The principle
The autoguidinge system is based on a recovery of the focal plane of the telescope by two identical achromatic doublets with a focal length of 50mm. The final magnification is 1: 1 which means that the slit guidance is as if the camera were at the focus of the instrument. The slit is inclined at an angle 15 ° relative to the horizontal to release the optical return from the central field, then a mirror causes the beam to exit at 90 ° from the main optical axis. The double achromatic doublet then makes it possible to focus the image of the slit on the guide camera.

The slit is an integral part of the UVEX, it is also described in the section dedicated to the spectroscope. This section regarding the slot remains valid when using the Shelyak Alpy guide cube.
The autoguiding system is produced by a reflective slot inclined at 15 ° to the optical axis. The slitting system is that of the basic module of the Alpy 600 from the company Shelyak which consists of a very thin blade (50µm) in nickel engraved with 4 slots (25µm 50µm 100µm and 300µm) with a 25µm hole very practical for UVEX settings. The size of the 4 slots is very useful for adapting the flow resolution ratio depending on the objects and the telescope used. The slit system is very fragile. It is a very sensitive point of the spectrograph that it will have to be handled with care and installed upside down as we will see later.



Each slot system consists of a 50 µm thick nickel sheet produced by electro-deposition. What constitutes a series of slots and “Clear” type holes, the light therefore does not pass through glass passing through the slot. This slot has huge advantages as described by Christian BUIL in an email exchange in April 2019
… If we summarize the interests of this solution:
- An economical solution
- A gain in transmission of around 8% and even stronger in blue and UV.
- No stray reflection (the double image in the guide camera which disturbs beginners and sometimes systems).
- An ink black in the heart of the slot, which will significantly improve the quality of guidance for all observers.
- A reflection coefficient of nickel higher than that of chromium (therefore, guidance on potentially weaker targets).
- More contrasting spectra (less diffusion) – Great flexibility (many slots on the same plate, and a key thing for UVEX, holes which simulate a star, which can be key for the adjustment of the spectrograph
- this also applies to the whole range of Shelyak spectros (?), but admitting all the same that this nickel slot is less pros than glass slots).
… If we summarize the interests of this solution:
- An economical solution
- A gain in transmission of around 8% and even stronger in blue and UV.
- No stray reflection (the double image in the guide camera which disturbs beginners and sometimes systems).
- An ink black in the heart of the slot, which will significantly improve the quality of guidance for all observers.
- A reflection coefficient of nickel higher than that of chromium (therefore, guidance on potentially weaker targets).
- More contrasting spectra (less diffusion) – Great flexibility (many slots on the same plate, and a key thing for UVEX, holes which simulate a star, which can be key for the adjustment of the spectrograph
- this also applies to the whole range of Shelyak spectros (?), but admitting all the same that this nickel slit is less pros than glass slots).

Be careful however not to forget to mount the slit upside down on the slit support provided for the UVEX, which is counter-intuitive. Indeed, the edges of the slit do not have the same cutting edge on the two sides at the base, this slit not being made for automatic guiding at the base.



The inclination of the slit allows all the light flux that does not pass through the slit to be returned to the guidance system at an angle of 30 ° to the vertical which releases it from the optical axis. The field of vision is taken up by a Thorlabs ME05-G01 1/2 “and 3.2 mm thick plane mirror tilted at 60 ° in the direction of the focusing system.

The beam is then made parallel by a first achromatic doublet Thorlabs AC127-050-A. A second doublet (same reference as the previous one) focuses the image of the slot on the CCD sensor of the guide camera.


The guidance system has a 1: 1 magnification ratio which implies that the size of the objects on the slit therefore at the focus of the telescope have the same size on the sensor of the guide camera.


The field of the guide camera therefore depends on the focal length of the instrument and also on the guide camera used, but the very particular geometry of the slot imposes constraints. First of all, the optimum field of sharpness is ensured at the center of the slit or overall the geometric deformations are not annoying as long as the object is located in the zone between the central screw and the screw n ° 2 visible in Chinese shadow ci -above.

Basically the area of sharpness is 4mm x 4mm which is compatible with the ZWO Asi 120N camera (4.8mm x 3.6mm sensor) but larger sensors can provide added value in terms of field which is not negligible when pointing. Even if the astrometric reduction can only be done in the area of the sharpness field because of the shadow of the two screws (right and left on the photo) a target appearing in the wider field allows manual refocusing on the slot.


The field is therefore limited by the size of the sensor for a sensor whose dimensions are less than 4 mm, which is not generally the case. For the majority of cases, the dimension therefore depends on the focal length of the instrument as can be seen in the table below.
Focal length (mm) | Central field in ‘arc |
500 | 28′ |
800 | 17′ |
1000 | 14′ |
2000 | 7′ |
2800 | 5′ |