CCD for direct imaging

The camera is typically mounted at the modified Cassegrain focus of the telescope, located at the end of one of the fork's arms. It is a Roper Scientific manufactured by Princeton Instruments.

This instrument is a 13.5 µm 2048 × 2048 pixels detector installed on a dewar cooled with liquid nitrogen. The chip is thinned and has Unichrome UV coating. The nitrogen loading bt pressure option has recently been added to this camera, which makes the process faster and safer.

The table below shows the main characteristics of the chip.

Characteristics of the Roper CCD
(by manufacturer's specifications)
rows × columns2048 × 2048
pixel size13.5 × 13.5 µ
chip size27.6 mm × 26.6 mm
gain #11.12 e-/ADU
gain #22.24 e-/ADU
gain #34.10 e-/ADU
read-out noise 100kHz<3.5 e-
read-out noise 1Mhz7.77 e-
dark current<1 e-/hour/pixel
A/D converter 16 bits
operating temperature-110 °C
read-out time4.5s a 1Mhz

Gain and readout noise measured at 100 Khz are 1.12 e-/ADU and 2.7 e- for gain #1, 2.18 e-/ADU and 3.14 e- for gain #2, and 4.20 e-/ADU and 4.39 e- for gain #3.

It is important to take into account that Fernández-Lajús et al. (BAAA 58, 2016) found for this detector a dependency of the read-out noise with the binning factor used.

Flat-Fields

To take dome flat-fields a white screen is available which is illuminated by 6 incandescent lamps, four 50-W and two 20W lamps. These lamps are controlled by three switches, each of which turn on two lamps of equal voltage allowing several combinations: 40W, 100W, 120W, 200W, and 240W.

The table below shows indicative exposure times for the dome flat fields with BVRI filters. The count values (in ADUs) are approximate, and include some 1000 bias ADUs. The diffuse light coming into the dome during the day increases the number of counts; however, it does not introduce irregularities apparent in the screen illumination.

FILTERLAMPEXPOSURECOUNTS
B4×50W + 2×20W15 s25000
V2×50W4 s45000
R2×20W5 s42000
I2×20W4 s50000

For U, the times are extremely long; therefore, it is advisable to take sky flat fields at sunset or dawn. If you still intend to take dome flat fields in U, make sure the lamps installed are quartz cap lamps.

Flats (and, in general, object images) must not be taken under any circumstances with exposure times lower than 3 seconds since the illumination is not even due to the shutter. A lower limit can be safely set at 4 seconds. This should be borne in mind especially when taking sky flats, as the intention is to reduce exposure times to prevent saturation.

Moreover, it is not convenient to take dome flats with an odd number of lights on, as the screen illumination is not uniform, which produces a significant gradient in the flat-fields.

Mounting and setting

The scale in this CCD is 0.15 "/px, with a useful 5.2 x 5.2 square arc minutes field. Due to oversampling, to properly sample stellar images it is convenient to use the CCD with a 5 × 5 binning. In this case, the resulting scale is 0.75 "/px.

If a focal reducer is used, the resulting scale is 0.45 "/px, with a useful circular 9 arc minute diameter field. As the focal reducer does not transmit the UV and its transmission falls sharply on the blue side of the B filter, it is not advisable to use U and B filters with a focal reducer.

Filter wheel

The filter wheel is double and and there are six compartments for 3 × 3 inch filters on each of them, which makes it possible to make combinations between the lower and the upper wheels. Currently, UBVRI wide band filters are available.

Filters sets available


(square, 5 cm. side) UBVRI SET
Name Width Band
UG2
2 mm
U
BG18
3 mm
U
BG12
2 mm
B
GG385
2 mm
B
GG495
3 mm
V
BG18
3 mm
V
KG1
3 mm
R
OG550
2 mm
R
RG6
1 mm
R
RG715
3 mm
I
RG780
1 mm
I


(square, 7.5 cm side) UBVRI SET for CCD
Name Width Band
UG1
1 mm
U
WG295
2 mm
U
S8612
2 mm
U
GG385
1 mm
B
BG1
2 mm
B
BG39
2 mm
B
GG495
2 mm
V
BG40
3 mm
V
OG570
3 mm
R
KG3
2 mm
R
RG9
2 mm
I
WG295
3 mm
I


Additional broadband filters
Name Size Width Quant.
BG38
1.5 cm × 4.0 cm
2 mm
2
BG38
2.5 cm × 2.5 cm
2 mm
1
BG18
2.5 cm × 2.5 cm
2 mm
2
BG18
2.5 cm × 2.5 cm
1 mm
3
RG715
1.5 cm × 4.0 cm
3 mm
3
(Light blue)
diameter 3.5 cm
...
2
(red < 6300 Å)
2.5 cm × 2.5 cm
2 mm
1
(red < 6100 Å)
2.5 cm × 2.5 cm
2 mm
1
(vgreen)
diameter 2.5 cm
3 mm
1


Set 1 Interferential filters
Size: 5 cm × 5 cm
Width: 4 mm
Name Central Lambda FWHM (*) Quant.
400
4020 Å
110 Å
1
450
4530 Å
100 Å
1
470
4700 Å
100 Å
1
490
4959 Å
80 Å
2
550
5500 Å
80 Å
1
590
5930 Å
90 Å
1
653
6530 Å
90 Å
1

(*) Approximate values.




Set 2 Interferencial filters (for comets)
Size: 2.5 cm in diameter
Width: 6 mm
Quantity: 1 of each
Molecule Central Lambda FWHM
(Continuous)
3650 Å
80 Å
CN
3871 Å
50Å
C3
4060 Å
70 Å
CO+
4260 Å
65 Å
(Continuous)
4845 Å
65 Å
C2
5140 Å
90 Å
(Continuous)
6840 Å
90 Å
H2O
7000 Å
175 Å

By the end of 1997, these filtres were measured at the laboratory, and the values listed were confirmed. However, in most cases the transmittance measured was too low, reaching less than 10% for some filters.



Set 3 Interferencial filters (Ionosphere)
Size: 7 cm in diameter
Width: 9 mm
Name Central Lambda FWHM Transmittance Quant.
703 3930 Å 70 Å 28% 1
5350A 5350 Å ... ... 3
5576A 5576 Å ... ... 1
5750A 5750 Å ... ... 1
5892A 5892 Å ... ... 1
6080A 6080 Å 20 Å ... 3
6299A 6300 Å 20 Å 58.5% 1
6300A 6300 Å ... ... 1

These filters have never been used in the direct camera of the telescope. Please contact the technical personnel if you wish to test them. The FWHM and T(%) values are measured. The remaining filters will be measured in the near future.




New Set Interferencial Filters
Size: 3 × 3 inches
Quantity: 1 of each
Central Lambda FWHM
6400 Å
10 Å
6585 Å
6 Å
6605 Å
6 Å
6719 Å
50 Å


Neutral Filtros
Width: 2mm
Density Magnitudes Size Quant.
3
7.5
2.5 cm × 2.5 cm
2
2
5.0
2.5 cm × 2.5 cm
2
1.0
2.5
2.5 cm × 2.5 cm
1
0.6
1.5
2.5 cm × 2.5 cm
2
0.1
0.25
2.5 cm × 2.5 cm
1
0.6
1.5
5.0 cm × 5.0 cm
1
0.4
1.0
5.0 cm × 5.0 cm
1
0.2
0.5
5.0 cm × 5.0 cm
1