Bias frames allow the "zero-point" of each CCD pixel to be established and subtracted from each science frame. They are normally frames of zero exposure time obtained in completely dark conditions. For PBT there are two complications.
To obtain bias frames with PBT:
Move the images for camera ccc to a (new) folder /starlight/biasyyyymmddccc/
To Be Done Investigate stability of bias over time scales of hours and days
Flat fields are images of a uniformly illuminated flat object which enable variations in camera sensitivity to be removed. These variations include pixel-to-pixel variations in CCD sensitivity due to imperfection inthe CCD manufacture and large-scale variations due to the camera optics. In the case of PBT, vignetting causes severe loss of sensitivity at the periphery of the image.
The normal way to obtain a flat-field is to take an image of the twilight sky or a white screen inside the dome. PBT only has the twilight sky option. Since the field of view of each camera is large (10 degrees), even the twilight sky is not perfect (there is a large brightness gradient from one side of the sky to the other after the Sun has gone down). The presence of clouds and/or stars renders a flat-field image useless. So does the presence of saturated pixels. So it is necessary to obtain the flat fields while the sky is still bright enough that no stars are visible, but not so bright that the images are saturated.
To obtain flat-field images with PBT:
Warning: On 2009/02/10 we experienced problems with flat-field images for camera 126 (spoc1). The images remain saturated long after the other cameras have reached astronomically low light levels, and there is severe vertical banding on the images. With reducing light levels, the saturation cleared from the center of the image, but persisted at the edges. With very low light levels, the camera returned to normal. It is not clear whether this is a persistent or intermittent problem.
On 2009/02/13 -- problem persists, though flat taken on uniform cloud at low light levels might be usable in extremis.
mean bias 2009/02/06
mean bias 2009/02/06
mean bias 2009/02/06
Calibration frames are stored on arpc55:/data/starlight/calibrations
Bias frames were taken in early February 2009. To create master bias frames, ftools was used to sum images and then divide the summed image by a constant, N. Within the set of bias frames taken on 2009-02-09, some had exposure time 0.001, others 0.002 and others 0.5. In the set taken 2010-02-13, all had exposure time 0.1. For 1261, 128 and 129, the higher exposure times were ignored. These master biases include only exposure times of 0.001 s. For 1262, only exposure times of 0.1 were included.
The bias frame created for camera 129 appeared as a dark frame with randomly scattered bright pixels at an average of about 30 counts above the background. On further examination, it became evident that these bright pixels appeared at the same x and y coordinates for camera 129 on the individual bias frames and on the individual flat field frames. It was decided that the bias should continue to contain these 'bad' pixels. Ideally, bias frames should be taken much more frequently, allowing a clearer view of whether problems like these are transient.
{| border=\"1\"
|+
'''Data on Master Bias Frames'''
!Camera Number for Which Bias Was Made
!Name of Master Bias
!Date and Time Frames Taken
!Exposure Times of Frames Used
!Number of Frames Used (N)
!Mean Pixel Value
!Standard Deviation of Pixel Value
|-
| align=\"center\" | 126
| align=\"center\" | meanbias126_20090206.fit
| align=\"center\" | 2009-02-06 @ 14:20-14:52
| align=\"center\" | 0.001
| align=\"center\" | 162
| align=\"center\" | 652.327
| align=\"center\" | 9.828
|-
| align=\"center\" | 128
| align=\"center\" | meanbias128_20090206.fit
| align=\"center\" | 2009-02-06 @ 14:20-14:52
| align=\"center\" | 0.001
| align=\"center\" | 129
| align=\"center\" | 2285.23
| align=\"center\" | 10.12
|-
| align=\"center\" | 129
| align=\"center\" | meanbias129_20090206.fit
| align=\"center\" | 2009-02-06 @ 14:20-14:52
| align=\"center\" | 0.001
| align=\"center\" | 115
| align=\"center\" | 2168.47
| align=\"center\" | 22.19
|}flat field 2010/02/11
flat field 2009/02/10
flat field 2009/02/10
Calibration frames are stored on arpc55:/data/starlight/calibrations
Flat fields were taken in early February 2009 and 2010. To choose suitable flat frames to make the master, every fifth frame in the set of flats was examined. Data was tabulated in a table with headers "Image Number", "Time", "Exposure Time", "
fimgstat was used to find the mean and maximum pixel value and the standard deviation of pixel values in the frame. Mean flats chosen to create final flats were: meanflat12620100211.fit, meanflat12820090210.fit, and meanflat129_20090210.fit. To create normalised flat fields, the mean flats were divided by their maximum pixel values. Smoothed flat fields were created using the normalised flat fields and a Gaussian function with a full width at half maximum of 10.
{| border=\"1\"
|+
'''Data on Flat Fields'''
!Camera Number for Which Flat Was Made
!Name of Mean Flat
!Name of Final Flat
!Date and Time Frames Taken
!Exposure Times of Frames Used
!Number of Frames Used (N)
!Mean Pixel Value
!Standard Deviation of Pixel Value
!Max Pixel Value
|-
| align=\"center\" | 126
| align=\"center\" | meanflat126_20100211.fit
| align=\"center\" | smoothnorm126.fit
| align=\"center\" | 2010-02-11 @ 18:00-18:03
| align=\"center\" | 0.01
| align=\"center\" | 41
| align=\"center\" | 12513.74
| align=\"center\" | 1294.283
| align=\"center\" | 28955.299
|-
| align=\"center\" | 128
| align=\"center\" | meanflat128_20090210.fit
| align=\"center\" | smoothnorm128.fit
| align=\"center\" | 2009-02-10 @ 17:35-17:47
| align=\"center\" | 0.01
| align=\"center\" | 136
| align=\"center\" | 24893.67
| align=\"center\" | 1985.530
| align=\"center\" | 29262.236
|-
| align=\"center\" | 128
| align=\"center\" | meanflat128_20100211.fit
| align=\"center\" | N/A
| align=\"center\" | 2010-02-11 @ 17:39-17:49
| align=\"center\" | 0.01
| align=\"center\" | 86
| align=\"center\" | 26744.99
| align=\"center\" | 3441.227
| align=\"center\" | 32718.691
|-
| align=\"center\" | 129
| align=\"center\" | meanflat129_20090210.fit
| align=\"center\" | smoothnorm129.fit
| align=\"center\" | 2009-02-10 @ 17:32-17:44
| align=\"center\" | 0.01
| align=\"center\" | 141
| align=\"center\" | 26479.12
| align=\"center\" | 3386.300
| align=\"center\" | 33086.387
|-
| align=\"center\" | 129
| align=\"center\" | meanflat129_20100211.fit
| align=\"center\" | N/A
| align=\"center\" | 2010-02-11 @ 17:46-17:51
| align=\"center\" | 0.01
| align=\"center\" | 45
| align=\"center\" | 19018.61
| align=\"center\" | 3132.683
| align=\"center\" | 25792.689
|}