What size diagonal should i use

[leon]

Hi All

When i first built my 8" reflector i was using .96 rack and pinion and .96 EP's.
Now that i have up-graded to 2" focouser and EP's, is my Diagonal to small.

Things just don't seem right, with the scope, also the small diagonal has a chip out of one side, could this effect the viewing as well.

Any advice would be much appreciated.

Thanks leon

[ving (David)]

diagonal in a reflector? do kyou mean secondary mirror?

[janoskiss (Steve H)]

Do you mean the secondary mirror of the Newtonian (rather than a mirror or prism "diagonal")?

Not likely, but it is possible that the secondary mirror is too small with the 2" focuser because that will support much wider FOV eyepieces. Here is how to check:

Once the secondary mirror is centred under focuser and scope is collimated, take your widest true field eyepiece, point it at a bright star placing it half way between the centre and the edge of field. Defocus the image winding the focuser barrel outwards (away from to secondary) until the star image turns into a shadow of your spider with the mirror behind it. If you can see the mirror clips in that image (assuming you have some), the secondary is large enough.

[Vermin (Tom)]

Here's a nifty bit of freeware for calculating secondary size: http://skyandtelescope.com/resources...icle_328_6.asp

[leon]

Yea sorry ving, my mistake i should have said secondary, but you know, the thing is on an angle.....You know, excuse, excuse. etc...

[33South (Chris)]

Did a quick check on google still plenty of references to newtonian diagonals maybe its a UK/USA thing.

I grew up calling it a diagonal, still do - maybe ingrained from days when the Newtonian was the primary choice and the secondary IS on the diagonal.

[ving (David)]

oh!..... potato!

[leon]

Hey Ving, in the link that Tom suggested i go to they even refer to the secondary as a diagonal, sorry just had to tell you...

[janoskiss (Steve H)]

I must admit that when you said you changed the focuser to a 2" and are wondering about the size of the diagonal I was confused too... I was thinking hard about the other sort of diagonal until the penny dropped