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No difference to TFoV despite increase in aperture?

Refractor Eyepieces
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#1 NiroMP

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Posted 05 March 2021 - 03:33 AM

I had tried using TFoV calculations avail on line but noted that the TFoV doesn't change when the aperture changes keeping other matters constant. For ex:

Scope A is 80mm aperture while focal length is 910mm

Scope B is 100mm aperture while focal length is 900mm

If the eyepiece used is 18 mm FL and AFoV is60mm (celestron x-cel) the resulting TFoV is 1.2 and 1.86.

 

So it's clear that the aperture is not a factor

Also not sure why this doesn't consider the eyepiece diameter? Say1.25 or 2 for example 

May be I have missed a thing?

 



#2 astrokeith

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Posted 05 March 2021 - 03:50 AM

So the difference between AFov and TFoV is simply the AFoV divided by the magnification, and magnification is telescope focal length divided by eyepiece focal length.

 

So very simple and you really dont need an on-line calculator. In this was the calculator is hiding from you the formula which is the problem.

 

And so aperture is not a factor in TFoV.

 

Now the eyepiece barrel diameter is determined by the manufacturer when they design for a particular focal length eyepiece and AFoV. So you will have noticed that wide AFoV often require 2" eyepieces (also with long focal length eyepieces). The calculator you are using assumes, quite rightly, that the AFoV is what is stated and the manufacturer has sized the eyepiece barrel appropriately (actually its the eyepiece field stop diameter that varies and needs bigger barrels to accommodate it.)



#3 Ernest_SPB

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Posted 05 March 2021 - 03:50 AM

Eyepiece limits FOV by it field stop, not by its setting diameters. If field stops (can be evaluated as 57.3*FL/AFOV) are the same - it does not mater diameter of EP barrel.



#4 NOLAMusEd

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Posted 05 March 2021 - 03:53 AM

If two scopes have the same focal length, you are going to get the same magnification and thus the same true field of view out of an eyepiece. However, the focal ratio of the scope with the larger aperture is going to be faster (steeper light cone) and thus brighter at that magnification. You gain an increase in exit pupil size in this case.

In another situation where you have two scopes with the same focal ratio but of different apertures, the bigger scope will have a longer focal length giving more magnification and a smaller true field with a given eyepiece, but at the equivalent brightness (exit pupil) of the smaller scope at the lesser magnification/wider tfov.
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#5 russell23

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Posted 05 March 2021 - 05:45 AM

A given eyepiece will give the same TFOV in two different scopes with the same scope FL.  For example, I have an 80mm f/11.4 with 910mm FL and a120mm f/7.5 with a 900mm FL.  My 30mm 70 deg AFOV eyepiece gives the same TFOV in both scopes. 

 

If you want a scope that gives more TFOV then you need to get a scope with a shorter FL.  So I have a 72mm scope with a 432mm FL.  That scope has less than half the FL of the other two so it gives over twice the TFOV with the same 30mm 70 deg eyepiece.  


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#6 TOMDEY

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Posted 05 March 2021 - 06:29 AM

I had tried using TFoV calculations avail on line but noted that the TFoV doesn't change when the aperture changes keeping other matters constant. For ex:

Scope A is 80mm aperture while focal length is 910mm

Scope B is 100mm aperture while focal length is 900mm

If the eyepiece used is 18 mm FL and AFoV is60mm (celestron x-cel) the resulting TFoV is 1.2 and 1.86.

 

So it's clear that the aperture is not a factor

Also not sure why this doesn't consider the eyepiece diameter? Say1.25 or 2 for example 

May be I have missed a thing?

That's the (first-order) optics of it. Barrel diameter and aperture have nothing directly to do with TFOV. But larger barrels allow larger glass inside that eyepiece, which can then accommodate larger TFOV by design (e.g. larger Field Stop). Faster feed and more aperture can provide brighter images, but that's unrelated to the geometric field.    Tom


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#7 Magnetic Field

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Posted 05 March 2021 - 07:03 AM

I had tried using TFoV calculations avail on line but noted that the TFoV doesn't change when the aperture changes keeping other matters constant. For ex:

Scope A is 80mm aperture while focal length is 910mm

Scope B is 100mm aperture while focal length is 900mm

If the eyepiece used is 18 mm FL and AFoV is60mm (celestron x-cel) the resulting TFoV is 1.2 and 1.86.

 

So it's clear that the aperture is not a factor

Also not sure why this doesn't consider the eyepiece diameter? Say1.25 or 2 for example 

May be I have missed a thing?

Go to https://www.televue....Tab=EP_ENZ-0306

 

and play around with the field stop diameter (it is given for each Tele Vue eyepiece).

 

Then use this to get an idea of the true field: Field Stop Diameter in mm / focal length of your telescope in mm * 57.267 (here '/' denotes division)

 

(57.267 is just a conversion factor and is related to 'radian' and degrees conversion).

 

E.g. Nagler DeLite 15mm has a field stop of 16mm that translates into 1 degrees true field: 16mm/900mm*57.267) for your 900mm focal length.

 

This is independent of you aperture.

 

You can independently verify the field by measuring how long it takes for a star to scuttle through your field (type into google 'star drift method + true field)'

 

Note: The following is just a rule of thumb (and most amateurs lack the skills to grasp it as so many other things): true field = apparent field / magnification.


Edited by Magnetic Field, 05 March 2021 - 07:05 AM.


#8 NiroMP

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Posted 05 March 2021 - 08:11 AM

The whole thing came since I started trying understand the impact of using 2 inch eyepieces for a wider view. Of course without actually using them and trying to make the decision of buying the 2 inch components from beginning. So now I understand that the aperture has no effect on the TFoV. Thanks for the clarity. 2 inch may be I will understand after using. 


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#9 TOMDEY

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Posted 05 March 2021 - 08:31 AM

Oh --- this graphic that I came up with shows it quite nicely. That overtly reveals how the hole size in the back of your telescope enters into it.    Tom

 

~click on~ >>>

Attached Thumbnails

  • 34 EYEPIECE GRAPH absolute field drawtube 11 jpg Tom Dey.jpg

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#10 25585

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Posted 05 March 2021 - 08:46 AM

Its the eyepiece exit pupil that will change between same FL but different aperture scopes.



#11 NiroMP

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Posted 08 March 2021 - 03:13 AM

Thanks all for helpful responses.

The conclusions I reached are:
1. Aperture only affects details but not the area you see.
2. At any given modification, say at 50x both scopes will have identical TFoVs.

Therefore as a total beginner I could retain TFoV even if a longer focal length scope is used by selecting EPs as appropriate to achieve a similar magnification.

Edited by NiroMP, 08 March 2021 - 03:15 AM.


#12 luxo II

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Posted 08 March 2021 - 03:51 AM

Therefore ... I could retain TFoV even if a longer focal length scope is used by selecting longer EPs as appropriate to achieve a similar magnification.

Correct. Likewise you could retain TFoV even if a shorter focal length scope is used by selecting shorter EPs as appropriate to achieve a similar magnification.

 

But there are catches:

 

At low power, the pupil of the eye imposes a limit on the exit pupil of the eyepiece - any bigger and the light doesn't enter the eye, its blocked by your pupil. The corresponding eyepiece focal length equals the focal ratio of the scope x the eye pupil (nominal 6 or 7mm).

 

At high power, the aperture of the scope defines the maximum resolution, the corresponding magnification being somewhere between x1 and x3 per mm of aperture depending on the type of object and the quality of the optics.

 

The best image quality at high magnification results from a long focal length scope matched to eyepieces that are easily manufactured to a high degree of polish, and accuracy.

 

- With shorter scopes (under f/5) for both refractors and reflectors it is difficult to control aberrations, and the scope may only be great for low power, and marginal for high power.

- If it's a reflector, the secondary obstruction is large at low focal ratios, this has undesirable consequences;
- Short eyepieces - under 10mm - impose significant difficulty making and polishing the surfaces to the required precision, and the precision required (ie tolerances) of the mechanical assembly are challenging.

 

if you are a lunar & planetary type there really IS a reason to use scopes f/12-f/15 or more. OTOH if you want wide fields, f/5 or under is your thing. But one scope cannot excel at both. The middle ground IMHO is f/7.


Edited by luxo II, 08 March 2021 - 04:03 AM.

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#13 Jon Isaacs

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Posted 08 March 2021 - 05:18 AM

- With shorter scopes (under f/5) for both refractors and reflectors it is difficult to control aberrations, and the scope may only be great for low power, and marginal for high power.

- If it's a reflector, the secondary obstruction is large at low focal ratios, this has undesirable consequences;
- Short eyepieces - under 10mm - impose significant difficulty making and polishing the surfaces to the required precision, and the precision required (ie tolerances) of the mechanical assembly are challenging.

 

 

A few thoughts:

 

- Short focal lengths make controlling aberrations more important.  But it is definitely doable.  A F/4 Newtonian with a Paracorr 2 to correct the coma can have a wide, coma free field, (greater than any 2 inch eyepiece available) and yet provide excellent views at high magnifications. The high magnification performance depends on the optical quality.  Like wise, a Maksutov-Cassegrain has a very fast primary mirror (F/3 or so) but with a properly made meniscus corrector and secondary mirror, the aberrations can be controlled. 

 

With refractors, it works the other way, you start out with a long focal length objective and then use a reducer/flattener to shorten the focal length.  Scopes like the NP-series TeleVues and FSQ's series Takahashi's use dual EDs elements to achieve wide, flat fields at F/5.  

 

- The size of the secondary depends on a number of factors.  For example, my 22 inch F/4.4 came stock with a 3.5 inch secondary, 16%.  Newtonians are the only designs capable of such small secondaries.  My 25 inch F/5 had a 14% secondary.  The secondary size is relatively unimportant as long as it is not too big.  

 

-  In terms of planetary performance, the eyepiece is the least concern, it basically depends on the size and quality of the optics in the telescope.  Getting the image to the focal plane is the most important thing.  Excellent eyepieces are available in nearly all focal lengths.  

The conclusions I reached are:
1. Aperture only affects details but not the area you see.
2. At any given modification, say at 50x both scopes will have identical TFoVs.

Therefore as a total beginner I could retain TFoV even if a longer focal length scope is used by selecting EPs as appropriate to achieve a similar magnification.

 

It is more complicated than that:

 

1. Aperture does not affect the field of view.  One way to think of this is to start with a large aperture telescope.  If you are looking through it and someone places a disk with a hole in the center over the front of the scope, this reduces the aperture, the image will darken and less detail may be seen but the image itself will be unchanged, at least in terms of field of view.  

 

2.  At that same magnification, the TFoV depends also on the AFoV of the eyepiece.  At 50x, a 50 degree eyepiece will provide a 1.0 TFoV, (50deg/50x = 1.0 deg) a 100 degree eyepiece will provide a 2.0 degree TFoV.  It is both the AFoV of the eyepiece and the magnification that determine the TFoV. 

 

In theory, you can achieve a similar TFoV by selecting a longer focal length scope and an longer focal length eyepiece of the same AFoV.   But there is a limit because the 2 inch eyepiece format limits the possible field of view.  

 

Consider two refractors, both with 2 inch focusers, one of the is a 100mm F/5, one of them is a 100mm F/10.  The F/5 is capable of a very wide field of view.  A 40mm eyepiece with a 65 degree field of view will provide a 5.2 degree TFoV at 12.5x.  

 

To achieve that same TFoV, the 100mm F/10 would require an 80mm eyepiece with a 65 degree field of view.  That is impossible with a 2 inch eyepiece because the widest field of view of an 80mm eyepiece that can fit in the barrel is about 32.5 degrees.  This is because the field stop, the ring you see at the edge of the eyepiece, has a maximum possible diameter of 46mm, about 1.8 inches. To achieve an 80mm 65 degree AFoV eyepiece would require a 4 inch eyepiece and such eyepieces are not made. 

 

Jon


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#14 NiroMP

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Posted 09 March 2021 - 05:34 AM

These were in the refractor segment actually and I am an ordinary man looking for magnifications no more than 140-150x and no less than 20x. So, do I need to be vary of exit pupils if its below 1? Say .75?
Also exit pupils higher than 5 are an issue? I havent measured mine 😳 which doesn't help much.

#15 NOLAMusEd

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Posted 09 March 2021 - 01:31 PM

These were in the refractor segment actually and I am an ordinary man looking for magnifications no more than 140-150x and no less than 20x. So, do I need to be vary of exit pupils if its below 1? Say .75?
Also exit pupils higher than 5 are an issue? I havent measured mine which doesn't help much.

 

The detriment to using an eyepiece that yields an exit pupil that is larger than your eye is capable of dilating to (whether restricted by age or the brightness of one's observing conditions) is not being able to "take in" all the light of the image. Think of it as using an aperture reducer on your telescope. An eyepiece that gives an EP that matches that of your fully dilated eye will maximize brightness and magnification. There are astronomers who use eyepieces that produce larger EP's because they don't mind sacrificing some brightness to get the widest true field available from their scopes.

 

As exit pupil shrinks, image brightness goes down, and once you get below 1mm  artifacts like floaters in your eye (especially when viewing bright targets like the moon) become more apparent. It's still useful to have one eyepiece in that range especially if you like splitting close doubles.



#16 Mitrovarr

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Posted 09 March 2021 - 03:34 PM

Also not sure why this doesn't consider the eyepiece diameter? Say1.25 or 2 for example
May be I have missed a thing?


This is taken into account if you use real eyepiece focal lengths and AFOV, because eyepieces with impossible dimensions (like a 30mm eyepiece with 82 degree AFOV in a 1.25" barrel) don't exist.

#17 Magnetic Field

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Posted 10 March 2021 - 02:01 PM

These were in the refractor segment actually and I am an ordinary man looking for magnifications no more than 140-150x and no less than 20x. So, do I need to be vary of exit pupils if its below 1? Say .75?
Also exit pupils higher than 5 are an issue? I havent measured mine which doesn't help much.

I suffer heavily from eye floaters (those pesky like worm ****).

 

But if you don't suffer from floaters I wouldn't worry too much if the exit pupil falls below 1mm.

 

I for one have got no choice, e.g. I used a magnification of 216x last year on Mars with my 4" f.6.4 which resulted in an exit pupil of 0.46mm. Definitely not ideal.

 

But eye floaters are worse on the brightly lit full moon.

 

For various reasons I cannot use anything bigger than 4" which would help me combat eye floaters.



#18 faackanders2

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Posted 10 March 2021 - 05:20 PM

I had tried using TFoV calculations avail on line but noted that the TFoV doesn't change when the aperture changes keeping other matters constant. For ex:

Scope A is 80mm aperture while focal length is 910mm

Scope B is 100mm aperture while focal length is 900mm

If the eyepiece used is 18 mm FL and AFoV is60mm (celestron x-cel) the resulting TFoV is 1.2 and 1.86.

 

So it's clear that the aperture is not a factor

Also not sure why this doesn't consider the eyepiece diameter? Say1.25 or 2 for example 

May be I have missed a thing?

The two scopes you are comparing have nearly the same focal length, therefore TFOV is nearly the same; but the larger aperture will show many more stars with its' extra light gatering capability.  Getting a shorter focal length scope will increase TFOV at expense of magnification.  Getting a longer focal length scope will increase magnification at expense of TFOV.  Eyepieces give you a flexible range of AFOV/TFOV/Magnification you can use with your scope, but your max and min limits are bounded by your scope.l


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