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Pinholes

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6 replies to this topic

#1 Old Don

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Posted 06 December 2019 - 01:13 AM

What causes pinholes in a Questar full aperture glass solar filter?



#2 luxo II

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Posted 06 December 2019 - 01:29 AM

Water vapour (humidity) and time (age).

 

Don't wash it or use solvents.



#3 BillHarris

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Posted 06 December 2019 - 09:34 AM

Get a black Sharpie and "dot over" the pinholes.

#4 Old Don

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Posted 07 December 2019 - 01:12 AM

If it is a solid glass filter how would humidity and age cause pinholes in the glass?



#5 RMay

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Posted 07 December 2019 - 01:47 AM

I believe it’s pinholes in the coatings, which is why some folks are noting that one can use a Sharpie to cover them, and since they’re pinholes, they probably don’t impact the overall effectiveness of the filter.

Ron

#6 luxo II

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Posted 07 December 2019 - 03:40 AM

If it is a solid glass filter how would humidity and age cause pinholes in the glass?

Ok ... the solar filters are a metal coating on glass, often with a coloured filter over that. Although to the eye a new coating may look perfect, invariably there are molecular-sized pinholes sufficient to let water molecules in.

The Van der Waals force holds the coating to glass but - with time - water molecules will find a way to get in between the coating and the glass and the pinholes grow slowly until eventually they’re visible as macroscopic holes. Same occurs with anti-reflective coatings and first-surface mirror coatings, by the way.

But if you look after the optics - and store them with desiccant - coatings can last a long time.

Conversely humidity - and unnecessarily frequent washing - will accelerate the rate of deterioration.


Edited by luxo II, 07 December 2019 - 06:23 AM.


#7 Old Don

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Posted 07 December 2019 - 03:47 AM

Very clear. Thank you for the extra explanation.




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