1. Fibre optic assemblies

The fore optics and fibre optic assembly have been designed specifically for the piccolo system and are built to order. The length of the nylon ferrules and of each leg of the assembly are specified by users at time of order and the assembly built accordingly. The fibre optic assembly may be a bifurcated assembly

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Figure 1 Single bifurcated fibre optic assembly schematic

or a double bifurcated assembly

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Figure 2 Double bifurcated fibre optic assembly schematic

The fibres are low OH and the user can select which diameter of fibre (400μm or 600μm) to use for the upwelling or downwelling fore optic. This needs to be specified at time of order so that the system can be set up and calibrated accordingly. Note fibre assemblies supplied after January 2021 are of a different design and TeamPiccolo will have advise on delivery which fibre leg has to go to which spectrometer. The fibres are stainless steel sheathed and have a black water resistant coating.

N.B. If the fibre optic inside the assembly, the fibre optic tip in the stainless steel ferrule or the electrical cables are damaged it has proven not to be cost effective to repair the damage. A new assembly will need to be purchased.

When two spectrometers are use, the double bifurcated fibre approach enables the Earth surface radiance (and exitance) to be measured simultaneously with the same support. For a discussion on the meaning of ‘support’ see Atkinson, P. M., Curran, P. J., 1995. Defining an Optimal Size of Support for Remote-Sensing Investigations. IEEE Transactions on Geoscience and Remote Sensing 33 (3), 768–776.

2. Fore optic assemblies

There are two fore optic assemblies. One for downwelling solar irradiance and one Earth surface upwelling radiance (and exitance). The upwelling fore optic has a  field-of-view (FOV) determined by the numerical aperture of the fibre used. This is approximately 25^o for the Piccolo fibres (for a discussion on why an approximate value is specified see Field Guide to Geometrical Optics, Greivenkamp, Society of Photo Optical, 2004). This FOV can be restricted by placing a Gershun tube assembly in front of the fore optic shutter. This option should be specified at time of ordering. The upwelling fore optic either has a optical glass dome or an optical glass planner window to protect the internal shutter. The downwelling fore optic has a cosine-corrected diffuser to integrate irradiance over 2π and an optical glass dome to protect the internal shutter. The shutters at stitched by the piccolo control electronic so that they are both closed to enable total dark current to be recorded of each opened in turn to let upwelling or dowwelling light enter as required.

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Figure 3 Photo showing upwelling fore optic (on left) with shutter evident and downwelling fore optic with cosine corrected diffuser

There is an ‘o’-ring on the outside of the main barrel of the fore optic to make a weather proof seal with the window barrel. Great care should be taken so that no damage occurs to this ‘o’-ring if the front barrel is removed and the refitted. Any damage could cause water to enter the system and interfere with measurements and destroy the shutter.

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Figure 4 Main body barrel with ‘o’-ring

There is a desiccant block within the fore optic main body barrel to reduce the possibility of moisture forming inside and causing damage to the shutters. There is also a weatherproof cable gland through which the fire optic cable passes (See further discussion below). The Piccolo is supplied with the fore optics assembled and fibre optic cables fitted. If it is necessary to disassemble the fore optics/fibre optics great care must be exercised a) not to damage the shutters – if the hold down screws are over tightened the shutter will not move freely, if they are too loose they may come free and interfere with shutter operation (it is best not to dismount the shutters without discussing with the Piccolo suppliers first) N. B. DO NOT TOUCH THE SHUTTER BLADE – ANY CONTAMINATION WILL CAUSE IT TO FAIL b)  to refit the cable glands correctly to ensure a weatherproof seal. N.B. the shutters are provided fitted to the shutter housing and should not be dissembled. There are anti-tamper marks.

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Figure 5 Upwelling fore optic assembly with plane optical glass window

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Figure 6 Downwelling fore optic assembly with domed glass window

Attaching fibre optic to shutter housing and connecting electrical cable to shutter

After threading each of the fibres that will be attached to the fore optics through the unassembled cable gland, the main barrel body and the desiccant block the fibre tip ferrule should be very carefully inserted into the shutter housing assembly threaded connector. The brass-coloured connector on the cable should then be tightened a few turns. A very small drop of lock tight (type 932) should be applied to the thread. GREAT CARE SHOULD BE  EXERCISED SO THAT NO LOCK-TIGHT GETS ONTO THE FIRE TIP – IF IT DOES THE FIBRE WILL NEED TO BE RETURNED TO THE SUPPLIER TO BE REPOLISHED AND THE SYSTEM RECALIBRATED -THERE WILL BE CHARGES INCURRED FOR BOTH OF THESE OPERATIONS. The brass-coloured connector should then be tightened onto the shutter housing connector until hand tight. The electrical connector can then be connected to the shutter.

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Figure 7 fibre optic assembly ferrule, brass-coloured threaded connector and electrical connector

Cable gland

The cable gland (RS part number 506-6169) is specified as IP68. To meet this specification it need to be fitted correctly. The gland nut should be on the inside of the fore optic barrel (note the same gland is used for the spectrometer enclosures and same fitting requirements apply) and the external connector with strain relief should be connector to the nut and tightened by hand. When hand tight a further 1/4 turn should be applied using a spanner. This will compress the nylon rib shown in Figure 7 by arrow A below against the barrel body to form the seal. The gland should be replaced rather than reused if it has to be detached from the barrel after use.  After the fibre has been threaded through and secured (hand tight and with one very small drop of lock tight Type 932) on the shutter housing fibre attachment thread) to the shutter housing (seen in Figure  5 and 6) and the shutter housing seated in the lower barrel the strain gauge with rubber seal inside the nylon clasp can be hand tightened.  

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Figure 8 Cable gland

The shutter housing can now be aligned with the locking pins or lug on the main barrel body and the fibre optic cable gently pulled tight and the strain relief gland connector locked in place to hand tight. The cosine corrected diffuse assembly can then be put in place and the large diameter threaded ring screwed down inside the shutter housing assembly to secure it in place. A very small drop of lock-tight can be used t make sure it does not loosen during use. GREAT CARE SHOULD BE EXERCISED SO THAT NO LOCK-TIGHT GETS ONTO DIFFUSER – IF THIS DOES OCCUR THE DIFFUSER WILL NEED TO BE REPLACED AND THE SYSTEM RECALIBRATED -THERE WILL BE CHARGES INCURRED FOR BOTH OF THESE OPERATIONS. The dome or window barrel can then be screwed to the main barrel and hand tightened. This will lock the shutter housing in place.

Fore optic component suppliers 

All fibre optic assemblies are manufactured in the UK by Alker fibre optic specialists < https://alker.co.uk/ >. Each assembly is manufactured to order and length of each leg and overall length need to be specified at  that time.

Fore optic and Shutter housings are manufactured by Peak Design <john@peakdesign.co.uk>.

The shutters are Unibliz ES6B (data sheet attached  here ) manufactured by  Vincent Associates of USA <https://www.uniblitz.com/>. The UK supplier is Acal-BFi <https://www.acalbfi.com/uk>, note a minimum order of 3 units may apply.  N.B. Unibliz advise that the shutters used in the Piccolos supplied with pre 2020 instruments (TS6B) are now obsolete . The ES6B  shutter should be used. However, there is a difference in external dimensions which requires that a modified shutter holding tray be used. A manufacturing drawing for this shutter tray is attached here.