The case of the self-soiling eyepiece

[lothman]

I have read that with a bit of practice, you can measure lengths to within 1% using an eyepiece reticule. That would be 0.1mm or 1 tick mark on a typical, 10 mm/100 tick scale. Conceivably, the 0.1mm play could add a one tick error and halve the measurement accuracy.

but if you use a 1mm/100 tick rectile with a 10x higher magnification and the rectile wobbles around 0.1mm than you have a 10% error.

If you have the possibility to measure Pixel in your digital Image you no longer need the rectile since pixel/mm stays constant at fixed magnification.

[jfiresto]

I was just trying a quick scale analysis to decide if I could discard the 0.1mm play as a significant term in the measurement error. I had in mind a feature measured against the full length of a 10mm eyepiece micrometer, as 10mm is somewhat less than half the field of view. The 0.1mm play could then displace the eyepiece's micrometer by 1% of the feature's micrometer image-length and similarly bias its calculated length. The play-induced error if I have to measure against a shorter length of the micrometer [sigh, no more magnification] would of course be worse!

Measuring images from a photo port camera avoids all that and documents the measurements, but I find it is sometimes faster and/or easier to measure with an eyepiece.

[rjlittlefield]

I was just trying a quick scale analysis to decide if I could discard the 0.1mm play as a significant term in the measurement error.

I don't understand the concern.

I would take a measurement with an eyepiece reticle by lining up the image with the reticle and counting tick marks, just like using a ruler. When using a ruler, I can start measuring at any tick mark and get the same answer. The same is true for the reticle.

So, as long as the reticle stays in one place long enough to make the measurement, then it seems to me that any amount of lateral shift between measurements will introduce exactly zero error, none at all.

What am I missing in this line of thought?

--Rik

[jfiresto]

I think I am the the wrong nothing to answer that. I have largely been a (computer) modeller. What that question needs is a practical observationalist or experimentalist.

I do know that if there is another, exogenous factor to confuse my measurements, i will find some way to unwittingly and irregularly excite that factor. Perhaps that is why I am modeller.

[ChrisR]

...
I do know that if there is another, exogenous factor to confuse my measurements, i will find some way to unwittingly and irregularly excite that factor. Perhaps that is why I am modeller.

That sounds like a healthy respect for what Donald Rumsfeld would have called "Unknown unknowns"

[viktor j nilsson]

I was just trying a quick scale analysis to decide if I could discard the 0.1mm play as a significant term in the measurement error.

I don't understand the concern.

I would take a measurement with an eyepiece reticle by lining up the image with the reticle and counting tick marks, just like using a ruler. When using a ruler, I can start measuring at any tick mark and get the same answer. The same is true for the reticle.

So, as long as the reticle stays in one place long enough to make the measurement, then it seems to me that any amount of lateral shift between measurements will introduce exactly zero error, none at all.

What am I missing in this line of thought?

--Rik

Ha! Yes, you're of course correct. I just accepted the idea that the tilting introduced a source of error that was either on or off, I didn't even think about it in practical terms. Of course, as long as the eyepiece remains stationary while taking the measurement everything should be fine.