerg believes this to be in its case, marked 'do not use'.

   Field-of-view: 2.5 arcmin diameter unvignetted, 50% vignetted at 5 arcmin (7 arcmin unvignetted field without a derotator).

   Throughput is believed to be ~ 75%, probably declining to ~ 65% in the UV (300 nm) and IR (1 micron). I can find no records at ING of on-sky measurements of optical-derotator throughputs (i.e. comparison of counts on detector with and without derotator).

3. UV/optical (30" slit) derotator

   There are only two air-glass surfaces in the derotator, and the derotation optics themselves are all-reflecting. On-axis image degradation is < 0.15 arcsec over the wavelength range 300-1000nm. The derotator was commissioned with UES in Dec 1997. The derotator is self-balanced and no counter-weights are needed.

   Throughput measured in the lab (April 1997) to be 93 - 95% over the wavelength range 300-1100nm. Re-measured 12/09 by Tibor, found to be 91% at 633nm and 90% at 543nm.

   The central 30 arcsec diameter of the slit is unvignetted, vignetting increases slowly outside this region (7 arcmin unvignetted field without a derotator). With a telescope focus of 97.8 mm, MFB/KMD found the unvignetted field of view to be circular, and of radius 50 arcsec, with partial vignetting out to 75 arcsec, where the image disappears completely. At a focus 103.8 mm, the unvignetted radius is again 50 arcsec, and the image disappears completely at radius 85 arcsec. It is 50% at radius 1.2 arcmin, 25% at radius 1.55 arcmin. These measurements were made by John Telting Mar 2000.

   In May 2016, the UV derotator external windows were cleaned and throughput was measured by Neil in the lab (91% - 93% @ 404 nm, 543 nm and 643 nm). The output window of the derotator was found to be scratched all over, which is increasing scattered light. The input window is scratched too, although less severely.

4. IR derotator

   This component is also called K-mirror, and consists in a reflecting prism and a flat mirror. Currently, the IR derotator can only be used with GRACE (the old configuration - using it with GHRIL - is not implemented in the current version of the TSC). The derotator is self-balanced and no counter-weights are needed.

   Field-of-view: 2.9 arcmin diameter unvignetted (7 arcmin unvignetted field without a derotator).

   The 3-Ag (over-coated) surfaces are all reflecting, and were recoated by Dave Jackson (RGO Cambridge) mid 1998. Realigned 5?/98, before ELECTRA run.

   The throughput was measured by Tom Gregory in August 1999 (by comparing counts from a star with and without derotator): 0.56 in B, 0.64 in V, 0.64 in R, +- 0.02, implying reflectivities of each surface 0.85.

   In ~ May 2000, Tom Gregory had the surfaces recoated and sealed the two apertures with CaF2 windows to protect the silvered surfaces and to cut down airflow through the derotator (which was degrading seeing). The throughput was measured May 2000, with a 633-nm laser to be 79+-1%, and on-sky in V band to be 75% (measurement error probably dominated by transparency variations). The throughput will be even better in the near-IR, since the windows are coated for minimum reflectance at 1.2 microns. As of June 2006, there have been no changes to these optics since the derotator was installed in GRACE in 2003.

   In March 2016, the IR derotator was inspected and cleaned by Emilie & Neil in the lab. Total throughput (including entrance window) was measured ~91% at 633nm. A detailed report can be found here. (password-protected access).

Focus

There's some confusion (noted by KMD 9/03) in the 1994 Observers' Guide about the focal lengths and arcsec/mm scales with optical and IR derotators and without derotator. The values given in the table on page 35 disagree with those implied in Section 9.2.

In March 2005, KMD/MFB measured the focal distances from the flange (on which the derotator is mounted). The Zemax models were created from the drawings by Tibor in December, 2009. There are some discrepancies between the measurements, the flange labels and the Zemax modelling. The focus positions can only be verified with on-sky measurements.

Derotator name      Telescope best    Focal distance
                    focus [mm]        from flange [mm]
              
None (GHRIL)        98.2             ~585
None (GRACE)        98.4             ~585

IR (GRACE)          98.4             ~450

UV/optical 30"      96.0              600
UV/optical 30"      97.8             ~630
UV/optical 30"      98.2              640
UV/optical 30"     103.8              768

Optical  wide       97.8              485
Optical  wide      103.8              615

Pointing correction

A beam travelling along the optical axis of the telescope emerges from the derotator with a small lateral shift and a small change of direction. The image of a star in the focal plane thus shifts, typically by < 1 mm (4.5 arcsec), and traces a circle of this radius twice as the derotator rotates through 360 deg (with a small offset between the circles). The telescope-control system includes a measured correction to compensate for this. The status of these corrections is given below. The quoted residuals are for 360 deg rotation of the derotator (i.e. 720 deg on the sky). These corrections are for use with the new (1998) Alpha TCS control system. It is not certain that the the corrections used with the old TCS will work properly with the Alpha TCS.

1. Optical 1 (wide-field) derotator: last (and first) measured by Robin Clark, 1999 Dec 27 for alpha TCS. Expected residual ~ 1 arcsec.
2. Optical 2 (wide-field) derotator: measured 990127 by FJG/MPF, installed in alpha TCS, residual 0.2 arcsec. (VAX TCS correction is from 1992?)
3. UV/optical (30"-slit) derotator (short-slit): measured 1998 for Alpha TCS? (VAX TCS correction is from 1998 FJG/DLK.)
4. IR derotator: measured by Robin Clark 7?/98 after the realignment, redetermined 1/8/00 after recoating of optics, shift under rotation now ~0.2 arcsec. (VAX TCS correction is from 9/96 JMB.)

Robin Clark's recipe for measuring the derotator corrections can be found here.

1999 Sep 15, last modified 2005 May 25.

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