Maximum FOV of a DOB and Widefield EPs

[Robh (Rob)]

I was outside during the day checking the apparent FOV of my 2 inch 30mm Meade eyepiece (specs say 70 degrees).
Looking through the eyepiece at arms length, you can see that its view forms a definite angle against the background panorama.
I put a garden stick in the ground 10 metres away to the left then another 10 metres away to the right, so that each stick located the left and right extremity of the FOV.
Measured the distance between the sticks and calculated the angle as 66 degrees. OK.

Then a thought struck me. My 12 inch DOB must also have an maximum apparent FOV. Sitting it horizontally, I located the sticks at 10 metres again, left and right, by looking through the focuser without an eyepiece.
Measured the distance between the sticks and calculated the angle as roughly 1.4 degrees.
Is there some formula to calculate this based on the mirror size and focal length?

The 30mm Meade EP produces a magnification of 1500/30=50. So the true FOV is 70/50=1.4 degrees which is pretty much the maximum FOV of the DOB. Coincidence of course, as I'd bought the EP after I'd bought the telescope. A different EP could have had a different apparent FOV.

But what if I'd used a 2 inch 31mm Tele Vue Nagler with apparent FOV 82 degrees. Mag=1500/31=48. True FOV=82/48=1.7 degrees which is wider than the maximum FOV of the DOB. It appears the extra FOV of the EP beyond the central 1.4 degrees gets no light and is in fact a waste.
Is this a correct assumption?
If correct, then the telescope's maximum FOV needs to be taken into account in buying a widefield eyepiece.

Curious, Rob.

[Alchemy (Clive)]

heres a bunch of formulae, might find what you want there, best of luck

clive

[Robh (Rob)]

Hi all,
After a lot of analysing, I've realised this is a little bit more complicated than I originally thought. For a 12 inch (300mm) DOB with focal length 1500mm, the current maximum true FOV is about 1.9 degrees.
I've set a maximum Exit Pupil of 7mm. As you get older, Exit Pupil will reduce to 6mm or 5mm. Low power 2 inch eyepieces are now pushing 80 degrees apparent FOV. The theoretical limit for apparent FOV is 180 degrees. The human eye has an Apparent and also True FOV nearly 180 degrees (magnification = 1). Sharpness, however, diminishes rapidly away from viewed centre.

Now go to attached Excel Spreadsheet for your own telescope. Was zipped on a Mac but should open as an Excel Workbook.
AFOV is the Apparent FOV of the eyepiece in degrees. TFOV is the True FOV in degrees, as seen through the eyepiece in the telescope. Eyepiece focal lengths have been round off to the nearest 5mm for convenience.
Enter your telescope's aperture diameter and aperture focal length. All the rest is calculated automatically. You can compare different eyepiece combos mms/Mag/Exit Pupil against AFOV.
Numbers in blue are the TFOV for each eyepiece setting.
If you want to enter a specific eyepiece focal length, say 12mm, simply change the 50mm to 12mm.
The maximum TFOV for your telescope will occur for Exit Pupil=7 and the highest AFOV for that eyepiece focal length. I've gone all the way to AFOV 180 degrees even though eyepieces are still generally way below an AFOV 120 degrees.
Hope you find it of some use.
Regards, Rob.

[Starkler (Geoff)]

If you're talking about the maximum usable fov from a newt, you need to calculate the useful illuminated field size which is a function of f-ratio, secondary mirror size and placement.

You can use your 31 nagler in any newt and the limiting factor is if the scope offers useful illumination (>70%) to the edge of of eyepiece field.

[Robh (Rob)]

I think I've more or less resolved this to my satisfaction.
See my post number 3 for the spreadsheet if you want it.
It applies to any type of telescope not just DOBS.
You can also use it to see the effect of different eyepieces on the true FOV.

Clive, thanks for the formulae. Some interesting stuff there. Still going through it.

Geoff, there are probably other factors I could have looked at like % illumination to the edge of the field but I wanted to keep it simple. I was just after the maximum true FOV, based on aperture size, aperture focal length, eyepiece focal length and apparent FOV, and exit pupil.

Regards, Rob.

[Coen]

From Cloudy Nights:
http://www.cloudynights.com/ubbthrea...all&PHPSESSID=

While not strictly FOV does have some more formulae for telescope comparisons etc. That might be of passing interest.

There is also: http://www.stargazing.net/naa/scopemath.htm