34 replies to this topic

[Kent10]

Where do you look in the field of view when you say it is sharper off-axis?  Do you look both left and right of the field center?  What about above and below the field center?  If it was cancelling some astigmatism, I would expect maybe the same "better sharpness" left and right of field center.  But then worse images above and below the field center.  Or vice versa, or some similar combination where opposites sides from the center give same result, and 90 degrees to those gives worse result.

I probably viewed in 2 directions wherever the handpad on the AP 1100 mount took me.  I'll try to be more aware of it and try all around next time I am out.

[stevenwav]

In my experience, I never saw this effect either - I will try looking for it as well. I know the original ads heralded sharp on-axis views.

[Kent10]

Thanks for sharing your experiences, too, Steve.  I will be glad if this is not consistently seen in all the Clave.  On the other hand, if the view is better and rivals my monos just slightly off axis that would be a good thing to know.

[Kent10]

Because it might be cloudy tonight, I decided to do some indoor testing.

 

I used my Tec 180FL on a DM-6 mount.

 

I started with some weaving on a curtain on the other side of the house.  I looked for a low contrast area.  The lighting was quite low because it was cloudy out.  I used the same 6mm Clave as I used outdoors under the night sky.  As with my tests outdoors, I thought I saw a brighter image and with more contrast when viewed slightly off-axis.  It was difficult for me to try many different areas off-axis because of the DM-6 mount so I mostly concentrated on one area off-axis.  Next, I used a 5mm Clave and saw the same.  Brighter and with more contrast off-axis.

 

Next, I used an artificial star.  As with outdoors under the night sky, the star appeared brighter and perhaps tighter slightly off-axis.

 

If I went too far off-axis but still not close to the edge, then I did not get this affect.  For me, it seems there is a sweet spot.  As I approached the edge, of course the contrast was poor and there were other aberrations.

 

Next, I used a $5 bill and concentrated on Lincoln's eye pupil.  There are white spots inside the pupil.  Same thing.  Brighter and easier to see the white spots slightly off-axis.  This was under low light.  Then I shined a light on the bill and I was not sure now that I could see a difference.  I assume that is because the contrast was greater.  It was much easier to see the white spots in the center.

 

Next, I printed a resolution chart and tried to find an area with low contrast.  That worked but for me the better results were with Lincoln's eye.

 

My results are not conclusive for me and I don't want anyone to feel I am done testing and only believe that all is better slightly off-axis.  But I do have more confidence that what I see is true ...  For me. 

 

I did a little more research and found another CN member who mentions better correction 30% off-axis in Claves and he has a 6mm listed in his signature.  I am going to contact him to ask if he has seen this affect or if he only read about it in Lord's article.  Will be interesting if he responds.

[Kent10]

A quick update.

 

It did clear tonight so I had some viewing.  I used the 5mm and 6mm Claves again as well as my Zeiss 6mm mono.  Seeing was not so good and so it was sometimes difficult to see any differences on and off-axis.  Early on I thought I saw differences in the brightness of stars on and off-axis.  It seems the brighter stars are easier to see a difference.  The double double was more difficult to see any difference perhaps because they are fainter stars.  That is why I wonder if differences are more easily seen in larger aperture scopes.

 

M13, M5 and M10.  Differences difficult to see probably because of seeing.  Though when using my Zeiss mono I easily saw more stars and they were more brilliant almost as if the 6mm mono was more like a 5.7.  When using the 5mm Clave I also saw more stars.

 

Another test is faint stars.  I found some areas in M13 and M11 that had faint stars.  Especially in M11 I thought off-axis in the Clave helped bring out those faint stars better.

 

Once again, just another test with no strong conclusions.  I did test all around the axis and the results seemed similar.

 

Still waiting to hear from our CN friend who has a 6mm Clave.

[RichA]

In my experience, I never saw this effect either - I will try looking for it as well. I know the original ads heralded sharp on-axis views.

It is very likely a problem in the human eye.  There have been reports of people seeing sharper in eyepieces when the eye is viewing off centre.  I remember one case where the sharpest view came for a person right on the edge which for simple eyepieces is impossible owing to off-axis aberrations in those eyepieces.

[Kent10]

I know I am stating the obvious, but you need quite decent seeing to see any differences.  Tonight the seeing was poor and it was difficult to see the airy disks.  I could not see any difference on or off-axis.

 

One thing I have been wondering.  Chris Lord describes the sharpness 30% off-axis for a reason.  Where does he get this?  I can't imagine he is just making it up.  Is it from a ray-trace diagram?  If only we could find and see where he is getting he results.

[stevenwav]

I know I am stating the obvious, but you need quite decent seeing to see any differences.  Tonight the seeing was poor and it was difficult to see the airy disks.  I could not see any difference on or off-axis.

 

One thing I have been wondering.  Chris Lord describes the sharpness 30% off-axis for a reason.  Where does he get this?  I can't imagine he is just making it up.  Is it from a ray-trace diagram?  If only we could find and see where he is getting he results.

 A little research online yielded this -  source is PEDRO RÉ -

 

 

"Chris Lord refers in his book: The true Plössl eyepiece, as manufactured by Carl Zeiss, and after W.W.II
by ets. Clavé and now Kinoptique, has the crown elements almost in contact (the separation can be
as little as 2 thousandths of an inch), and the eye doublet has a shorter focal length than the field
doublet. This widens the apparent field to 45˚ at f/6 at the expense of eye clearance (0.7Fe). In its best
form, this design is distortion free, and has no detectable lateral colour, even at f/4. Fields are dark
and ghost free, and contrast is excellent. However, unlike the Abbé Orthoscopic and its derivatives,

where longitudinal spherical correction is zero on axis, the asymmetric form of the Plössl leads to a
zonal correction and the sharpest imagery does not occur on axis but some 30% towards the edge of
the field of view. At low to medium powers this is of no consequence, but it is noticeable at high
powers (exit pupils less than 1.5mm) (…)."

Edited by stevenwav, Yesterday, 11:15 AM.

[Kent10]

Thanks, Steve.  A few years ago, when I started collecting Clave, I read all I could find and contacted several "experts."  I use them a lot but never noticed the better sharpness off-axis until a few days ago.  Then I recalled that I read something about that but couldn't recall where.  So I looked in my saved bookmarks and found Chris Lord's reference to the 30% off-axis.  Now I am trying to prove it to myself.  But I am wondering where Chris Lord got his information.  Did he see what I am seeing?  If only he would see this thread and share his thoughts.

[Kent10]

BTW, Chris Lord is a member of CN and used to post quite frequently.  Unfortunately for us, he hasn't posted in a while.