High magnification and wide FoV?

[exoplanet]

This may well be a stupid question, so please feel free point that out.

Looking at these EPs Baader Planetarium

They have a 68degree FoV and with FL as little as 3mm. So based on my limited knowledge the 3mm FL (with my 1200mm scope FL) gives a magnification of 400x - but how can that be reconciled with a wide FoV? surely a high magnification inherently means a narrow FoV?

In my head I picture Jupiter at 400x as a pretty large disc and in a wide FoV as as a fairly small disc, surely you cant have both at the same time?

Feel free to now point out my errors.

Thanks

John

[GSR]

Hi John- 400x magnification with 68 degree FOV will give you a true field of view of 0.17 degrees.

To work out your true field of view through any eyepiece divide the apparent FOV (in this case 68) by the Magnification (400)

Hope this helps

[moonraker1234]

Hi

Yes that is a real problem! Large magnification is not always best, and can result in blurred or indistinct images, unless resorting to a larger scope, which may compensate, at least in part!

[johnh]

Widefield eyepieces are for lower magnification as they use lots of elements in their construction so more light scatter and lower magnification gives brighter image. For planets I use a 5mm TMB Supermonocentric with minimum number of elements giving brighter crisper image that the widefield will not give but the field is 30deg which is very narrow - like looking down a straw, thats the trade off. 400x is way too much for a 8-10" scope 200-250x is sensible in good seeing.

John.

[exoplanet]

Apparent FoV and True FoV - got it. So would it be fair to say that the apparent FoV is sort of like a "starting" point and you use the magnification factor to work out what you will really get?

That makes much more sense.

johnh - thanks for the info, I wouldnt use 400x on my scope, just using that as an exteme example!

[astrobase1]

Hi Exoplanet

Where I live light pollution is relatively high and the seeing generally bad , as a result the smallest eyepiece I can get good quality clear views with is about 6mm which gives 200x magnification in my scope . I use the Baader Orthos which only have an FOV of 40 degrees , TFOV 12 arcmin which for planets and double stars seems perfectly adequate ( I,d like to magnify more but dark skies and good seeing are a rare luxury )

I am sure someone on this site knows the exact size of Jupiter but it still seems pretty small even in a 40 degree lens .

A wider field of view means if you are manually tracking the object will stay in your field of view for longer but it still moves pretty damn quickly and the higher the mag the worse it gets .

[John]

With un-driven scopes (like mine !) using eyepieces with a wide angle of view helps keep objects in the field of view for longer and with less "nudging" required.

The apparent field of view (AFoV) is a specification of the eyepiece, ie: 30, 40, 50, 60, 70,82 or even 100 degrees. True field of view (TFoV) is the amount of sky that you actually see when you use a particular scope / eyepiece combination and can be worked out by dividing the AFoV of the eyepiece by the magnification it is giving in the scope it's being used in.

For example, if I use a 6mm Baader Genuine Orthoscopic, which has a 43 degree AFoV, in my 6" mak-newtonian which has a focal length of 900mm the magnfication is 150x and the TFoV is .286 degrees. With my 6mm Tele Vue Ethos eyepiece, AFoV 100 degrees, in the same scope, the magnification is still 150x but the TFoV is now .666 degrees - 233% larger. Jupiter (for example) remains the same angular size in both cases but, with the wider angler eyepiece, it is surrounded by much more sky.

[johninderby]

Yes you can have high mag and wide FOV, but it will cost you. With the advances in glass and anti-reflection coatings the latest premium eyepieces with 7 or 8 or even more elements are just as good as the old specialist planetary eyepieces for light scatter and light transmission rates. Things really have moved on.

My latest high power acquisition (don't ask the price ) is an Ethos SX 3.7mm 110 degree FOV eyepiece, and I would rate it as just as good if not better than my old TMB monocentrics for sharpness and contrast. Viewing the moon with this eyepiece is an amazing experience.

John

[ollypenrice]

I'm not dead sure that the question has been answered though. Try this explanation; in a pair of EPs of x mm focal length the object will appear the same size. In the widefield there will be more space visible around it. Is that what you were asking?

Olly

[spaceboy]

Hi Exo

For example: In a 10" F/4.8 A 24mm panoptic of 68' AFOV gives 67.5' TFOV at 1000Yrds which offers near the 68' maximum of the Eyepiece. Put in a 19mm Panoptic with the same 68' AFOV and you will only get 53.5' TFOV at 1000Yrds

So the 19mm Pan would not offer the best TFOV for the money in this scope but the 24mm and above do what they say on the tin and more.

When a manufacturer makes eye pieces they make them to cover all telescopes and not any one scope in particular. So where we see the 19mm Pan would not be best in a 10" F/4.8 so a choice of different AFOV eye piece may be better suited to that focal length. This link was given me in a previous thread Tele Vue Optics: Calculator and only really comes in handy if you want to buy TV's. But at least it can give you an idea of how the FOV changes and which focal lengths can be optimal in any given scope.

Hope it helps

SPACEBOY

[pvaz]

...With my 6mm Tele Vue Ethos eyepiece, AFoV 100 degrees, in the same scope, the magnification is still 150x but the TFoV is now .666 degrees - 233% larger...

That proves it! Ethoi are the devils work to clean out our wallets!

[spaceboy]

That proves it! Ethoi are the devils work to clean out our wallets!

True but it has been said to me by a seller of Ethos that the eye can only take in a 70' FOV. I have had it argued by Ethos fans that the eye unaided you would be able to see up to approx 150' or so. This I agree with totally. I think it a concern for someone who sells Ethos at £400+ would encourage a customer to purchase a £200+ EP.

I can only assume it is some thing to do with the exit pupil thing again. Think of it like this.... I have a 10" F4.8 it is advised not to have a 35mm ep as light would be wasted as my pupil cannot dilated large enough to make use of the light exiting the ep and my view shouldn't be any better than lets say a 30mm ep. And this tends to be advice followed by all.

Is this the same but in reverse?? Unaided I can see around 150' and most people could see a car coming at 70+ MPH from most directions but put Tyson 3ft in front of me and for sure I could get a right hook and not even see it coming. So we can assume that where when an ep is 5mm from your eye that the maximum FOV will be 70'. Just as the 19mm and 24mm TFOV can differ over AFOV depending on magnification. Our eyes I guess react the same way depending how close or how far you are from an object.

Now when you look through a Ethos the view will undoubtedly be awesome compared to the 50' plossl you had when you 1st got into the hobby but how much is the eye actually making use of?? I went for Naglers purely for tracking purpose but even at 82' I find myself looking through the key hole so to speak. My eyeball is like a marble rolling around the bottom of a pint glass. The perfect combination of magnification, scope, FOV and eye relief can give you an equal TFOV to AFOV in the 24 mm pan in my case.

On the link previously posted the widest TFOV from a Ethos in my setup never reaches 100' anyway. At best is the 21mm offering 90.5 which is again going to be amazing but another marble moment.

A test we did in school all them years ago was to put a dot on one side of a piece of paper and a dot on the other. You stare straight forward at the one dot with one eye closed and then slowly move the paper towards you. At some point you hit your blind spot.... just happens to be where the right hook comes from as well Granted no-one would turn an Ethos down if offered one and even if your eye doesn't actually see the full FOV with out having to loose part of it while looking over the other side or that although the center looks great you peripheral vision makes the outer edge blurry they do have advantages for manual dob tracking, but what a price to pay not to have to keep nudging

This is my opinion and I have been proved wrong in the past but having given it some thought that shop keeper made scense all be it I couldn't understand why he sells them in the 1st place

[johninderby]

Re: Ethos and FOV

Forget the FOV of the Ethos, it's the quality of the view that makes them so good. The sharpness and contrast are what I remember most about using them.

John

[pvaz]

Re: Ethos and FOV

Forget the FOV of the Ethos, it's the quality of the view that makes them so good. The sharpness and contrast are what I remember most about using them.

John

Couldn't put it better. True you can't take the hole FoV but the pleasing side of it is you get ortho-like quality and it almost seams like you have a window in front of you, instead of a key hole.

This is my wish list right now:

- 18 and 12.5 mm BGOs (preparing for a long FL scope)

- 14 or 16" SW flextube

- Ethos 17mm (or maybe the 21mm and sell the Nagler 31T5 and bigbarlow)

Time to save... It will take a while.

[Chris H]

The basic eyepiece design such as a Plossl has a 50 degree field of view any ep with a larger field of view such as the one you have looked at are described as wide field. Yes the field of view with a 3mm ep will be small even with a 68 degree ep but not as small as with a 50 degree FOV. Jupiter at x400 for example will be the same size whatever type of ep is used it's just the amount of space visible around it that varies from ep to ep. Hope this makes sense.

[John]

Re: Ethos and FOV

Forget the FOV of the Ethos, it's the quality of the view that makes them so good. The sharpness and contrast are what I remember most about using them.

John

Precisely John - I wish I'd not used an Ethos in the example in my post to illustrate FoV now