2" and 1.25" FOV, querie!

[bomberbaz]

Ok so maybe I am missing something here but having done some looking about can't find a definitive answer. 

Why is the 1.25 eyepiece seemingly restricted to maximum 32mm/50 degree fov shrinking to 40mm/40 fov although lower down the 1.25 focal length we can go all the way up to 110 degrees. 

And is the theoretical max fov for a 2" 82 degrees (eg 31 nagler) and is this also the same set of rules!

I know field stop comes into it but it is getting all physics and maths and not really my strong point so maybe someone can give me the idiots guide.

Oh btw, did a little testing of max tfov also, if a scope is only able to accept 1.25 eyepieces then the eyepiece giving the best TFOV would be a 13mm Ethos which is 20% ish more than a 32mm plossl. Although a tad more expensive  ​ 

Steve

[Stu]

Hi

You are right in linking it to the field stop of the different formats, basically the larger barrel of the 2" allows a wider fov than the 1.25"

The calculation is:

Field stop/focal length of scope * 57.3

Assuming a scope of 1000mm focal length, the following results apply:

32mm TV Plossl = 27/1000*57.3= 1.55 degrees

24mm Panoptic is the same as it has the same field stop as the 32mm Plossl. These two give the maximum in a 1.25" format.

The 13mm Ethos comes in at 1.28 degrees, smaller because it's field stop is narrower at 22.3mm

An example from the 2" range, the 21mm Ethos has a field stop of 36.2 and its fov in my scope example is:

36.2/1000*57.3 = 2.07, showing the benefit of the wider barrel.

A 55mm Plossl in 2" format has the widest possible field stop in 2" I believe at 46mm, but so does the 41mm Panoptic so these two will show the same amount of sky.

Stu

Sent from my iPhone using Tapatalk

[bomberbaz]

cheers stu, kinda made sense to me, think I will give up on the maths side of this, never my strong point tbh 

[bingevader]

Why is the 1.25 eyepiece seemingly restricted to maximum 32mm/50 degree fov shrinking to 40mm/40 fov although lower down the 1.25 focal length we can go all the way up to 110 degrees. 

The use of aFOV versus the tFOV numbers which always get quoted with eyepieces and can be confusing.

The 110° you mention is the aFOV which denotes the arc, in degrees, of light that will enter your eye.

The tFOV gives the arc, in degrees, of the sky that you are actually able to see with a given eyepiece.

So an 8mm 100° eyepiece maybe able to receive a beam of light that is 100° wide, but will actually see about 0.66° of the sky.

[John]

When I started in the hobby I used to think that wide / ultra wide eyepiece designs somehow bent the light to achieve fields of view that would overcome the limits of the physical size of the eyepiece barrel. Of course I was wrong as I found to my cost when I bought a 1.25" 32mm Celestron Erfle eyepiece that claimed a 65 degree apparent field of view. In reality the field stop of that eyepiece was the same size as that of a 32mm plossl and the actual apparent field no larger - around 52 degrees. So no "magic" in eyepieces !   

[bingevader]

It is fascinating though, if a little confusing.

At the shorter focal lengths, the eyepieces almost appear to "overcome the physical limits", with a 100° EP doubling the tFOV of a 50° EP.

It's only when you get up to the maximum field stop and realise that it is this that limits the maximum true field of view, that it begins to make sense.

Using Stu's example (with a 1.25" EP and a maximum field stop of 27mm in a 1000mm FL 'scope), 1.55° is as wide a tFOV as is possible, how you get there may vary (EPs with a longer FL/smaller aFOV/lower magnification or EPs with a shorter FL/larger aFOV/higher magnification), but it can't be exceeded.

Thanks John and Stu.

[Stu]

It's only when you get up to the maximum field stop and realise that it is this that limits the maximum true field of view, that it begins to make sense.

I think this point only clicked for me when I compared the results of the two methods of calculating fov for different eyepieces ie

Apparent field of view /

(Scope focal length / EP focal length)

Which is basically:

Apparent field of view / magnification

vs the method which uses the field stop

ie

(Fieldstop/focal length) * 57.3

With a short focal length eyepiece, these formulas both give very similar results. As you approach the maximum fov for each format, the afov/mag formula starts to become inaccurate and show a wider fov than the fieldstop method. The fieldstop method is the more accurate of the two.

Eg for a 24mm Panoptic, the first method gives 1.63 degrees and the second 1.54

For a 4.5mm Delos, the first method gives 0.324 degrees and the second 0.32, a much closer result because the barrel size is not play a role in limiting the fieldstop

Hope that's clear, and my maths is right (all done on a smartphone looking up info off Televue site), but the principle remains correct

Stu

Sent from my iPhone using Tapatalk

[bomberbaz]

ah I think I get it now (clink clink) sound of pennies dropping

[bingevader]

The field stop is a much more useful figure to have, I wish (for Christmas maybe! ) all manufacturers would include it as standard.

A question arises, from this rather useful discussion, to those of you with these EPs with enormous aFOVs.

I can understand at the higher magnifications why an EP with 100° aFOV would help with drift on a static 'scope.

But what are the pros and cons at the lower magnifications?

I know that if I drop down to my 18mm EP from the 24mm or 32mm the image can be dimmer and less pleasing.

Is it "better" to have a lower mag, smaller aFOV EP or a higher mag, larger aFOV EP if the resulting tFOV would be the same?

I can't afford one, I'm just interested in how the different EPs perform.

Or are you 'scopes large enough that it doesn't matter!

[Stu]

Very true about the field stop measurement. One reason why I always use Televue as an example for these sorts of things is that they publish all the details very clearly.

Regarding your other question, this is where exit pupil becomes important. It depends on the size of your scope and the sky conditions you are observing under.

Your exit pupil should be not larger than your fully dilated pupil which decreases with age. I usually use 6mm as a maximum but 7mm is perhaps accepted particularly for young things with good eyesight ;-)

Under very dark conditions, it is possible to use larger exit pupils to good effect. However when used with any light pollution, there is a tendency for the sky background to be washed out, reducing contrast.

The remedy to this is to use a shorter focal length eyepiece to reduce the exit pupil size and darken the sky to improve contrast. This obviously also reduces the field of view so that's where the desire for 100 degree afov eyepieces comes in.

If we take a 250mm f4.8 scope ie 1200mm focal length, we get the following:

With a 32mm TV Plossl we get a magnification of x37.5, a field of view of 1.29 degrees and an exit pupil of 6.6mm

Whilst this is basically ok, with the large exit pupil under skies with any light pollution it may appear washed out.

Upping the stakes somewhat to a 21mm Ethos (!), we get a magnification of 57.1, a field of view of 1.73 and an exit pupil of 4.4mm

The ultra wide field eyepiece gives you a wider field of view, higher magnification and a smaller exit pupil so the sky background will be darker and the contrast better.

Here are the fovs for comparison, the smaller is the 32mm plossl and the larger, obviously, the 21mm ethos.

Stu

Sent from my iPhone using Tapatalk

[alan potts]

Stu,

All very nicely puts clear and easy to follow. I was just wondering if it would be worth editing both your posts here to make a sticky for the beginner section?

Alan.

[YKSE]

But what are the pros and cons at the lower magnifications?

I know that if I drop down to my 18mm EP from the 24mm or 32mm the image can be dimmer and less pleasing.

Is it "better" to have a lower mag, smaller aFOV EP or a higher mag, larger aFOV EP if the resulting tFOV would be the same?

These are very interesting questions, actually.

Getting higher in magnification (smaller exit pupils), you're reducing the total surface brightness seen in the eyepiece, both the sky background and the observing objects. For point objectss like stars, the reduced total surface brightness with higher magnifications will enhance the constrast, and make dimmer stars more easily seen, On the other hand, for low surface brightness objects like many expansive galaxies and nebulae, the higher mag will not enhance the contrast at all, it'll be washed out together with the background. That's the reason why M33 is easier seen in a pair of binoculr than in a telescope in backyard, simply because a telescope gives too high magnification.

Some more reading for those interested:

http://www.rocketmime.com/astronomy/Telescope/SurfaceBrightness.html#EPexample

[Stu]

It's certainly very true that for low surface brightness objects, there is no substitute for dark skies. If the sky background is brighter than the object then nothing you do will make it visible.

As said, under very dark skies, using a larger exit pupil is quite possible for widefield objects.

Sent from my iPhone using Tapatalk

[John]

This thread sort of sums up why it's good to have a range of eyepieces available. There is no substitute for having options when observing so you can find out for yourself what delivers the best views on a particular object and occasion. Sometimes you find that what actually works the best is not what the theory / maths suggests

[bingevader]

Yes, it was the surface brightness problem I was getting at.

So really, for these 100° EPs, I need darker skies, a bigger 'scope (not this Christmas!), or both!

Some more reading for those interested: http://www.rocketmime.com/astronomy/Telescope/SurfaceBrightness.html#EPexample

An interesting article.

I liked the way he kept referring to the equations as "easy" and "simple"!

Nothing wrong with the etx90 either.

[Stu]

Yes, it was the surface brightness problem I was getting at.

So really, for these 100° EPs, I need darker skies, a bigger 'scope (not this Christmas!), or both!

An interesting article.

I liked the way he kept referring to the equations as "easy" and "simple"!

Nothing wrong with the etx90 either.

Under a dark sky, a big scope really flies, but as said they cannot do magic ;-). If the sky background is too bright then even the biggest scope won't show you low surface brightness objects. Dark skies rule in that instance....

Sent from my iPhone using Tapatalk

[jetstream]

The nice thing about ES 82's is the fact that the focal lengths come close to the Ethos with close field stop diameters. ie 21E=36.2mm FS and the 24mmES 82 = 33.5mm FS diameter. Those ES 82's work well for what Stu is talking about as long as the exit pupil is matched to the objects viewed and the filter used. This is part of Al Naglers pioneering "Majesty factor". My hat is off to him. But to get a similar effect the ES (& others) can be used to at a reduced cost but without the huge 100deg AFOV.

Unfiltered extended objects respond well to about 2mm exit pupil. Eventhough the sky and the object are dimmed with increasing mag, the eye/brain sees larger objects better, up to whatever mag the (roughly) 2mm exit pupil produces in your scope. This obviously has some leaway....http://clarkvision.com/visastro/m51-mag/index.html and is object dependent.

John's right.....put them in the focuser and try them all...

[YKSE]

An interesting article.

I liked the way he kept referring to the equations as "easy" and "simple"!

Nothing wrong with the etx90 either.

I agree that the use of math in that article was not the best one, It is simply trying to explain that

1. The maximum surface brightness is obtained in naked eye, or in telescope with eyepiece given same exit pupil as our max dark adapted pupil.

2. Telescopes only make objects bigger, not brighter.

3. The graph about surface brightness in the article is assuming the max dark adapted pupil of our eyes is 7mm, so 100% surface brightness with EPs given 7mm exit pupil, while an EP given 2mm exit pupil reduces the surface brightness to 8%, etc.

As others already pointed out, the math is just a guideline, there're much more differences in our eyes.

Dark sky is without doubt the best cure for low surface brightness objects, but we have our limited locations to transport us to, it can be of help that that we know something about the objects we're going to observe, such as size and surface brightness. Just as the link Gerry posted above, our eyes are quite limited, only objects bigger than certain sizes with certain brightness can be identified, that's why we choose different magnifications to reach size-brightness balance. For example, for small stella-like nebulae, vi need to use higher mag to get a positive ID, while for extended low surface brightness objects, the best chance we see them are in low mag EPs with exit pupil close to our maximum pupils.

Just my two cents, as they usully say, YMMV - You Mileage May Vary.

[Stu]

Loads of good info coming through in this thread :-)

[bomberbaz]

Loads of good info coming through in this thread :-)

Yes well as the OP I deserve bragging rights for having such a thought provoking & questioning mind and also any sponsorship money or similar fringe benefits need to come my way too 

[Stu]

Yes well as the OP I deserve bragging rights for having such a thought provoking & questioning mind and also any sponsorship money or similar fringe benefits need to come my way too

Agreed ;-)

Sent from my iPhone using Tapatalk

[bomberbaz]

Agreed ;-)

Sent from my iPhone using Tapatalk

Ok so what you giving me then, I notice your sig says you have too many black & green eyepieces where-as I can nevr have too many   

[bingevader]

I don't have anything above a plossl.

I've thoroughly enjoyed this thread.