Shelyak LISA Spectroscope (VIS)

The Shelyak LISA (VIS): A Spectroscope for f/5 Systems

The LISA is engineered specifically for capturing spectra of faint and diffuse deep-sky objects where high light-gathering power is more critical than ultra-high resolution. It is the most sensitive instrument in Shelyak's range, dedicated to comets, novae, supernovae, faint variable stars, the redshift of distant galaxies or quasars and the confirmation of planetary nebulae.

An f/5 Design with a 0.68x Internal Reduction

The optical path of the LISA begins with light from an f/5 telescope beam passing through the slit. A collimator with a 130mm focal length transforms this light into a parallel beam before it strikes the diffraction grating. After dispersion, an 88mm objective lens focuses the spectrum onto the camera sensor. The slit image is therefore multiplied by 88/130 = 0.68 on the sensor. Shelyak credits this internal reducer and the low resolution for the LISA's sensitivity on faint objects.

R~1,000 Resolving Power from 400 nm to 700 nm

With the 23 µm slit, the LISA reaches a resolving power (R) of about 1,000; R ranges from about 600 to 1,000 depending on slit and mode. The VIS (Visible) configuration covers 400 nm to 700 nm, and the range even extends into the near UV and near IR.

A 4 mm Mirrored Slit with Four Interchangeable Widths

The LISA's slit assembly is central to its functionality. The slit is a mirror slit: the field around it is sent to the guiding camera port, so you can position the target accurately on the 4 mm long slit and hold it there.

• Included Slits: The standard interchangeable slit carries 15, 19, 23 and 35 µm widths, so you can favour resolution or brightness.
• Autoguiding Port: The reflected field of view is directed to a C-mount port with a 17.5 mm backfocus, where a guiding camera (not supplied) lets you autoguide the telescope.
• Optional Slits: For specialized applications, Shelyak offers optional wide slits and photometric slits.

Matching the LISA to Your Telescope's Focal Ratio

To achieve maximum throughput, the LISA is designed to be fed by an f/5 light cone. You can install it on telescopes that are natively f/5. On Schmidt-Cassegrain or Ritchey-Chrétien telescopes, generally f/10 or f/8, use a focal reducer to bring the focal ratio close to f/5. A Schmidt-Cassegrain takes an f/6.3 reducer and an SCT T-adapter, with a longer spacing between the LISA and the reducer to reach f/5.

M42, C-Mount, and 2" Nosepiece Connections

The LISA provides standardized mechanical interfaces for easy integration into your existing setup.

• Telescope Connection: The instrument includes a 2" nosepiece for use in standard 2" focusers. The nosepiece can be removed to reach a female T-mount thread (M42x0.75mm). For a Schmidt-Cassegrain (2" external thread) you need the optional BA0027 variable T-adapter.
• Imaging Camera Connection: The acquisition camera attaches via a male M42x0.75 T-thread. The backfocus is 54.9 mm; the manual says the T-mount thread can also take a DSLR with a T-ring.
• Guiding Camera Connection: The autoguider port uses a standard C-mount interface with a 17.5 mm backfocus.

Sensor Matching for Acquisition and Guiding

The acquisition camera mounts on the T-mount thread at 54.9 mm backfocus, and the guiding camera (not supplied) on the C-mount port at 17.5 mm. The manual adds that all Lhires III camera adapters fit the LISA, since both instruments have the same spectrum port backfocus.

The LISA includes two adapter rings to help achieve correct spacing for cameras with different backfocus requirements. For ATIK cameras you need the 13mm ring (13mm mechanical backfocus), supplied with the LISA; the second adapter is for cameras with 17mm backfocus.

Integrated Features for High-Precision Wavelength Calibration

Accurate scientific results depend on precise wavelength calibration and flat-fielding. Light can be injected through a calibration window (tungsten lamp, neon lamp, energy saver lamp), and a white screen controlled by an electro-magnet can be moved to send this light to the slit; when it is in front of the slit, it stops starlight entering the spectrograph. The optional calibration module adds an Argon/Neon lamp, with many emission lines over the whole LISA range, and a tungsten lamp for flats.

From Acquisition to Final Spectrum with Demetra

Shelyak provides the free Demetra software, and the LISA manual also names IRIS, AudeLA, RSpec and VisualSpec. The process includes standard steps like bias and dark subtraction, flat-fielding to correct for instrumental response, and geometric correction of the 2D spectrum. Then come sky background subtraction, extraction of the 1D profile (the manual describes optimal extraction) and wavelength calibration with your reference lamp spectrum. The final step is to apply an instrumental response correction, typically derived from observing a reference star of a known spectral type, to produce a scientifically valid spectrum.

Contributing to Novae, Supernovae, and Comet Research

The LISA is a tool for collaboration between amateur and professional astronomers: novae, supernovae, comets, symbiotic stars and the confirmation of planetary nebulae. Many scientific publications are based on LISA observations, with examples such as an ATEL on H-alpha in SS433 and spectroscopy of classical novae. Advice and examples are shared on the spectro-L discussion list and the ARAS forum.

A 1.4 kg Instrument Ready for the Field

The LISA weighs 1.4 kg without a camera; the manual reminds you to check the total weight capacity of your mount. Focusing can be started during the day with an energy-saver lamp or a neon lamp, as the manual describes.