Expert: Paul Wagner - 4/30/2008

Question
QUESTION: I want to build a telescope and the primary mirrors that are available here in egypt are not alot and the best one is 8 inches f/15 so can u please help me build it

ANSWER: HI Hady

I can give you lots of advice, but the first advice I will give you is that you need to find a source for a mirror.  You can grind your own, or buy one, but that's where the whole thing starts.  Once you have the mirror, the rest can be made pretty easily by a carpenter or craftsman---nothing too complicated.

So.  Where can you get a mirror?  There are some good companies in the USA that sell them, but I don't know if they will ship to Egypt.  You probably want something with a focal length of about 60 inches or so--less than two meters.  Otherwise, the scope is too long to be convenient.  I would not buy an 8 inch f'15 mirror for this reason.

Try these website to get a better idea of what you need:

http://members.aol.com/sfsidewalk/dobplans.htm

http://www.e-scopes.cc/

http://members.aol.com/raycash/sidewalk.htm


And a site the sells good scopes:
http://www.telescope.com/control/category/~category_id=telescopes;jsessionid=922

I hope that helps!

Paul Wagner

---------- FOLLOW-UP ----------

QUESTION: hey Paul I found an 8 inch mirror focal length 800 ml so can you please help
me and tell me what the secondary mirror should be

ANSWER: Hi Hady

here is a webpage that has all the information you need!

http://bi-staff.beckman.uiuc.edu/~melockwo/diagonals.html

Paul Wagner

---------- FOLLOW-UP ----------

QUESTION: Ok so what do i do?

Answer
HI Hady

You will need to design your telescope to determine how far away from the focal point the secondary mirror will be. You will need at least four inches, just to get outside the field of view of the primary mirror.  I'd add another three inches or so...

8.00" F/ 4.00, d2 =  5.00"     6.00"     7.00"     8.00"       
  FIF = 0.00":        1.25"     1.50"     1.75"     2.00"       
  FIF = 0.25":        1.46"     1.70"     1.95"     2.19"     
  FIF = 0.50":        1.67"     1.91"     2.14"     2.38"     

From this table, I would used something like the 2.14-2.38 inch diagonal.  That's about 55-60 mm.

But you really need to read through a book or website about the design of telescopes so that you understand this stuff!

Paul Wagner