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Thanks for the advice, a friend ground the 8 inch I have now with me looking on and having a go every now and then. I wanted to grind my own mirror but thought that doing another 8 inch wouldn't really add a great deal beyond my current scope – seeing wise, obviously there is the experience of grinding a mirror. After reading a couple of books mostly by Texereau or Berry, the advice given was that an eight would be a good place to start and is a very common recommendation, but they also noted that this was due the fact that the eight inch is relatively easy to handle physically and can be ground a lot quicker than larger mirrors. The point was also made that smaller mirror provides a nice basis for longer focal ratios – removing the difficultly associated with parabolising a mirror. This is OK on a smaller mirror because the tube length does not become unmanageable for a Newtonian. It seems to be these factors that lead to the recommendation of smaller mirror as these are less likely to discourage first timers.
The recommendation was also made by Texereau that an adult may start on a 10 inch (with a longish focal ratio) although this will take longer to grind.
My thought was go a 12 inch (admittedly will be heavier and take longer to grind) but keep the focal length long (removing the tricky parabolising needed with short focal lengths) and have a scope which provides the experience of grinding a mirror and also provides something over the current 8 inch I am using now – going from the 8 to a friend’s 12 you can really notice the difference. As noted, I derive the greatest pleasure (dare I say it…) observing the moon and can (and have) sit for hours just looking at our moon – this is the seed that started all this.
The focal length was a concern but think that I will be suitably motived to carry a step ladder to the back yard. I live in a part of the country where we don’t get a cloud for 6 months of the year and therefore any viewing night is long, dark and almost magical. I was concerned about the possibility that a tube of this length would bow just enough to cause problems.
Thanks again all
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