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Why long focal length telescopes are considered to have smaller field of view

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#1 yvatyan

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Posted 02 July 2020 - 07:19 AM

I've read at several places that long focal length telescopes due to their longer focal length are not suitable for watching nebulaes, but I don't understand where is this statement originated .

 

Using formulas I get through the web:

  1. Actual FOV = FOVe / M,
  2. M = FO / Fe,

where Ffocal length of objective, Fe focal length of eyepiece, FOVe Field Of View of eyepiece and M is magnification.

 

I get for example this:
If I take 68 degree 20mm eyepiece for 2000mm telescope I get 100x magnification and 0.68 degree FOV the same if I take
68 degree 10mm eyepiece for 1000mm telescope.

 

So what is case to take long focal length scopes for planetary viewing and short ones for large deep sky objects.

 

 

Thank you



#2 Jim Davis

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Posted 02 July 2020 - 07:35 AM

You get a 68 deg "apparent" field of view, not actual field of view. The apparent field is how wide it looks to your eye, not how much of the sky you are actually seeing.

 

The actual field of view is called "True" field of view. For the 10mm, the TFOV would be 0.70 deg, and the 20mm would give 1.40 deg. This is how much of the angle of the sky you are viewing. So, you will be seeing 1.40 deg of the sky, magnified to look 68 deg wide.

 

If you have a shorter focal length, you will see more of the true field of view at a lower magnification, but still magnified to look 68 deg wide to your eye.


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#3 kathyastro

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Posted 02 July 2020 - 07:36 AM

Every scope has a maximum field of view that is independent of the eyepiece selection.  If the eyepiece supposedly offers a FOV that is wider than the scope's maximum, the view will be vignetted down to the scope's maximum.

 

FOVmax = 57.3 * FS / FL

 

where FS is the field stop diameter, and FL is the focal length.  The result is in degrees.  The field stop is constrained by the barrel diameter: 46mm for a 2" focuser, and 27mm for a 1.25" focuser.  SCTs and Maks are also constrained by the diameter of the baffle tube.

 

A 2000mm scope with a 1.25" focuser cannot give you more than a 0.77 degree FOV.

A 1000mm scope with a 1.25" focuser cannot give you more than a 1.55 degree FOV.


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#4 sg6

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Posted 02 July 2020 - 07:36 AM

The scope FoV is basically Aperture/Focal Length. You are talking a final FoV with objective and eyepiece.

Also the theory is just one side, slight other aspect is precticallity.

 

You can put an eyepiece in a 500mm scope, say a 25mm 60 degree EP and so get 20x and a field of 3 degrees. Nice.

To get the same field on a 2000mm (8" SCT) scope you need a magnification again of 20x and in a 2000mm scope that means an eyepiece of 100mm.

 

Theory says they are the same, have you found a 100mm 60 degree eyepiece?

 

In a way many do not use a long focal length scope on planets, I point a 90mm ED at them.

For DSO's you often just need to find the things, that means low magnification and that relates back to shorter focal length scopes, which in turn produces a wider field.

 

Try this: Scope focal length 6 meters, how would you get a FoV to see say all M42, about 1.25 degrees in size?

 

At a guess 95% of people on CN cannot see all of M31 in their scope, M31 is 3 degrees - Why? And lots use a "wide field" DSO scope.


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#5 BFaucett

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Posted 02 July 2020 - 07:53 AM

Field of View Calculator

https://astronomy.to.../field_of_view/

 

Bob F. smile.gif



#6 Tony Flanders

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Posted 02 July 2020 - 07:57 AM

I've read at several places that long focal length telescopes due to their longer focal length are not suitable for watching nebulaes, but I don't understand where is this statement originated

 

...

 

So what is case to take long focal length scopes for planetary viewing and short ones for large deep sky objects?

First of all, let me make it clear that I do not agree with the statement that long-focal-length telescopes are unsuitable for viewing nebulae, and I disagree even more with the statement that short-focal-length telescopes are unsuitable for viewing planets.

 

However, on the whole, short focal ratios are preferable for deep-sky viewing, because they allow a wider range of choices in magnification. That particular factor is irrelevant for the planets, which are always viewed at high magnifications. So the fact that you have additional choices on the low-power end is irrelevant for planets.

 

Here's why short focal ratios offer a wider choice of magnifications. High magnifications are achieved by making the focal length of an eyepiece shorter. Rather than using a 20-mm eyepiece, you can use a 10-mm eyepiece to achieve twice the magnification. There is no limit to how small you can make an eyepiece's focal length, so you can always achieve as high a magnification as you want. There are other factors that limit magnification on the upper end, including the telescope's aperture, its optical quality, and the thermal stability of the telescope and the atmosphere. But there are no physical limitations.

 

To make the magnification lower you need to make the eyepiece's focal length longer. And beyond a certain point, that requires making the eyepiece's physical size bigger as well. A conventional 32-mm Plossl fits into the 1.25-inch focusers common in most inexpensive telescopes; a conventional 55-mm Plossl does not. And since the largest common focuser size even in expensive amateur telescopes is 2 inches, there is a limit to how low a magnifcation -- and hence wide a field -- you can achieve with a local-focal-length telescope.

 

So, in your example, if you wanted to achieve a true field of view of 1.36 degrees, you could use that 20-mm eyepiece in your 1000-mm focal-length telescope to achieve 50X. But to achieve the same in your 2000-mm focal-length telescope, you would need a 40-mm eyepiece. And a 40-mm eyepiece with 68-degree AFOV doesn't fit into a 1.25-inch barrel. It does (just barely) fit into a 2-inch barrel, so you could indeed get 1.36 degrees out of your 2000-mm focal-length telescope if it has a 2-inch focuser. But that's the limit; you cannot get more. With the 1000-mm scope, by contrast, a 2-inch focuser would let you go all the way up to a 2.72-degree field of view.

 

I am, by the way, saying exactly the same thing as kathyastro above, but explaining it in a different way. Her explanation is more precise; mine may (or may not) make more sense to some people.

 

The fact remains that most nebulae are much smaller than 1.32 degrees. So the inability to achieve an ultrawide true field of view in that 2000-mm focal-length scope, while perhaps annoying, is certainly not the end of the world.


Edited by Tony Flanders, 02 July 2020 - 08:02 AM.

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#7 yvatyan

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Posted 02 July 2020 - 11:09 AM

Thank you all, I think I've understood the real life limitations.

 

So, wanting larger TFOV for large DSOs by making 

  1. Fe larger we stuck in
  • eyepiece upper focal length limit (40mm for 1.25" barrel)

      2. FOVe larger we stuck in

  • TFOV limit that depends on barrel size (the view stop) and scope focal length, (27mm stop for 1.25" barrel giving 1.55° TFOV for 1000mm scope),
  • FOVe limit (for 40mm eyepieces it is even lesser then 50°).

 

Wanting higher magnification for planets by making

  1. Fe smaller we stuck in
  • brightness limit that depends on objective size,
  • eyepiece focal length low limit (I can't find lesser 2.3mm).

 

TFOV limit can be increased by going to higher diameter barrels, e.g. 2".

 

Summarizing, high focal length telescopes achieve max magnification with smaller eyepieces, but also has smaller TFOV limit.



#8 Alan French

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Posted 02 July 2020 - 12:43 PM

True Field 400 to 2000.jpg


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#9 Tony Flanders

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Posted 02 July 2020 - 02:47 PM

Thank you all, I think I've understood the real life limitations.

 

So, wanting larger TFOV for large DSOs by making

  1. Fe larger we stuck in
  • eyepiece upper focal length limit (40mm for 1.25" barrel)

      2. FOVe larger we stuck in

  • TFOV limit that depends on barrel size (the view stop) and scope focal length, (27mm stop for 1.25" barrel giving 1.55° TFOV for 1000mm scope),
  • FOVe limit (for 40mm eyepieces it is even lesser then 50°).

 

Wanting higher magnification for planets by making

  1. Fe smaller we stuck in
  • brightness limit that depends on objective size,
  • eyepiece focal length low limit (I can't find lesser 2.3mm).

 

TFOV limit can be increased by going to higher diameter barrels, e.g. 2".

 

Summarizing, high focal length telescopes achieve max magnification with smaller eyepieces, but also has smaller TFOV limit.

Technically, you can fit an eyepiece with arbitrarily long focal length in a 1.25-inch barrel, but only at the price of decreasing the apparent field of view. The longest eyepiece with an AFOV around 50 degrees that fits in a 1.25-inch barrel is 32 mm, which is why 32-mm Plossls are so common. 40-mm Plossls have an apparent field of view around 40 degrees.

 

Ultimately, the product of the two -- 32x50=1600 or 40x40=1600 -- is limited by the size of the field stop.

 

Although I'm not aware of any eyepieces with focal lengths shorter than 2 mm, it is easy to achieve shorter effective focal lengths by combining a Barlow lens with less exotic eyepieces. But since there are few telescopes faster than f/4, and a 2-mm eyepiece in an f/4 scope yields 2X per mm of aperture -- about as much as can be usefully deployed -- there isn't much call for shorter focal lengths.

 

The limit on high magnification per unit of aperture has more to do with diffraction effects than with image brightness. Targets like Mercury, Venus, Mars, and the Moon are bright enough to take very high magnifications, but the image is inevitably fuzzy at 2X per mm of aperture due to the fact that point sources such as stars turn into quite large diffraction patterns -- extended circles surrounded by rings -- rather than appearing as points. All those overlapping diffraction patterns end up looking fuzzy.

 

Incidentally, there is no law that you can't make eyepieces with barrels wider than 2 inches. In fact giant 19th-century refractors such as the Yerkes 40-inch f/19 often employ precisely such eyepieces to achieve reasonably wide fields of view despite those extremely long focal lengths. But those eyepieces are custom-made, and very expensive.



#10 bignerdguy

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Posted 02 July 2020 - 04:33 PM

One factor i see wasn't discussed here is light gather ability.  Longer focal length scopes are usually the larger aperture scopes with either larger lenses or mirrors and as a result have the ability to gather a LOT more light.  Most of your short focal length scopes, barring the odd shorty reflector are going to be refractors. The amount of light you can gather is going to depend on the size of the aperture of the mirrors or lenses. The light path on a refractor is shorter since it basically passes straight through from one side to the other.  A Newtonian reflector will bounce it from the back end of the tube back to a smaller mirror on the front where the image will be compressed and then out to the eyepiece and as a result not only have a longer focal length but be that much brighter.  SCT's go even longer and higher power because image compression is that much greater.  Needless to say the larger the aperture the more light you gather and the longer the focal length the higher the magnification will be with each eyepiece.  

 

Short focal length scopes are great for wide field low power views or photos.  No they cant go too high because there is a usable limit as to how much magnification is usable, not just possible.  For each scope there is an upper limit as to how high it can magnify before it can no longer focus.  That's why marketing on some short focal length scopes can sometimes be misleading when they say they can go to XXX magnification, sure they can GO to that but you cant always see anything at that high a power with a short focal length.

 

Long focal length scopes will be great for higher magnification and light gathering but at the expense of actual field of view.  Like the others said the "apparent" field of view is around whatever the eyepiece advertises but only at what magnification you are getting, so on a long focal length scope you will see less of the field with the same eyepiece than if you put it in a shorter focal length scope.



#11 Tony Flanders

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Posted 03 July 2020 - 05:01 AM

The light path on a refractor is shorter since it basically passes straight through from one side to the other.  A Newtonian reflector will bounce it from the back end of the tube back to a smaller mirror on the front where the image will be compressed and then out to the eyepiece and as a result not only have a longer focal length but be that much brighter. SCT's go even longer and higher power because image compression is that much greater.


This part of your explanation is rather confused. Refractors can have either long or short focal lengths, depending on how deeply curved the objective lens is. The tube is then adjusted to fit whatever focal length the lens has.

 

It is hard to make optically excellent lenses with short focal ratios, so by and large, refractors tend to have longer focal lengths than Newtonians of equal aperture. An f/5 refractor, with focal length 5 times the aperture, is considered to be extremely "fast," i.e. has an abnormally short focal length -- and it usually has optical problems to match, unless it uses very fancy glass and complex optical design. An f/5 Newtonian, by contrast, is considered just about average.

The secondary mirror of a Newtonian is flat, optically neutral, and has no effect whatsoever on the image. It certainly does not "compress" it, whatever that means. Its only purpose is to move the focal plane outside the tube, where it's easier to attach a camera or eyepiece. The Palomar 200-inch reflector is big enough to fit an observer inside the tube, so it can dispense with the secondary mirror entirely.

SCTs have long effective focal lengths due to the negatively curved secondary mirror, which acts like a Barlow lens.

 

But yes, aperture does indeed matter greatly, which is why this whole subject is usually couched in terms of focal ratio rather than focal length.


Edited by Tony Flanders, 03 July 2020 - 05:07 AM.

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#12 StarWager

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Posted 03 July 2020 - 10:31 AM

This thread is great to read. Helped me understand the general statements about focal ratio/length and viewing planets vs DSO.

Since I just acquired a 480mm refractor, after using my 1200mm dob for 3 years, I am seeing practically how the shorter focal length and smaller aperture impacts my EP collection! The focal ratios are basically equal, at f/5.9 and f/6.

I still don't understand what the ratio by itself impacts .. What aspects of viewing are "equal" in my 2 vastly different scopes, based on the focal ratio being equal?

Thanks!

#13 kathyastro

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Posted 03 July 2020 - 11:31 AM

What aspects of viewing are "equal" in my 2 vastly different scopes, based on the focal ratio being equal?

For eyepiece viewing, no a lot.  The amount of coma will be similar.

 

If you were to get into AP, exposure times would be the same.



#14 Tony Flanders

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Posted 03 July 2020 - 03:05 PM

I just acquired a 480mm refractor, after using my 1200mm dob for 3 years, I am seeing practically how the shorter focal length and smaller aperture impacts my EP collection! The focal ratios are basically equal, at f/5.9 and f/6.

I still don't understand what the ratio by itself impacts .. What aspects of viewing are "equal" in my 2 vastly different scopes, based on the focal ratio being equal?


The answer to that is: A whole lot less than most people imagine. The three most important characteristics of a telescope are aperture, aperture, and optical quality. The focal ratio and the design (i.e. refractor vs. reflector vs. catadioptric) are much less important.

 

The main way that focal ratio affects you is in the choice of eyepieces. The range of useful exit pupils for a telescope is roughly 0.5 mm to 7 mm. In an f/6 telescope, that means that your eyepieces -- or eyepiece-plus-Barlow combos -- should range roughly from 3 mm to 42 mm. As you have discovered, the same set of eyepiece will work very nicely for all f/6 scopes, regardless of aperture.

 

In an f/10 telescope, by contrast, the equivalent focal lengths would be 5 mm to 70 mm. As that example proves, you may have trouble at the long end of the spectrum due to the unavailability of eyepieces in that focal length. But f/6 scopes sit solidly in the middle, where it's really easy to get the full range of useful magnifications with widely available eyepieces.

 

As far as image quality is concerned, focal ratio, all by itself, means nothing whatsoever.

 

It is true that certain designs, such as the achromatic refractor, have problems at short focal ratios. But those problems are all fixable by using more complex and/or expensive designs. To fix false color in refractors, you use more expensive glasses. To fix coma in reflectors, you use coma correctors.



#15 Alan French

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Posted 03 July 2020 - 03:36 PM

As Tony said f/ratio is not very important, but it does provide a nice "eyepiece shortcut," at least if you think of eyepieces in terms of exit pupil, something that has always made sense to me.  

 

The exit pupil produced by an eyepiece can be calculated simply from the f/ratio: Exit Pupil = Eyepiece Focal Length divided by Telescope F/ratio. So an f/8 telescope produces a 1mm exit pupil, a nice size for lunar and planetary viewing, with an 8mm eyepiece. Want a 2mm exit pupil? You'll need twice that focal length, or 16mm. Half a millimeter exit pupil, halve it, 4mm.

 

Clear skies, Alan


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#16 Dave Mitsky

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Posted 03 July 2020 - 04:18 PM

Incidentally, there is no law that you can't make eyepieces with barrels wider than 2 inches. In fact giant 19th-century refractors such as the Yerkes 40-inch f/19 often employ precisely such eyepieces to achieve reasonably wide fields of view despite those extremely long focal lengths. But those eyepieces are custom-made, and very expensive.

Expensive, yes, but there are 3" eyepieces offered on the general market so I wouldn't call them custom-made.

 

https://explorescien...oducts/100-30mm

https://agenaastro.c...piece-80mm.html

There are also the Siebert lines with barrels larger than 2 inches.

 

https://www.sieberto...80mm 4 eyepiece


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#17 Dave Mitsky

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Posted 03 July 2020 - 09:46 PM

Here's a photo of a 30mm Explore Scientific 100-degree eyepiece that I took at Cherry Springs State Park.

Attached Thumbnails

  • 3-inch 30mm ES.jpg


#18 StarWager

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Posted 04 July 2020 - 10:54 AM

If you were to get into AP, exposure times would be the same.


Thanks! the exposure time makes sense, and down the road I will try AP as well.

#19 StarWager

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Posted 04 July 2020 - 11:07 AM

Tony and Alan,

Thanks so much for your responses regarding exit pupil size and focal ratio. I had just literally started documenting exit pupil size for my scope/EP combinations a couple weeks ago. Before that, I had no understanding of exit pupil. Now I am getting it, how the exit pupil and amount of light entering my eye for an EP will be the same in my 2 scopes, even tho the magnification is way different.

I am not used to thinking of EP's in terms of exit pupil yet, but am learning.

sw

#20 Ray_T

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Posted 07 July 2020 - 12:38 PM

 

The limit on high magnification per unit of aperture has more to do with diffraction effects than with image brightness. Targets like Mercury, Venus, Mars, and the Moon are bright enough to take very high magnifications, but the image is inevitably fuzzy at 2X per mm of aperture due to the fact that point sources such as stars turn into quite large diffraction patterns -- extended circles surrounded by rings -- rather than appearing as points. All those overlapping diffraction patterns end up looking fuzzy.

 

 

Interesting, the specs for my new telescope (203 mm, 1200FL, f/5.9) indicate maximum magnification of 406x. This struck me as being too high to see anything well. I figured maybe half of this magnification would provide really good viewing, so I splurged and purchased the Meade 5.5mm (82 degree AFOV) as my goto eyepiece for maximum magnification (218x with TFOV of .38 degrees). Does this make sense as more realistic when considering magnification limitations? Basically, I decided on 1x aperture. Thanks.



#21 Tony Flanders

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Posted 07 July 2020 - 01:05 PM

Interesting, the specs for my new telescope (203 mm, 1200FL, f/5.9) indicate maximum magnification of 406x. This struck me as being too high to see anything well. I figured maybe half of this magnification would provide really good viewing, so I splurged and purchased the Meade 5.5mm (82 degree AFOV) as my goto eyepiece for maximum magnification (218x with TFOV of .38 degrees). Does this make sense as more realistic when considering magnification limitations? Basically, I decided on 1x aperture. Thanks.

In one word: yes. The bigger the telescope, the more likely it is that the limiting factor for high magnification will be the atmosphere rather than diffraction effects. There are certainly nights when your telescope could usefully use magnifications above 218X. But it's a fair bet that such nights are few and far between on the east coast of Vancouver Island. Realistically, you're likely to use a 5.5-mm eyepiece much more often than any shorter eyepiece.


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#22 Sketcher

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Posted 07 July 2020 - 02:59 PM

A relative simple answer to the topic question: "Why long focal length telescopes are considered to have smaller field of view":

 

If you use the same eyepiece with two telescopes of different focal lengths, the true field of view with that eyepiece and the longer focal-length telescope will be smaller.than the true field of view of the same eyepiece used with the shorter focal-length telescope.

 

Furthermore, when going to extremes, the widest true-field-of-view possible with any given eyepiece barrel diameter will be greater with a shorter focal-length telescope than it will be with a longer focal-length telescope.

 

In practical terms, some telescopes simply have too long of a focal-length to be able to see all of the Pleiades or all of the Andromeda Galaxy in a single field of view when using a 1.25-inch (or a 2'inch, etc.) barrel-size eyepiece.

 

So, in the above context, the reason why long focal length telescopes are considered to have smaller true fields of view is because their true fields of view actually are smaller -- with any given eyepiece.




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