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digital_davem

Understanding the field of view of a telescope

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Whilst this can be calculated it relies on knowing features of the scope that aren't readily available.

What you are referring to is the size of the illuminated field, this is very important to imagers as it determines the size of sensor they can use and whether they will experience vignetting.

Scopes designed for imaging normally state the illuminated field size in the specs. They may even go so far as to state a fully illuminated and corrected field size, this will be the size sensor you can use without suffering CA.

The limiting factor for the illuminated field size depends on the scope, for a Newtonian design it is the size of the secondary mirror, you can use a bigger secondary to get a bigger field but this will then block more light from reaching the primary.

For refractors the limitation is the last baffle or the focuser draw tube, you need the focuser so not much you can do about that.

For Cassegrain scopes it is usually the primary baffle that limits the field size, shortening the baffle would allow stray light into the focuser so it is not really practical to change this.

The size of the field will be further effected if you add more optics into the light path, a Barlow lens will make it bigger whereas a reducer will make it smaller, hence you can introduce vignetting with a reducer.

Obstructions such as 1.25" filters may also make the field smaller.

CA can cause a secondary limitation, whilst the field may be fully illuminated the star shapes may be too distorted to be useful for imaging (unless you like Star Trek style warp drive effects!).

I hope that helps more than confuses.

/Dan

Sent from my iPad using Tapatalk

That makes sense. I was thinking about this before I went to sleep. The drawtube in my focuser has a 36.4mm diameter. So no matter how big the patch of light produced by the objective, the largest diameter circle the eyepiece/camera/unaided eye could see is 36.4mm.  If the eyepiece has a bigger apparent field of view, all it would see at the edges is the inside of the drawtube. So, in practice even the fanciest EP has an absolute limit defined by the diameter of the drawtube (or less if there are baffles in the drawtube. That is why as you note, bigger barreled EP in bigger drawtubes can potentially have a wider apparent field.

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I think how this must work in like this. The objective projects a nice circular image at the focal point.  Your eyepiece photographs this circle. If your eyepiece has a wide apparent field of view you see all of the image circle including the fuzzy edges.  If your eyepiece has a narrower field of view you see only the central region of the objective image circle.  In fact (just a guess) I reckon that orthos and plossls and the like actually have a much wider apparent field of view that you see in a practical eyepiece. However the edges of that apparent field of view have embarrasingly terrible image quality so the manufacturers include a fixed aperture stop in the eyepiece to block off the fuzzy regions, That would explain why the edge of the visble circle is always a knife edge sharp line.  Real optical circles just sort of blurrily fade to black at the edges, that sharp circle has to be artificial.

I think your understanding is correct.

The sharp edges is the knife-edged field stops in eyepieces. For simpler eyepieces, the aberrations in the edges can be too bad to be useful, therefore, any type of eyepieces have a critical focal ratio (faster scopes' steeper light cones are more challenging) as limit, the size of field stops are set according to telescope focal ratio, focal length of eyepiece, type of eyepiece. With some eyepieces, you can remove or replace field stop with bigger ones, then you get wider field of view, but not as clean in the edge.

Complicated design of eyepieces handles the light cone in the edge much better, therefore wider field stops for the same focal length. The down side is the weight and size, such an eyepiece can be of size of a 60mm short acchromate refractor :smiley:

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...

But it does not seem to explain my central condumdrum of why telescopes can do the impossible!  With my camera I can use a telephoto to get a close up view or I can use a wide angle to get a lot in. But I cannot swap out my telephoto to another telephoto of the same focal length that will also show me the wide view at the same time. The  two are mutually incompatible. Yet in eyepiece land I appear to be able to buy a high magnification eyepiece that has a wider view than a low magnification eyepiece of a different design. That's pretty counter-intuitive, you must admit.  Again I come back to my idea that the objective must project a large circle most of which is wasted by cheap eyepieces. Only the expensive multi-element designs can "see" the outer regions of the objective's image circle.

...

I think part of the problem here is that if you want to think in terms of photographic optics then you have the analogy the wrong way round - the lens on a camera is analogous to the primary optics of the telescope - not the eyepiece. Swapping eyepieces is, rather, the equivalent of swapping camera backs. And that's something that you can indeed do with the right camera system - LF cameras and many medium format camera systems have (or perhaps I should say had) this capability, and changing the back for one of a different format will indeed enable you to 'see' more or less of the image projected by the primary optics. In fact, if you take the ground glass back off a large format camera and then use a loupe to examine the aerial image you would, in effect, have yourself a refracting telescope.

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Ahh, the majesty factor! I love it :grin:

The field stop controls the actual field of view and "majesty eyepieces" hide it from view :laugh:

Most of these eyepieces have very large FS diameters for their magnification, which gives more field and the large apparent FOV adds that well loved "looking through the porthole" look.

For the field comparisons any 2 EP's should have the same FS, regardless of AFOV ie my 17E vs my 18ES.... 30mm FS vs 25mm FS... obviously the Ethos give a bigger view and also that great immersive experience. Of course all this is just my opinion :grin:

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If we want to look at it from an imaging point of view it is all incredibly simple. (For once!  :BangHead:)  The size of the astronomical object as projected onto your chip and measured (say) in mm is governed by one single term, the focal length. The focal length and only the focal length governs the size of the Horsehead Nebula on your chip. Phew! Now, how much of the horsehead can you really image at this focal length? This is governed, optically, by the corrected circle of your optics. It is governed photographically by the size of your chip. (Its resolution, if you insist, is governed by the size of your pixels with the caveat that both guiding and seeing may break down before your theoretical resolution is reached. And the size of the image on your screen will rise as the number of pixels will rise - but will they contain any useful extra information as they do so? Ah!)

Now to visual. How Al Nagler (TeleVue) manages to extract an enormous and exquisitely corrected observable FOV from the same focal length EP as normally has a much smaller FOV I do not know. I do know that he does so - and I know that other manufacturers are quick to copy. I have an optically 'fair to middling' half metre Dob, F4.1. It isn't a work of optical wonder but it is quite nice. At the very limited field stop of a Plossl the edge stars look like a hangover seen from the inside. Put in a TeleVue Ethos and the field opens up enormously and yet, as if by magic, the edge stars are fine. No hangover. As I say, I don't know the optical explanation for this, only the delightful results.

Olly

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If we want to look at it from an imaging point of view it is all incredibly simple. (For once!  :BangHead:)  The size of the astronomical object as projected onto your chip and measured (say) in mm is governed by one single term, the focal length. The focal length and only the focal length governs the size of the Horsehead Nebula on your chip. Phew! Now, how much of the horsehead can you really image at this focal length? This is governed, optically, by the corrected circle of your optics. It is governed photographically by the size of your chip. (Its resolution, if you insist, is governed by the size of your pixels with the caveat that both guiding and seeing may break down before your theoretical resolution is reached. And the size of the image on your screen will rise as the number of pixels will rise - but will they contain any useful extra information as they do so? Ah!)

Now to visual. How Al Nagler (TeleVue) manages to extract an enormous and exquisitely corrected observable FOV from the same focal length EP as normally has a much smaller FOV I do not know. I do know that he does so - and I know that other manufacturers are quick to copy. I have an optically 'fair to middling' half metre Dob, F4.1. It isn't a work of optical wonder but it is quite nice. At the very limited field stop of a Plossl the edge stars look like a hangover seen from the inside. Put in a TeleVue Ethos and the field opens up enormously and yet, as if by magic, the edge stars are fine. No hangover. As I say, I don't know the optical explanation for this, only the delightful results.

Olly

Is there an easy way to know the diameter of the optical circle produced by the objective?  And what percentage of it is usable? 

My f/16.5 refractor seems very gentle on EPs.  My £17.99-a-piece Antares plossls are pin sharp out to the field stop as far as I can tell. But those exotic wide field EP do seem to have an remarkable reputation. I'm wondering how large an optical circle they need to do their stuff. If I wanted something with a wider view than plossls, how much wider a view will the optical circle of my scope support before I'm just looking at the fuzzy edges? Is this only possible with fast, short focal length scopes? I've been doing some calculations based on formulae I found on a web site that, if correct, suggest the max true field of view of my scope with any EP is 1.2 degrees...

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So there are 3 ways you can achieve a wide angle view with a bigger true field of view: 

- don't change the eyepiece but change the telescope to a shorter tube

- keep the telescope but use an EP with a longer focal length

- keep the telescope and don't change the EP focal length but swap the EP for a different design that has a wider apparent FOV

Given my current theory of how this probably works, the third option of using a fancy EP comes at a (technical cost): the extra field of view is making use of the outer edge of the objective's image circle which is of poorer quality than the central region.  This is analogous to what happens if you put a lens designed for APS-C sensors on a camera with a full frame sensor: you can get a wide angle view but chances are the sweet spot of the image circle is too small for the big sensor and the edges of the frame will be a lot fuzzier than the centre of the frame. And if you go too far, the lens circle won't even cover the full frame centre and you will end up with a circular image in the middle of blackness.

Your sensor analogies are quite correct. A 13mm Orthoscopic eyepiece (40° aFOV) is a micro 4/3, a 13mm Plossl (50° aFOV) is an APS-C, a 13mm Nagler (82° aFOV) is a 35mm full frame, and a 13mm Ethos (100° aFOV) is a medium format. The 13mm is always 13mm but the different aFOV gives different fields of view.

For astrophotography with the camera at prime focus (telescope used as lens, without an eyepiece in the optical path), yes it is possible that a telescope produces an image circle smaller than the camera sensor and you will get vignetting. To my knowledge, there is no easy way to calculate the image circle given by a particular telescope, but models specifically targeted towards astrophotography will sometimes quote the image circle in the specifications.

With visual observing, using an eyepiece, you won't see conventional vignetting, but you will end up with the 'exit pupil' (image circle projected by the particular telescope/eyepiece combination) being larger than the pupil diameter of your eye so some of the light gathered by the telescope is lost.

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Your sensor analogies are quite correct. A 13mm Orthoscopic eyepiece (40° aFOV) is a micro 4/3, a 13mm Plossl (50° aFOV) is an APS-C, a 13mm Nagler (82° aFOV) is a 35mm full frame, and a 13mm Ethos (100° aFOV) is a medium format. The 13mm is always 13mm but the different aFOV gives different fields of view.

For astrophotography with the camera at prime focus (telescope used as lens, without an eyepiece in the optical path), yes it is possible that a telescope produces an image circle smaller than the camera sensor and you will get vignetting. To my knowledge, there is no easy way to calculate the image circle given by a particular telescope, but models specifically targeted towards astrophotography will sometimes quote the image circle in the specifications.

With visual observing, using an eyepiece, you won't see conventional vignetting, but you will end up with the 'exit pupil' (image circle projected by the particular telescope/eyepiece combination) being larger than the pupil diameter of your eye so some of the light gathered by the telescope is lost.

I received my camera adaptor today. Just tried it out on the moon.  It doesn't focus with the diagonal but it is fine straight through. The 99% moon doesn't quite fit in the m4/3 field of view, with the top slightly clipped.

here's a snap:

post-47707-0-36747100-1448490276_thumb.j

Interesting that sharpness isn't consistent across the globe. Why is that do you think?

Even a quick snap demonstrates that I need to do something about the tripod. If i can get some 3/8" threaded rod and some nuts and washers, I should be able to directly bolt the head to the legs without the flex of the centre column.

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Lunar and planetary imaging suffer a lot from the seeing conditions. This is why video is often used for imaging them. The individual frames are then stacked to take the parts that have the best focus.

/Dan

Sent from my iPad using Tapatalk

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I received my camera adaptor today. Just tried it out on the moon.  It doesn't focus with the diagonal but it is fine straight through. The 99% moon doesn't quite fit in the m4/3 field of view, with the top slightly clipped.

here's a snap:

attachicon.gif1.jpg

Interesting that sharpness isn't consistent across the globe. Why is that do you think?

Even a quick snap demonstrates that I need to do something about the tripod. If i can get some 3/8" threaded rod and some nuts and washers, I should be able to directly bolt the head to the legs without the flex of the centre column.

 That's a really nice shot. The colour is good :)

Your telescope is showing some spherical aberration & field curvature, which is quite normal for an uncorrected instrument. Curved image plane on flat sensor = not sharp across the entire frame. You can buy field flatteners to correct for that, they are normally 2" but they may be available in 1.25" fitting. They are normally matched to the focal ratio of the telescope but I am afraid f/16 is outside my area of experience. I normally use Newtonian telescopes and camera lenses rather than refractors, and I am a deep sky nut rather than lunar & planetary.

Anything you can do to make the mount more stable is a good thing, even hanging a weight off it would help. If you are getting flex in the centre column, quite likely with such a long telescope, stiffening that up will be a big help.

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