Probably stupid question about focal lengths

[Mr niall]

Hello there

probably a stupid question with an obvious answer but I've been lurking around this forum for weeks and can't find an answer sorry! Now I'll pre-empt this by saying I'm a total mathematical  failure and that is probably the crux of the problem but.....

I want to build a small dobsonian for the kids as my skyprodigy 70 just isn't cutting the mustard. Now I think I understand the basics of focal lengths and the such - but as far as secondary mirror location goes (and I've checked NEWT etc and I'm still not completely clear!),  am I right in thinking that if a primary mirror has a focal length of, say, 800mm then the secondary mirror isn't actually placed 800mm further up the tube, but that 800mm distance includes the light bouncing off the secondary mirror and up into the focussing hole? All the focussing hole stuff confuses me too tbh; where exactly is that 800mm point supposed to be in relation to the eyepiece? I.e. Just below it, or above it, or at the bottom of the lens? I don't get that because I thought to focus the eyepiece you moved the eyepiece up and down anyway? Why is focussed height so important? Is it a product of the internal diameter of the tube?

 

sorry, I've probably given myself as a total fool. I've read stellafane and a few others about 6 times and I don't get it so that probably says more about my level of intelligence than the quality of their instructions!

Many thanks!

[Charic]

Welcome to the SGL.

This will be interesting, not just a noob question or stupid question, and if you don't ask you won't find out,  but I'm certain your assumption is  correct, that the focal point, where the image is created, is re-directed  by the secondary mirror  to a point along the focuser tube.
Some folk here have already built their own scopes, they will keep you right!
 

 

[Kropster]

Yes, the secondary just bends the light path and doesn't change the focal length.

Not sure about where the focal point falls in relation to the eyepiece.

[hughgilhespie]

Hi Mr Niall,

Actually your questions are good ones and the answers aren't all that obvious.

First, the focal length of your mirror is fixed and can't be changed so for your say 800 mm focal length you have to have exactly that between the mirror and the focal plane of the eyepiece. But this is a Newtonian so you are going to bend your light cone from the mirror by 90 degrees so that it pokes through the side of the tube. The total length is still 800 mm but now you need to know where to put the secondary mirror so that the focal plane of the main mirror is in the right place - and where is the right place?

You need to know that different eyepieces can have their focal planes in different places. For example, Televue Plossls focus 0.25 inches INSIDE the focusser . The Explore Scientific 24 mm ES82 focusses 0.09 inches INSIDE the focusser. This variation is why you need a moving focusser. You are always (for astronomy anyway) focussed at infinity but changing eyepieces needs to have a moveable focus.

Next you will need to know the dimensions of your focusser, the thickness of your telescope tube and the internal diameter of your telescope tube and how much movement your focusser allows. From these you can calculate how far you need your cone of light to poke out through the side of your tube so that it gets to say the end of the focusser draw tube when it is half-way extended. Actually, doing a drawing is probably better than doing calculations.

Finally, when you know the distance from the centre line of the telescope tube to the end of your main mirror light cone you subtract this number from your main mirror focal length and that tells you the height to put your secondary mirror. It will also let you calculate the size of secondary mirror you will need.

So, all in all, not that simple or obvious. There are lots of good books on making telescopes and you may want to get hold of one before you start. 

Regards, Hugh

[Dave Lloyd]

Hi Mr niall, welcome, you've set yourself quite a task. 

[Davey-T]

Welcome to SGL, this is a good place to ask all and any questions from building scopes to the meaning of life.

As Hugh said best get a book or two and learn about mirror quality and such before spending any money, there are forums devoted to ATM (amateur telescope makers) and also lot's of on line help.

Good luck and keep a pictorial record and post on here to help others.

Dave

[Mr niall]

Thats amazing guys, thanks very much - its much clearer in my head now. I will indeed keep a pictorial record too thats a good idea! 

Niall