Shelyak UVEX Motorized Spectroscope

Shelyak UVEX Motorized: Remote Spectroscopy

The motorized UVEX is the ultimate version of the UVEX, allowing remote operation. You can remotely control the central wavelength (grating angle), adjust the spectroscope's focus, and switch the external calibration module.

This makes it easy to take dark, flat and calibration images remotely, and the Demetra version for the motors module adds image sequences that link several series of images.

A Crossed Czerny-Turner System for a 350 nm to 1µm Range

At the heart of the UVEX is a crossed Czerny-Turner optical design. Its main optics are two spherical mirrors, a collimator (M1) and an objective (M2), and these mirrors are what let the UVEX go far into the blue (UV) and red (IR), since a mirror is purely achromatic.

To counteract the natural astigmatism of this design and produce a sharp, thin spectrum, a cylindrical lens is placed near the camera sensor, concentrating more light on each pixel.

An R=1700 Resolving Power with the 600 l/mm Grating

The UVEX ships with a 600 lines/mm grating blazed at 500 nm, which delivers a resolving power (R) of approximately 1700 at 650 nm. Resolving power is R = λ/Δλ; these figures are theoretical, calculated with the standard 23µm slit, and a wider slit reduces them.

The instrument is designed for flexibility, allowing you to swap out the standard grating for optional higher or lower-resolution gratings (150, 300, 1200, and 1800 l/mm are available). Each grating comes with its own vernier. Remember to set the grating density in the software, since the angle-to-wavelength relation depends on it.

6 Manual Positions from 25µm to a 3mm Hole

The UVEX features a multi-position slit module that is adjusted manually at the instrument. While grating and focus are motorized, slit selection requires physical access. The slit is changed with the supplied Phillips screwdriver, and offers six positions:

• Four Slit Widths: 25µm, 50µm, 100µm, and 300µm. Narrower slits give more resolution, wider slits let in more light. Other slits are optional, and Alpy slits fit the UVEX.
• 25µm Hole: Mainly for test purposes.
• 3mm Hole: The "slitless" position.

Optimizing for the F/8 Focal Ratio

The UVEX is optically optimized for a telescope focal ratio of F/8. It is particularly suitable for Ritchey-Chrétien (RC) telescopes up to 400mm in diameter at F/8.

For the manual UVEX, Shelyak adds that you can go up to F/5 without vignetting, at the cost of a slight degradation in quality.

M42x0.75 Threads and a 13mm Back Focus Constraint

The UVEX connects to your imaging train via a female M42x0.75 T-thread on the included guiding module adapter. The UVEX attaches to the telescope through the guiding module (sold separately), and this adapter thread can take any optics or device. On the camera side, an adapter ring for ZWO cameras is included; adapters for QHY and Atik cameras are available from Shelyak.

A critical consideration is the camera back focus. Due to the compact optical layout, the acquisition camera must have a maximum back focus of 13mm. Check your camera's backfocus before choosing it.

Sensor and Back Focus Requirements for UVEX

Besides the 13mm maximum backfocus, note that the spectrum on the sensor is about 20mm long; a smaller sensor records part of it at each grating angle.

The recommended camera for the UVEX is the ZWO ASI533 MM Pro, and the Demetra version for the motors module takes CMOS cameras into account with its CMED correction step.

Motorized Integration with External Modules

The "Motorized" designation refers to three core remote functions. The first two, grating angle and focus, are driven by two stepper motors in the motors module. The third is the ability to control an external calibration module. A dedicated control cable is included for this purpose, so the motors module can switch the external calibration module between Sky, Dark, Flat and Calibration; this control is equivalent to the SPOX module.

Three buttons and LEDs on the module also allow local control during installation and tests. The motors keep their position in memory, and the grating can rotate indefinitely. If only USB is connected, the controller starts but the motors cannot move until the 12V supply is plugged in.

DEMETRA and the Open USIS Communication Protocol

The motors module comes with a new version of DEMETRA, which is recommended because it controls the UVEX transparently during acquisitions. It adds calibration "by set line" (the dispersion law is set once and readjusted on a well-identified line, useful from UV to IR where reference lines are few), motors module control, image sequences, and CMOS camera support through the CMED correction step.

For users who want to integrate the UVEX into custom automation platforms, communication uses the USIS protocol, a fully documented serial protocol, so you can develop your own tools with no restriction. Shelyak also provides the USIS Controller program (Windows) for direct control without DEMETRA, and says it may work on ASCOM and INDI drivers depending on demand.

Observing Projects

With interchangeable gratings and coverage from the near UV to the near IR, the UVEX suits many projects.

Typical science targets include:

• Be Stars: Monitoring the H-alpha emission line profile to track changes in the star's circumstellar disk.
• Variable Stars: Following changes in their spectral features.
• Far UV and IR: Using calibration by set line to work at the ends of the range, where reference lines are few.

A 1.46 kg Instrument for Robotic Observatories

The UVEX Motorized, without cameras, has a mass of 1.46 kg and dimensions of 153 x 154 x 133 mm. The manual gives 2.53 kg with an acquisition camera, the calibration and guiding modules and a guiding camera.

Shelyak's recommended steps: install the main camera (13mm maximum backfocus), get first spectra on a table with any light source, calibrate the grating angle and focus motors, then add the guiding and calibration modules (sold separately), run a full observation with an optic in front of the UVEX, and only then install it on the telescope. The 12V / 1A power supply and the USB 2 cable are not included.