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Old 26-10-2018, 09:10 PM
Madanie7 (Brendan)
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FOV calculators

Hi,
When I add my equipment to field of view calculators like astronomy.tools and to SkySafari and then add, say, the moon as the viewing object. It's doesn't quite look like what I see in the eyepiece.

For example, the software says I could fit the whole moon in the fov with my 13mm Nagler easily by when I look through the eyepiece it's virtually touching the edge.

Is it something I am doing incorrectly?

It's not the end of the world but it would be nice to use this a a means to help decide on eyepieces.

Cheers
Brendan
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Old 26-10-2018, 11:10 PM
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billdan (Bill)
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I guess it depends on how close your eyeball is to the eyepiece. As you move your eye further away from the eyepiece the image gets larger. So maybe these FOV calculators expect your eyeball to be touching the eyepiece?



Cheers
Bill
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Old 27-10-2018, 06:15 AM
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mental4astro (Alexander)
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There's one thing that you can do for yourself as a calculator. An EP will have as a spec its Apparent Field Of View (AFOV), and depending on the scope you are using a certain magnification. With this youcan work out its True Field Of View (TFOV), which is what you want. The equation is simple:

AFOV ÷ Magnification = TFOV.

As an example, let's take an EP of 30mm with an AFOV of 68°, and a scope of 1000mm focal length (aperture has nothing to do with FOV, only itsfocal length).

First, the magnification this EP/Scope combo gives is 33.33X. Then, the TFOV = 68° ÷ 33.33 = 2.04°

This is roughly four times the size of the full Moon.

Now, same scope, but a 5mm 82° EP:

Magnification = 200X. TFOV = 82° ÷ 200 = 0.41°

I occasionally use a calculator of sorts with Sky Safari. It has a function where you can input the size of the TFOV an EP will give, and the app provides a fine circle of the size of the TFOV. But I only use it as a planning tool, not in helping me decide what EP to use. This is because wirh different objects there are many factors that affect which EP is most efficient for it, seeing, transparency, magnitude and its size (some objects may be described as "bright", but if its size is large, what you see through the eyepiece will be dim), and also here un the field, aperture (how much extension of the DSO you can actually see).

FOV calculators can be useful, but not a panacea for in the field eyepiece selection. Try and error and experience will be your best eyepiece selector when push comes to shove

Alex.
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Old 27-10-2018, 10:16 AM
Madanie7 (Brendan)
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Thanks gents.
All taken on board and probably should have said it's not just to select eyepieces but it's handy to see what something will look like in the scope.


So, If I take the 13mm Nagler with 82° AFOV.
In the 6se I get 115x so the TFOV is 0.71°

If the moon is 0.50° that would mean I would clearly see the moon in the eyepiece, yeh?

When I put the eyepiece in the scope, the moon is right at the outer edge of the eyepiece . It just doesn't seem to replicate that's all.

Cheers
Brendan
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Old 27-10-2018, 12:15 PM
astro744
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Use the formula
Angular diameter (deg) = field stop diameter of eyepiece (mm) x 57.3 / focal length of telescope.

See http://www.televue.com/engine/TV3b_page.asp?id=214 for Tele Vue eyepieces. For 13mm T6 Nagler, FSD=17.6mm and for telescope focal length of 1500mm;

Angular diameter = 17.6 x 57.3 / 1500 = 0.67deg.

With the Moon at perigee you can have close to 0.55 deg so that can leave (0.67-0.55)/2 = 0.06 deg all around the edge of the Moon. (Note Moon is not at perigee at present but getting closer these past few days. This is still 3.6 arc min or 216 arc sec or about 7 Jupiter diameters (present diameter) so have a look at Jupiter at 115x and approximate what seven look like. Is the the gap between the Moon and the edge of field? If not and from how you describe it then something else is at play and the most obvious is that the telescope is not working at f10 and your effective focal length is greater than 1500mm. This can happen with an SCT as it is a complex curved two mirror system but it won't happen by itself. What can affect the effective focal ratio/length is if you had a longer path to the eyepiece such as adding a 2" diagonal or using a binoviewer require the SCT primary to be focus differently by considerably shifting its position, (this is how SCTs focus).

I would trust the Tele Vue eyepiece field stop diameter specifications and use these for TFOV calculations as the AFOV/magnification is close but doesn't take into account angular distortion.

I'm not sure why you are getting a larger image than the focal length would give you and the only reason I can think of is that your effective focal length is greater.

Perhaps do a search for "calculating SCT effective focal length or focal ratio". (Note aperture is fixed).
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Old 27-10-2018, 01:08 PM
Madanie7 (Brendan)
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Quote:
Originally Posted by astro744 View Post
Use the formula
Angular diameter (deg) = field stop diameter of eyepiece (mm) x 57.3 / focal length of telescope.

See http://www.televue.com/engine/TV3b_page.asp?id=214 for Tele Vue eyepieces. For 13mm T6 Nagler, FSD=17.6mm and for telescope focal length of 1500mm;

Angular diameter = 17.6 x 57.3 / 1500 = 0.67deg.

With the Moon at perigee you can have close to 0.55 deg so that can leave (0.67-0.55)/2 = 0.06 deg all around the edge of the Moon. (Note Moon is not at perigee at present but getting closer these past few days. This is still 3.6 arc min or 216 arc sec or about 7 Jupiter diameters (present diameter) so have a look at Jupiter at 115x and approximate what seven look like. Is the the gap between the Moon and the edge of field? If not and from how you describe it then something else is at play and the most obvious is that the telescope is not working at f10 and your effective focal length is greater than 1500mm. This can happen with an SCT as it is a complex curved two mirror system but it won't happen by itself. What can affect the effective focal ratio/length is if you had a longer path to the eyepiece such as adding a 2" diagonal or using a binoviewer require the SCT primary to be focus differently by considerably shifting its position, (this is how SCTs focus).

I would trust the Tele Vue eyepiece field stop diameter specifications and use these for TFOV calculations as the AFOV/magnification is close but doesn't take into account angular distortion.

I'm not sure why you are getting a larger image than the focal length would give you and the only reason I can think of is that your effective focal length is greater.

Perhaps do a search for "calculating SCT effective focal length or focal ratio". (Note aperture is fixed).
Thank you very much. I will do some research
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