Newt. focal lenght

Ok here's one for you ATM folk and also those in the know.
I am planning on buying a mirror set and building a dob., probably a 12
incher but that doesn't matter at this point. What I don't know is at what
point do I want the mirrors focal point to be, with a given focal length of
lets say 48" do I want to make this measurement from the surface of the
mirror to.....well where? I assume it is somewhere within the focuser
however am not sure exactly how far in. I could buy a book and probably will
but at this time I am just in the planning/dreaming phase. Any help will be
appreciated!

Thanks,
Tom Wales

What I don't know is at what point do I want the mirrors focal point
to be, with a given focal length of lets say 48" do I want to make
this measurement from the surface of the mirror to.....well where?

In order to cater to a wide range of eyepieces, it's usually
recommended to allow 0.5" to 1" of spare focuser in-travel. In other
words, the focal point should be 0.5" to 1" above the face of your
focuser, when *fully racked in*. However, the larger you make this
number, the more you sacrifice field illumination.

The Televue Nagler 22 and 31 are two eyepieces that immediately come to
my mind as requiring generous in-travel. If you plan on using either
of these, I recommend 0.75" of spare travel.

If you haven't already done so, try downloading the nifty free program
Newt to help with your project. The program help file also contains
some useful explanations.

Cheers,
Ritesh

Tom, there's a nice little freeware program out there in the ether
called Newt. Google for it. You plug in the parameters and it will tell
you all of the measurements for your scope. Nice gui, too.

Cheers - Chris

Tom Wales wrote:
Ok here's one for you ATM folk and also those in the know.
I am planning on buying a mirror set and building a dob., probably a 12
incher but that doesn't matter at this point. What I don't know is at what
point do I want the mirrors focal point to be, with a given focal length of
lets say 48" do I want to make this measurement from the surface of the
mirror to.....well where? I assume it is somewhere within the focuser
however am not sure exactly how far in. I could buy a book and probably will
but at this time I am just in the planning/dreaming phase. Any help will be
appreciated!

Thanks,
Tom Wales

"starburst" wrote in message
...
Tom, there's a nice little freeware program out there in the ether called
Newt. Google for it. You plug in the parameters and it will tell you all
of the measurements for your scope. Nice gui, too.

Cheers - Chris


Thanks you guys, googled it up and loaded on to my computer, this program
while very simple... ROCKS! Makes total sence of it all and very user
friendly.

All the best, - Tom