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Reflection and wave factor?


Gal

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Hello, 

So I have a question about what actually 1/4 and 1/6 PV means? I have seen that Skywatcher telescopes have 1/4 and 1/6 PV and that GSO telescopes have 1/12 wave but it also states that this is RMS so is this the same as Skywatcher measurement or is it different, and which one is better. Also saw that the GSO has around 93% reflection, does anyone know how much reflection do the Skywatcher telescopes have, for example: http://www.teleskop-express.de/shop/product_info.php/info/p3888_Skywatcher-Explorer-250PDS---10--f-4-7-Newton---2--1-10-Crayford.html

link to the GSO webside: http://www.gs-telescope.com/content.asp?id=142

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Perhaps this will be of some use as to a practical guide, explains it better rather than me writing about it. :)

http://www.oldham-optical.co.uk/Optical.htm

Pretty sure that the GSO is similar to sky-watcher i.e no better than 1/4 PV ( I think ), see article for explanation of terms sometimes where the RMS value is quoted.  I've never seen reflectivity values for sky-watchers, but l'd say somewhere in the 88 - 92% region perhaps ??. I'd like to know that data too. There is some uncertainty over the current crop of GSO mirrors and their coatings as to how well they last compared to the sky-watchers. The GSOs are often quoted at 93% - 94%. seeing I am interested in the GSOs too, but as yet not got an answer on the current status with the mirrors in them. I was told that the older GSOs were prone to degrade somewhat faster compared the SWs of today, but perhaps this has changed with the newer models.

Hope that helps a bit :)

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This is a very complex area as the above link demonstrates !

Roughly speaking 1/12 wave RMS translates to around 1/4 wave Peak to Valley (PV) which means the mirror or lens diffraction limited and the minimum needed for a scope to perform to it's theoretical limits I believe. 

Strehl Ratio is another measure that you will see used and some folks think thats a more useful way to express the accuracy of a mirrors figure. Here is some more reading on that:

http://www.rfroyce.com/standards.htm

Another thing to remember is that frequently what is being referred to is just the primary mirror. The secondary mirror tends not to be measured and yet has an impact in optical performance of course.

At the end of the day though it's how the scope performs that matters. All the data can be interpreted a number of different ways and the testing equipment can throw up different results from the same mirror if even one tiny factor is changed.

It's a bit of a minefield I reckon  :rolleyes2:

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