Strange artifact in multiple eyepieces

[vlaiv]

I'll try to describe what is happening, because it is quite strange, I believe it is not associated with any particular eyepiece (so Mods, please move this topic if necessary), nor scope for that matter.

I've noticed a strange thing in my high power eyepieces under certain circumstances. I was observing Moon tonight with 7mm TMB clone (not sure if it is TS or SW model - no labeling on it) and with 11mm ES82. Scope used was SW ST102 F/5 achro. Barlow used was APM x2.7 barlow. And this artifact was quite pronounced tonight. I've seen it before but I always assumed it is smudge on the eye lens of eyepiece. I have long eyelashes and I often end up with a bit of grease here and there. But tonight I decided to try and check what was all about. I cleaned both eyepieces in the middle of the session - some of the light scatter in couple zones went away but effect was still there - so it is not the dirt. It is not due to floaters either.

Let me describe it: It looks like sort of vignetting but in center of the field - a small zone around 5 degrees (apparent) wide, where view is just a bit darker then surrounding area. Also when I try to place that bit on anything that I'm observing it looks like there is slight blurring of the subject but I can't be sure because thing moves when I move my eye. It moves in opposite direction from eye movement - as one would expect if something is present on eye lens.

Both eyepieces displayed this behavior, but only when using barlow. Not sure if it is related to magnification, and if it still there but much less pronounced when eyepiece is used without barlow. It is not like kidney bean - that can be completely dark, this effect is always such that you can see thru it, but image in that zone gets darker.

Then I started thinking at what times previously and with what eyepieces have I seen it before. I'm certain that another eyepiece is affected with this - Baader BCO 6mm. When I was observing Sun in white light with Hershel wedge in this scope it was really visible and bothered me. It also appeared in 8" F/6 dob with 7mm TMB clone when observing Jupiter, but much less noticeable than with Moon and Sun.

One more thing I should mention - It looks like it is not always exact center of the field that is affected. I noticed this tonight. If I rotate eyepiece - it keeps exact position, but if I move scope so that moon moves in FOV, it changes position a bit. It is still almost at center but behaves like some sort of reflection - it tends to move a bit to opposite than moon is related to center of FOV.

Only thing that I'm not quite sure if was always present and can be source of the problem is barlow lens. I think that I saw it with two different barlows - GSO simple x2 - model that you can unscrew barlow element, and APM x2.7 - also removable element. Only thing that might be the same all over if every time this has appeared is barlow "body" - I use same barlow housing from GSO but with 1.25" extension and APM x2.7 element at the moment.

So closest thing to a "solution" to this mystery is that barlow housing is not properly blackened and has some reflections - but I would expect flare and brightening in general rather than darkening of center and slight blurring in that zone.

Anyone has any idea what this could be?

[Paz]

I don't know what it is but reading your notes I thought what you suggest in the last paragraph. Are you able to try rotating the Barlow whilst keeping everything else constant and see if it moves with the Barlow.

Another thought was Barlows increase eye relief, so could it be your eye is slightly  in front of the exit pupil when the Barlow is in? You could try moving  the distance from eye to eyepiece directly in and out and see if it comes and goes with distance.

My only other thought is have you got a shorter focal length eyepiece than 7mm that does not have this problem. If so would that rule out certain potential culprits (I don't know if it would, I'm just wondering).

[vlaiv]

13 minutes ago, Paz said:

I don't know what it is but reading your notes I thought what you suggest in the last paragraph. Are you able to try rotating the Barlow whilst keeping everything else constant and see if it moves with the Barlow.

Another thought was Barlows increase eye relief, so could it be your eye is slightly  in front of the exit pupil when the Barlow is in? You could try moving  the distance from eye to eyepiece directly in and out and see if it comes and goes with distance.

My only other thought is have you got a shorter focal length eyepiece than 7mm that does not have this problem. If so would that rule out certain potential culprits (I don't know if it would, I'm just wondering).

I did not try to rotate barlow (unfortunately clouds just rolled in and the moon is setting so I can't test it now), but I did try to back off and spot doesn't change. Well it does change but in rather strange way, when I back off, apart from image shrinking in the small circle and spot also but not to proportion - it then covers more and more of visible FOV (maybe it is just that FOV is getting smaller because of distance), streak like features also become apparent - also darker like the spot.

It looks a bit like this (it reminded me of it so I looked up such image on internet):

So imagine replacing blue/red glow with actual moon image, and center ball being darker area, and "lightning" spikes as also being darker (but still see thru). "Lightning spikes" are not as numerous as in image, and are a bit thicker than these in image, but shape (not straight but have waviness to them and originate at central dark spot and go outwards) pretty much looks like that.

[John]

The only time I've experienced something like that effect was when I was testing eyepieces and found that a 7mm Baader Genuine Ortho had a similar "zone" to the one that you describe towards the centre of it's field of view.

On closer examination I found that, due to a fault in manufacture, the lens groups within the eyepiece were touching instead of being separated by a small air space. A thin lens spacing ring had been omitted allowing the curved surfaces of the two lens groups to be in contact.

 

[25585]

I have had a similar anomaly with one side of a pair of binoculars. I tried both eyes, the thing was seen both times. 

What caused it I finally realised, was an area of condensation that formed in the middle when cold - back in a warm house it disappeared, but it reappeared next cold outing.