My guess? When they were experimenting with how to machine mass produce these mirrors the made a really good mirror for the 12" and 10" at those F ratio's by sheer luck and said "yep! Save those settings for those apertures from now on!"
Reasoning is because GSO also makes mass produced mirrors but they are at clear F ratio's F4 F4.5 F5.
Making a mirror is an involved process. Even with mass production instruments, like Skywatcher, the figure needs to be monitored in order to produce the required shape. But to produce a mirror of better quality, not just something punched out and see you later, each mirror needs to be monitored. When the mirror is approaching the desired focal length/ratio, the grinding process is adjusted in preference for figure over a single exact focal length/ratio. The production of a decent mirror has a trade off with cost and technical requirements. This trade off is there will be a variation in final focal lengths between finished mirrors. Hopefully small, but sometimes significant.
I have seen 12" 'f/5' mirrors vary in focal length by as much as 125mm. All of these are sold a 'f/5' though. Not one of the many f/5 and f/4 telescopes I have made has ever had the exact same true focal length. Each and every telescope had different length poles compared to its same focal ratio siblings.
This creates significant issues for dobs where the focuser is in the optimal place in one instrument, but it is not in another. It can have insufficient in-travel or out-travel. We as end consumers are left to deal with this. Yeah, sure the factory COULD adjust each and every solid tube length to suit, but this would blow out the cost - not just production cost, but packing too as each tube would be needing different sized boxes, would then pack as a bulk group differently, and the costs just keep piling up. There are short cuts that need to be taken with mass production.
For these particular Skywatcher dobs, Skywatcher is in a position to be able to select the appropriate mirrors for these instruments. The focal length tolerance is a whole lot less in truss/pole instruments than solid tube scopes - with solid tubes, we can fiddle around with moving the primary mirror up and down in its cell, or adding extension tubes, etc. The OTA of these collapsible scopes are mass production pieces. The components are all rattled off exactly the same. Then for these scopes to work the way they need to, Skywatcher selects the mirrors that fit this narrow range of suitable focal lengths.
Alex.
Last edited by mental4astro; 20-10-2015 at 11:10 AM.
