Measuring Fieldstops

[Figgis]

Hello All.

                I've seen what Baader advertise as the Fieldstop measurements on their range of Hyperion Eyepieces but wonder what the rule of thumb is for measuring them or indeed checking what they say is Gospel.  Any and all comments welcome.

[Zermelo]

Hello, could you say a bit more about what you're after?  Is it that you want to calculate the true field of view?

If you check out consolidated lists of eyepiece specs like this one, you'll see that more often than not, official field stop figures aren't available. You can work backwards from other measurements with formulae, but I believe that the result is only approximate because of the different distortions that might be present in the eyepiece design. You can measure the fieldstop with callipers if it's accessible (or if you're prepared to disassemble). I've read that there are a few eyepiece designs where this isn't very informative for TFOV purposes, because the field stop positioning is not the determining factor in those cases.

With any luck, you'll get a response from someone who knows about these things, of which there are a few on here.

[Louis D]

If you have a Tele Vue eyepiece, their online specs are quite accurate in my experience.  You can start with that eyepiece as the baseline for your measurements.  First, setup a ruler or yard/meter stick a good distance (20 to 40 feet minimum) from your telescope.  Next, put the TV eyepiece in the focuser, focus on the ruler, and note the left to right distance from field stop edge to field stop edge.  Next, substitute your unknown field stop eyepieces and measure the displayed distances.  Since it's a direct linear relationship between distance on the ruler and field stop diameter, all you have to do is divide the TV field stop number by the distance it showed on the ruler.  This becomes your conversion coefficient from ruler distance to field stop diameter.  Now, just multiply each ruler distance by that coefficient to calculate each eyepiece's field stop diameter.  Due to measurement errors, you shouldn't expect FS diameter accuracy better than about 0.3mm in my experience.

This method is completely independent of eyepiece magnification distortion across the field of view or location of the FS within the eyepiece.  If you don't have any TV eyepieces to boot strap the process, you might try a 32mm Plossl which generally has a ~27mm field stop diameter and is completely measurable with calipers to get better accuracy.  In fact, any simple positive-only eyepiece like a Plossl, Kellner, Ortho, Huygens, RK, RKE, etc. will work as long as you have calipers accurate to sub-millimeter distances to generate your baseline coefficient.  In fact, if you have multiple coefficient measurements from multiple eyepieces, you can average them for better accuracy.

[Don Pensack]

7 hours ago, Figgis said:

Hello All.

                I've seen what Baader advertise as the Fieldstop measurements on their range of Hyperion Eyepieces but wonder what the rule of thumb is for measuring them or indeed checking what they say is Gospel.  Any and all comments welcome.

There is a fairly easy way.

1) pick a star on or very near the celestial equator.

2) time exactly how long it takes to drift across the field, through the center, from edge to edge.  Do 3 timings and pick the longest one (that one is likely to have passed dead center)

3) convert minutes and seconds to decimal minutes (example, 4 minutes 12 seconds = 4.20 minutes).

4) divide by 3.99.  The figure derived = the field in degrees.  Now you know the exact field of view in your scope with that eyepiece.

 

Then, calculate the field stop using this formula:

Field stop = (True field x telescope FL in mm) / 57.296

FS = (TF x TFL) / 57.296

The degree of accuracy is not exact, but you can get within 0.1mm, usually.

 

A calculated way to checking the figure derived is the following formula:

FS = (AF / 57.296) x EPFL

AF = apparent field

EPFL = eyepiece focal length.

This formula is almost never right because manufacturers lie about the exact apparent fields of the eyepieces and because focal lengths are often slightly off (maybe +/- 0.1mm)

But it gives a rational check number to see if your measurement is in the ballpark.

[Figgis]

Thank you Don.

                          I had your equation written down incorrectly, yours is perfect.