FORS2 CCD upgrade

Table of Content

In General
Detector Characteristics
New Field Geometry
User Interfaces
Wavelength Coverage

In General

ESO is working on the replacement of the present 2kx2k, 24µm pixels detector of FORS2 with a mosaic of two MIT 2kx4k, 15µm pixels red-enhanced chips with the first tests starting in late 2001. The goal is to get a much better sensitivity in the 700-1100 nm wavelength domain. Furthermore a lower readout noise will be achived at much higher readout speed then with the Site CCD used with FORS2 so far. The detector upgrade including all software changes on the instrument side will be ready for operation in April 2002 with a temporary installation of the mosaic during the bright time in October/November 2001 for tests and commissioning.

This page summarizes the changes expected to come with the CCD upgrade without being very accurate since most parameters are still to be measured but it should be accurate enough for proposal preparations. There will be no changes for the observing block preparation of next observing Period 68 (October 2001 to March 2002)!

Detector Characteristics

High Time Resolution - HIT modes

One important change in the new configuration will be that the CCDs are read out in dispersion direction. As a consequence the high time resolution modes will have to be modified such that spectroscopy will be done with the cross disperser grisms. This cannot be offered in time for Period 69 and has to be followed up later on.

CCD Readout Modes

As a general rule only 2x2 binned readout will be offered (100kHz for spectroscopy and 225kHz for imaging) to obtain homogenous FORS archive data and to reduce calibration times. This will result in image scales of 0.25 arcseconds/pixel in standard resolution and 0.125 arcseconds/pixel in high resolution mode. Unbinned readout modes for special applications (225kHz) must be explicitely requested and justified in the proposals.

CCD Readout Time and Readout Noise

The new MIT CCD can be read out faster then the SITE CCD at lower RON. As a consequence of the changed readout direction there will be some overhead time required to rotate the images. Therefore we request users to consider 40s including the readout time and the so far unknown overhead time in the P69 proposals for every single CCD readout.

Performance of the old SITE CCD with the expected performance of the new MIT mosaic. Overhead times for wiping, CCD setup and image rotation are not included.

observing mode	readout speed	binning	RON in e-	readout time
Site 2kx2k (old)	50 kHz 1-port	1x1	5.8	85s
Site 2kx2k (old)	50 kHz 4-port	1x1	5.8	21s
MIT 2x2kx4k spectroscopic	100kHz 2-port	2x2	<3	25s
MIT 2x2kx4k imaging	225kHz 2-port	2x2	<4	10s

CCD Response

This figure displays the detector response of the old SITE (solid line) and the new MIT mosaic (dotted line). A significant gain in performance is expected in the near infrared at lambda > 7500Å.

New Field Geometry

With the standard resolution collimator the field will be restricted by the geometry of the MOS unit which is permanently in the field of view and of approximately the same size as the 6.8 arcminute field of view of the previous configuration. In high resolution mode, the field of view will increase from 3.4' x 3.4' to 4.2' x 4.2'.

The CCD mosaic is mounted off-axis to ensure that the target will fall on the upper (master) CCD and to ensure that the gap will fall between two MOS slits.

Even though the enlarged detector size will be vignetted in the focal plane, there will be more freedom in selecting slits in a larger x-axes range for a given wavelength range! In the example below the wavelenth range was set to the nominal wavelength range (545 - 810nm) of grism 600RI with the old SITE CCD. The inner blue box (between the two |X| |X|) is the additional space within slits can be placed with the MIT mosaic:

The new field geometry for observations with the standard resolution collimator. The field of view will be limited in the focal field by the MOS unit (large blue rectangle).

The new field geometry for observations with the high resolution collimator. The field of view will be limited by the size of the two CCD (two green rectangles).

User Interfaces

Exposure Time Calculators

The Exposure Time Calculator can be optionally used with both detector response curves.

The following photometric zero points were calculated with the ETC for a zero magnitude star at zero airmass. The calculated performance can be compared among the two CCD configurations and with data taken on the sky:

Expected changes of the photometric zero points

Filter	Calculated	Calculated	Measured
	MIT	SITE	Aug.18, 2001
z	27.15	26.31
I	27.63	27.26	27.23
Rsp	28.20	28.13	28.13
V	28.00	28.00	27.95
B	27.60	27.64	27.58
Usp	23.96	24.70	24.71

FIMS - Mask Preparation Software

FIMS will only support the modes for which mask preparation is absolutely required: MOS, MXU and imaging with occulting bars. All other observing modes like imaging, long slit and Echelle spectroscopy are supported with "fast mode" which should be more straight forward to prepare from the point of view of an observer. FIMS will display the CCD projected to the sky (green rectangles) and the field stop geometry in the focal plane of the telescope (blue rectangle).

Observations and Target Acquisitions

All target acquisitions will be done based on acquisition images with the upper "master CCD". As a consequences users will be forced to select reference stars and reference slits only on the master CCD. The lower "slave CCD" will be merged based on transformations calculated from pinhole masks by the FIMS mask preparation software and stand alone routines to be provided to the FORS users.

Wavelength Coverage

Grism name +number	Wavelength range (SITE)	Dispersion	Wavelength range (MIT)	Order separation
	[nm]	[Å/mm]	[nm]	filter

GRIS_600B+22	345 - 590	50	330 - 621	none
GRIS_600I+25	690 - 910	44	663 - 939	OG590
GRIS_300V+20 (1)	330 - (650)	110	330 - (650)	none
GRIS_300V+20 (1)	385 - (750)	111	385 - (750)	GG375
GRIS_300V+20	445 - 870	112	445 - 870	GG435
GRIS_300I+21	600 - 1100	108	600 - 1100	OG590
GRIS_200I+28	560 - 1100	162	560 - 1100	OG550 (cemented)
GRIS_150I+27 (1)	330 - (650)	225	330 - (650)	none
GRIS_150I+27 (1)	385 - (750)	230	385 - (750)	GG375
GRIS_150I+27 (1)	445 - (880)	230	445 - (880)	GG435
GRIS_150I+27	600 - 1100	230	600 - 1100	OG590
holographic:
GRIS_1400V+18 (3)	469 - 573	20.8	456 - 586	none
GRIS_1200R+93 (3)	590 - 715	25.0	575 - 731	GG435
GRIS_1028z+29 (3)	790 - 930	28.3	773 - 948	OG590
GRIS_600RI+19 (3)	545 - 810	55	512 - 845	GG435
GRIS_600z+23 (3)	770 - 1036	54	737 - 1070	OG590
2nd order:
GRIS_600I+25 (2)	380 - 475	19	369 - 488	FILT_465_250+82
GRIS_600z+23 (2,3)	404 - 535	20	389 - 546	FILT_465_250+82

(1) If used without or with the listed order separation filter, the orders will overlap above the given wavelength.
(2) Second order.
(3) Based on a volume phased holographic grating.

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Sun Sep 9 14:13:33 CLT 2001