FOV and focal reducer
Posted 18 February 2004 - 10:43 AM
Posted 18 February 2004 - 11:06 AM
TFOV = AFOV/mag
There are some great definitions and information in the Astronomy FAQ thread in the "Off Topic" category.
Posted 18 February 2004 - 11:57 AM
From the new focal ratio, you can calculate your new effective focal length. Instead of 2000mm (assuming 8", f10 scope), your focal length is now 1260mm (63% of 2000).
Put in your 25mm Plossl, and mag = 1260/25 = 50x. Like Dennis said, take AFOV/mag = 52 deg/50x. You get 1.04. A little different than Dennis said, but we're close.
1 degree is A LOT more than .64 degrees; you'll see about 2.5x more area of sky.
Posted 18 February 2004 - 02:35 PM
1260mm/40mm = 31.5x mag
44*/31.5 = 1.4*
Also, would a 2x barlow effectively give the 25mm EP without the focal reducer a TFoV of 0.32* due to doubling the focal length? Just trying to wrap my head around all this. Thanks guys.
Posted 18 February 2004 - 03:29 PM
8" = 2032mm (8 x 25.4)
F/6.3 would yield 1280mm
Plossl has 50* AFOV.
Jeff, your calculations are based on 2000mm and a 52* AFOV. Are my numbers wrong? Please understand, I don't mean to split hairs. I just want to learn this info properly. Thanks!
Posted 18 February 2004 - 04:18 PM
Once you use up all the light coming into a 2" eyepiece, you can't get any wider TFOV (True FOV). You'll just get a vignetted image, like looking into a tube.
There's a really good table of formulas at:
TFOV can be calculated as:
TFOV in degrees = eyepiece / focal length x 57.3
Televue lists the largest 2" field stop as being 46mm (basically the entire eyepiece barrel), so:
TFOV = 46 / (8 x 25.4 x 10) x 57.3 = 1.3 degrees
The Celestron focal is slightly larger than a 2" drawtube, so if you measure the telescope side ID and plug into the above formula you'll get the maximum TFOV through the reducer.
For example, a 50mm Plossl has the same 46mm field stop as a 41mm Panoptic, so they would both give a TFOV of 1.3 degrees, despite the usual TFOV/AFOV calculations.
The TFOV/AFOV calculations can be used with impunity once the field stop is smaller than the maximum light cone.
Posted 18 February 2004 - 04:40 PM
Posted 18 February 2004 - 04:56 PM
Suk...thanks for clearing all that up. Yes, I knew I was ignoring the issues you mentioned.
Posted 18 February 2004 - 05:10 PM