RATCam

• Specifications/performance
• Zeropoints
• Filter Set
• Standards
• Colour Terms
• Saturation & Charge Persistence
• Pipeline
• Phase 1 Information
• Phase 2 UI Instructions

Introduction

RATCam is the Liverpool Telescope optical CCD camera and was funded by PPARC. It was built jointly by JMU (responsible for software and integration), TTL (responsible for the filter wheel mechanisms) and Astro-Cam at SDSU (responsible for the Dewar, cooler and controller).

Specifications & Current Performance

Zeropoints

The following approximate zeropoints are per photoelectron. Remember to take account of the gain if you are considering counts.

Band	Zeropoint (one photoelectron)
Sloan u' (w.r.t. Landolt U)	21.3
Bessell B	24.4
Bessell V	24.3
Sloan r' (w.r.t. Landolt R)	24.5
Sloan i' (w.r.t. Landolt I)	24.1

Filter Set

There are a total of eight core filters, which have been chosen to maximize scientific utility to both stellar and extragalactic astronomers combined with high throughput. The core filter set is given in the table at right. Filter transmission curves can be downloaded by clicking on the filter name. Note that the transmission curves are measured at 20 degrees Centigrade and in a collimated beam, and therefore will need to be corrected for the actual temperature at the time of observation and the f/10 beam of the telescope. Observations may also be made in "clear" mode, i.e. unfiltered.

There are two factors of which to be aware when using the H-alpha filter:

• The plate scale when using the H-alpha filter is approximately 0.138 arcsec/pix in contrast to 0.140 arcsec/pix in the other filters.
• The H-alpha filter generates weak ghost images from any bright sources which appear in your field. The secondary image can be located by linearly extrapolating a vector from the instrument's optical axis through the primary image of the star in length by a factor of 1.0620. Flux in the secondary image is 1% of the primary.

Standards

Standard fields for RATCam are based on the Landolt series of standards (link to Landolt paper on NASA ADS). They are spaced every few hours of RA and are observed in all bands every two hours by the robotic control system. If you require standards beyond what are specified here you must request them explicitly on your Phase 2 sequence definition and will be charged the observing time for then.

Clicking the name of the field will bring up an image of the field with north at top and east to the left. All images are taken from the Digitised Sky Survey "POSS-1 Red" archives. Annotated stars' magnitudes are V-band, taken directly from the Landolt paper.

Name	J2000		V
	RA	Dec
PG0231+051	02h 33m 40.0s	+05° 18' 40.0"	(x) 16.105±0.0068 (A) 12.772±0.0008 (B) 14.735±0.0030 (C) 13.702±0.0014 (D) 14.027±0.0029 (E) 13.804±0.0046
RUBIN 149	07h 24m 14.4s	-00° 33' 04.0"	(x) 13.866±0.0022 (A) 14.495±0.0066 (B) 12.642±0.0021 (C) 14.425±0.0052 (D) 11.480±0.0019 (E) 13.718±0.0064
PG1047+003	10h 50m 07.6s	-00° 01' 07.3"	(x) 13.474±0.0039 (A) 13.512±0.0047 (B) 14.751±0.0050 (C) 12.453±0.0094
PG1525-071A	15h 28m 13.0s	-07° 15' 54.0"	(x) 15.053±0.0000 (A) 13.509±0.0000 (B) 16.403±0.0000 (C) 13.530±0.0000 (D) 16.301±0.0021
MARK-A	20h 43m 58.3s	-10° 46' 10.0"	(x) 13.258±0.0019 (1) 15.911±0.0040 (2) 14.540±0.0028 (3) 14.818±0.0024
PG2331-055A	23h 33m 48.0s	+05° 46' 10.7 "	(x) 15.182±0.0057 (A) 13.051±0.0021 (B) 14.744±0.0035

Colour Terms

Preliminary colour terms have been calculated for RATCam to allow transformation from instrumental to catalogue magnitudes.

(1)	g_inst	=	g_cat + A_g * (g_cat-r_cat)
(2)	r_inst	=	r_cat + A_r * (r_cat-i_cat)
(3)	i_inst	=	i_cat + A_i * (r_cat-i_cat)
(4)	z_inst	=	z_cat + A_z * (i_cat-z_cat)
(5)	B_inst	=	B_cat + A_B * (B_cat-V_cat)
(6)	V_inst	=	V_cat + A_V * (B_cat-V_cat)
(7)	R_inst	=	R_cat + A_R * (V_cat-R_cat)
(8)	I_inst	=	I_cat + A_I * (V_cat-I_cat)

where:

	f_inst	=	instrumental magnitude in filter f: • g,r,i,z from Sloan -- Allyn Smith et al, "The u'g'r'i'z' Standard-Star System", AJ, v123, Pp2121-2144, 2002 • B,V,R from Landolt -- Landolt, A, "UBVRI photometric standard stars in the magnitude range 11.5-16.0 around the celestial equator", AJ, v104, Pp340-371, 1992
	f_cat	=	catalogue magnitude in filter f
	A_f	=	colour term in filter f, given by: Filter A err(A) g' -0.029 0.013 r' 0.034 0.007 i' 0.053 0.011 z' 0.09 0.008 B -0.07 0.007 V 0.052 0.01 R 0.2 0.009 I 0.085 0.006

Saturation & Charge Persistence

Objects that are heavily saturated (i.e. above 65000 counts and bleeding) can leave persistent "ghost" images on subsequent frames. The residual image fades as a roughly linear function of time. There is unfortunately nothing we can do about this, and the best approach is just to make sure it does not happen in the first place. It is therefore important that observers carefully check their fields to ensure that no bright objects near their target will be grossly saturated in their requested integration time.

The telescope operations staff may at their discretion reduce exposure times or remove targets from the observing database. In such cases we will contact the PI to discuss how best to meet their science objectives without endangering the equipment or other observers' programmes.

Pipeline

Basic instrumental reductions are applied to all RATCam images before the data are passed to users. This includes bias subtraction, trimming of the overscan regions and flat fielding. A library of the current calibration frames is maintained as part of the data archive and updated daily so that images are always reduced using the latest available flat-field image available at the time. Each of the operations are performed as described below.

Bias Subtraction

There is insufficient repeatable structure for bias frames to be useful. Bias subtraction is therefore based purely on analysis of the underscan region. Linear regression is used to determine a fit to the bias counts as a function of pixel row number and values deducted across the image according to this smooth function. Experience shows that RATCam does have a small ramp in the bias down each column and this first order fit is required. No attempt is made to remove any bias gradient in rows across the image.

Overscan Trimming

The overscan regions are trimmed off the image leaving a 2048x2048 (assuming on-chip binning was not used) pixel image.

Dark Substraction

This is not currently performed though the facility exists in the reduction pipeline. Experience has shown that when the camera is at normal operating temperature, dark current is not significant. If the camera temperature fluctuates for any reason, the dark current is sufficiently variable that a dark would need to be obtained with every frame and library dark frames are no use.

Flat Fielding

The appropriate master flat field is selected from the library to match the filter and binning configuration of the current exposure. In fact the library holds reciprocal flat-fields normalised to unity because of the computational efficiency of multiplying rather than dividing. The image data are therefore multiplied by the library flat.

Each twilight the instrument control software (ICS) attempts to update the oldest master flats in its library, by taking 3-5 raw sky flats for each filter/binning combination, giving preference to the most used. There usually isn't enough time to take sky flats for all combinations in one twilight session, so the next oldest in the list is attempted in following nights and so on. Usually the update is complete after 2-3 nights and the process starts again.

The master flat is derived from the median of each sky flat after each has been normalised to the common mean count level. New master flats, and their corresponding ratio image of new/old masters, are inspected for any corruption, inclusion of stars, etc.

Bad Pixel Mask

No cosmic ray rejection or bad pixel mask is applied since it is important for users performing accurate photometry to know exactly what masking has been applied. However, bad pixel masks have been generated and kindly made available by the Angstrom project, a gravitational lensing programme underway on the LT and RoboNet:

• unbinned bad pixel mask (13KB gzipped FITS, unzips to 8MB)
• 2x2 binned bad pixel mask (5KB gzipped FITS, unzips to 2MB)

Vignetting

The filter wheel slightly vignettes the optical beam to a different extent for each filter. In the extreme corners of the worst affected bands (i' and g') the flux is reduced by up to 15% compared to the unobstructed beam. In the other filters, obscuration is about 5% in the very corner of the observed field, falling to negligible values between 10 - 20 arcsec from the field edge.

The vignetting generally flat fields out very well and is rarely obvious in the reduced data, sometimes leaving distortions only in the range 3 - 5 arcsec from the field corner. Even where the data is well flat fielded though, noise characteristics of photon counting statistics could be affected in these regions of the frame.

Fringe Frames

We currently do not perform any automated defringing on CCD data before it is loaded into the archive. In order to help you defringe your own data, linked below are prepared master fringe frames created by stacking multiple deep integrations of blank fields. The master fringe frames are updated infrequently because the fringes on the CCD have been found to vary only on timescales of months. If you need access to the individual integrations which go into building these master frames, they are publicly available from the data archive. Simply select RATFringe from the Proposal ID drop-down list. You can therefore extract the most recent fringe frame from the archive at any time.

Legacy Fringe Frames. Prior to July 2006 SDSS-I, binned 2x2 (4MB gzipped FITS, unzips to 8MB) SDSS-Z, binned 2x2 (2.3MB gzipped FITS, unzips to 4MB) Legacy Fringe Frames. Created July/August 2006 SDSS-I, binned 1x1, stack of 46 x 300 second exposures, 13MB gzipped fits SDSS-Z, binned 2x2, stack of 27 x 300 second exposures, 3.1MB gzipped fits SDSS-I, binned 2x2, stack of 27 x 300 second exposures, 3.1MB gzipped fits SDSS-R, binned 2x2, stack of 27 x 300 second exposures, 2.6MB gzipped fits, fringes barely visible. SDSS-Z, binned 1x1, stack of 27 x 300 second exposures, 11MB gzipped fits Legacy Fringe Frames. Created September 2007 from data obtained Jan - Aug 2007. SDSS-Z, binned 1x1, stack of 36 x 300 second exposures, 15MB gzipped fits SDSS-Z, binned 2x2, stack of 33 x 300 second exposures, 4MB gzipped fits SDSS-I, binned 1x1, stack of 18 x 300 second exposures, 15MB gzipped fits SDSS-I, binned 2x2, stack of 34 x 300 second exposures, 4MB gzipped fits Legacy Fringe Frames. Created December 2007 from data obtained Sep - Dec 2007. These are best suited to any data obtained in 2007 after the September mirror recoating and servicing. I.e., the majority of semester 07B. SDSS-I, binned 1x1, stack of 216 x 300 second exposures, 15MB gzipped fits SDSS-I, binned 2x2, stack of 18 x 300 second exposures, 4MB gzipped fits SDSS-Z, binned 1x1, stack of 18 x 300 second exposures, 15MB gzipped fits SDSS-Z, binned 2x2, stack of 18 x 300 second exposures, 4MB gzipped fits Legacy Fringe Frames. Created Feb 2008 from data obtained Jan - Feb 2008. SDSS-I, binned 1x1, stack of 90 x 300 second exposures, 15MB gzipped fits Created Jan 2009 from data obtained throughout 2008. SDSS-I, binned 1x1, 90 x 300sec exposures from 08B, 15MB gzipped fits SDSS-I, binned 2x2, Rebinned version of above 1x1 data, 4MB gzipped fits SDSS-Z, binned 1x1, 72 x 300sec exposures from 08A and 08B, 15MB gzipped fits SDSS-Z, binned 2x2, Rebinned version of above 1x1 data, 4MB gzipped fits Created Jan 2010 from data obtained throughout 2009. SDSS-I, binned 1x1, 112 x 300sec exposures from 09A-09B, 15MB gzipped fits SDSS-I, binned 2x2, 93 x 300sec exposures from 09A-09B, 4MB gzipped fits SDSS-Z, binned 1x1, 122 x 300sec exposures from 09A-09B, 15MB gzipped fits SDSS-Z, binned 2x2, 36 x 300sec exposures from 09A-09B, 4MB gzipped fits

FITS Header Error Codes

Error codes are written into the FITS headers for specific error flags brought up during processing. These error flags are stored in the header keywords L1STATOV, L1STATZE, L1STATTR, L1STATFL and L1STATDA and follow this convention:

• negative values are failure states (special case: -1 = operation not attempted)
• positive values are warning states (special case: +1 = no errors or warnings)

Tables of failure and warning states are given below - but please note these are not exhaustive lists.

FITS HEADER FAILURE STATE CODES
Code #	Process Name	Error String
-12	DpRT_startup	Input is not a valid LT filename.
-14	DpRT_startup	According to filename flags, input has already been processed.
-15	DpRT_startup	Run_mode is not valid (%d). See dprt.h for valid values.
-18	DpRT_startup	Invalid exposure type flag.
-35	DpRT_zero	Cannot find zero (bias) frame file.
-37	DpRT_dark	Cannot find dark frame file.
-38	DpRT_flat	Mean counts in flat <= 0. Processing abandoned.
-40	DpRT_flat	Cannot find flatfield file.
-60	DpRT_fringe	Cannot find fringe file.
-61	DpRT_fringe	Fringe and data frame sizes do not match.
-63	DpRT_fringe	Correlation scaling failed. No defringing performed.
-73	DpRT_dark	Correlation scaling failed. No dedarking performed.
-261		Error opening FITS.
-263		Error getting header keywords.
-264		Non-square binning.
-268		Error reading image array.
-354	DpRT_make_bias	Could not open working directory.
-356	DpRT_make_bias	Failure to allocate memory for **fits_pointers.
-357	DpRT_make_bias	Failed to open FITS file.
-358	DpRT_make_bias	Failed to allocate memory to median_array or new_bias_array.
-359	DpRT_make_bias	Failed to allocate memory to temp_array.
-360	DpRT_make_bias	Failed to allocate memory to temp_array[%d].
-361	DpRT_make_bias	Failed to read image data from FITS file number %d.
-363	DpRT_make_bias	Poor stats in new_bias.
-453	DpRT_make_flat	Could not read file (outer loop).
-454	DpRT_make_flat	Could not open directory %s (inner loop).
-462	DpRT_make_flat	Fewer than %d good flats from which to make master flat.
-452	DpRT_make_flat	Could not read existing flat frame.
-356	DpRT_make_flat	Failure to allocate memory for **fits_pointers.
-364	DpRT_make_bias	New bias is highly deviant from the old one: mean abs dev = %.
-454	DpRT_make_flat	Could not open directory.
-457	DpRT_make_flat	Failed to open FITS file.
-458	DpRT_make_flat	Failed to allocate memory to median_array or [mean|median]_flat_array.
-459	DpRT_make_flat	Failed to allocate memory to temp_array.
-460	DpRT_make_flat	Failed to allocate memory to temp_array[%d].
-461	DpRT_make_flat	Failed to read image data from FITS file number %d.

FITS HEADER WARNING STATE CODES
Code #	Process Name	Error String
32	DpRT_startup	CCD not at thermal set point.
52	DpRT_output	No filter calibration data for filter.
53	DpRT_output	No filter calibration data.
62	DpRT_fringe	No correlation between data and fringe. Defringing will use simple exposure time scaing.
64	DpRT_fringe	Correlation scaling failed. Simple exposure time scaing will be used.
72	DpRT_dark	No correlation between data and dark. Dedarking will use simple exposure time scaing.
220	DpRT_init	No '.' character in flatfield filename.
225	DpRT_init	Divide-by-zeroes were safely trapped.
226	DpRT_init	Failed to trim the flat field.
230	DpRT_init	Failed to open FLATLIB directory.
232	filter_params	Failed to open filter parameter lookup table.
233	filter_params	Failed to parse filter config line.
231	filter_params	Failed to allocate name memory for filter.
234	filter_params	Filter config line has a name > 49 chars.
235	filter_params	Filter scale factor or ZP read from file is not valid for filter.
236	filter_params	Failed to allocate filter memory.
251	DpRT_init	Failed to allocate memory for *photstar_data. Calibration data will not be available.
252	DpRT_init	Failed to open photstar parameter lookup table.
351	DpRT_make_bias	Could not open log file.
352	DpRT_make_bias	Could not read existing bias frame.
353	DpRT_make_bias	Could not read file.
355	DpRT_make_bias	Too many good bias frames were found.
362	DpRT_make_bias	New bias is highly discrepant from library bias.
366	DpRT_make_bias	More bias frames were discarded than kept.
367	DpRT_make_bias	Fewer than %d good bias frames.
363	DpRT_make_flat	Poor stats in new [mean|median] flat.
364	DpRT_make_flat	New flat is highly deviant from the old one.
365	DpRT_make_flat	No suitable flat frames found in directory. %d were read but discarded.
453	DpRT_make_flat	Could not read file (inner loop).
500 - 507	DpRT_init	NULL pointer in dprt_*_lib.

Phase 1 Information

To the calculated exposure time (T_e) the following overheads must be added:

• Acquisition time T_a - time taken to slew the telescope on target. This is 60 seconds.
• Autoguider acquisition - only if you nominate to use the autoguider. This is 45 seconds.
• Filter change time T_f. This is 5 seconds.
• Readout time T_r. This is 10 seconds.

Example Suppose you have a programme where you have 20 objects which you wish to observe for 10 nights in a row, with an exposure time of 60 seconds, through three different filters. In this example, the observer has decided not to use the autoguider.

For one 60s exposure only (i.e. only one filter), the total time T1 is  T1 = Ta + Tf + Te + Tr = 60 + 5 + 60 + 10 = 135 s However for three exposures of the same object, where a different filter is used each time (nf = 3), the total time spent on the object Tobj is:  Tobj = Ta + nf ( Tf + Te + Tr ) = 60 + 3 (5 + 60 + 10) = 285 s Note that the acquisition time is only applied once; no slewing is required after the 1st exposure because the telescope is already on target. For twenty different objects (nobj=20) slewing between each object, per night for ten nights (nN=10), the total time Ttot is:  Ttot = nN nobj Tobj = 10 x 20 x 285 = 57000 s = 15.83 hours As the unit of allocation is integer hours, you should therefore apply for 16 hours of time.

Phase 2 UI Instructions

See here for instructions on using the Phase 2 GUI to program the LT to use RATCam:
RATCam Phase 2 Instructions