newbie with many questions

[ernie00]

Hello,
I'm new to astronomy, been reding quite a bit on this website, very intresting stuff but a lot of it I don't quite understand. hehe

A few years ago a friend of mine gave me a 4.5 inch mirror with a smaller one 1 inch. Along with some information 78 cm between the Primary and secondary mirror of 16 cm.. HE told me I could play a little with it as long as I kept the 94 cm between the eyepiece to mirror.

So few years later, I'm ready to build the telescope.. I have no clue on what I am doing, looked at some picutres on the internet to give me some idea and go from there.

Also he told me I needed to buy an eyepiece... it sounded simple but after looking and reading lot of stuff that I can't understand I'm really clueless on what to get. All I want is to look at the moon and stars and go from there. Any suggestions?

Thanks a lot in advance
Eric

[RickJ]

First mixing units makes things confusing.

Let's say the mirror is 114mm and the focal length of the mirror is 940mm. That makes it an f/8.25 mirror, likely spherical which is fine for this type of scope. It will work great.

The mirrors are for a Newtonian telescope. The concave 114mm mirror sits at the bottom of a tube and reflects the image back up the tube. Problem. It's inside the tube so if you put your face over the tube to see it you shadow the mirror and see nothing but your face very out of focus. So a small flat mirror, the 25mm one you mention, goes in the center of the scope and reflects the light out the side if the tube so you can see the image without blocking all the light. The mirror does block 25mm worth of the center of the mirror but this is only about 5% of the light (25x25)/(114x114). Now the closer the small flat mirror is to the main mirror the larger it has to be to intercept the light cone but the larger the fully illuminated field of view is. A 25mm secondary (that's the name for the small mirror -- the main one is called the primary of course) is about right for this size scope. Though he is telling you to put it 780mm up the tube from the main mirror which will put the image 160mm from the centerline of the telescope tube. Assuming the tube is 130mm in diameter and the wall of the tube is 5mm thick that would put the image (160-65-5) 90mm outside the tube ( 3.5") which defines the focuser you will need. If the focuser can't put the focal point of the eyepiece at that point you can't bring the scope to focus. So while his dimensions fit some focusers they don't fit all. It's best if the focuser is a bit short as you can always add extension tubes but there's not much you can do if it is too tall but try and saw it off in such a way it still works. Or move the primary mirror up the tube closer to the secondary. That may require rebalancing the scope on its mount.

A link to all this is:
http://www.catseyecollimation.com/designie5.html

Be sure to enter all factors in the same units. It will tell you far more than you really need to know. Maybe someone knows a more basic site. Or use the diagram to get the idea of what you are doing.

You will also need to align the mirrors. At your focal ratio a simple sight tube is all you need. Actually a film can put in the eyepiece holder upside down, bottom up, aht has a hole in the center of the bottom to look through like a peep sight (no lid) is all you need to put your eye in the right place. Then follow the instructions here:
http://starizona.com/acb/basics/using_collimating_newt.aspx

Besides the focuser, you'll need a spider and secondary holder for that secondary mirror, a mirror cell for the primary and some sort of finder to help you point the scope. Then there's the mount to consider. Or maybe he gave you all this as well. In any case the mirrors are just the start.

Rick

[ernie00]

Thanks for the info very very intresting...

the telescope is build out of a MDF box instead of a tube, couldn't find an abs tube to match it close to the size of the mirror so I took another route. So the wall are much thicker than 5 mm. I've made the primary mirror support to be adustable in height so I have about 15 mm of adjustment on up and 15 mm on down.

I guess the best thing to do is to being the telescope to the shop near my house and they could guide me towards a proper eyepiece.

[Reesa]

Hi,

I'm also new to astronomy, and decided to make my own scope. It kinda sounds like you're already done, but just in case, here are a few sites that I got information from.

http://http://members.aol.com/davetrott/page11.htm

http://http://members.aol.com/davetrott/page14.htm (http://members.aol.com/davetrott/page14.htm)

http://http://members.aol.com/sfsidewalk/intro.htm (http://members.aol.com/sfsidewalk/intro.htm)

I'd love to see it when you're finished