Conveners:
Jeff Baumgardner (jeffreyb@bu.edu) (Boston University),
Josh Semeter (joshua.semeter@sri.com) (SRI)
2004 Wednesday 30 June 0930-1130 AM
The first topic will concentrate on how to use commercially available components to build a state of the art instrument. Examples will include all-sky and narrow field imagers as well as imaging spectrographs. The second topic will discuss the state of the art of narrow band interference filters, how not to over specify what you need from the manufacturer, and how they perform in different optical designs. Some discussion of how to calibrate data taken using these filters will be included if time permits. The third topic, CCD detectors, will review the current status of the technology and give real examples using commercially available cameras.
Each of these topics will be presented as a 15-20 minute or so tutorial, followed by a discussion period. After the three main areas are presented, and discussed, a summary topic may be introduced: Predicting the end to end sensitivity of a typical imaging instrument.
Interference filters are used in Fabry-Perot spectrometers, monochromatic imagers, grating spectrometers, and photometers. In order to better understand the characteristics and limitation of these filters, a review of the manufacturing steps involved in producing these filters was presented. Graphs illustrating the band-pass shape for a typical filter used in different optical systems were shown. One important property of these filters, the shift toward shorter wavelengths as the angle of incidence increases, was discussed at length. Output from a computer model used to predict the shape of the band pass at various input angles in collimated as well as converging beams was shown. For the range of angles usually encountered in imaging systems, the area under the band-pass curve remains the same but the transmission of the filter to a given wavelength can change dramatically, causing calibration errors. A plea was made for frequent measuring of the band-pass of filters used in the field, and not relying on the curves sent with the filters from the manufacturer.
Josh Semeter (SRI) gave a tutorial on the current state of the art of detector systems. Depending on the phenomena to be studied (e.g. fast moving auroral forms, or faint airglow spectra), different kinds of detectors are needed. Intensified CCDs can have fast readout times (> 30 fps) suitable for narrow field aurora but suffer from limited resolution and dynamic range. A new type of fast readout CCD was described-the EMCCD (electron multiplying CCD).This device has gain stages on the ccd chip producing intensifier like images without the lag associated with the phosphor and with potentially higher resolution.. Currently these devices are only available with 512 x 512 chip sizes. The EMCCD uses a frame transfer technique that may cause smearing of bright, fast moving images. Discussion centered around how the community could get some of these expensive ($40k) cameras to evaluate them for use in aeronomy. A presentation concerning the Signal to Noise ratio in images was made using detector parameters such as read out noise, dark signal and quantum efficiency.
The forth tutorial discussed general optical design parameters of imaging systems. Jeff Baumgardner (Boston Univ.) used two designs for monochromatic imagers to illustrate design constraints and characteristics of these systems when used with interference filters. Some ray tracings were shown illustrating the use of field lenses to properly illuminate each pupil in such systems. The use of field flattener lenses was also discussed. The discussion of the pros and cons of the two designs generated some lively audience participation.
The last tutorial was a short description of the OH temperature Mapper used by Mike Taylor (Utah). This instrument makes use of all of the technology discussed in the previous tutorials. Consequently, there were lots of questions from the audience concerning the design details and data reduction techniques. It was noted that the calibration of such a device is difficult to achieve over the whole sky.
The workshop ended before the topic of predicting the end to end sensitivity of an instrument could be addressed. Many participants in the optical workshop indicated that they would also be attending the calibration workshop in the afternoon, and that some of the calibration issues surrounding all-sky cameras would be addressed at that time.
The workshop had lots of audience participation and definitely not an "AGU" style, however, there could have been even more input from the room. Speakers should ask questions of the room to try and start dialogs, rather than wait to be interrupted. Some attendees asked if there will be a follow-up workshop next year, especially concerning the photometric reduction of all-sky images.