Weird "soft shadow" through eyepiece

[Aftoforakias]

Hello to the forum once again.

I have noticed something odd using my 20mm erecting eyepiece on my telescope (astromaster 130 eq) and although I've looked around,

I'm still searching for a suitable solution for my issue. Here are the details:

Looking straight through the eyepiece I get a soft-like "gradient" darkness/obstruction in the middle. It is not a well-defined shadow of the

secondary mirror like when I'm out of focus. I'm assuming it has to do with the secondary, but I'm not sure.

The focus itself is correct. On the edges of my view I still see well-focused objects but the obstruction is there in the middle.

Now, when I look into the eyepiece at an angle, the obstruction is gone.

Note that the obstruction seems to pan counter to where my view is panned when looking inside the eyepiece.

The end result is that I can never seem to find a "sweet spot" where I see the whole fov my eyepiece can provide, together with the distinct round

boundaries of the eyepiece. At best, I can see around 25-30% of the fov and the rest is obscured.

I do not have the same problem at all using my 10mm eyepiece.

I thought the 10mm would be more difficult to use, but this particular issue makes my 20mm almost unusable..

I am available for any clarifications. Please help!

Thanks in advance,

Spyros

[Ricochet]

Is this issue during the day or at night? If it is during the day, the "problem" is likely to be seeing the central obstruction in the 4mm exit pupil when your own pupils are not dilated to 4mm. 

[lenscap]

Hi Spyros,

The Celestron 20mm erecting eyepiece contains a set of "Amici" prisms which flip the image to allow terrestrial use, but which may be causing the problem you describe.

If you wish you can remove the slotted retaining ring at the bottom of the eyepiece and extract the cage containing the prisms. The eyepiece will then produce an inverted image, like any normal astronomical eyepiece, and the view will be much improved and noticeably brighter without the prism set.

The field of view will still be just as narrow though, about 30 degrees. Upgrading to an inexpensive Plossl eyepiece (52 degrees afov) would be a worthwhile improvement.

Clear skies!

[Aftoforakias]

@Ricochet I am not using my telescope during the day so I couldn't tell you if it appears then (but now that you mention it I should have tested it).

@lenscap Your advice makes sense. I will try removing the Amici prism since I'm not using it for terrestrial use anyway. It is nice to know that this will also improve my view..

It could be that the prism has moved from its initial position or there is some tiny debris producing this obstruction, right?

I don't mind the narrow fov, I just want to see the whole circle "frame" when looking down the eyepiece, with clear margins around it like all the other eyepieces I've ever used.

Thank you both for the fast replies!

I'll let you know as soon as I have some results,

Spyros

Edited May 22, 2020 by Aftoforakias

[Aftoforakias]

Update:

I just tested the eyepiece again in broad daylight and the same thing happens. I then removed the Amici prism and that seems to have fixed it!

The eyepiece now is only left with the eye lens and the doublet. The improvement in image quality is really significant.

(I did not really appreciate the fact that the prism seems to not be made of glass..)

The only difference is that now the eyepiece must not be all the way in its slot in order to get a clear focus with the telescope's stock focuser.

But it's pretty easy to pull the eyepiece back a bit and lock it in place there. (Really only pulling it out less than 2cm is enough.)

The eye relief doesn't seem to have changed, btw.

Finally I can see the whole frame! Thanks a ton!

Spyros

[Louis D]

4 hours ago, Aftoforakias said:

The only difference is that now the eyepiece must not be all the way in its slot in order to get a clear focus with the telescope's stock focuser.

Yes, you significantly shortened the optical path length through the eyepiece by removing the prisms.  As such, you're having to substitute empty air for that length by pulling the eyepiece out of the focuser a bit.

Glad to hear it also significantly improved the view through the eyepiece.

Here's a trick to see the secondary shadow more sharply.  Pull your eye back from the eyepiece in the daytime, and the secondary shadow should shrink with the field of view and sharpen up in the center.  If you move your head left, it should move right and vice-versa.