focal length

[quimby44]

hi there just a quick question might be a silly one but here goes  i have a 76 x 700 newtonion reflector do i just divide the mm ep by 700 or do i add the length to mirror and back to ep, its 640mm to the mirror then its 550mm from mirror to my ep so have i got a 700mm or 1190mm focal length cheers stephen

[Moonshane]

no you are quite right. 700mm in this case is the focal length and focal length of scope divided by focal length of eyepiece (both using the same qualities (i.e. both mm or both inches) = magnification.

[Moonshane]

ps

I note that you recently used your scope for solar projection. this is not recommended for newtonian designs as not only does the eyepiece get hot (as you found to your cost) but you can damage the secondary too (and the primary if bits drop off the secondary). visual grade solar film is the best way for newts.

[quimby44]

cheers i wont be doing that untill i get some solar filters could have been a expensive mistake need to check my telescope so going to get a lens first then some filters think i will give solar a miss for now