Pete Yeomans said:
Hi, I am trying to work out the details of an 8" f7 Dobsonian before building and have a few questions regarding the size range of secondary mirrors. Using NEWT, I get a range of mirror sizes from 1.2" upward that appear acceptable. What should the diameter of the 100% illuminated field be if I'm using Naglers and Pentax lenses of 14mm or less? At 1.2" it's only 0.0041,while @1.5" it is 0.4363 which seems to be a huge jump for a small increase in size of obstruction.
Regards, Pete
It appears to me that you need to be using a secondary at least 1.5 inches in diameter.
With the 1.2 inch diagonal, the fully illuminated field is apparently .0041 degrees, this is only 15 arc-seonds, half the size of Jupiter. This means that you would only achieve full brightness and resolution in the very, very center, Jupiter would not even be fully illuminated.
The 0.4363 degree fully illuminated field is about 26 arc-minutes, almost the size of the moon, acceptable for a planetary scope. A 13mm Nagler in your scope will provide 109x and a TFoV of approximately 0.75 degrees, the edges will not be fully illuminated but that is OK.
It is good realize that an undersized secondary means that you are not seeing the full mirror so that not only is brightness compromised but so is the resolving power.
Jon
