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13.2: Information for example case study involving detector characterization

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    45718
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    Discussion of a Detector Characterization Laboratory

    Scott D. Johnson

    Our goal is to characterize a number of scientific grade detectors for use in an astronomical setting. The main purpose behind this exercise is to understand a portion of what each engineer in the major disciplines might do in designing a laboratory. Our objective will be to design this advanced laboratory. We will need to investigate cutting-edge material to achieve this. For the sake of clarity some subjects will have to be cursorily studied in class, but should be more fully studied outside of class.

    A report is to be produced about the laboratory design itself, not about what the various engineers would do (which might be discussed in class but is not to be in a report - not everything talked about in meetings is report worthy). For example we might want to design a very low vibration laboratory and civil engineer might work on how to do that, we will report on the results of the investigation not on what the civil engineer did to get these results.

    Why design a laboratory to characterize detectors? Why not let the company give you the characterization? Or why not hire someone? Because science in general has very cutting-edge engineering/science requirements. Regular companies generally do not look at cutting-edge requirements because that is not their business (it is not very profitable in the short term). Companies that make detectors maybe able to give you some characterization information but they are in the business of selling detectors to everyone and cannot afford to do detailed tests for astronomical purposes. Besides would you buy a product from someone based on their telling you about the product without independent verification? Is that how you buy a car???

    Optics considerations:

    Let us say for purpose of an example that we looking to chose something that has about a 20 µm pitch and at least 1024x1024 pixels, with a spectral range of 100 nm to 14000 nm (note this spans from UV to IR...a very large range). This however is not fully possible, so we will need to consider how to achieve this with multiple strategies. We would like to consider all possible ranges of detectors from CCDs to CMOS devices. 

    We do not have a specific focal length or diameter of the telescope as the astronomer is going to leave that up to us in the first design phase.

    If we assume an effective focal length of 5 m1 and a 20 µm pitch, we get a sampling resolution of about \(\text{resolution} = \arctan(\frac{20 \times 10^{-6}}{5}) \rightarrow 0.8 \ \text{arcsec} \) (good for IR, but not for UV/Vis) and a field of view of \(\text{FOV} = \arctan(0.235/5) \rightarrow 14'\). In general this does not meet any specification given on the problem poser. So we will need to do better than this. So what have you figured out so far?

    To assure ourselves that we are not diffraction limited we will need to make sure our diameter of this example telescope is at least large enough to handle the 0.8 arcsec we wish to achieve. For practical purposes (since theory is in a perfect world) we will aim for 0.4 arcsec.

    Given that we will calculate what we think would be a good diameter of the telescope. This will also give us an idea our our optical needs. We will need to discuss this with the astronomers to assure ourselves they will have the appropriate optics. The system will be more diffraction limited at the red-end of the spectrum, therefore we will only do this calculation for the 14000 nm wavelength. Using a standard equation2 to calculate this.

    \(0.4 \times \frac{2 \pi}{360 \times 60 \times 60} = \frac{1.22 \times 14000 \times 10^{-9}}{x}\)

    x = 8.8 meters

    Therefore an effective 5 meter focal length with a 9 meter diameter telescope should work. This diameter is very large. Can it be done? Note that the f-number on this would be awful, so we must think about a much larger focal length. The larger the number the better the light we should be able to get in the telescope. This leads to a design issue not expressed in the problem poser. We would like high resolution but also we would like an f-number say around 30 to 50 (f/50). The following calculation do not allow for this. This will need to be investigated more fully.

    So we want to characterize a detector given a possibly large focal length and optics with a f-number of maybe f/50. It is unlikely we will be able to design a lab that will be able to handle optics as large as those prescribed so we will need design f/50 optics with a few "tricks." We will need an optical engineer to work with us on this design.

    It maybe possible to find a detector that achieves a low dark current as required in the specifications, but it is more likely we will need to cool the detector to achieve a very low dark current. If we do need to cool, we will need to investigate what temperature is necessary to achieve the specified dark current. A cooler of some sort might be required for this project. There are many different type coolers that are used in astronomical missions, some require power some do not. If we choose a cooler that needs power we have to remember that in any satellite-type telescope (like Hubble) there are limited power requirements. This may require a redesign of a cooler for size and power. We will need an electrical engineer, a mechanical engineer, a physicist and possibly a chemical engineer to help us on this portion of the laboratory.

    While we might find a detector that says it achieves a read noise of ~2 e-, it is very likely that clocking will need to be modified to achieve this. If this requires slow clocking, this may cause some problems with our dark current accumulation. Our laboratory will need electronics that is designed for flexibility so that we can test different read out speeds (clock speed) along with different temperatures. There are data acquisition systems that might be able to handle this, but it likely will have to be at least partially redesigned. We will need to both and electrical engineer and a computer scientist on our team to help us with this work.

    It is clear that a lot of testing will need to be done and we will need a laboratory to do our specific tests. We could redesign and reuse an existing laboratory. Rather then reusing a laboratory space (which could be done) for this assignment it would be more instructive to actually design it. Since we intend this laboratory to be useful beyond the current work, we would like a well designed scientific-type laboratory. We will need the services of a civil engineer to pick the best site and design the best usable building. We will need a mechanical engineer to help us design (or purchase) specialized machinery within the building. We will need an electrical engineer to advise on how to keep the electronics as noise free as possible.

    Finally we will act as a system engineers to design the whole system for characterization. We will design for this project, but keep in mind that we wish to utilize what we design after the project is completed. The resources are likely to be pretty expensive and doing a one-time project does not make sense for a laboratory.

    Comments and Web Sites:

    Vibration considerations

    • Microphonics is electric noise due to vibration.
    • A good page on the pros and cons of vibrations systems (but a little biased): (Herzan tutorial)
    • BTW, vibration is such an important topic you can find it in courses for mechanical engineers, civil engineers, and electrical engineers. Some schools include vibration in their two year control systems course (or signals and systems;cross-listed into all disciplines in some schools) and some schools have a course dedicated to vibration.

    Building Site considerations

    Vibration isolation tables

    Telescopes

    Cryogen System

    • Good Dewars for a number of scientific applications: IRLABS

    Pulse Tube

    Technical Equipment

    • Sells a number of technical products that are useful for a laboratory: Newport Corporation
    • Sells a number of technical products that would be useful for a laboratory: Vere Inc.

    DAQ


    1This is a rather small focal length for a true massive astronomical project. Why? Because this is just a help aid, it is up to you to figure out the real answers.

    2Rayleigh criterion for diffraction for a circular aperture.


    13.2: Information for example case study involving detector characterization is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.