Jump to content

what's a field stop?


Recommended Posts

hi there

the field stop is what gives a nice crisp black circle when looking through the eyepiece. without one the view will be unaffected but you will get a fuzzy black edge instead.

it would appear not to be the case with the eyepiece in question but in some designs (like Televue Panoptics) the whole optical assembly is held in by the field stop and/or retaining ring and it should not be removed.

Link to comment
Share on other sites

In general the field stop is that chromed tube screwed to the bottom end of the eyepiece. The bit that you slide into the diagonal.

Light can enter a lens from all angles. The field stop limits this. In the case of the eyepiece just those from the direction of the objective are able to enter the tube.

Usually the field stop on an eyepiece is bigger then the angle that the eyepiece has, so you do not see it. If you plugged a 40mm plossl in then the eyepiece can see a greater angle then the tube (field stop) so you become aware of a defined circle that limits your view.

Try it out by looking up a toilet role tube. Your eye can see a wider view then the tube allows, so the tube is acting as a field stop.

If there isn't one then curious how the eyepiece will fit on the diagonal, but they are probably easy to get hold of. This assumes that they have a common thread to attach them to the main eyepiece body.

Link to comment
Share on other sites

Perhaps the person selling the eyepiece really means instead that the barrel is missing? If the field stop is missing then in many cases it will mean there's nothing to hold the lens elements in place.

In any case it suggests that the seller didn't look after it very well, or perhaps has cannibalised it for a project.

I'd avoid it.

(p.s. the field stop is NOT the same as the barrel. The field stop is the narrowest aperture through which light enters the field lens. In a plossl this is a little threaded cylinder which holds the lens elements in place, which you can see by looknig up through the barrel. But maybe the seller doesn't know this.)

Link to comment
Share on other sites

Thanks for the link. I don't own any orthoscopic eyepieces and will defer to someone who does, but I'm wondering what's holding the lens elements in place, and whether they might have been dislodged by the removal of field stop. And why it's missing.

I'd also want to check out if there's any import duty to pay (maybe not since it's secondhand but you never know). And remember that unless you have a .965" focuser, you'll need an adapter (though that's no big issue of course).

(Edit: thinking about it, it's possible that the field stop was removed or altered so as to give a wider field of view - given that this design has a relatively narrow field. If done carefully, then the only difference should be a noticeable degradation of image at the edge of field: the part that is normally blacked out by the field stop. But the worry would remain, in my mind, that this tinkering might have displaced the lens elements. And the seller admits that performance has been compromised. Though fair play to him: he admits that he paid several hundred Australian dollars for a lemon.)

Link to comment
Share on other sites

just when you think you've found a bargin...same ep's on the web are around £150 ish

It's that magic "Zeiss" name that does it :icon_scratch:

The more recent 1.25" orthoscopics known as the ZAO's and ZAO II's fetch as much as £400 each. They are reputed to be the best planetary eyepieces that money can buy ....

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.