Ortho Collection - taking shape

[Louis D]

11 hours ago, Mr Spock said:

Interesting. A while ago I had a 10mm Radian which I compared to my 10mm NLV - they were about the same. Comparing the 10mm NLV to the 9mm ortho the latter is significantly brighter when looking at my white test chart. In photographic terms maybe ⅓ a stop difference or more.

Both the Radian and LV/NLV lines used rare earth glass types that seemed to make for slightly dimmer viewing in each as compared to the XL/XW and much later Delos/Delite lines.  I see the same effect in my NT4 eyepieces as well, so they probably use rare earth glasses as well.  My 27mm Panoptic shows a bit of this as well.

[Don Pensack]

18 hours ago, jetstream said:

This transmission stuff is something I don't understand...the difference in measured transmission doesn't always reflect what the eye notices IMHO. The optical experts say the levels of difference cant be noticed and yet I personally do see it.

The 10BCO is my king of "transmission" i.e. object detection. The Nagler 3-6 zoom is near the bottom of "transmission" to my eyes even though it is a great lunar/planetary eyepiece.

All I know is that the top DSO observers all use orthos at high mag for faint threshold objects. It is an absolute bonus that these orthos, Circle T included offer top tier views of the moon and planets.

I have a UO Tani 4mm ortho that is unbelievably good, so good that it takes the Vixen 3.5mm HR to knock it off the stump, and that is quite an accomplishment.

Our nighttime vision sensitivity peaks at about 500nm (dropping from a daytime peak at 550nm--this is known as the Purkinje Effect).  And peak sensitivity at night is about 450-550nm with the range from ~425 to 600nm.

Ergo, the spectrum of transmission makes a difference in the perceived brightness of the image.

If one eyepiece has a transmission of 99% at 750nm and the other a transmission of 89% at 750nm, that would be a profound difference in the lab, but pretty much invisible to the eye.

What is the transmission in the 450-550nm range?  That would be more telling because we could see it.

That is why we can see fainter blue-white stars than we can red stars, and the difference is large.

Eyepieces with types of glass that filter the violet-blue end of the spectrum will appear to be brighter on red giants, but have poorer transmission for the blue spiral arms of galaxies.

This is likely the case with lanthanum oxide glass, which tends to yellow the image.

 

Plus, focus of the star image plays a role.  Not all eyepieces have identical spot diagrams, i.e. focus the image to a point smaller than the Airy disc, leaving only scope size and seeing the determinant of the visibility of faint stars.

Lens polish enters into this, since a slight amount of light scatter may obscure the faintest stars.  It's the same with coatings.  If the coatings are poorly applied or not matched to the index of refraction of the glass, this can have

an effect on transmission and scatter.

And how the image in the eyepiece is modified by the steepness of the light cone from the scope can play a role in determining the sharpness of the image, and sharpness is also related to the ability to see faint details.

This is why excellent seeing seems to make small details in DSOs more visible.  Light transmission didn't change, but sharpness did.

 

And, there is the psychological effect of having a very dark field outside the field stop in a narrow field eyepiece that makes the object in the field appear brighter.  This doesn't work for all observers, but it does for some.

 

We equate the faintness of the image we see or can see as equating to transmission, but it isn't simply a matter of light transmission through the eyepiece.  The perceived brightness or dimness of a particular object in the eyepiece

is related to multiple factors.  Otherwise, you can't explain why a 4 element eyepiece with 2 groups (4 air-to-glass surfaces) is outperformed by a 5 element eyepiece with 3 groups (6 air-to-glass surfaces).

When we say "poor transmission" we are probably referring to multiple factors.

 

It's the same with contrast by the way.  Contrast has a definable lab measurement, but that is not how we use the term.  It, too, is related to a confluence of many factors.

 

[jetstream]

4 hours ago, Don Pensack said:

When we say "poor transmission" we are probably referring to multiple factors.

Agree that differences in "transmission" and "contrast" must have multiple factors involved.

These days I default to the try it and see method, wading through many eyepieces but there are a few that stand out- Delos and Vixen HRs on the top of the list as well as the Docter 12.5mm UWA.

Hyperwides have the Ethos, Nikon HW and the too good for its price 20mm APM at the front of the pack, to my eyes.

And then I throw a Circle T in the focuser and a smile appears- 50 bucks and razor sharp, contrasty views. We are lucky to have such a good selection of eyepieces available thats for sure.

[Mr Spock]

First clear lunar night tonight. The 4mm is really sharp and they eye relief is comfortable - better than expected. The jump from the 7mm (x171) to 4mm (x300) didn't seem that large. I'll still be looking for a 6mm and 5mm though