2- Autoguiding

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.

Optical guidance system on 1: 1 magnification slot

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.

Shelyak slot system here installed on the spot for the Alpy 600, to be mounted upside down for the UVEX
“Alternative” slit of the basic version of the ALPY 600. Source Shelyak.

Standard slit present in the basic version of the ALPY 600 – 1 hole of 25 µm and slots of 25, 50, 100, 300 µm. Source Shelyak

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).
“Standard” model which equips the ALPY600 spectroscope in Basic version, installed upside down on the support in 3D printing of the UVEX

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.

Slit at the place, bad configuration the slit is very wide, we cannot effectively guide in this configuration the lips do not reflect the light of the star once in the slit.

Slit upside down, good configuration we notice that the slit is surrounded by a very fine characteristic line. Be careful not to trust the writing direction (35 µ) here because of the mirrors affected at the time of acquisition by the camera software.

The spectrograph side the diagram above is the one where the width of the slits is written in µm, but the writing is also visible on the other side upside down. Source from Shelyak technical documentation.

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 1/2 “guide mirror and its support in 3D printing, the part protected by a blue plastic film is to be installed in front is the aluminized part of the mirror.

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.

1/2 “Thorlabs Achromatic Doublets (x2). Take care when mounting in the support, the two doublets must be mounted head to tail, the bulging parts towards the inside of the tube.
The two achromatic doublets mounted in the SM05L05 Thorlabs support, the references are oriented outwards.

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.

Image of the Caliberx n ° 2 guide field, Zwo Asi 120N guide camera, note that the border around the slot is clearly visible, and that the latter is very fine, this means that the slot is well mounted upside down on its support
Field of the Be EW Lac star, Perl Vixen fluorite 90/810 + Caliberx n ° 2, Zwo Asi 120N – 9s exposure, the field here is 19.5 ‘x 14.5’ limit magnitude 15, condition of urban shooting. When pointing in the direction of the Milky Way many stars appear in the guide field in a few seconds of poses, which allows an astrometric reduction and a precise and sure aiming of spectroscopic targets even up to a magnitude 15!

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.

Position of the sharpness field on the slit system.

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.

L’attribut alt de cette image est vide, son nom de fichier est image-12.png.
Nebula M42 – C11 Edge + Calibrex + Asi ZWO 178MM 5s of 4×4 binning pose.
L’attribut alt de cette image est vide, son nom de fichier est sn2020ue.jpg.
Supernovae SN2020 eu – C11 Edge + Calibrex + Asi ZWO 178MM 20s of 4×4 binning pose 500/900 gain. The field is extended here, the central screw is visible entirely on the left and half of the screw n ° 2 is visible on the right. We can see the hole at the top left. The field at the top and bottom of the central screw can be used for pointing.

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
50028′
80017′
100014′
20007′
28005′

One thought on “2- Autoguiding”

  1. Hi
    I very interested in building a UVEX 3 and a Calibre. There is a lot a good information on this site and you should all be proud of what you have achieved.
    At the moment I have been putting a list of materials together to get a final price of construction and the make sure of availability.
    One thing I cannot find is the Shelyak slit 50µm disk. I am unable to find it on the Shelyak web site. Would it be possible to send me a link to the correct web page?
    Many thanks
    Regards
    Graham

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