I was puzzling this out the other night and I still can't seem to feel right about it. To work out the effective field of view when observing, the equation is: Apparent field of view of the eyepiece/magnification, ie. 52 degree 10mm plossl used in a 1000mm FL scope gives an effective field of view of 0.52 degrees. True? This is regardless of the OTA aperture. And this is the part that isn't sitting right with me. Surely having twice the aperture will affect the effective field of view. It just stands to reason to me that if I have a bigger hole in the wall I can see further to the side than through the smaller hole.
It is the design of the eye piece that allowes object to be focused at certain focal lenghts, a 31mm nagler with an 82deg field gives a mag of 59 times and an aparent field of view 83.4 arc mins whilste a 13mm nagler with a 82 deg field and magnification of141 times gives 35.00 arc mins, so it comes down to the design of the eyepiece
Paul, just do the sums. I just tried it with my 10", 254mm @ 1140 mm long dob f4.5 & a 6" 150mm @ 1000mm long ,f6.6. & 17.5" 445mm @ 2000mm, f4.5 truss dob. Naturally the AFOV will change.
Mine with a 32mm E/Pc, 52deg field gives me 36x mag@ 1.44 degrees AFOV.
6" with the 32mm E/Pc, 52deg field gives 31x mag @ 1.67 degrees AFOV.
17.5" with the 32mm E/Pc, 52deg field gives 62x mag @ .83 degrees AFOV.
Not exactly sure of prob. L.
Oh I don't have any problems with the sums, RAJAH and Ron. I can work them out np. Its just that the sums don't seem reasonable to me. And I accept that eyepiece design has a lot to do with it, but it just seems "natural" to me that the bigger the hole I'm looking through the wider the my field of view will be. Maybe someone artistic here can draw a diagram that will explain it to me.
Paul it should become clear if you understand the following:
1: Your mirror or objective lens, regardless of aperture, focuses all collected light of any single distant point to a single point at the focal plane.
2: The width of the focal plane that you get to see is determined by the field stop of the eyepiece.
3: The true field of view is the effective fov of the eyepiece divided by the ratio of primary FL to eyepiece FL.
To paraphrase from an exellent TV science show (and this is showing my age) "Why is this so?" I've worked out the Effective Field of all my eyepieces with various scopes I've have/had, using the formula you're suggesting. NP. But I've never questioned the reasoning behind the formula. And then the other night this annoying little voice in my head asked "Why doesn't the OTA aperture come into the figuring?" And you know how persistant that little voice can get.
I've done a bit of net surfing but haven't really come up with an acceptable answer. So I was hoping someone here could provide a simple, easy to understand answer. I accept that it doesn't and I am quite happy to work with the formula I know, but that damn little voice still wants to know "why doesn't it?"
I think I know where your coming from Paul, your imagining that if you had a wall that had a 5cm hole and a 50cm hole and if you stood the same distance away (ie same focal length) then you would certainly see more through the 50cm hole. The problem is how that hole translates to the optical train. My guess is that you see the hole as the aperture of the telescope, when in reality the hole is the field stop diameter of the eyepiece. Of course I might be wrong about your thought processes, but I hope this helps anyway
Yes Andrew, that's more along the lines of my thought. What confuses me is that the objective, as Geoff says, focuses what passes though the objective to a single point (am I reading that the right way?). So the wider the objective the "sky field" (probably the wrong term but its how I think of if) that is focused. Like your hole in the wall analogy. Does that make sense?
What confuses me is that the objective, as Geoff says, focuses what passes though the objective to a single point (am I reading that the right way?).
Yes, you read that as I intended. A small or a large objective of the same focal length will still focus all light it collects to a point at the focal length in the same way, just the bigger one will focus more light to the same point.
In the picture all the rays from a distant source come to focus at a single point. This is the position of the focal plane .
If the viewed object is x degrees off centre, it will be the same angle off centre at the focal plane. Now for our eyepiece with the much shorter FL, the angular displacement at the focal plane is much greater. This is how magnification works.
It doesnt matter what size the objective is, it can be a pinhole and the true field of view is the same.
You could model it in mechanical terms with a lever with its pivot at the position of the objective, the length of the lever is the focal length.
The end of the lever is pivoted to a short lever of length of your eyepieces focal length. A small movement in the "objective lever" would create a much greater anglular movement in the "eyepiece" lever.
Ok I'm starting to visualize this, and its starting to make sense. One clarification Geoff. Is your analogous lever pivotted at the objective or the focal plane?
Quote:
Originally Posted by Starkler
You could model it in mechanical terms with a lever with its pivot at the position of the objective, the length of the lever is the focal length.
The bit about the lever I get, but then your saying
Quote:
Originally Posted by Starkler
The end of the lever is pivoted to a short lever of length of your eyepieces focal length. A small movement in the "objective lever" would create a much greater anglular movement in the "eyepiece" lever.
The lever has two pivot points? Or the end of the longer lever "articulates" the shorter lever? Like a hinge?
Lol I was about to edit and delete what I said about levers ....
Yep, the the end of the longer lever "articulates" the shorter lever Like a hinge.
I think your confusion comes from thinking of the objective as a window that you look through, when in fact it behaves like a pinhole and as said by Andrew above, the real window is the field stop of the eyepiece. How much sky you see depends how far the eyepiece field stop is from this "pinhole".
Thanks Geoff. Sometimes I pick up things really quickly , sometimes I'm as porous as a brick The idea of the levers helped. I visualized that in conjunction with a couple of diagrams I found and that little voice started going "Ok, Ok. Maybe I was wrong" " Finally
My guess is that you see the hole as the aperture of the telescope, when in reality the hole is the field stop diameter of the eyepiece. Of course I might be wrong about your thought processes, but I hope this helps anyway
Andrew,
Thats exactly right.
The "maximum possible TFOV of any eyepiece is given by the formula
field stop diameter (in mm) x 57.3 (1 radian) / focal length of objective.
This is simple physics and geometry. You can't exceeed this True field size. Some manufacturers falsely exaggerate the AFOV of their eyepieces. The only accurate way to calculate the TFOV of an eyepiece is to use the "drift timing method" or where possible measure the internal field stop.
For those interested in calculating the TFOV of some of their eyepieces using the drift timing method, just too see how
much "horse _ _ _ _" some manufacturers come out with, here is how you do it.
Select a star as close as possible to the celestial equator. Position the star so that it is just "outside" the FOV but set to pass across the center of the FOV of the eyepiece (if using an eq mount turn the RA drive off) using a watch or stopwatch measure the time it takes for the star to drift across the center of the FOV from edge to edge.
A star on the celestial equator will drift 15.04 arc minutes per minute of time. This translates to a simple "approximation formula"
You take the drift time in seconds and divide it by 4 to get the TFOV in arcminutes. Multiply this by 60 to get TFOV in degrees. You then back calculate to the eyepieces AFOV by multiplying the TFOV in degrees as a decimal by the magnification the eyepiece gives in that particular telescope. This will give the eyepieces AFOV in degrees.
Televue are the only company that "publish" all of their eyepiece field stop data and you will find calculating TFOV based on the field stop data closely approximates the actual field of view measured using the drift timing method. Not all eyepiece manufacturers can support the same claim and accuracy as Televue.
I know Mike, you and I had a great conversation with them a while back when we were talking about how you managed to find the time to do everything you do. It's just that every now and then I like to burst out and add a bit of colour. Besides I prefer "emoticon" rather than "smilies" That says it all.
Thanks for that John. I've read about the drift method, but never bothered to check the eyepiece's claimed field. I'm a very trusting soul. If we ever get some more clear nights and my camera stops nagging me I'll do what you've suggested and compare reality with my calculated FOV
I've read about the drift method, but never bothered to check the eyepiece's claimed field. I'm a very trusting soul.
They dont even tell the truth about focal length !
I had a 26mm eyepiece that measured to be pretty close to 30mm by checking the size of the exit pupil. So whilst a drift test would support the manufacturers claims re FOV, in fact the focal length was understated making the eyepiece appear to have a wider apparent fov than reality
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Question regarding the effective field of view.
I've worked out the Effective Field of all my eyepieces with various scopes I've have/had, using the formula you're suggesting. NP. But I've never questioned the reasoning behind the formula. And then the other night this annoying little voice in my head asked
Sometimes I pick up things really quickly 
The idea of the levers helped. I visualized that in conjunction with a couple of diagrams I found and that little voice started going "Ok, Ok. Maybe I was wrong" 
" Finally 
