Layer Oriented Wfs Lowfs

The timing for the LOWFS mode is more complex (see Fig. 2). Not only do the two CCDs read with different frame rates (1:2 or 1:4), but both CCDs require different binning factors. The FIERA controller makes use of local microsequencers on each clock and video board. These microsequencers are loaded, triggered and monitored by the DSP controlling the complete readout sequence, allowing each board to have different timing. The only constraint is that the lengths of these clock patterns must be identical.

The issue of different horizontal binning factors for the two CCDs was addressed by a clock pattern that bins four pixels on one CCD, but only two pixels on the other by omitting clock transitions on the latter. As the standard FIERA software expects an equal number of pixels to be read from all CCDs, dummy pixels are read from the CCD with the higher binning factor. They are ignored by the subsequent image processing routines. A similar method is applied for the vertical binning. While the first CCD is subject to four line shifts two of them are skipped on the second device.

Different frame rates are executed in such a way that a virtual frame is defined which contains two or four frames, depending on the frame ratio of 1:2 or 1:4. While each frame of the CCD with the higher rate is "clocked", the CCD with the lower rate skips one out of two or three out of four. The result is a frame with many dummy pixels which are ignored by the software.

Table 3. Timing budget for SHWFS and LOWFS modes.

Mode

SHWFS 1x1

SHWFS 1x1 windowed

LOWFS 2x2

LOWFS 4x4

No. active hor. pixels

94

64

94

94

No. active ver. pixels

84

64

84

84

Binning hor.

1

1

2

4

Binning ver.

1

1

2

4

No. active pixels

7896

4096

7896

7896

Inactive hor. pixels

0

1920

0

0

Inactive ver. lines

0

20

0

0

System timing

ys

ys

ys

ys

Pixel period

1,60

1,60

1,60

1,60

Horizontal shift

0,60

0,60

0,60

0,60

Vertical shift

2,00

2,00

2,00

2,00

Results

ys

ys

ys

ys

Frame transfer

84,00

84,00

84,00

84,00

Clear line

56,40

56,40

56,40

56,40

Read active pixels

3158,40

1638,40

789,60

197,40

Binning overhead

0,00

0,00

296,10

222,08

Skip inactive pixels

0,00

288,00

0,00

0,00

Skip lines

0,00

20,00

0,00

0,00

FIERA overhead

n/a

600,00

n/a

n/a

Frame period

3298,80

2686,80

1226,10

559,88

Max. frame rate [Hz]

303,14

372,19

815,59

1786,11

Figure 2. Layer oriented readout sequence.

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