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focal plane in eyepieces

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#1 Guest_**DONOTDELETE**_*

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Posted 26 December 2004 - 02:11 PM

Just out of curiousity, where, in a focused eyepiece, does the focal plane of the scope land?
Thanks
Stev E.

#2 Jim Easterbrook

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Posted 26 December 2004 - 02:51 PM

My opinion is that it should be in line with the "shoulder" of the eyepiece where the barrel meets the body. Most of my eyepieces are within a couple of milimetres of this, but one or two are way off. I know no good reason why all eyepieces shouldn't be parfocal.

#3 David Knisely

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Posted 26 December 2004 - 04:43 PM

Just out of curiousity, where, in a focused eyepiece, does the focal plane of the scope land?
Thanks
Stev E.


The focal plane of the telescope itself is always at the instrument's main focal point ("prime focus"), but for your eye and an eyepiece, there isn't exactly a "focal plane" except inside your eyeball where the final image is formed. What you see when you look in the eyepiece is light coming from various shallow angles to the optic axis which your eye lens then focuses on your retina and your eye interprets the same way as it does when it is just looking at non-telescopic scenery. You can use eyepiece projection for photography which, in that case, puts the focal plane some distance behind the eyepiece inside the camera. Point your telescope at the moon sometime and hold a white piece of paper behind the eyepiece. Then, you can focus the eyepiece until an image appears sharply on the card. At that location, you have a "focal plane" via eyepiece projection, but it probably won't be quite the same eyepiece focal position as it is when you are looking into the scope. If you want to know the eyepiece location when it is focused for your eye, the eyepiece is some distance behind the focal point of the telescope, with the exact amount depending on the focal length of the eyepiece and how well your eye accomodates and focuses. Theoretically, if the eyepiece were a single-element "thin" lens, it would be located approximately one eyepiece focal length behind the focal point of the telescope, but eyepieces are quite complex optical systems, so this isn't often true. Clear skies to you.

#4 Mike Hosea

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Posted 26 December 2004 - 05:57 PM

The eyepiece has a focal plane, the telescope has a focal plane, and your eye has a focal plane. Of course "plane" is a misnomer in all three cases since the focal surface isn't usually flat. In fact, when it is flat, it's worth pointing out (e.g. a flat-field APO). Anyway, the telescope focuses light onto its focal surface, which should be within "reach" of the eyepiece in the focuser. The eyepiece focal surface location varies quite a bit. With Pentax XLs and XWs it is about even with the barrel flange, aka the "shoulder" of the eyepiece where the barrel ends and the rest of the eyepiece begins. For most 1.25" Tele Vue eyepieces it is about a quarter of an inch below the flange, but it varies. In the eyepiece specification chart on the Tele Vue web site, the effective location of the focal plane of each Tele Vue eyepiece is given as the F measurement in the table.

I've seen eyepieces where the focal plane was near the end of the barrel. This configuration requires a lot of out-travel of the typical focuser, or a barrel extension. At the other end of the spectrum would be something like the 31mm Nagler which has its effective focal plane location about 3/8" above the barrel flange. Anyway, wherever it happens to be is where the telescope focal plane would land if the system and 20/20 eyeball are focused at infinity. If the eyeball is not 20/20, an adjustment is made in one direction or the other.

Note that I have occasionally used the term "effective". If the eyepiece has optics in front of the focal plane, e.g. Naglers, Radians, and such, the focal plane of the eyepiece can be anywhere inside the eyepiece that the designer wants it. Exactly where that is would be completely irrelevant and useless information for us. What we need to know is where the focal plane of the eyepiece is effectively when considering the eyepiece as a "black box".

If you would reveal more about why you are asking, we might be able to offer more pointed information rather than all these generalities.

#5 Starman1

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Posted 26 December 2004 - 05:59 PM

My opinion is that it should be in line with the "shoulder" of the eyepiece where the barrel meets the body. Most of my eyepieces are within a couple of milimetres of this, but one or two are way off. I know no good reason why all eyepieces shouldn't be parfocal.


Economy (it's less costly to produce a shorter eyepiece in a long focal length and have the field stop at the end of the barrel than it is to make the eyepiece taller and have the field stop be at the "shoulder" of the barrel)

Tradition (The appearance of eyepieces is a very important part of marketing. Very long eyepieces look "odd"--not like "normal" eyepieces)

Sellability (if long FL eyepieces were all very long, they might not sell--one of the problems with the Speers-Waler series--and if all short FL eyepieces were very short, people might think they should cost less due to a smaller amount of materials used. Unorthodox appearances are often commented on in reviews, and it can hurt the sales for a company producing a good quality, but unusual looking, eyepiece)

Design (eyepieces with internal Barlows, or unusual lens configurations, especially in the 2" size, could become too heavy if strict adherence to parfocalizing were rigorously followed. Some large eyepieces would be even more massive, and less usable)

I can see a company making at least an attempt to parfocalize all their 1-1/4" eyepieces, but so long as we have focusers, I see no reason to attempt to do so with 2" eyepieces so long as they all come to focus in, say, a 1-1/2" to 2" range (though this may cause problems for the ATM).

#6 Guest_**DONOTDELETE**_*

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Posted 26 December 2004 - 10:37 PM

Thanks all for the info. What got me thinking about this was the prospect of modifing a spare finder scope to accept a 1.25 ep to make it more comfortable to use. Being more or less fixed focus except for threading the objective in or out, I want to place the ep close to the center of the focal range.
Steve E.

#7 Mike Hosea

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Posted 26 December 2004 - 10:59 PM

Well, good, that helps a lot. Assuming this eyepiece won't have a built-in Barlow for a purpose like that, check for the field stop in the barrel of the eyepiece you want to use. That should be where the focal plane of the eyepiece is. You'll want to place the focal plane of the finder scope right there.


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