Shelyak Fiber Injection & Guiding Unit
Collecting the light from a target star is key in the spectrography process, and so special care must be taken when collecting that light. The Shelyak FIGU, or Fiber Injection and Guiding Unit, was built specifically for this purpose. In a nutshell, here is how it works:
The Fiber Injection & Guiding Unit is the interface between your telescope and the eShel Spectrograph. It can be installed by using the 2” adapter provided or, alternatively, you can use a T-adapter. When using an f/10 SCT, Shelyak recommends the use of a 0.63X focal reducer and a Variable T-Adapter with the 15 mm extension tube in place. Backfocus of the FIGU is about 45 mm.
During operation, a mirror reflects the field of view seen by the telescope into a C-mount or CS mount guide camera, allowing you to easily center the object. A small hole (around 50μm, about the same diameter as the optical fiber) is in this mirror, and that hole is aligned with the fiber so that all the light that goes through the hole will be collected in the fiber.
When the star you wish to observe is exactly aligned with the hole, it disappears from the guiding image. In other words, the less you can see the star on the guiding image, the more light is collected in the fiber. Of course, several parameters have a direct impact on the light collection: sky seeing, telescope focal length, guiding precision, telescope focusing... all these elements must be adjusted (well... not easy to adjust the seeing!) to get the maximum light in the fiber – and then increase the S/N (signal to noise) ratio at the end of the process.
The guiding port has a 1.5x magnifier optical relay inside the unit which makes the field of view smaller than in prime focus. The typical guiding image field of view is only slightly vignetted in a circle of about 7 mm in diameter.
A moving mirror is installed in the guiding unit, controlled by an electro-magnet. This sends the calibration light to the acquisition fiber. Calibration light is transported to the guiding unit through a secondary optical fiber, called the calibration fiber. The magnet is activated by a special power supply from the calibration unit which provides 13V for a brief time then a sustained 6V. It is also controlled by software when you acquire spectra or calibration frames.
The acquisition fiber is a 50μm (core diameter) silica-silica multi-mode fiber. Connectors are of the FC type; they are tightened by a nut, and positioned by an index so that when you remove the connector, you'll put it back at exactly the same angular position. The optical fibers are protected in a metallic/plastic casing to prevent any damage, specially important in severe environments. The casing also prevents external light from entering the fiber. The acquisition optical fiber connects the back of the Fiber Injection & Guiding Unit to the eShel spectrograph.
The calibration fiber is the same type (multi-mode silica), but is bigger (200μm diameter) to get more light. It is also provided with FC connectors. The calibration optical fiber connects the calibration unit to the side of the Fiber Injection & Guiding Unit. When a power source is on (Flat or Thorium-Argon) and the mirror properly activated, you will clearly see the hole in the guiding image – this can be useful to locate the hole at night.
- Additional Information
SKU SL-PF0008 Manufacturer Shelyak Spectrograph or Accessory? Spectrography Accessory Spectrography Series Accessory eShel Warranty 1 Year Warranty
- Included Items
- Shelyak Fiber Injection and Guiding Unit Console
- 50um Acquisition Optical Fiber with FC Connector
- 200um Calibration Optical Fiber with FC Connector
- 2" Telescope Adapter
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