crunching numbers

I was curious what the true field of view (TFOV) was through the DDO big 'scope. So I bugged Paul and crunched a bunch of the numbers.

telescope data:

• focal length ratio = 17.4
• mirror diameter = 74 inches (1.88 metres)

"low" eyepiece data:

• stated magnification = 300x
• stated focal length = 100 mm
• design = Kellner
• AFOV = 40°

"high" eyepiece data:

• stated magnification = 600x
• known focal length = 56 mm
• make = Meade
• design = Super Plössl
• AFOV = 50°
• this is the one on loan from the CAO

So, I get the following numbers...

• focal length of telescope = 32.5 metres!
• max. theoretical magnification = 3700x!
• lowest = 269x (assuming a 7mm pupil)
• magnification of low eyepiece = 325x
• mag of Meade Super Plössl = 580x
• TFOV of low eyepiece = 7.4 arcminutes (0.123°)
• TFOV of Meade Super Plössl = 5.2 arcminutes (0.086°)

Afterthoughts...

This explains why it is difficult to view through the eyepiece at low power. The cone of light emitted from the eyepiece is bigger than the average pupil.

Paul said he thought the Kellner had an AFOV of 46°. That doesn't seem right to me. In fact, when I use 40°, I still get a very wide (er, wider) field of view. Paul said with the 100mm eyepiece he can just barely fit the ε (epsilon) Lyrae pairs in the field. They're 210" or 3.5' apart... So, we need to do a bit more checking with that one.

Wow.

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Edited 24 Jul '22. The 17.4 number is not length! It refers to the focal ratio! The focal length is noted correctly below.