New replacememt Home Magnet for Observatory Dome was received today from Pulsar Observatories.   This was a replacement for the original magnet that had become badly corroded and became unreliable for homing the observatory (photo below left).  The original had a cut or flaw in its outer casing and water had entered the magnet and corroded it over the course of a couple of years. The new magnet (photo below right) was installed and successfully used for an overdue recalibration of the dome. The new magnet also allowed ASCOM Conform tests for the Dome to pass its Find Home Test where it had previously failed.

Photo showing the old corroded magnet		Photo showing the new replacement Home Magnet

Update 2021-12-05
Around one year since the original home magnet was replaced with a new one, it has been found that one side of the new home magnet has begun to corrode.   Whilst the magnetic strength of the magnet still seems strong, the dome appeared to be unable to find the home magnet during an attempted dome recalibration at end of S939 session (2021-12-05) or when requesting the dome to go the Home position (2021-12-06)

Photo showing the new magnet after 12 months

Update 2022-07-13
Magnet checked today (2022-07-13) and whilst it continues to corrode it still works for finding the dome's Home position.

The Magnet Supplied by Pulsar is a 10mm diameter x 5mm thick, very strong Neodymium magnet,  like these ones from Amazon :
 F645-N52-10 Magnet Expert 10mm dia x 5mm thick Ultra High Performance N52 Neodymium Magnet - 3.2kg pull ( Pack of 10 ) by first4magnets™
 https://www.amazon.co.uk/Magnet-Expert-thick-Performance-Neodymium/dp/B00GFMQ5OM   (£10.48)

- Corrosion issue is due to basic rusting process (metal/Fe + water + oxygen). 
- The magnet is inevitably exposed to water from dew/condensation created during nightime observing sessions.
- Neodymium magnets are apparently known for their poor resistance to corrosion, unless they are waterproof versions that are coated by a plastic layer or shrink-wrapped in a PVC cover (which the Pulsar supplied magnets are not)
- The position that the Home magnet needs to be placed in my system, where it sits in direct contact with metal embedded in the reinforced rubber trim, might enhance the rate of corrosion either directly by some electrolysis effect or by causing damage/scratching to the magnet's 'protective' casing.
- Based on original supplied magnet, the corrosion will eventually progress to the point that it is no longer strong enough to be detected by the Dome Unit/Encoder.

Tests performed with an alternate 10mm diameter magnet :
 
 10 x Cylindrical Magnets 3mm Thick By 10mm in Diameter Round Disc Magnets Very Strong  1.2kg pull by Harlington Group
  https://www.amazon.co.uk/dp/B08426C287/ref=pe_27063361_485629781_TE_item  (£3.95)
  (the received magnets have a better casing than the pictures & reviews on product page)

- One regular 3mm magnet doesn't have sufficient strength to allow Home position detection
 
- Two regular 3mm magnets used together (6mm) do however have sufficient joint strength to allow Home position detection
 
- Two regular magnets may or may not prove to have better corrosion resistance than the Pulsar Supplied Neodymium magnet 
    (this doesn't appears to be the case  based on extent of corrosion found on 2023-11-17)

Update 2023-11-17.
After replacing one of the dome's Drive Wheels/Connectors on 2023-11-17 (which had become loose/badly worn) and before the dome was recalibrated,  the dome's home magnet was checked and was found to be almost totally corroded with minimal remaining strength and is non-functional - the dome was unable to find Home Position using it.  This was happened on two previous occasions in 2000 and 2022. The Home Magnet is believed to corrode in my Observatory due to an electrolysis effect caused by its contact with metal beading in the rubber trim attached to the edge of dome's roof flange combined with frequent wetting from dew drops running down the inside of the observatory dome. The pieces of the corroded magnet were removed and replaced by a new magnet (actually by a new pair of magnets,  see Home Magnet Tests, 2022-07-13). After successful calibrating the dome the new magnets were removed and carefully stored ready for some future use - repeated experience shows that they can't be left in position for extended periods without corroding.