NIR imaging background and observations

When one is observing in the near-infrared one is always fighting against the strong and variable sky background, which has significantly more influence than in the optical. It has contributions from OH airglow in the J, H, and K-bands, moonlight (either directly or reflected off clouds) especially in the J-band, and from thermal emission from the telescope and sky in the K-band which varies with temperature and humidity. Although the 10-30$\%$ variations in strength with the background caused by these factors do not strongly limit the S/N of observations (except at K for large changes in temperature), they greatly complicate both the creation of mosaics of large regions and accurate surface photometry. Because of the possible rapid shifts in the background time shifts of 60-90 seconds are used. Because saturation of the CCD chip can take place quickly, the exposure times are very short, usually less than 10 seconds.
Like in the optical, data reduction requires accurate correction for the small additive effect of internal lumination, charge generation and charge leakages (dark frames), the large additive effects of sky illumination (sky subtraction frames), and the multiplicative effects of position dependant pixel sensitivity (flatfielding frames). This requires to take various sets of calibration images.
The primary goal of flatfielding images is to correct for pixel-to-pixel sensitivity variations across the area so that relative intensities of objects imaged in different parts of the array are accurately recorded. Flattening the sky background is a secondary effect, although this should also be achieved if the array responds similarly to stellar continuum light and sky emission. Three flatfielding strategies are possible: A set of sky images can be combined to form a sky flat frame, or the same can be done using twilight images. The images of an illuminated screen within the dome can be combined to form a dome flat-fielding frame. The dome flat field could be determined daily taking exposures of the telescope dome with and without illumination from a quartz lamp. The final flatfield frame is the image difference of the two exposures, normalising to its mean.
During the observation one typically shifts the sky frame in a circular pattern around the object frame, with each frame first on the sky, the next on the object and the following frame again on the sky, but now an other part. For the CA set this was done in an OFF-ON pattern (off the source, i.e. on the sky - on the source). For the UKIRT set this was done in an ON-ON-OFF-OFF pattern. The different sets were not fitting to each next set to form a singular sequence. For the data reduction the sequence of images were re-ordered to an OFF-ON pattern so our reduction programs could handle the data similarly as the CA set. To be able to end with a sky frame again the last sky frame was often used again so no object frames were lost unintentionally.

TABLE 1

OBSERVATION SAMPLE

Galaxy	RA	DEC	Type	T	$d_{25}$	$i$	$v_{rot}$	$v_{\odot}$	$v_{vir}$	D
	(2000.0)	(2000.0)			[$'$]	[$^o$]	[km s$^{-1}$]	[km s$^{-1}$]	[km s$^{-1}$]	[Mpc]
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)
NGC 2424	07h40m39.3s	+39d14m00s	SBb	3.5	3.4	86.5	195.5	3353	3243	45.7
NGC 2591	08h37m25.5s	+78d01m35s	Sc	3.0	3.0	85.5	117.8	1323	1583	22.3
IC 3322A	12h25m42.6s	+07d13m00s	SBc	4.0	2.3	88.5	126.3	995	1055	14.9
NGC 5290	13h45m19.2s	+41d42m45s	Sbc	3.1	3.4	88.5	220.9	2573	2817	39.7
NGC 5348	13h54m11.2s	+05d13m38s	SBbc	3.5	3.5	86.5	67.7	1451	1524	21.4
NGC 5981	15h37m52.7s	+59d23m38s	Sbc	2.8	2.5	86.5	251.1	1764	2813	39.6
NGC 0973	02h34m20.1s	+32d30m20s	Sb	4.0	3.9	89.5	269.2	4855	4948	69.7
UGC 3186	04h51m46.0s	+03d40m05s	Sc	2.0	1.6	-	108.5	4578	4509	63.5
NGC 1886	05h21m48.1s	-23d48m37s	Sbc	3.8	3.2	86.5	154.6	1755	1544	21.7
NGC 2424	07h40m39.3s	+39d14m00s	SBb	3.5	3.4	86.5	195.5	3353	3243	45.7
UGC 4277	08h13m57.2s	+52d38m53s	Sc	4.9	3.3	-	271.4	5459	5650	79.6
IC 2531	09h59m55.5s	-29d37m04s	Sc	7.5	6.6	89.5	222.1	2472	2315	32.6
NGC 4179	12h12m52.1s	+01d17m59s	S0	-1.9	4.3	-	-	1239	1269	17.9
FGC 2339	21h44m39.4s	-06d41m21s	Sc	6.2	1.9	88.5	85.2	3098	3096	43.6
IC 5249	22h47m06.2s	-64d49m56s	SBcd	6.7	4.0	89.0	98.1	2364	2111	29.7
Notes: (1) Galaxy. (2) Right Ascension. (3) Declination. (4) Galaxy Type. (5) Morphological Type Code. (6) Diameter at $\mu_B$ = 25 mag arcsec$^{-2}$. (7) inclination. (8) Rotational velocity. (9) Heliocentric velocity. (10) Systemic velocity with respect to the Virgo Cluster. (11) Distance based on $H_0$ = 71 km s$^{-1}$ Mpc$^{-1}$.

For the CA set at the beginning or end of each night dome flatfielding and dark images were taken. The observations were typically done in 2-3 sets of 28 image frames. In the CA observations the exposure time in the J-band was 10$\times$6 seconds (which were summed and averaged on the chip) integration time per frame; in the K'-band this was 30$\times$2 seconds per frame, so the sky was monitored every 60 seconds.
For the UKIRT set three sets of 10 frames with a total exposure time of 120 seconds per final image. The observations were typically done in sets of 10-15 image frames. In the first night four standard stars were observed, both in the K and J-band: P9105, P9122, P9138 and P9148 (taken from the catalogue by Persson (1998). In the second night the same stars were used, except for P9148. The stars were observed 1-3 times for 1$\times$15 seconds in J and 1$\times$10 seconds in K' during the night.

TABLE 2

OBSERVATION DATA

Galaxy	Filter	Date	Run-	$t_{exp}$	Coadds	Seeing
			ID	[min]	#$\times$[s]	[$''$]
IC 3322A	J	04022004	CA	56	56$\times$60	0.9
IC 3322A	K'	04022004	CA	24	24$\times$60	1.3
NGC 2424	J	05022004	CA	29	29$\times$60	0.8
NGC 2591	K'	07022004	CA	53	53$\times$60	0.9
NGC 5290	J	03022004	CA	42	42$\times$60	0.8
NGC 5290	K'	03022004	CA	38	38$\times$60	1.2
NGC 5348	J	07022004	CA	34	34$\times$60	0.9
NGC 5981	J	05022004	CA	61	61$\times$60	0.8
IC 2531	J	19122000	UKIRT	28	14$\times$120	0.9
IC 2531	K'	19122000	UKIRT	30	10$\times$120	0.9
NGC 0973	J	19122000	UKIRT	30	15$\times$120	0.6
NGC 0973	K'	20122000	UKIRT	30	15$\times$120	0.9
NGC 1886	J	19122000	UKIRT	30	15$\times$120	1.0
NGC 1886	K'	19122000	UKIRT	30	15$\times$120	1.0
NGC 2424	J	20122000	UKIRT	20	10$\times$120	1.3
NGC 2424	K'	20122000	UKIRT	30	15$\times$120	1.8
UGC 3186	J	20122000	UKIRT	30	15$\times$120	1.3
UGC 3186	K'	20122000	UKIRT	26	13$\times$120	1.3
UGC 4277	J	19122000	UKIRT	30	15$\times$120	0.7
UGC 4277	K'	20122000	UKIRT	30	15$\times$120	1.3
FGC 2339	R	02082000	ESO	30	3$\times$600
IC 5249	R	01082000	ESO	30	3$\times$600
NGC 4179	V	051999	ESO	65	1$\times$600, 1$\times$480, 1$\times$360	1.6
					1$\times$300, 9$\times$240