# Thread: Can someone do some TFOV math for me?

1. ## Can someone do some TFOV math for me?

I tried to figure the TFOV myself. Is the following correct?

I looked up what I think is the correct formula. AFOV divided by magnification. The formula gave a TFOV of 0.5 degrees for the two scopes below with the parameters that I set. Is this correct? I thought that the Mak would have a more narrow FOV than the refractor. Did I miss some necessary parameter?

Infinity
https://telescopeobserver.com/review...oto-telescope/

Suppose that each had an eyepiece that gave about 120X magnification.
Suppose that the eyepiece had about 60* apparent FOV.

Is that all the necessary parameters to give a reasonable comparison?

Anyway, what I am trying to do is get an idea of how the TFOV of a Mak and a refractor compare. I've read that the Mak has a more narrow FOV, but the calculation above makes it look like you can easily make them have the same FOV.

Harry

2. ## Re: Can someone do some TFOV math for me?

The formula for maximum FOV of a refractor is
1 Radian (57.3) multiplied by 27mm or 46mm (clear aperture of a 1.25" or 2" diagonal) divided by the focal length of the scope.

So for example, my 150mm f/8 refractor with a focal length of 1200mm and a 2" diagonal
57.3 x 46 / 1200 = 2.19° FOV

3. ## Re: Can someone do some TFOV math for me?

If you want to use the formula TFOV = AFOV/magnification, you first need to calculate the magnification based on the focal length of the scope. The two scopes have different focal lengths, so they will have different magnifications using the same focal length eyepiece. The formula assumes the same EP. You can, as you say, make both scopes have the same TFOV, but not with the same EP.

So let's say you are using a 20mm EP with a 60 degree AFOV.
The Meade Infinity has a focal length of 600, so the magnification is 600/20 = 30
TFOV = 60/30 = 2
The Celestron SLT has a focal length of 1250, so mag is 1250/20 = 62.5
TFOV = 60/62.5 = .96

4. ## Re: Can someone do some TFOV math for me?

I was just typing that I realized I needed to different focal length eyepieces to get 120X for the different scopes.
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So, for the Meade Infinity refractor 102mm X 600mm.

57.3 x 27 divided by 600 = 2.58* maximum FOV.

If the same formula applies to a Mak, then -

57.3 x 27 divided by 1250 = 1.24*

I should say about the original question, that I realize two different focal length eyepieces would be needed to achieve a 120x magnification for the different scopes.

For the 102mm x 600mm refractor I would need a 5mm eyepiece to get 120X mag.

For the 1250mm Mak, I would need a 10.4mm eyepiece.

It would seem that I can get a good FOV out of a Mak for birdwatching. I can't achieve as large an FOV as with a refractor, but probably good enough.

Harry

5. ## Re: Can someone do some TFOV math for me?

Here is a good scope/ eyepiece calculator for you if you like. Just input the scope and eyepiece numbers.
N.A.A. Telescope Math Calculator

6. ## Re: Can someone do some TFOV math for me?

You have to do two calculations: one for the eyepiece and one for the scope. The one for the eyepiece is based on AFOV and magnification, and the one for the scope is what Gabby described. The actual TFOV is whichever of the two is less.

7. ## The Following User Says Thank You to KathyNS For This Useful Post:

Voyageur (07-03-2017)

8. ## Re: Can someone do some TFOV math for me?

And different eyepieces not only have different focal lengths and AFOV, they have different eye relief (how far your eye needs to be from the lens) and different exit pupil (the size of the circle of light produced).

If you observe with eyeglasses, generally you need more eye relief. Some people hate close eye relief; I don't mind it and in fact don't like excessive eye relief as it's harder to keep my eye centered if I'm miles away from the top of the lens! You'll need to find this out for yourself.

The maximum exit pupil you will want should be no larger than the maximum amount your eye dilates in the dark. As we age, this value gets smaller. An exit pupil larger than your dilated pupil wastes the light coming through the EP and your view may be dim.

There is a formula for exit pupil also. Check out this Simple Formulas for the Telescope Owner

I need a cold drink!

9. ## The Following User Says Thank You to Voyageur For This Useful Post:

michael131313 (07-03-2017)

10. ## Re: Can someone do some TFOV math for me?

Originally Posted by KathyNS
You have to do two calculations: one for the eyepiece and one for the scope. The one for the eyepiece is based on AFOV and magnification, and the one for the scope is what Gabby described. The actual TFOV is whichever of the two is less.
Thank you for making that important distinction, Kathy!

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