RATCam

RATCam was decommissioned at the end of February 2014. The information presented here is for historical purposes only. The common user, visible light CCD imager is now IO:O

Introduction

RATCam was the Liverpool Telescope's main workhorse optical CCD camera and was funded by PPARC (now STFC). 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 & Performance

  Detector 2048x2048 pixel EEV CCD42-40, back-illuminated non-aimo AR coated broadband chip
  Pixel size 13.5 microns
  Pixel scale approx. 0.135 arcsec/pixel (unbinned)
  Field of view 4.6x4.6 arcmin
  Read noise < 5 electrons
  Pattern noise None measurable
  Dark current None measurable
  Binning 1x1, 2x2, 3x3, 4x4
  Readout time ~10 sec (1x1), ~5 sec (2x2)
  Windowed modes Contact us
  Bad pixels 67 dark point defects; one bad column
(these are manufacturer's spec)
click here for bad pixel masks
  Gain (1x1) 2.34 electrons/count
  Gain (2x2) 2.13 electrons/count
  Quantum Efficiency

Wavelength (nm)

Quantum Efficiency* (%)

350

41.8

400

78.8

500

93.8

650

86.6

900

28.3

* Measurements made at -85°C. RATCam's operating temperature is -107°C however.

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

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Filter Set

Filter Name

Wavelength Range
(Angstroms)

Sloan u'

3150 - 3900

Sloan g'

4100 - 5500

Sloan r'

5560 - 6890

Sloan i'

6930 - 8670

Sloan z'

8510 ->

Bessell B

3780 - 4830

Bessell V

4950 - 5950

Halpha
[GIF]

6517 - 6617

There were a total of eight core filters, which were 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 could also be made in "clear" mode, i.e. unfiltered.

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

  • The plate scale when using the H-alpha filter was approximately 0.138 arcsec/pix in contrast to 0.140 arcsec/pix in the other filters.
  • The H-alpha filter generated weak ghost images from any bright sources which appeared in the 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 were based on the Landolt series of standards (link to Landolt paper on NASA ADS). They were spaced every few hours of RA and were observed in all bands every two hours by the robotic control system. If observers required standards beyond what were specified here they requested them explicitly in their Phase 2 sequence definition. Note: The time needed for these observations had to be included in applications for telescope time (since observers were charged for any time spent observing extra standards for their programme.)

Clicking the name of the field brings 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
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

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
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
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
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
23h 33m 48.0s
+05° 46' 10.7 "
(x) 15.182±0.0057
(A) 13.051±0.0021
(B) 14.744±0.0035

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Colour Terms

Preliminary colour terms were 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 were heavily saturated (i.e. above 65000 counts and bleeding) could leave persistent "ghost" images on subsequent frames. The residual image faded as a roughly linear function of time. There was unfortunately nothing that could be done about this, and the best approach was just to make sure it did not happen in the first place. It was therefore important that observers carefully checked their fields to ensure that no bright objects near their target would be grossly saturated in their requested integration time.

The telescope operations staff occasionally at their discretion reduced exposure times or removed targets from the observing database. In such cases they contacted the PI to discuss how best to meet their science objectives without endangering the equipment or other observers' programmes.

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Pipeline


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

Bias Subtraction

There was insufficient repeatable structure for bias frames to be useful. Bias subtraction was therefore based purely on analysis of the underscan region. Linear regression was 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 showed that RATCam did have a small ramp in the bias down each column and this first order fit was required. No attempt was made to remove any bias gradient in rows across the image.

Overscan Trimming

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

Dark Substraction

This was not performed though the facility existed in the reduction pipeline. Experience showed that when the camera was at normal operating temperature, dark current was not significant. If the camera temperature fluctuated for any reason, the dark current was sufficiently variable that a dark would need to be obtained with every frame and library dark frames were of no use.

Flat Fielding

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

Each twilight the instrument control software (ICS) attempted 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 wasn't enough time to take sky flats for all combinations in one twilight session, so the next oldest in the list was attempted in following nights and so on. Usually the update was complete after 2-3 nights and the process started again.

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

Bad Pixel Mask

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

Vignetting

The filter wheel slightly vignetted the optical beam to a different extent for each filter. In the extreme corners of the worst affected bands (i' and g') the flux was reduced by up to 15% compared to the unobstructed beam. In the other filters, obscuration was 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 fielded out very well and was rarely obvious in the reduced data, sometimes leaving distortions only in the range 3 - 5 arcsec from the field corner. Even where the data was well flat fielded though, noise characteristics of photon counting statistics could be affected in these regions of the frame.

Fringe Frames

Automated defringing was not performed on CCD data before it was loaded into the archive. To help you defringe your own historical data, linked below are prepared master fringe frames created by stacking multiple deep integrations of blank fields. The master fringe frames were updated infrequently because the fringes on the CCD were found to vary only on timescales of months. If you need access to the individual integrations which went into building these master frames, they are still publicly available from the data archive. Simply select RATFringe from the Proposal ID drop-down list.

 

  • Legacy Fringe Frames. Prior to July 2006
  • Legacy Fringe Frames. Created July/August 2006
  • Legacy Fringe Frames. Created September 2007 from data obtained Jan - Aug 2007.
  • 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.
  • Legacy Fringe Frames. Created Feb 2008 from data obtained Jan - Feb 2008.
  • Created Jan 2009 from data obtained throughout 2008.
  • Created Jan 2010 from data obtained throughout 2009.

FITS Header Error Codes

Error codes were 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.

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Phase 1 Information

RATCam operated in a very similar manner to IO:O. Users calulating the amount of time required for a complete observation followed the same algorithm as for that instrument, but using the following numbers:

  • Acquisition time Ta - 60 seconds (time taken to slew the telescope on target).
  • Autoguider acquisition - 45 seconds (only if you nominate to use the autoguider).
  • Filter change time Tf - 5 seconds.
  • Readout time Tr - 10 seconds.

Phase 2 UI Instructions

Historical instructions on using the Phase 2 GUI to program the LT to use RATCam:
RATCam Phase 2 Instructions