Flat field data at a low count rate was acquired with both the 75 mm and 40 mm intensifiers incorporated in the BigMIC system. Subsets of typical images are shown in Fig. 8.3 and Fig. 8.4 respectively. Centroiding to 1/8 of a CCD pixel was used giving a data acquisition pixel size of
10x10 pm^. The counts acquired in both images are approximately equal.
F ig u r e 8.3. F lat field im age o b tained with the 75 mm in tensifier. T he size o f this im age is 5 1 2 x 2 5 6 sub-pixels w hich corresponds to 5.1 2 x 2 .5 6 mm^ at the p h otocathode plane.
F ig u r e 8.4. F lat field im age o b tain ed with the 40 mm in ten sifier. T he size o f this im age is 5 1 2 x 2 5 6 sub-pixels w hich corresponds to 5.1 2 x 2 .5 6 mm^ at the photo cath o d e plane.
The 75 mm images show a number of artefacts:
A very large number of defects
From the number of defects seen in Fig. 8.3 it is estimated that ~50,000 defects will be present over the whole detector area. All of these defects have approximately the same size. Their measured width in the accumulated data is -50 |im. Because of the width consistency it is believed that they originate from defects in the MCP pores as opposed to the photocathode.
The only other possibility visualised was that they originated in the fibre taper that couples the intensifier to the CCD. This, however, was discounted by very high resolution analysis of the fibre taper. Whilst there are some defects in the fibre taper’s stmcture, these are at a far lower frequency than those present in the 75 mm image and confined to single broken fibres. Since each scintillation on the output phosphor of the intensifier is -120 |im FWHM and is thus imaged onto -40 fibres, one bad fibre will make very little difference to the event profile as captured by the CCD.
In which MCPs are the defects present? If the defects are in MCPl then they would cause a loss of data from just the associated pores and the maximum size of the defect in the acquired image would be equivalent to the distance between surrounding pores - i.e. -24 pm. Due to electron collimation effects (Fig. 8.5) MCP2 undersamples the charge cloud from MCPl and hence dead pores in this MCP will then, in acquired images, have a greater effect due to the larger pore spacing leading to a maximum defect size of 64 pm.
Thus it is believed that the defects originate in MCP2. It is not known whether these were present in the MCP as received from Galileo, or were introduced in the processing or re processing at Photek. To overcome this problem in future intensifiers, it will be very important to analyse the MCPs individually before processing.
Hexagonally packed MCP sections with differing sensitivity
It was seen (Fig. 8.3) that the counts accumulated in one of the hexagons are noticeably higher than in the surrounding hexagons. Analysis of the data shows that -20% more counts are obtained in this area. This artefact has not been noticed with any previous intensifier and it is believed that the resistivity of this hexagon is slightly different leading to the gain difference.
There is one other big difference between this intensifier and previous generations which leads us to this conclusion; it does not have a saturated pulse height distribution. With a saturated pulse height distribution all events above a threshold (placed in the valley of the distribution curve) are counted with equal weight and any variations in gain across the MCP area excluded. With no valley in the distribution curve small variations in gain do become noticed due to the arbitrary position of the threshold.
Imaging of the structure of the 25 |im pores of MCP2
This is noticeable as a high frequency modulation in Fig. 8.3. It was found that the modulation level introduced into a flat field is -50% and that the period is -32 jam (the pore to pore spacing on MCPs 2 and 3) implying that imaging of the MCP2 pores is occurring.
This conclusion is backed up by the data from the 40 mm intensifier, where the only difference between the two intensifiers is the pore spacing on MCP2 and MCP3. Here a basically smooth flat field is obtained (Fig. 8.4). A low frequency modulation which could be attributed to a non-uniformity in cathode response or uneven light illumination is the only artefact present.
Why does imaging of the MCP2 pores occur? It is believed that this is due to an electron lensing affect created by the end-spoiling (the distance the electroding on the MCP face penetrates into pores) of MCPl. This is supported by data from Galileo Electro-Optics Corporation (1995) which states that: “End-spoiling on the output face of the MCP acts as an array of microlenses which provide a strong collimating effect on the electrons exiting each pore”.
The effect of this electron lensing is shown schematically in Fig. 8.5, where it can be seen that the electron cloud from MCPl which is confined to a single pore is undersampled by MCP2 and thus a modulation will inherently be introduced.
iiisii
(b)
F ig u r e 8.5. S chem atic representation o f M C P 2 sam pling o f M C P l. T he shaded circles rep resen t the pores o f M C P l w hile the em pty circles rep resen t those o f M C P 2. (a) 10/12 g m pores on M C P l and 2 0/30 g m p ores on M C P2, (b) 10/12 g m pores on both M CPs.