History

As some users of HIRES may be aware, the collimator position corresponding to best focus for HIRES has, in the past, shown some evidence for drift. Although the problem has been characterized as a drift in collimator position, it is believed that the drift has actually been either in the camera stage, the dewar position, or perhaps even in the position of the chip within the dewar. The drift has been by roughly the same amount, regardless of instrumental setup. i.e. it is the same for all combinations of cross-disperser, collimator, filter, etc.

To correct for this focus drift, observers will be given a new default position of the camera stage. Those who look this deeply into their FITS headers may notice that CAFRAW now has a value of -65000 compared to the old value of -30010. The drift will continue to be monitored and studied and if waranted, CAFRAW will be adjusted accordingly.

Monitoring the problem

To better characterize the HIRES focus drift, we intend to maintain a record here of the value of cofraw corresponding to best focus. It is our intention to do this sufficiently often to properly characterize how the drift behaves with time. To ensure a homogenous data set the instrumental set up listed below will be used for all measurements. A single measurement consists of a series of Th/Ar spectra taken with the ThAr1 lamp over a range of values of cofraw. The exposure time is 2 seconds. The spectra will be stored in /s/sdata120/hireseng/focusdrift. Since the weight of LN2 in the 3.8 liter dewar could conceivably affect focus position, this number is recorded as well.

The IRAF task specfocus which is in the nmisc package was used to analyse the Th/Ar spectra. This task measures the widths of spectral lines in a series of images at various focus positions and determines a "best" focus value. The parameters corwidth and level were set to 10 and 0.5 respectively. The image can be split up into sub-regions and measured separately, or a single "best" focus can be found for the whole image. Since splitting the chip yields some information about the deviation of the chip from the focal surface a vertical division by 3 is done as well and values recorded for the top, middle, and bottom thirds of the image.

1. filter1 = KV 370, filter2 = clear.
2. B1 decker (0.57 X 3.5)
3. red collimator, red cross disperser
4. echelle angle = 0.0314
5. cross disperser angle = -0.4560
6. 1 X 2 binning
7. ng3 filter in front of Th/Ar 1 lamp

Measurements of “best focus” cofraw

Figure

Table

Date	Spectra	dwrn2lv	cafraw	whole chip		top third		mid third		bot third
				cofraw	fwhm (pix)	cofraw	fwhm (pix)	cofraw	fwhm (pix)	cofraw	fwhm (pix)
Mar 31,2001	hires0001-0007	25.5	-65,000	+103,000	2.35	+70,900	2.35	+107,000	2.23	+105,000	2.28
Apr 16,2001	hires0068-0074	64.4	-65,000	+100,000	2.31	+70,000	2.30	+130,000	2.26	+96,000	2.22
May 23,2001	hires0075-0081	78.0	-65,000	+113,000	2.34	+86,700	2.41	+113,000	2.21	+112,300	2.28
Jul 12, 2001	hires0082-0090	61.7	-65,000	+102,500	2.28	+50,000	2.28	+121,000	2.21	+81,100	2.21
Sep 13, 2001	hires0091-0099	53.5	-65,000	+111,500	2.31	+100,000	2.39	+87,500	2.21	+113,000	2.27
Nov 2, 2001	hires0100-0108	90.0	-65,000	+99,100	2.31	+71,900	2.42	+91,300	2.28	+134,000	2.24
Jan 21, 2002	hires0109-0117	79.9	-65,000	+95,000	2.34	+85,000	2.42	+92,500	2.23	+81,500	2.27
Mar 12,2002	hires0118-0126	83.5	-65,000	+95,000	2.30	+69,800	2.30	+95,000	2.23	+50,000	2.28
May31,2002	hires0127-0135	69.0	-65,000	+83,700	2.24	+103,000	2.20	+69,500	2.30	+83,700	2.15
Jul 30, 2002	hires0136-0144	67.8	-65,000	+50,000	2.21	+50,000	2.31	+50,000	2.20	+50,000	2.30
Aug 7, 2002	hires0184-0198	101.5	-65,000	+92,200	1.99	+47,500	2.02	+121,000	1.91	+72,500	1.93
Sep 16, 2002	hires0199-0211	103.5	-65,000	+82,500	2.23	+65,000	2.11	+118,000	2.34	+82,500	2.35
Nov 19,2002	hires0212-0224	76.5	-65,000	+76,300	2.35	+110,000	2.33	+113,000	2.26	+76,300	2.54
Jan 25, 2003	hires0225-0237	100.0	-65,000	+151,000	2.37	+137,000	2.27	+170,000	2.40	+151,000	2.34
Jan 29, 2003	hires0238-0250	88.6	-65,000	+147,500	2.37	+118,000	2.31	+170,000	2.37	+140,000	2.34
Mar 26,2003	hires0251-0261	17.8	-65,000	+125,000	2.30	+124,000	2.27	+145,000	2.30	+117,000	2.26
May 2, 2003	hires0262-0272	95.6	-65,000	+103,000	2.34	+72,500	2.18	+142,000	2.35	+75,800	2.29
Aug 7, 2003	hires0273-0283	76.6	-65,000	+84,500	2.29	+66,500	2.14	+133,000	2.34	+87,500	2.25
Jan 14,2004	hires0284-0294	52.2	-65,000	+122,500	2.35	+103,000	2.19	+170,000	2.38	+85,600	2.33
Mar 2, 2004	hires0295-0305	100.0	-65,000	+150,000	2.62	+133,000	2.68	+170,000	2.47	+148,600	2.62
Jun 1, 2004	hires0001-0011	66.0.0	-65,000	+72,500	2.62	+91,300	2.68	+103,000	2.47	+76,200	2.62