The ALTAIR field lens is designed to provide better Strehl ratios and sharper image quality at large distances from the guide star, essentially reducing the anisoplanatism of the system.
The first tests of a prototype field lens were taken in July and August 2005 and showed large improvements in AO correction, especially at large distances from the guide star. Initial results were so dramatic that the field lens was immediately offered for use. The final field lens was installed in 2006B. This document summarizes what investigators need to know to decide whether or not to use the field lens.
- Key points
- Field lens overview
- Reasons not to use the field lens
- Reasons to use the field lens
- Examples of images with and without the field lens
Projects in which the science target is separated from the (natural or laser) guide star by more than 5 arcseconds, or where the region of interest extends more than 5 arcseconds from the guide star will benefit greatly from using the field lens. These projects should almost certainly have the field lens "in" (but read the caveats below).
Projects where the science target is bright enough to be guided on directly may be slightly adversely affected by the field lens as the current lens results in a few percent reduction in throughput. These projects should consider having the field lens "out".
PIs should specify in the Phase I Tool and must specify in the Phase II Observing Tool whether they would like the field lens "in" (being used) or "out" (not in the beam).
Field Lens Overview
The field lens is a convex lens designed to conjugate ALTAIR to the ground level, rather than at high altitude. The overall benefit of this re-conjugation of the system is to provide better Strehl and sharper image quality at large distances from the guide star, essentially reducing the anisoplanatism of the system. The field lens is designed to be "optional" as it may be placed into the beam or out of the beam with little overhead.
Initial testing of the field lens has shown that it improves correction even at very far distances from the guidestar, up to 25 arcseconds (the limit of the field lens coverage). Without the field lens, high quality correction is limited to about 5 to 7 arcseconds from the guide star. Use of the field lens should allow a factor 20 to 30 increase in the sky area available to ALTAIR.
The information listed on this page is taken from tests undertaken on two nights only of the prototype lens. It is likely that under different seeing conditions, the improvements seen with the field lens may not be as dramatic as those shown here. As more information is learned about the characteristics of the current prototype field lens, updates will be made to this webpage.
Reasons NOT to Use the Field Lens
Passage of the beam through the field lens results in a small loss of light. The current field lens has an antireflective coating which reduces the loss of light through the lens to a few percent.
A measurement of the new field lens flat field has not been made, but it should be better than the old prototype field lens. The prototype field lens had some large-scale flat field effects at the level of less than a few percent. GCAL cannot send light through ALTAIR, so we cannot do closed-dome flats with ALTAIR in the science path. Without the field lens, this is not a large problem because ALTAIR has very little effect on the flat field as compared to NIRI only. With the field lens, users will see a low-level, large-scale flat field roughly centered on the field.
The plate scale with the prototype field lens in has not been carefully evaluated. It appears to be ~2% smaller with the field lens in, about 21.4 milliarcseconds per pixel at f/32. If PIs need precision better than 1% for the plate scale, it is recommended that they take separate astrometric standards. In addition, PIs interested in astrometric precision should consider the NIRI/ALTAIR distortion described on the NIRI pixel scales page.
Reasons to Use the Field Lens
Strehl is greatly increased when more than 5 arcsec from the guide star. This plot summarizes some of the Strehl improvements seen in poor seeing conditions (July 2005). From this plot, you can see that 10 arcseconds from the guide star in K-band, the field lens cuts the Strehl loss from about 45% down to 7% even in poor seeing conditions. More detailed measurements are pending. In good seeing, we expect the improvement to be even greater (see images below, which were taken in good seeing).
The PSF is much more stable over the field (see images below).
Overall signal to noise is increased dramatically even when up to 25 arcseconds from the guide star. A 25 arcsecond separation is the limit of the field lens coverage.
The following images of M33 visually demonstrate the performance increase with the use of the field lens. These were taken under good seeing conditions (August 2005). The width of the strip corresponds to about 45 arcseconds on the sky. The guide star is the nucleus of the galaxy, left of center.
Field lens OUT, K-band
Field lens IN, K-band
Field lens OUT, H-band
Field lens IN, H-band
Field lens OUT, J-band
Field lens IN, J-band