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manualOperation

LN/MCTDetector

Manual Version 1Revision A

December 19, 1996

iii

Table of Contents

Chapter 1 General Information ........................................................................... 5

Description..........................................................................................................................5General Features .................................................................................................................5Environmental Conditions ..................................................................................................7

Chapter 2 Pumping Down................................................................................... 9

Introduction.........................................................................................................................9Valve Configuration..........................................................................................................10

Achieving the vacuum .........................................................................................10

Chapter 3 Detector Setup.................................................................................. 11

Connecting the detector ....................................................................................................11Mounting...........................................................................................................................12

Chapter 4 Filling the Dewar .............................................................................. 13

Introduction.......................................................................................................................13Operating Temperature .....................................................................................................13Data Collection .................................................................................................................13Filling the Dewar ..............................................................................................................14

Dewar options ......................................................................................................15

Chapter 5 Operation .......................................................................................... 17

Introduction.......................................................................................................................17Optics ................................................................................................................................17Shutter ...............................................................................................................................17Software ............................................................................................................................17Binning for Single or Multiple Regions of Interest ..........................................................18

Defining a single region.......................................................................................19Definition of multiple regions..............................................................................19Graphical input (the Mouse button).....................................................................20Storing the access pattern ....................................................................................20

Vignetting..........................................................................................................................20Baseline Signal..................................................................................................................21

iv LN/MCT Detector manual Version 1.A

Chapter 6 Focusing ........................................................................................... 23

Imaging Systems...............................................................................................................23Spectroscopy Systems.......................................................................................................23

Chapter 7 Setting the Gain................................................................................ 25

Introduction.......................................................................................................................25Cover Removal .................................................................................................................26Setting the Gain.................................................................................................................26

Appendix A Specifications ............................................................................... 29

Appendix B Changing the Filter ....................................................................... 31

Introduction.......................................................................................................................31Procedure ..........................................................................................................................31

Index ................................................................................................................... 37

FiguresFigure 1. Model 7280 MCT-320 IR Detector. ...................................................................6Figure 2. Controller interface connector..........................................................................11Figure 3. Focal-plane position .........................................................................................12Figure 4. Dewar ports and valves. ...................................................................................14Figure 5. The Pixel Access dialog. ...................................................................................18Figure 6. Removing the electronics enclosure.................................................................25Figure 7. MCT circuit board and JP2 jumper block. .......................................................27Figure 8. Items handled in course of changing the filter. ................................................32Figure 9. Spectrometer adapter. .......................................................................................32Figure 10. Sideview of adapter and split mounting ring..................................................33Figure 11. Window area after removing adapter. ............................................................33Figure 12. Window area after removing nose assembly..................................................34Figure 13. Window area after removing thermal shield. .................................................35Figure 14. Window area after removing filter retaining ring. .........................................35

5

Chapter 1General Information

Description

The PI Model 7280 MCT-320 IR detector features a 320 × 240 liquid-nitrogen cooledfocal plane array (FPA). The HgCdTe (MCT) array achieves high sensitivity combinedwith repeatable and stable performance at cryogenic temperatures (77 K). With a 30 µmpixel size, the liquid nitrogen cooled MCT detector achieves high sensitivity andexcellent resolution in thermal imaging and spectroscopic measurements in the 1 to 4.3micron range (nominal). The Model 7280 MCT-320 detector uses PI’s large capacitydewar, which has a hold time of 60 hours or more with the 320 × 240 array.

The front enclosure containing the MCT array is open to the vacuum jacket thatsurrounds the internal LN dewar. The array is mounted on a cold finger in direct contactwith the LN dewar. The dewar is filled through a sealable top opening, and has twopressure relief valves operating at 1 and 10 psi to safely vent N2 gas.

The electronics enclosure contains the preamplifier and array driver board. Thisarrangement keeps all signal leads to the preamplifier as short as possible and providescomplete RF shielding.

The MCT-320 detector operates with a specially modified ST-133 Controller. Thecontroller is operated as described in the manual with two exceptions. First, there is notemperature control (the array is simply cooled to just above LN temperature and theTEMP LOCK light is always out). Second, the RS-170 (EIA) or CCIR standard videoprovided at the controller’s VIDEO is not provided. Images acquired with the MCT-320can be displayed on the computer monitor only.

PI’s WinSpec runs the MCT as a standard two-dimensional array as described in theWinSpec manual. The only special consideration for the MCT is that binning, if selected,must be done in software.

General Features

• Compact and lightweight design allows easy interfacing with various spectrometersand alignment with optical systems.

• Maximum user safety: Three different valves protect against pressure buildup eitherin the LN container or in the vacuum vessel.

• Minimum requirements for frequent pumping: Various adsorbents and desiccants areadded to trap contaminants.

6 LN/MCT Detector manual Version 1.A

Filter Plug

Safety pressure reliefvalves for venting nitro-gen gas

Nylonplug

Vacuum valve

ElectronicsHousing

Pressure relief valve, opens ifpositive pressure occurs in thevacuum enclosure

Vacuum enclosure

Sapphirewindow

TypicalSpectrometerAdapter

• A safe pumping interface: PI uses a helium-leak tested valve that ensures ease ininterfacing to vacuum pumps with a minimal chance of opening to the atmosphere.

• A sapphire window separates the FPA from the outside and maintains the vacuum.

• A filter holder (accepts one inch optical filters) is incorporated directly in front ofthe MCT array.

Figure 1.Model 7280MCT-320 IR

Detector.

Chapter 1 General Information 7

Environmental Conditions

• Storage temperature ≤55°C

• Operating environment 30°C > T > -30°C

Relative humidity ≤50%. High humidity climates may require continuous flushing of thespectrometer’s exit port with nitrogen.

Never remove the detector’s front window when the detector is cold; ice will formimmediately, destroying the array. Operations requiring contact with the device can onlybe performed at the factory.

Never operate the detector cooled without proper evacuation. This could destroy theMCT array!

WARNINGS

9

Chapter 2Pumping Down

Introduction

A high quality vacuum must be maintained in the MCT-320 detector. A pressure of ~10mTorr is needed to prevent condensation and contaminants from collecting on the FPA,which could ruin it.

All dewars are pumped for 72 hours at the factory while being baked at the maximumallowable temperature. This procedure removes impurities, especially water vapor, fromthe vacuum chamber. In addition, the vacuum vessel contains two traps to adsorbcontaminants over the lifetime of the dewar (years). Finally, the PI design ensurestrapping of any remaining contaminants on very low temperature surfaces to ensure aclean array surface.

All dewars are helium-leak tested to ensure a total seal. The vacuum valves are of thehighest commercial vacuum grade. Each valve is individually tested and certified by themanufacturers for its vacuum integrity.

Your vacuum system must have a trap (ideally cryogenic) placed between the detectorand the pump to prevent contamination due to backstreaming from the pump.

Note: New detectors will experience a higher outgassing rate than detectors that havebeen in operation for several months. Therefore, if a detector does not retain LN for theminimum holding time (see Chapter 4), it should be pumped. In addition, frequentpumping (once a week) improves the LN hold time efficiency, but is not required.

Operating the detector without proper vacuum may cause serious or irreversible damage.Do not operate the detector unless the vacuum chamber is evacuated. The detector wasshipped evacuated properly, but may have lost vacuum during shipping due to vibrationand other mechanical disturbances.

Note: If you don’t have an appropriate vacuum pump and a cryogenic trap, you maypour LN into the dewar and carefully monitor the outside temperature of the dewarwalls. If it is very cold and/or ice accumulates on the surface, this will indicate a loss ofvacuum.If this happens, empty the dewar immediately and call PI for advice on how toproceed.

CAUTION

WARNING


Valve Configuration

Do not attempt to pump down the detector with liquid nitrogen present in the dewar.Carefully pour out any remaining LN and wait for the detector to reach roomtemperature. Pump down according to the instructions below.

Achieving the vacuum➧ Remove the nylon plug on the open end of the vacuum valve. Save the plug.

➧ Connect this end of the vacuum valve to the vacuum system. This is a ¼″ Swagelockconnector. Do not open the vacuum valve yet.

➧ Begin pumping. The vacuum equipment should first be pumped down to areasonable level before the vacuum valve is opened.

➧ When a reasonable vacuum level is reached (~20 mTorr), open the vacuum valve byturning the brass knob counterclockwise a few turns.

➧ If you are using a simple forepump (rotary), pump down to 5-20 mTorr. If you havea dual pump station, e.g., forepump/diffusion pump or forepump/turbo pump, pumpdown to 10-5 to 10-6 Torr for better performance. Initially, overnight pumping maybe required to reach these levels.

➧ When evacuation is complete, firmly tighten the vacuum valve by hand. Do notovertighten. The vacuum block is now sealed.

➧ Remove the vacuum system from the Swagelock connector. Replace the nylon plugto prevent accidental loss of vacuum.

WARNING

11

Chapter 3Detector Setup

Connecting the detector

Each detector is supplied with a cable to connect to the controller. With the controllerpower OFF, connect one end of the cable to the controller’s DETECTOR connector,located on the Analog/Control module (see your controller manual). Tighten the screwsin place. Connect the other end of the cable to the detector (see Figure 2), and tighten thescrews to secure the connection.

Controller cableconnects tothis connector

Figure 2.Controller

interfaceconnector.


Mounting

Available adapters allow the MCT-320 Detector to be used in many differentspectroscopic and imaging systems. These adapters should normally be specified whenthe detector is ordered. Figure 3 below shows the location of the focal plane with respectto the the detector’s mounting flange and with respect to the standard K1 adapter, one ofthe many available adapter types.

0.561 inches FP to Nose Face

0.501 inches FP to Adapter

FP

Figure 3.Focal-plane

position

13

Chapter 4Filling the Dewar

Introduction

The Model 7280 MCT-320 detector uses liquid nitrogen to reduce the temperature of theFPA. The liquid nitrogen is stored in a dewar which is enclosed in a vacuum jacket forminimal external thermal losses. The chip temperature is not thermostatically controlledand is set at approximately 83-85°K. The large capacity (1.5 liters) dewar gives a holdtime of 60 hours or more.

Deep temperature cycling causes mechanical stressing of the array. To minimize thepossibility of the array being cracked due to excessive temperature cycling, the dewar’ssupply of LN should be continuously maintained. Note that a large-capacity automaticLN feeding system is available (see Dewar Options on page 15).

To assure the longest possible array life, do not allow the dewar’s supply of LN to beexhausted if at all possible.

Situations may arise where allowing the array to come to room temperature is simplyunavoidable, such as if the vacuum has to be repumped. However, all unnecessarytemperature cycling should be avoided.

Operating Temperature

As previously mentioned, the array temperature cannot be set. The temperature settingprovisions in the WinSpec software are not functional when operating with the MCT-320 Detector.

Data Collection

To minimize dark-current effects, we recommend that the system be turned on beforestarting to fill the dewar. This will allow the array to be continuously cleaned during thefilling procedure and so prevent charge saturation from occuring. However, this is tureonly if the vacuum performance of the dewar has been checked.

➧ Begin with the detector blocked off. For an imaging system, set the lens at thesmallest possible aperture (largest f-number). For a spectroscopic system, close theentrance slit of the spectrometer completely.

WARNINGS


➧ Set the software to the FreeRun and Asynchronous modes (consult the softwaremanual if you are not familiar with these modes). Choose a fast exposure (1 ms), andbegin data collection.

➧ Continue data collection until completion of the filling operation. Collection maythen be stopped

The controller must always be left on.

Filling the Dewar

Even minimal contact with LN can cause damage to eyes and skin. Avoid contact withthe splashing that will invariably accompany pouring LN into a room temperature dewar.

Dewar port cap

Retaining nut

Pressure relief valves

➧ After the detector has been properly evacuated, loosen the retaining nut (Figure 4) afew turns, then remove the LN dewar port cap by pulling it straight out.

Always be careful when removing the LN port cap if there is LN present in the dewar.Pressure due to nitrogen gas can cause the cap to fly out when the retaining nut isloosened, possibly spraying you with liquid LN.

➧ It is recommended that an LN transfer dewar with a pouring spout be used to transferLN from the storage tank to the detector. If you are going to use a funnel, place athin vent tube into the dewar through the funnel to reduce splashing due to boilingLN.

➧ Pour approximately 100 ml of LN into the dewar. Stop for 5-10 minutes until youobserve a “geyser-like” vapor burst from the dewar opening. This burst is normaland has to do with reaching a thermal equilibrium between the LN and the dewarcontainer surfaces.

CAUTION

WARNING

Figure 4.Dewar portsand valves.

CAUTION

Chapter 4 Filling the Dewar 15

➧ Fill up the dewar (approximately 1.5 liters). To test the LN level, insert a straightpiece of wire (a cryogenic “dip stick”) into the dewar briefly, then remove it. TheLN level will be indicated by the condensation on the wire.

➧ Once the dewar has been filled, replace the filler cap and hand-tighten the retainingnut.

The pressure relief valves (Figure 9) underneath the protective covering willoccasionally emit a plum of N2 gas and mist. Continuous hissing indicates that thevacuum in the dewar jacket is probably inadequate. In this case, first remove all LNfrom the dewar, then reconnect the detector to the vacuum pump.

Dewar optionsPrinceton Instruments also offers an automatic LN feeding system where the LN flowsfrom a large tank (31-liter capacity) to the dewar every preset period of time, e.g., sixhours. The entire filling procedure is fully automatic and only requires refilling the tankevery 2-4 weeks.

CAUTION

17

Chapter 5Operation

Introduction

Although operation of the MCT detector is straightforward, it does require attention toseveral important considerations.

• The controller must be connected and running before filling the dewar.

• The controller must continue to be connected and running for the entire time thedewar is filled.

• For maximum array life, the dewar’s LN should be replenished regularly to preventthe array from undergoing extreme temperature cycling.

Other factors important in making measurements with the MCT-320 are discussed in thefollowing paragraphs. Focusing is described in Chapter 6.

Optics

Note that ordinary glass lenses cannot be used for IR imaging. Instead it is necessary touse lenses of germanium, II/VI materials or sapphire. Up to approximately 2 µm, moststandard glass lenses will be sufficient unless they are coated with special anti-reflectionfilters.

Shutter

The basic Model 7280 MCT-320 Detector is not normally supplied with a shutter.

Software

The Model 7280 MCT-320 Detector operates with PI’s WinSpec application software asdescribed in the WinSpec manual. The selected array type should be PI 240×320 MCTand the selected controller type should be SpectroMAX.

Experience has shown that the values in the first data set taken with the MCT array willbe high, but that they drop sharply to the correct values with the second and subsequentdata sets. When accumulations are necessary, the number of spectra (WinSpec) or thenumber of images (WinView) selected in the Experiment Setup dialog box should betwo or higher, and the data from the first spectrum or image should be discarded.Operating with multiple Cleans does not remedy this effect.


Note that hardware binning is not available with this detector. Binning, if selected, mustbe done in software, and is selected from the Experiment Setup menu. The followingparagraphs from the WinSpec manual describe how to set up both single and multipleregions of interest.

We recommend that all data collections be made with using WinSpec’s background-subtraction feature (see Chapter 5 of the WinSpec manual). This is necessary because ofthe high background signal typically encountered in IR measurements.

Binning for Single or Multiple Regions of Interest

Open the Pixel Access dialog from the expert version of the Experiment Setup dialog.This dialog is shown in Figure 5.

Note: You can acquire a full image without losing your access pattern. Open theExperiment Setup dialog and uncheck the checkbox to the left of the Pixel Definitionbutton. Now collecting data will result in full-frame images. To switch back to youraccess pattern, just check the Pixel Definition checkbox.

The Pixel Access dialog allows you to define one or more rectangular subregions of theCCD for readout. The remaining pixels will be discarded.

In the WinSpec pixel access dialog, you define the area of interest in the spectraldirection by a start and an end pixel and a group factor. In the Y dimension of the CCD(along the slit) you define a start coordinate and the height of each access pattern. Eachpattern you define is read out and saved as a separate strip in the .SPE file.

Figure 5. ThePixel Access

dialog.

Chapter 5 Operation 19

Defining a single region➧ The Pixel Definition dialog is accessed from the Experiment Setup dialog by

selecting Acquire, Experiment Setup.

➧ In the Experiment Setup dialog, check the box located to the left of the PixelDefinition button, then click on the button. This will cause the WavelengthGrouping/Slit Binning dialog to appear.If you later uncheck the box located to theleft of the Pixel Definition button, definitions in the Pixel Definition Dialog aredisabled and the complete CCD will be read out.

➧ For defining only one region for readout, you must make sure that the Storedparameter shows 0 or 1. If a higher number is displayed, you must click the Clr Allbutton. The Mode text (below “Stored”) must show Edit which is the case when theedited region is region 1.

➧ Enter the desired region dimensions by keyboard or by selecting the area of interestand using the Mouse button.

➧ Exit the dialog by clicking on the OK button. If you exit the dialog using the Cancelbutton then a default region of interest is restored. This default region has the fullWavelength dimension of the CCD. In the Slit direction the center strip is taken witha height of 1.

Note: When you press OK to exit the dialog, the binning parameter settings may notimmediately take effect. If they do not, it will be necessary to click on OK to leavethe Experiment Setup screen and then to re-enter it again by once again clicking onExperiment Setup. At this point you must choose software binning by clicking onthe checkbox to the left of the Software Binning button. Whenever Pixel Definitionis checked, Software Binning must also be checked for binning to occur asprogrammed.

➧ To enter the full dimensions of the CCD chip at any time, click on the Full button.This does not affect the Strip Start and Strip Height values.

Definition of multiple regionsTo define more than one access region, the Store button must to be used. Before adding anew region, you should check the Number of Patterns Stored parameter. If this is 1, thenthe pattern you see in the dialog is the only one defined so far. If Number of PatternsStored is 3, for example, then there are three regions already defined, including the oneyou see in the dialog.

➧ If more than one pattern exists, go back and forth to view the previously storedpatterns by using the “<” and “>” buttons. These regions can only be viewed and theboxes to change them are therefore grayed out. Clicking the Store button stores apattern immediately. Clicking on Cancel if patterns have already been stored onlycloses the dialog.

➧ To delete a previously stored pattern use the “<” and “>” buttons to locate thepattern and then click on Clear. The remaining patterns are renumberedappropriately.


Note: Because of hardware limitations, for all patterns after the first one the WavelengthStart Pixel and End Pixel of the first pattern is automatically used. See the section belowfor other limitations of multiple access regions.

Graphical input (the Mouse button)In many cases the selection of an access region is easily determined by looking at a fullimage from the detector. It is easiest to enter such access regions graphically.

➧ First uncheck Pixel Access in the Experiment Setup dialog, and click on Run. Thisacquires a full image of the CCD. If you already have an image, open this image anddisplay it on screen.

➧ In the Experiment Setup dialog, check the Pixel Access box and click the PixelAccess button to open the Pixel Access dialog. If there are already access regionsstored, you will now see them displayed as rectangles in the displayed image.

➧ Use the mouse to drag an ROI box over the desired region in the image display.Click the Mouse button of the dialog to enter the ROI information. Change the StripStart or Strip Height value if necessary. Click Add to add the region to any existinglist of regions, or click OK if this is pattern 1. The corresponding values of the ROIbox are then stored as access region.

The display of the selected areas of interest is automatically updated when patterns areadded or cleared.

Storing the access patternLike most other dialog settings the access pattern is stored in the file SESSION.DATwhen you exit the program. These settings are loaded when you start the program thenext time.

You can also save the access pattern in an ASCII file and reload it again. In this way youcan define many different access patterns or experiment setup settings and load themwhen you need them.

➧ To save the experiment setup or access pattern you must first close all data windows.

➧ Then Select File, Save As and select Access to save only the access pattern. Enterthe file name for the pattern. ACCESS files automatically get the extension .ACC.

Vignetting

All arrays have been tested for uniformity and do not exhibit any vignetting (reduction ofresponse) at the extreme ends of the array. If you do measure such reduction in responseacross the array, it may be the result of one or more of the following conditions:

• Condensation of water on the edges of the array window has occurred. This shouldnot happen unless the cooling/pumping instructions, previously mentioned, were notfollowed or if the dewar has sprung a leak (a rare situation).

The array is held with a special mask designed to minimize reflection and stray light. Inaddition, a special cooled f/4.0 aperture is mounted in front of the array to minimize

Chapter 5 Operation 21

background collection. Note that most spectrometers have a throughput of f/4.0 orhigher.

If vignetting is observed, it is possible that your experiment exceeds these angularconstraints. PI measures the array response with a collimated uniform light source toprevent such false bias results.

Baseline Signal

Dark charge levels will depend on the exposure time setting of the array. The arrayoperates in the “snap-picture” mode where the MCT array is opened for a precise time tocollect signal. Immediately after this exposure (gating) period, the charge is injectedonto the underlying substrate and all charges from all pixels are read out.

The readout rate of the entire array is approximately 12 frames per second. Dark chargewill also be a function of “software binning.” The larger the number of pixels that arebinned together, into a spectrometric strip, or into a super-pixel (for low resolutionimaging), the higher the dark charge. Typical dark-charge levels at around 85 K are lessthan 200,000 electrons/pixel/second. The full-well cpacity for each pixel isapproximately 5,000,000 electrons.

Another, more important issue, is the background signal. At long wavelengths, roughlyabove 1.8 µm, the spectrometer, the lenses, etc, becomes the dominant sources of“unwanted” signal. This background signal determines the maximum exposure (gating)time of the MCT detector. For example, with a 2.5 µm (short wavelength) pass filter, themaximum exposure time due to background (297°K) is approximately 50 ms. With nofilter at all, and with only a f/4.0 cold-limiting aperture, the maximum exposure time isonly 1 ms. This is particularly important to remember when the detector is used inconjunction with a fast pulsing laser, i.e., above 12 pulses/second. Because the array canonly be read out at 12 frames per second, normally one would synchronize thecontroller/detector shuttering (gating) to the laser repeition rate. However, if the laserrepetition rate is above 12 Hz, e.g., 120 Hz, the exposure time would have to be 100 msto capture all 120 pulses. Unfortunately, background limitations prevent such operation.Consequently, the user may capture only a fraction of the transient spectra, not all ofthem. Even if longer exposures are possible, this will increase background levels andreduce the signal-to-noise performance. Unfortunately, this is the level of the MCT arraytechnology at this time.

If you observe a sudden change in the baseline signal you may have excessive humidityin the detector vacuum enclosure. Turn off the controller, remove the liquid nitrogen,and pump the detector for 30 to 60 minutes. If problems persist call the factory.

CAUTION

23

Chapter 6Focusing

Detectors for both imaging and spectroscopic applications must be focused formaximum resolution. Imaging applications would normally require adjusting the lensand possibly the lens adapater as well, depending on the design of the specific adapterused. Spectroscopic applications demand both focusing and alignment of the spectrum.

Imaging Systems

➧ If the software is not yet running, set it to the Freerun and Asynchronous modes.

➧ Slowly uncover the lens. If the image becomes washed out, cover the lens quickly,and choose a shorter exposure.

➧ Adjust the exposure until a suitable value is found. Check the brightest regions of theimage to determine when the full scale of the A/D converter is being used.

➧ Place a suitable target in front of the lens. An object with text or graphics works best.

➧ Set the focus adjustment of the lens to the correct distance between the camera andthe object. If the lens mount also has provision for focusing, adjust the lens-mountfocus for the best obtainable image. Then touch up the focus using the lens focusadjustment.

Microscope adapters follow a similar procedure, except in this case the front part of thelens mount should not need adjustment. See the adapter literature for focusing directions.

Spectroscopy Systems

Note: If you purchased an optical-fiber adapter and cable, install them only after theregular alignment procedure has been successfully completed. Consult the Optical FiberAdapters Manual for specific instructions. PI doesn’t offer IR lenses (above 1.8 µm) atthis time.

➧ Place a suitable IR emission source in front of the entrance slit. With an argon lamp,you may be able to measure lines out to perhaps 1.9 microns. In addition, IR LEDsare commercially available at reasonable prices, as are IR lasers (but at higherprices).

➧ If the software is not yet running, set it to the Freerun and Synchronous modes(consult the WinSpec manual if you are unfamiliar with these modes). Begin datacollection.


➧ Bin the entire array into one strip using Software Binning - Full . Strips tailored tothe intended measurement can be set up after completing the initial alignment andfocusing. See page 18 for a discussion of single and multiple-region binning.

➧ Rotate the spectrometer grating to approximately the zero order counter position. Asyou rotate the grating, a line should be seen moving across the screen. Note that thedisplayed spectrum may lag the current real spectrum by as much as one exposuretime, requiring that the spectrometer be adjusted very slowly when working with along exposure time.

➧ Rotate the grating to the first (or second) order counter position and check theillumination of the entrance slit. If a lens is used to focus the lamp’s light, try tomatch the f-number of the spectrometer in order to fill up the grating as uniformly aspossible.

➧ Observe an image of the spectrum. Spectral lines should be absolutely vertical orhorizontal. If the lines do not run horizontally or vertically, slowly rotate the detectoruntil spectral lines become aligned.

➧ Focusing the detector is achieved differently on different spectrometers. On modelswhere the adapter is made of two pieces that slide together, focusing is achieved byslowly sliding the detector in and out of the exit focal plane. One-piece adapters relyon a focusing adjustment on the spectrometer. See the spectrometer manual fordetails.

➧ If the spectral lines are not this well resolved, first check the focus. Then try rotatingthe detector with respect to the spectrometer.

Alignment and focusing may need to be repeated for optimal alignment. When complete,tighten the screws that attach the adapter to the spectrometer to prevent accidentalmisalignment. To tighten the set screws on the two-piece adapter, first stop datacollection. The top of the spectrometer may need to be opened to access the set screws.

25

Chapter 7Setting the Gain

Introduction

A jumper block on the MCT IR Detector circuit board allows the gain to be set to fourdifferent levels from 286 to 2960 electrons per count. The gain of the detector isgenerally set so that the full range of the A/D converter (12 bits; 4095 counts) and thewhole well capacity can be used. Users who consistently measure low-level signals maywish to increase the gain, which gives more counts for the same signal level and reducessome sources of instrumentation noise. Users who measure high-level signals may wishto reduce the gain so that the number of counts for a given signal level becomes smaller,allowing larger signals to be processed. To gain access to the jumper block that controlsthe gain, it is first necessary to remove the electronics enclosure cover, as describedbelow.

Remove these screwsto release cover

JP2 JumperAssembly

Figure 6.Removing the

electronicsenclosure.


Cover Removal

The electronics enclosure cover is secured by four screws, identified in Figure 6. Whenthese screws are removed, the cover can be pulled off, giving access to the circuit boardand gain-setting jumper block as shown in the figure.

To minimize risk to the equipment, the power should be OFF when performing thisprocedure.

Setting the Gain

The gain setting depends on the position of the three pin-shorting blocks in jumperassembly JP2 as shown in Figure 7. When a block is inserted over the upper two pins,the corresponding capacitor is ON. When inserted over the lower two pins, thecorresponding capacitor is OFF. The gain is maximum when all three are OFF andhighest when all three are ON. The table beneath the circuit diagram shows the gain foreach of the possible jumper configurations.

In reconfiguring the JP2, it will prove convenient to use needle-nose pliers to removeand insert the three pin-shoring blocks.

Once the desired gain has been established, reinstall the electronics enclosure cover. Itmust be installed because it provides essential shielding.

CAUTION

Chapter 7 Setting the Gain 27

PRINCETON INSTRUMENTSTC M 3100 I.R. PREAMP BD1700-0245 REV C

JP1

ON

OFF

3 2 1JP2

If the jumper is inserted on the upper two pins (configuration shown),the jumper section is ON. If the jumper is inserted on the lower twopins, the jumper section is OFF. The table shows the gain for allpossible settings.

ON

OFF

3 2 1JP2

JP2 Section

1 2 3Conversion Gain

(µV/e-)Maximum Signal

in CountsGain

(e-/count)

OFF OFF OFF 1.5 4095 286

OFF OFF ON 0.24 2950 1620

OFF ON OFF 0.37 4095 1200

ON ON ON 0.15 1740 2960

Note: Factory default setting is OFF-ON-OFF.

Figure 7. MCTcircuit board

and JP2 jumperblock.

29

Appendix ASpecifications

Array: 320 × 240 HgCdTe

Pixel Size: 30 µm

Wavelength Range: 1 to 4.3 microns (nominal)

Dark Charge: <200,000 electrons/pixel/second (85°K)

Full Well Capacity: ~5,000,000 electrons per pixel

Aperture: f/4.0 cooled aperture in front of array minimizes background collection.

Filter: 1″ optical filter can be installed directly in front of array (Appendix B)

Window: Sapphire.

Cooling: LN

Dewar: 1.5 liter top-filled dewar with 60 hour hold time

Array Temperature: 83-85°K

Controller: Specially modified ST-133

Maximum Integration Time: 0.5 seconds

Temperature control: disabled

Composite video: disabled

Software: WinSpec or WinView

Readout Rate: 12 frames per second

31

Appendix BChanging the Filter

Introduction

Directly in front of the array is a filter holder that accepts a standard one inch opticalfilter. The filter installed at the factory is ordinarily specified at the time of ordering. Foroptimum flexibility and convenience, the MCT detector has been designed so that thefilter can be changed. Users have the option of returning the unit to the factory to have adifferent filter installed, or of doing it in the field according to the directions in thisAppendix.

The filter should only be changed with the power off and with no LN2 in the detector;the detecor should be at room temperature. This procedure should only be performed bypersonnel with the necessary knowledge and skill to perform this delicate procedurewithout risk of damage to the array or other detector components.

Although the procedure is fairly straightforward, it must be performed with great careand attention to detail. It involves opening the vacuum chamber to the atmosphere anddirectly exposing the array. Even a minor mishap could result in irreparable damage tothe array that would not be covered by the warranty.

After the filter is changed and the detector reassembled, it will be necessary to repumpthe unit to restore the vacuum.

The procedure must be done in a dry atmosphere to prevent adsorption of water to thedetector parts. Water molecules, once they have adhered, are very difficult to “pump-out.”

All personnel involved in changing the filter must use gloves that will prevent body oilcontamination and/or fiber or other dirt contamination.

Unless you have the necssary vacuum equipment and are prepared to repump thedetector, do not even consider changing the filter yourself. Instead, contact the factoryand arrange to return the unit so that the filter can be changed under proper conditions byexperienced personnel. The operation will be done quickly at moderate cost and withoutrisk to the detector.

Procedure

The underlying assumption is that the detector is equipped with some kind ofspectrometer adapter. Although the adapter depicted in the illustrations is commonlyused, other types could have been used instead, in which case the adapter-removal part

WARNING

CAUTIONS


of the procedure would have to be adjusted to meet your specific requirments. Figure 8shows the various items that must be handled in the course of changing the filter andshould serve as a convenient reference. Additional photographs are provided as guidancein performing the separate steps of the procedure.

Split Mounting Ring

Split Mounting Ring

Typical Spectrometer Mount

SapphireWindow

Nose

NoseRetaining Ring

ThermalShield

FilterRetaining Ring

Spring Washer

Filter

Spectrometeradapter

Mounting screws

Mounting screws

➧ Remove the spectrometer adapter. The adapter shown in Figure 9 is clamped to themounting flange by two split mounting rings secured by six screws. These screwsare accessible through six holes in the adapter as shown in Figure 9. Figure 10 is a

Figure 8. Itemshandled in

course ofchanging the

filter.

Figure 9.Spectrometer

adapter.

Appendix B Changing the Filter 33

sideview that shows the adapter, screws, split mounting ring and screws moreclearly. Figure 11 shows the detector with the adapter removed.

Mounting flangeSplit mounting ring

Spectrometermounting adapter

Mounting screws

Nose retaining ring

Sapphire window

Nose

Flange

➧ Rotate the nose retaining ring exactly one turn counterclockwise. Note that there arefour holes in the ring which allow accessory tools to be used to assist in rotating thering. Take care not to rotate the ring too far it will come free and you don’t want thatto happen until after the next step.

Figure 10.Sideview of

adapter andsplit mounting

ring.

Figure 11.Window area

after removingadapter.


➧ Open the vacuum valve slowly, opening the vacuum chamber to the atmosphere.

➧ When there is no longer a vacuum present, rotate the nose retaining ring furthercounterclockwise until it comes free of the flange. Then lift out the ring and noseassembly and set them aside. Be careful! The nose assembly may just fall out whenthe retaining ring is removed.

The detector should appear as shown in Figure 12.

MCT array

Thermal shield

O-ring

Mountingscrews

➧ Remove the thermal shield, which is secured by four screws as shown in Figure 12.Removing the thermal shield gives access to the filter retaining ring, as shown inFigure 13.

➧ Grasp the filter retaining ring and rotate it counterclockwise until it comes free. Thespring washer and filter will either fall out at that point or can be very easilyremoved, leaving the detector as shown in Figure 14.


after removingnose assembly.

Appendix B Changing the Filter 35

Filter retaining ring

Recess for spring washer

Recess for filter

➧ Install the new filter. If the new filter is thinner than the one removed, place shimson top of the filter (a package of shims was supplied with the detector) as necessaryto establish the correct thickness. Then insert the spring washer and install the thefilter retaining ring, tightening it down so that the filter is held snugly.

➧ Reassemble the detector to where it was at the start of the procedure.

➧ Pump the detector to restore the vacuum as described in Chapter 2.


after removingthermal shield.


after removingfilter retaining

ring.

37

IndexAdapters

mounting, 12cable

detector, 11Cable connections, 11Controller, 5Description, 5dewar, 9, 14Dewar filling procedure, 13Features, 5Filter changing instructions, 31Focal plane position, 12focusing, 23Gain

setting criteria, 25Gain setting procedure, 25holding times, 5

Imaging applications, 23LN, 14LN feeding system, 15Mounting adapters, 12optical-fiber adapter, 23outgassing, 9Setup, 11Shutter, 17Spectrometer

focusing, 23Spectroscopy applications, 23trap, 9Vacuum

pumping, 10requirement, 9

WinSpec application software, 17

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