Curious abut blue colour from Eyepieces

[djpaul]

Hi All

 

Not been on here in ages but i finally got a chance to go out tonight with my scope

I have noticed when viewing the moon this evening with both of my Delos 10mm and 17.3mm and Sky watcher 32mm wide view eyepieces i get a thin blue line on the edge of the field of view from the moon.

Infact the sky watcher starts to get the line before it reaches the edge of the field of view

I then tried my Takahashi and Teleview 3-6mm zoom and i dont get the same thin blue line.

 

Im just curious as to what causes this and wondered if its just because they are wide view eyepieces

 

Any ideas what causes this

 

Paul

[John]

Many wide angle eyepieces show a thin bright rim around the field stop. It's know as the "ring of fire" effect although it's never that prominent I've found. Naglers show it, Ethos do to a much lesser extent. I've also seen in in other brands UWA and hyper wide eyepieces as well. It only seems to be there when viewing in daylight or when viewing a bright, extended object. I believe it's caused by diffraction of some type from the edge of the field stop

[John]

Here is a link from another forum that sheds a little more light on this (ha ha !):

http://www.cloudynights.com/topic/530975-what-actually-causes-ring-of-fire-in-31-nagler-and-others/

[djpaul]

Many thanks for replying John

Dont get me wrong the delos and skywatcher panaview are amazing eps, but i just wanted an answer as to what causes this

funny i hadnt noticed this before as i have had these eps for over a year now.

Actually i remember buying my first decent eps from you about 2 years back ....TV plossls 15,20mm and 32mm and my ( 2.5 powermate which i still have )

Paul

[YKSE]

As John has stated, many wide field eyepieces have this thin color ring at the field stop, some thiner or fainter than others. Bill Paolini's review on Delite 5,9 and 15mm has some picutures showing this color ring in some eyepieces.

[Don Pensack]

The thin ring of color at the edge is due to an aberration caused by the coatings that results in a thin line right at the field stop.

It can be blue, or green, or amber, or some other color, depending on coatings used.  I don't recall seeing a red, yet.

Multi-layer coatings result in this, and it is, more or less, unavoidable in wider field eyepieces.

This is NOT the same as what is referred to as the "Ring of Fire" noted in eyepieces like the Tele Vue 31mm Nagler. which is a broad swath of field (about the last 10° of field in that case)

that is colored orange (a strong tint).  It is a chromatic aberration of the exit pupil and is different than the thin ring of color at the field stop.

That makes the 31mm Nagler inappropriate for use as a daytime eyepiece, and may or may not bother some observers on the Moon.

For all other deep-sky use, the "Ring of Fire" is essentially unnoticeable.  There are other eyepieces with this same issue, but the 31mm Nagler was the one after which the phenomenon was named.

[John]

1 hour ago, Don Pensack said:

The thin ring of color at the edge is due to an aberration caused by the coatings that results in a thin line right at the field stop.

It can be blue, or green, or amber, or some other color, depending on coatings used.  I don't recall seeing a red, yet.

Multi-layer coatings result in this, and it is, more or less, unavoidable in wider field eyepieces.

This is NOT the same as what is referred to as the "Ring of Fire" noted in eyepieces like the Tele Vue 31mm Nagler. which is a broad swath of field (about the last 10° of field in that case)

that is colored orange (a strong tint).  It is a chromatic aberration of the exit pupil and is different than the thin ring of color at the field stop.

That makes the 31mm Nagler inappropriate for use as a daytime eyepiece, and may or may not bother some observers on the Moon.

For all other deep-sky use, the "Ring of Fire" is essentially unnoticeable.  There are other eyepieces with this same issue, but the 31mm Nagler was the one after which the phenomenon was named.

Thats interesting Don. I've seen a very thin ring of colour around the field stop of quite a few wide and ultra wide eyepieces but I've not noticed a wider band of field tinted even with my Nagler 31. To be fair though I don't think I've used it in daylight or on the Moon other than perhaps long ago when I first got the thing