II

II. The PUMA Instrument
II.1. Description
The PUMA instrument has been designed and built by a team of astronomers and engeneers of the Instrumentation Department of IAUNAM. It consists basically of a focal reducer, a scanning Fabry Perot, a filter wheel, a calibration system and a CCD detector. All movable parts are remotely controlled from the command room downstairs. The FP can be removed from the optical path allowing direct imaging with interference filters. This systems also permits future development with other optical elements such as grisms or multi-slits.
Basically, PUMA permits:

Direct imaging of extended objects in various lines: Ha, [NII](6584 A), [SII](6717 A, 6731 A), [OIII](5007 A).

Interferograms of objects in the same lines.

The complete list of inference filters is available here .

II.1.1. Mechanical design

The mechanical system consists in an aluminum structure fixed to the telescope. The flexion should be maintained inside a range of half pixel size (10 mu in our case). To avoid this problem when the telescope is in extreme position, it was decided to deflect the beam at 60° with a mirror.
PUMA has 2 field diaphragms, a calibration lamps wheel, a filter wheel with 8 positions.
The figure II.1 shows the system design of PUMA. A pictures gallery can be found here.

Figure II.1 (From R. Langarica et al 1997)

II.1.2. Optical design

The PUMA instrument is based on a focal redent is based on a focal reducer 2:1 which brings the initial focal ratio f/7.9 to f/3.95. The scale plate is 24.6 "/mm. Using Tek CCD detectors, with a pixel size of 24 mu, the pixel size on the sky is 0.59 "/pix. The total field of view of PUMA is 10' x 10' with the 1024 x 1024 Tektronix CCD detector.

The focal reducer is formed with a collimator and a camera. Both optical surfaces used special optical characteristics like PSK3 and FK54. This kind of glass is necessary to ensure a perfect apochromatism from UV (3600 Å) to near infrared (8650 Å). Glasses also have been treated against reflects.

II.2. The Fabry Perot interferometer

The etalon is the heart of PUMA instrument. It is located on the pupil between the collimator and the objective. The performance of a scanning FP is defined by the ability to keep the 2 plates of the interferometer perfectly parallel during the scanningring the scanning. This is realized through piezo-electric wedges. The CS100 controller of Queensgate Instrument is in charge of the tuning and the set up of parameters of the servo system and is also controlling the parallelism and the spacing of the 2 plates with a time constant of 0.5s and a step of 0.5nm.

Tests run with the FP, in optical bench and at the telescope reached the following characteristics for a pupil size of 50 mm at a wavelength of 6563 Å (Ha):

Finesse	24
Interference order	330
Free Spectral Range	19.89 Å (equivalent to 908 km/s)
Spectral Resolution (channel resolution)	0.414 Å (equivalent to 19 km/s)

Table II.1: FP characteristics

Tests made at the telescope, showed that this etalon has a great stability and the parallalon has a great stability and the parallelism is stable during all the observing run if a constant flux of nitrogen gas arrive to the etalon.

II.3. Electronic control
PUMA electronic control is mainly divided in 2 parts: the positioning of the different mobil parts (filter wheel, calibration lamps wheel) and the control of the interferometer trough. For those purposes we use 2 programmable peripherical interfaces 82C55 inside a microchip "Octagon".
The figure II.3.1 gives the schematic view of the connections between the focal reducer, the acquisition system and the CS100 controller.
Figure II.3.1 Connexions set up (From R. Langarica et al 1997)

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