1m performance information

The NMSU 1m is an altaz telescope located at the Apache Point Observatory.

Camera

The current imaging camera uses a 2048x2048 E2V CCD with 13.5 micron pixels, giving a pixel size of about 0.467 arcsec. The CCD is in a LN2 dewar, which has a hold time of about 14 hours in the side-mounted mode required for the Nasmyth port; note that this may require a fill in the middle of the night during the winter (fills take about 20 minutes). There is an automatically commandable fill system. The CCD electronics were provided by Bob Leach; the gain is set around 1.9 e/DN. The detector is linear up to about TBD DN. Readout noise should be around 5 electrons, but we currently have some low level pickup noise which makes this a bit worse. Readout time is about 20 seconds for the full array; subarray readout is possible.

Filters

We have a 10 slot filter wheel for 2 inch square filters. Currently, a UBVRI and a u'g'r'i'z' filter set are mounted in the wheel. We also have access to a set of narrow/medium band emission line filters owned by Rene Walterbos.

Photometric performance

Photometric zeropoints are around 21, 22.8, 22.9, 22.85, and 22.1 for UBVRI, where zeropoint is magnitude that gives 1 DN/s. Exposure times to reach a given S/N depend on the sky background; because of the relative poor image quality (see below), typical stellar photometry uses apertures of radius 2-5 arcsec, so there is significant sky contribution.

We can do all-sky photometric calibration observations when conditions permit, and have done this both in UBVRI and ugriz. Generally, all sky photometric rms residuals on high quality nights are in the 0.01-0.02 mag range, although we have not yet been able to achieve this level of accuracy for U band observations.

Pointing performance

Pointing is adequate, but not great; small scale structure on the drive surfaces is likely responsible. Typical pointing models give rms pointing performance of about 20 arcsec; for whatever reason, actual performance using the pointing models is somewhat worse. Given a 16 arcminute field of view, this does not cause trouble for putting objects in the FOV. For robotic operations, the standard mode is to reset the coordinates on a nearby bright SAO star for every object, so generally, objects observed in this mode end up quite near the center of the chip.

Tracking performance

Because of the relatively poor pointing, tracking is relatively poor, so guiding is essentially always needed to preserve image quality for even moderate length exposures, and to keep the object in the same position over a series of exposures. The telescope has an offset guider with a 1024x1024 Finger Lakes CCD system, in a guider which can move radially in the field. The guider FOV is large enough so that guide stars can almost always be found at the default radial position.

Image quality

Unfortunately image quality is relatively poor. We have seen images as good as almost 1 arcsecond, but generally images are in the 1.5-2.5 arcsec FWHM range. There are likely multiple causes for the poor image quality, and we hope to be able to make improvements. Problems probably include thermal inertia of the telescope/mirrors (image quality almost always improves as the night goes on), dome seeing, and poor alignment of the telescope (in particular, the current tertiary is very difficult to adjust).

Calibration


Flat fields are taken periodically of the twilight sky; no internal flat field screen is available. The flats appear to be relatively stable in time, so it appears to be possible to use flats constructed at significantly different dates than the observation dates.
The E2V detector has fringing when observing at longer wavelengths. It is possible to subtract out the fringing pattern to a large extent with fringe frames, although one needs to account for the changing proportion of the fringe component depending on the amount of moonlight.
There are nonuniformities in shutter timing across the field that are apparent for exposure times less than a second or so. It should be possible to correct for these with shutter shading calibration.