Minimum limit to telescope magnification

billfish678 · February 18, 2009, 12:06am

snailboy:

It’s an 8" Newtonian reflector as I mentioned earlier. It has a focal length of 1219 mm (f/6). I have 4 eyepieces: 25 mm, 15 mm, 10 mm, and 6.5 mm, along with filters. Like I said before, I’ve looked at getting a ~40 mm eyepiece for viewing larger objects and perhaps finding certain things like Uranus and Neptune. (I’ve found both of them before, but it’s difficult since they look like stars. It would be easier with a wider viewing range though.) I’d get more use out of one than the 6.5 mm. I’m just wondering what my practical lower limit for magnification would be.

And I know a few things about amateur astronomy. I’ve had this telescope for several years and before it, I had a small refractor. It’s not a new hobby to me. It’s just the details of the optics that I’m not so familiar with. Just so you know.

No problem. Practical stuff first. Do you have a 1.25 inch focuser or a 2 inch focuser?

snailboy · February 18, 2009, 11:56pm

1.25

Napier · February 19, 2009, 4:51pm

>Okay, I didn’t know that. But is there a simple explanation for why that is so?

Don’t think I saw this stated quite so simply in the thread, so:

If your telescope magnification is X, the angles of rays exiting the eyepiece will vary by X times whatever the angles of rays entering the objective do. And, the bundle of parallel rays exiting the eyepiece will have a diameter that is 1/X times the diameter of the bundle of parallel rays entering the objective.

But as I think about it it isn’t obvious what to say about why that is. If I think of it, though, I’ll post it.

billfish678 · February 20, 2009, 1:08am

Okay,

In that case about the longest focal length eyepiece you can go is 35mm.

If you really want a wider field of view, its almost cheaper to just buy a 4 inch to 6 inch f6 or so mirror and build yourself another smaller telescope.

I’d be tempted to buy a 4 inch f6 mirror to use with your 25 mm eyepiece rather than a 35 mm eyepiece.

A second much smaller scope is quite handy for traveling, camping, and impromptu observing sessions.