Quote:
Originally Posted by Merlin66
Yeah but.....
If you draw a 45 degree ellipse (conical) say an f5 cone..and with the same minor diameter, draw a 45 deg "cylinderical section" when you overlay them you'll see the differences!!
Believe me, I've done it...... |
You are right. 45 degree cross sections of a cone with typical telescope F-ratios versus a cylinder are different; however, both are ellipses but with different aspect ratio. As Steffen stated, you can think of a cylinder as a cone with its vertex at infinity. Bear in mind that a conical 45 degree cross section will look perfectly circular only if viewed from its cone's vertex. For a cylinder, that would be infinity. Having said that, there is nothing sacred about the secondary mirror looking "perfectly" circular. If the secondary mirror is perceived to be centered and round then it is good enough.
See attachments. The first illustration is for a cylinder. The ellipse will have an aspect ratio of sqrt(2) (~1.414) between its major and minor axes. The second illustration is for a cone. The ellipse will have an aspect ratio greater than sqrt(2) between its major and minor axes due to offset. Since the cylinder has an "F-ratio" equal to infinity, its offset is zero. The third illustration shows the different but that is for an F1.5 scope. For a typical F5 scope the different will be less than 1%. The difference is too small to consider. It is in the shadow of other potential collimation errors. I would not worry too much about the cone/cylinder cross section impact.
Jason
EDIT: Updated the illustrations