grinding a primary

[abouttogrind]

Hi all,

I would like to grind a mirror but not sure of the focal length I should be aiming for. I really enjoy gazing at the planets and the moon and have been told that I should aim for a longer focal length….but have not been given any specific advice. I would like the scope to be a dedicated planetary scope and was thinking in the order of f13-15. Or is this just ridiculous for a Dob? [I currently have a 8 inch (f10)]

I am aware that this will make the tube very long especially if I go for a larger mirror (thinking 12 inch). Does any one have any advice?

Thanks...

[rmcpb (Rob)]

Going to need a long ladder!!

The longer focal length would be great for a planetary scope but if you are making it as a Dob then it has to be useable so some trade offs would be necessary I think or you will end up with a great scope that is never used.

Why do you need such a large aperture for a planetary scope anyway?

Cheers

[abouttogrind]

Hey Rob,

Thanks for your reply. The size wasn't a real concern, I live in a very dark part of the country (so I don't need to go to a dark site) and being a resonably strong lad the size isn't as much of an issue. Given the views of the eight inch I was thinking that the additional magnification and light gathering ability would be great.

Just trying to decide whether or not this is a completely crazy idea - my wife thinks so. Also thinking that I would go with a 'string truss' design for added stability - considering that the focal length will be in the order of 4 metres

Thanks again...

[rmcpb (Rob)]

Not sure that something with that length lends itself to the string truss design. The angles on the strings would be very fine and possibly not stable enough. In this case a tube design would be more useable I would think.

If you don't have to move it then maybe if it could be set up in a permanent observatory complete with viewing platform it would be practical but I still think it would be too long.

[Orion]

Have you ground a mirror before?

[ving (David)]

I know little on this topic but would like to take the opportunity to welcome you to the forum

sounds like you are setting up quite an observatory there
I wish you the best of luck with your project.

[ving (David)]

by the way, I have no idea how much you know about astronomy so I hope this doesnt insult... a 2x barlow will double your FL. also a aperture mask will inrease it too but allow less light in...

*hides*

[astro_south (Andrew)]

Vingo.....
Not wanting to be picky, but FL won't change with an aperture mask - "Focal Ratio" (FL/aperture) will change though. You are correct about the image being dimmer.

[ving (David)]

oh yes! sorry... my silly, but thats what i meant tho may bad

[rumples riot]

I would think that a 12" f10 would be a good planetary scope but the focal length will be very long (3.05m) . The 8" f10 that you have now is pretty good as a planetary scope, what about getting a 10" f8 scope. focal length is only a bit over two metres.

A 12" f13 scope is 4 metres long, and not usable. You might want to consider grinding a 8 with a long f ratio. To work out how long your focal length will be simply times your diameter of your mirror in mm by the f ratio. as in 305mm (12") x f10 = 3050mm (that is your focal length).

So as you can see the larger the F ratio the longer the focal length.

Personally I would keep the f10 that you have and do some flocking modes to it. See Mikes project on this.

[abouttogrind]

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

[rmcpb (Rob)]

Would a tube of this length be prone to movement by wind, especially noticable at high magnifications?

You can see I don't like the idea of such a huge scope

[abouttogrind]

I get that feeling Rob

Thankfully wind isn’t an issue where I live (normally unbearably still).

I thought that a truss design (if up end up down this path) would remove this concern. As for the strings, I thought crossing them (not in the main "tube" - don’t want to cause unnecessary diffraction) would increase the angle and hopefully address any stability concerns.

Having considered your advice about the truss system, the other possibility may be to use a sonotube and then brace it on the outside.

Thanks for playing the Devils advocate thought

[rmcpb (Rob)]

I'll have to come up with more problems to get that title I think