Ethos 13mm

[Stargazer_00]

First I'll start by saying that this eyepiece is amazing and I love the views it gives.  Hands down my favorite eyepiece I've ever used.

Now on to the purpose of the thread.  I've noticed in use that the outer field, say outer 25%, is noticeably brighter in both my scopes than the centre.  This is not evident in my Delos or zoom.  I haven't tried my 18/24 recently but I don't recall it in these either.

Can anyone explain what this is?  I used to see it in my old ES eyepieces before I sold them on and figured it was something in the design of the eyepiece which caused it, like an internal reflection or something.

Clearly this isn't what is happening on an Ethos eyepiece so I'm wondering if this effect I'm seeing is something else.  I'm thinking at this point it's actually the secondary shadow.  The secondary obstruction itself causing the centre field to be dimmer/darker than the outer, unobstructed, field.

Does anyone else get this same effect? 

[Damo636]

I have to say I've not noticed that effect with any of my Ethos Graham. I believe its referred to as edge of field brightening and the topic comes up quite often on Cloudy Nights. I wouldn't have thought it was anything to do with the secondary obstruction at that exit pupil but couldn't be sure?

Edit: Some interesting reading here...

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4861218/page/0/view/collapsed/sb/5/o/all/fpart/all/vc/1

[alan potts]

Graham,

I have never noticed it before but will be looking for it from now on. I think it is because the FOV is so wide it is show light polutions from the towns and cities on the horizons around you, what say you?

Alan

[Stargazer_00]

I think it is very likely to do with light pollution and it is very distinctive.

going to read that thread damo has linked and see if anything on there gives a clue to what it is

[Pig]

Very interesting, it will be nice to know the outcome guys.

[Moonshane]

this might be one reason I don't use it / like it on the moon. hate it on the moon in fact.

[John]

I've not noticed this effect with my Ethos 13 and I've had it for a couple of years now. I'll look out for it when it's next in the drawtube of my 12" newtonian. Seems to be an effect seen in a number of eyepiece types, from the thread that Damo links to above.

[Pig]

Quite a few interesting comments in Damo's link.

I like the way someone refers to it as a phenomenon