explored should range from autonomously closing the loop on the spacecraft to deliberately varying the focus in order to attempt phase retrieval on the ground and uploading of new transfer matrices for the feedback loop. The interferograms from the wavefront sensor must be recorded and returned to the ground to enable thorough analysis.

Image Quality

A top priority is to evaluate the quality of the images. With the baseline equipment, evaluation will be primarily indirect, based on the output from the optical wavefront sensor. The quality of the images must be evaluated under a variety of conditions, including various positions in the field of view that can be reached through the fast steering mirror and a variety of thermal conditions.

To explore the effects of thermal environment, the image quality should be studied under a variety of orbital configurations. If the DOD phase of the mission is carried out in low Earth orbit, then these tests should first be done in the low Earth orbit and then repeated in the Molniya orbit. In addition, the image quality should be assessed after the DOD mission during a period in which the telescope is pointed so as to let it cool to the minimum possible operating temperature.

In addition to evaluating the images with data from the wavefront sensor, it is also important to evaluate actual images from both the InSb array and the fine-tracking array. Although neither of these detectors is ideal for analyzing the images, they do provide important information about the images to supplement that available from the wavefront sensor, and they also provide an end-to-end test of the system.

Thermal Background

The next critical test is to determine the thermal emission characteristics of the telescope and other components of the system. Like the test for image quality, this demonstration should be done both in the initial orbit and in the Molniya orbit to understand the changes in thermal background as a function of the orbital characteristics. The task group’s initial estimates are that, when in low Earth orbit, the telescope’s thermal emission will be too high for deep-infrared observations. However, a Molniya orbit, in which substantial passive cooling can occur, should allow such observations.

The ATD/NTOT’s design has not been optimized for thermal control. In addition, it is very different from traditional infrared-optimized telescopes on the ground. Thus, there are no detailed analyses available that can be used even as a guideline for estimating its performance. If designed from the outset for low thermal background, it would likely be quite different from the baseline system described in Chapter 2. Thus determining the thermal emission characteristics is critical for understanding the astronomical capability of the ATD/NTOT. Low thermal background is essential for carrying out the near-infrared cosmological survey discussed in Chapter 5.

Evaluation of the baseline telescope’s thermal environment would be carried out using images obtained with the InSb array. It is not clear to the task group whether this detector will have sufficient sensitivity to fully characterize the telescope’s emission under all possible circumstances. It will certainly be adequate for characterizing the emission whenever the system is, in some relative sense, warm. Furthermore, the thermal background seen at the focal plane is likely to vary as the fast steering mirror follows the motion of the target with the field of the telescope. These variations must be evaluated in order to assess the limits that they place on detecting faint sources at infrared wavelengths.

Field Distortion Stability

Since the ATD/NTOT is fundamentally different from most ground-based telescopes, the stability of its field distortion must be evaluated. This factor is important because it directly affects the ability to make long exposures. Optical stability also affects the ability to detect moving targets and to register successive images of the same field to remove cosmic-ray hits (something which may be essential because the ATD/NTOT’s eccentric orbit will take it well outside the shielding of Earth’s magnetosphere).

A particular concern is that the field distortion is very likely to change as the fast steering mirror moves the

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement