WHT Nasmyth derotators

Four derotators are available:

No.  New name                 Old name          Notes

1    Optical 1 (wide-field)   GHRIL optical     Formerly used by SCAM.
2    Optical 2 (wide-field)   UES old           No longer used.  Similar to 1.
3    UV/optical (30" field)   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 for the Utrech Echelle Spectrograph, formerly mounted at the opposite Nasmyth focus (now hosting the AO enclosure GRACE). The GHRIL and GRACE mounting flanges are identical, and lie at the same distance along the optical path.

Derotators can only be changed during the day (crane required). The correct focal-station name must be selected in TCS when a derotator is changed.

Individual derotators

1. Optical 1 (long-slit) derotator. The optics are similar to those of derotator 2. This is the derotator usually used by visiting instruments mounted at the Nasmyth focus, e.g. ESA SCAM/STJ, or Rafael Rebolo's FASTCAM.

   The throughput was measured by Tibor Agocs on 10/7/08, to be 0.50 at 543 nm. After cleaning it, the throughput was 0.55.

2. Optical 2 (long-slit) derotator. Field of view 2.5 arcmin. throughput is believed to be ~ 75%, probably declining to ~ 65% in the UV (3000 A) ad IR (1 micron). It comprises a flat mirror and a pair of fused silica prisms + 5 mm of BK7, and is described in La Palma Technical Note 9.
   As of 12/09, Juerg believes this to be in its case, marked 'do not use'.
3. UV/optical (30" slit) derotator. Throughput measured in the lab (April 1997) to be 93 - 95% over the wavelength range 3000 - 11000 A. Re-measured 12/09 by Tibor, confirmed to be > 90%.
   On-axis image degradation is < 0.15 arcsec over the wavelength range 300 - 1000 nm. The central 30 arcsec of the slit are unvignetted, vignetting increases slowly outside this region. 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 made by John Telting Mar 2000.
   There are only two air-glass surfaces in the derotator, and the derotation optics themselves are all-reflecting. Image degradation < 0.15 arcsec over range 3000 - 11000 A.
   The derotator was commissioned with UES in Dec 1997.
4. IR derotator. The field of view of 2.9 arcmin. The 3 Ag (overcoated) 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 Aug 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 thoroughput 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 Jun 2006, there have been no changes to these optics since the derotator was installed in GRACE in 2003.

Throughput

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).

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 GHRIL flange (on which the derotator is mounted) as follows:

Derotator           Tel. focus     Focal distance 
                                   from flange
                    mm             mm

None                98.2           585
UV/optical 30"      98.2           640
UV/optical 30"      97.8          ~630
UV/optical 30"     103.76          768
Optical 2           97.8           485
Optical 2          103.76          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") 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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