Program

• Goal: To nail down the transformation from Smith calibrated MegaCam observations to AB magnitudes.
• Method: On photometric nights, observe stars with very well calibrated SEDs (from CALSPEC) on the same chips as Smith standard stars were observed that same night.
• Targets: Currently observable targets are given below.

Many of the gritty details of the observing plan can be found in the Phase 2 file under the program 06BE92. For further information, contact Alex Conley( conley@astro.utoronto.ca).

Eventually, we would like to observe at least one of these stars in all four filters (griz). More than one star would be very useful, as it would provide some ability to check the results. However, it is not necessary to observe all of the stars, nor is it necessary that they be linked with all of the SA standard star fields. Furthermore, it is not necessary that multiple filters be observed on a given night, as long as the filter observed was one for which standard star observations were obtained. It isn't strictly necessary that DEEP field observations were obtained on the same night, although it would be convenient.

Exposure times are in the 1-7 second range; some of these stars are fairly bright, so defocusing may be necessary as noted below. The dimmer stars, which do not require defocusing, are preferred. Because they are bright, observations during twilight are perfectly acceptable.

Right now, SA 95 and SA 92 are of interest although SA 104, SA 105, and SA113 are partially supported. At the moment, SA 92, 95 and SA 101 seem to be the standards that are being observed by MegaCam, but SA 101 is not suitable because almost all of the Smith standards in this field are saturated. SA 95 is preferable to SA 92.

Thus, an observational sequence consists of:

1. Choose a filter for which standard star observations were or will be obtained on the current night.
2. Choose one of the three current targets from below.
3. Determine which standard star field of SA 92, SA 95 or SA 110 was or will be observed that night. If multiple standards were taken, SA 95 is preferable.
4. Look up the combination of pointings for the combination of the CALSPEC star and the standard field. These are given below, but are also present as defined OGs in the PH2.
5. Defocus the telescope if necessary, as specified for the target star and filter.
6. Go to each pointing and take an exposure of the appropriate length.
7. If there is remaining time, go back to step one and do another filter.

The actual exposure time is given below for the current targets. If the seeing is very good, for some of the targets defocusing may be necessary. This is noted on a target by target basis.

Current targets

At the moment (Dec 20, 2006) there are two possible targets: G191B2B and HZ4. This are both mid-night target. G191B2B is moderately bright, and may require defocusing, but HZ4 is sufficiently dim that none should be necessary. For these reasons, HZ4 is preferable.

These should be observed on a night where either SA95 or 92 was or will be observed. The exact pointings depend on which of these is active. Note that the pointing offset information is also in the PH2.

Object	Priority	RA (J2000)	DEC (J2000)	V	Exposure	Defocus As a function of seeing	Finder	Pointings	Time
HZ4	High	03:55:21.70	09:47:18.7	14.51	7/7/7/7 sec (griz)	None	chart	See PH2	Middle
G191B2B	Medium	05:05:30.6	52:49:56	11.77	2/2/2/3 sec (griz)	sometimes	chart	pointings	Middle

This stars are all quite blue, so defocusing (if necessary) is most important to the blue. The defocus calculations can be seen below.

Purpose

The purpose of this program is to improve the calibration of the Legacy Survey data to the AB system. The standard star observations taken every night allow the Legacy observations to be tied to the Smith system, but this is not an AB system. One route to handling this problem is to try to calculate the difference between the Smith system and an AB system, which is also being pursued. A more direct route is to observe stars with well known SEDs using MegaCam and use synthetic photometry to compare the AB zeropoint with the Smith zeropoint for that night. Smith observations are still necessary and useful because they are available on all nights and at a wider range of chip positions.

The particular choice of CALSPEC stars is driven by the fact that the best available SED of Vega and BD +17 4708 are both tied to the CALSPEC program, so systematic uncertainties tying the final AB magnitudes to Landolt magnitudes will partially cancel. Furthermore, these SEDs are expected to be more reliable than ground based calibrations.

DEEP fields

For quick reference, the DEEP fields are located at:

Field	RA (J2000)	DEC (J2000)
D1	02:26:00.0	-04:30:00
D2	10:00:28.6	02:12:21
D3	14:19:28.01	52:40:41
D4	22:15:31.67	-17:44:05.7

CALSPEC stars

The CALSPEC web page can be found here. There are several types of stars. There are 4 fundamental H WDs that are used to define the CALSPEC calibration. These are by far the most desireable objects to observe. They are:

Name	RA (hms)	DEC (dms)	V	B-V	Finding Chart	Notes
G191B2B	5:05:30.6	52:49:56	11.77	-0.33	chart
GD71	05:52:27.51	15:53:16.6	13.03	-0.25	chart	Landolt star
GD153	12:57:02.37	22:01:56.0	13.35	-0.29	chart
HZ43	13:16:22.0	29:05:57.0	12.91	-0.30	chart	Has companion star within 3" Don't observe

None of these stars is particularly equatorial, so they aren't all that close to any of the Landolt SA fields. All coordinates are J2000.

There are also a set of secondary standards that have been tied, with STIS, to the calibration of the above objects. These are slightly less desireable, but still pretty useful.

Name	RA (hms)	DEC (dms)	V	B-V	Finding Chart	Notes
HZ4	03:55:21.70	09:47:18.7	14.51	+0.09	chart
Feige 34	10:39:36.7	43:06:10	11.18	-0.34	chart	Is Smith star
HD93521	10:48:23.51	37:34:12.8	7.04	-0.27	chart	Too bright! Don't observe.
HZ21	12:13:56.42	32:56:30.8	14.69	-0.33	chart
HZ44	13:23:35.37	36:08:00.0	11.67	-0.29	chart
BD+28 4211	21:51:11.07	+28:51:51.8	10.51	-0.34	chart	Smith star
Feige 110	23:19:58.39	-05:09:55.8	11.83	-0.30	chart

Two of these (Feige 34, BD+28 4211) are Smith standard stars, which is of some utility, but both are isolated standards.

In addition, there are 3 solar analogue stars that have been placed on the CALSPEC system in the optical. The fact that these are solar analogues is not of interest to us, but the fact that they have very different colours than the WDs is useful. The price is that their SEDs are considerably more complicated.

Name	RA (hms)	DEC (dms)	V	B-V	Finding Chart	Notes
P041C	14:51:58.19	71:43:17.43	12.01	+0.60	None
P177D	15:59:13.59	47:36:41.8	13.47	+0.63	None
P330E	16:31:33.85	30:08:47.1	13.01	+0.62	None

Note that the CALSPEC page lists some additional stars not given here. These do not have STIS based calibration, and hence are not of interest to us.

Standard star sequences

Here we give pointings for various combinations of CALSPEC stars and standard fields. SA 107 is not useful to us because the unsaturated standards are too close to the edge of the field. Note that the Phase 2 file provides pointing information in the form of offsets which is probably more useful for observers.

GD153

This information is also present in the PH2 as defined OGs.

Field	Npointings	Chips	Pointing1	Pointing2	Pointing3	Pointing4	Pointing5
SA104	2	17, 0	RA 12:58:51.99 DEC 21:59:38.5	RA 12:55:16.52 DEC 21:49:52.4
SA105	1	13	RA 12:57:03.37 DEC 21:54:21.0
SA110	4	25,15,2	RA 12:58:22.33 DEC 22:09:46.7	RA 12:57:46.22 DEC 21:56:18.5	RA 12:56:07.00 DEC 21:43:40.1	RA 12:56:02.97 DEC 21:41:58.6
SA113	5	22,13,15,5,4	RA 12:58:03.37 DEC 21:53:17.9	RA 12:57:31.62 DEC 21:41:00.5	RA 12:57:14.17 DEC 21:03:36.2	RA 12:57:10.98 DEC 21:57:22.8	RA 12:57:07.71 DEC 21:41:55.3

This table gives the pointings needed if GD153 is the CALSPEC star for various standard stars. The Chips column names which chips we are trying to place the CALSPEC star on, and the pointing columns give the position to point the telescope at to achieve this goal.

G191B2B

This information is also present in the PH2 as defined OGs.

Field	Npointings	Chips	Pointing1	Pointing2	Pointing3	Pointing4	Pointing5
SA92	3	12,23,31	RA 05:04:59.30 DEC 52:37:56.4	RA 05:05:57.53 DEC 52:59:08.6	RA 05:05:28.02 DEC 53:06:01.5
SA95	4	12,15,15,20	RA 05:04:36.95 DEC 52:50:54.4	RA 05:06:33.11 DEC 52:35:52.2	RA 05:06:25.76 DEC 52:35:40.3	RA 05:04:56.40 DEC 52:42:06.5

This table gives the pointings needed if G191B2B is the CALSPEC star for various standard stars. The Chips column names which chips we are trying to place the CALSPEC star on, and the pointing columns give the position to point the telescope at to achieve this goal.

Feige 110

This information is also present in the PH2 as defined OGs.

Field	Npointings	Chips	Pointing1	Pointing2	Pointing3	Pointing4	Pointing5
SA92	3	12,23,31	RA 23:19:27.89 DEC -05:21:33.4	RA 23:20:26.12 DEC -05:00:21.2	RA 23:19:56.61 DEC -04:53:28.3
SA95	4	12,15,15,20	RA 23:20:53.55 DEC -05:24:11.50	RA 23:21:00.90 DEC -05:23:59.60	RA 23:20:53.55 DEC -05:24:11.50	RA 23:19:04.74 DEC -05:08:57.40
SA104	2	17, 0	RA 23:18:21.48 DEC -05:12:13.3	RA 23:18:12.54 DEC -05:18:12.5
SA105	1	13	RA 23:19:59.4 DEC -05:17:30.8
SA110	4	25, 15, 2, 2	RA 23:21:18.35 DEC -05:02:05.1	RA 23:20:42.24 DEC -05:15:33.3	RA 23:19:03.02 DEC -05:28:11.7	RA 23:18:58.99 DEC -05:29:53.2
SA113	5	22, 13, 15, 5, 4	RA 23:20:59.39 DEC -05:18:33.9	RA 23:20:27.64 DEC -05:30:51.3	RA 23:20:10.19 DEC -05:08:15.6	RA 23:20:07.00 DEC -05:14:30.0	RA 23:20:03.73 DEC -05:29:56.5

This table gives the pointings needed if Feige110 is the CALSPEC star for various standard stars. The Chips column names which chips we are trying to place the CALSPEC star on, and the pointing columns give the position to point the telescope at to achieve this goal.

BD +28 4211

This information is also present in the PH2 as defined OGs.

Field	Npointings	Chips	Pointing1	Pointing2	Pointing3	Pointing4	Pointing5
SA104	2	17, 0	RA 21:53:00.69 DEC 28:49:34.3	RA 21:49:25.22 DEC 28:39:48.2
SA105	1	13	RA 21:51:12.07 DEC 28:44:16.8
SA110	4	25, 15, 2, 2	RA 21:52:31.03 DEC 28:59:42.5	RA 21:51:54.92 DEC 28:46:14.3	RA 21:50:15.70 DEC 28:33:35.9	RA 21:50:11.67 DEC 28:31:54.4
SA113	5	22, 13, 15, 5, 4	RA 21:52:12.07 DEC 28:43:13.7	RA 21:51:40.32 DEC 28:30:56.3	RA 21:51:22.86 DEC 28:53:32.0	RA 21:51:19.68 DEC 28:47:18.6	RA 21:51:16.41 DEC 28:31:51.1

This table gives the pointings needed if BD+28 4211 is the CALSPEC star for various standard stars. The Chips column names which chips we are trying to place the CALSPEC star on, and the pointing columns give the position to point the telescope at to achieve this goal.

Defocus

Because some of these stars are relatively bright, it may be necessary defocus the telescope in order to observe them. Below are the results of some calculations done by Nicolas Regnault giving the amount of defocus needed for various magnitudes of stars, as well as the necessary defocus values for various CALSPEC stars.

Defocus requirements for Feige110 (microns)

Defocus given in microns.

Seeing	g	r	i	z
< 0.6"	700	500	200	None
< 0.9"	400	None	None	None
> 0.9"	None	None	None	None

Defocus requirements for G191B2B (microns)

Defocus given in microns.

Seeing	g	r	i	z
< 0.6"	800	600	300	None
< 0.9"	500	200	None	None
> 0.9"	None	None	None	None

Defocus requirements for BD+28 4211

Defocus given in microns.

Seeing	g	r	i	z
< 0.6"	1000	700	400	None
< 0.9"	700	500	None	None
< 1.2"	400	None	None	None
> 1.2"	None	None	None	None

The curves are shown for two different levels of seeing, 1.3 arcsec (sigma=3) and 0.9 arcsec (sigma=2). For four second exposures, it is not necessary to defocus the telescope for stars dimmer than about magnitude 12, and for 2 seconds about 11.3.

Most of these stars are quite blue, so if it is not necessary to defocus for g it should not be necessary for riz. We can summarize the exposure times for which defocusing is necessary in a table as well.

Magnitudes for which defocusing is necessary

Exptime	1.2"	0.9"
1 sec	9.7	10.55
2 sec	10.4	11.25
3 sec	10.9	11.75
4 sec	11.2	12.05
6 sec	11.6	12.45
8 sec	12.95	12.80

These are in the Vegamax system.