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Equipment Discussions >> Eyepieces

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Kent10
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Reged: 05/08/12

Dark Spot in Pentax XO 5mm
      #5530861 - 11/20/12 07:16 PM

Hi:

I tried my new Pentax XO 5mm today on the sun. I was comparing it with my Pentax XW 5mm. I noticed in the XO only that there was a soft round dark spot directly in the middle about half the diameter of the field of view though my eye wasn't close enough to get the entire field. I didn't notice it in the XW. I am using a Tec 160FL refractor so it isn't a central obstruction. I tried many other EP's and didn't notice the same. Does anyone know what this is. I looked at the moon yesterday and don't recall seeing it and I may try again tonight. I was using a Baader Herschel Wedge for the sun. Thanks, Kent


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Deep13
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Reged: 01/25/05

Loc: NE Ohio
Re: Dark Spot in Pentax XO 5mm new [Re: Kent10]
      #5531016 - 11/20/12 08:50 PM

Dust speck?

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George Blahun Jr
super member


Reged: 08/28/07

Loc: Republic of Quaker Hill, CT
Re: Dark Spot in Pentax XO 5mm new [Re: Deep13]
      #5531128 - 11/20/12 09:51 PM

Kent, I have not experienced the condition you describe when using my 5mm XO pair in my MKV binoviewer and looking at any object, including the sun with my Baader wedge and TEC 140. I normally pull back a bit from the EP to avoid eyelashes hitting the lens. Could it be a floater in your eye? Have you tried viewing with your other eye?

George


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Kent10
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Reged: 05/08/12

Re: Dark Spot in Pentax XO 5mm new [Re: George Blahun Jr]
      #5531140 - 11/20/12 10:01 PM

Thanks guys. I don't think it is a dust speck because the EP is brand new and I checked it for dust specks. Also the dark spot is a perfect circle. I just got in from viewing the moon and could not see the spot this time. So I will have to test it again on the sun to see if I can figure out what it is. George, I was pulling back and trying not to blink. I also tried both eyes and it was the same. It was not a floater because the spot was perfectly round. It was faint but noticeable. Thanks again. I am going to figure it out eventually. I tried the 2.5 XO which I also have but it was not usable today with the seeing. I tried it on the moon and it was better but that high power was not worth using. I am not sure how often I will get to use it but it is nice to have and try from time to time.

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Kent10
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Reged: 05/08/12

Re: Dark Spot in Pentax XO 5mm new [Re: Kent10]
      #5531141 - 11/20/12 10:02 PM

George you use the XO as a pair. Wow. I know people recommend not going below 10mm in the BV. How do you find the 5mm in the BV. Any problems? Thanks.

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George Blahun Jr
super member


Reged: 08/28/07

Loc: Republic of Quaker Hill, CT
Re: Dark Spot in Pentax XO 5mm new [Re: Kent10]
      #5533133 - 11/21/12 11:34 PM

Kent, I have had no problems with the 5mm XO in the Baader MK V. My Kokusai Kohki 8mm will occasionally require me to rotate one EP to get a perfect merge, but it's usually just a slight turn off of what I believe is either a slightly high or low point on one of the EP barrels. The other evening CT had a rare excellent seeing night and I could make out detail inside and around the GRS while viewing Jupiter with the 5mm XO.

George

Edited by George Blahun Jr (11/22/12 10:33 AM)


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astrodon
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Loc: Portland, OR, USA
Re: Dark Spot in Pentax XO 5mm new [Re: George Blahun Jr]
      #5533973 - 11/22/12 01:34 PM

I wonder if the lenses are too tight inside and some which are supposed to be air-spaced are making contact? Or maybe a spacer is missing and the lenses are making contact? I had this happen with a 20mm Celestron Chinese Silvertop once.
Diagram of the lens third item in Bill P's write-up on the 5mm XO:
Bill P on the 5mm XO


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BillP
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Reged: 11/26/06

Loc: Vienna, VA
Re: Dark Spot in Pentax XO 5mm new [Re: Kent10]
      #5534087 - 11/22/12 03:21 PM

Kent,

Take a look at this thread, then perform the same test and see if you see a darker area in the middle. FYI, I do not with my XO.

If you look into the eyepiece and do not see any spots on any of the lenses, then another thing to check is if there is any spot or grain of grit anyplace within your diagonal/wedge. Maybe there is a small grain of dirt just in the wrong spot. Since many eyepieces, when placed in a focuser, do not always align precisely the same, it is easy enough for one eyepiece not to have a small piece of grit on the diagonal show, and another to have it show. It's a long shot, but gotta check everything.

Also, as George pointed out, rotate the eyepiece in the focuser. If it is eyepiece related then the spot will rotate around also.

Edited by BillP (11/22/12 03:22 PM)


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Kent10
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Re: Dark Spot in Pentax XO 5mm new [Re: BillP]
      #5534691 - 11/23/12 12:52 AM

Bill/Astrodon: Thanks for the ideas. I am out of town at the moment. Happy TG! But when I get back and have time I will try what you suggest. Now that I think of it, I did use my rocket blower on my diagonal because I saw a black piece of something inside. Maybe that was it but I will do the other test against white as well and check again when I view the sun. I appreciate your help.

George: I wish I could get some good seeing. I have dark skies but Jupiter is above my house and I think the seeing is not so good right now.


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Kent10
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Reged: 05/08/12

Re: Dark Spot in Pentax XO 5mm new [Re: Kent10]
      #5551304 - 12/02/12 11:52 PM

I wanted to update everyone on my progress. I viewed the sun again today. I found out this faint round black spot in the middle occurs on other eyepieces as well as the 5mm XO. I think it was just more obvious on the XO because of the smaller FOV perhaps and maybe the higher power. Also it was a smaller circle so easier to see than in the 10mm Pentax XW. So then I thought it might be the Baader Wedge that I am using that is producing the spot or perhaps it is a reflection of my pupil. I was using the continuum filter which is reflective and wondered about that too. Tried the wedge without that filter but the spot was still there. I then took the wedge apart to look inside to clean it. It is fairly new so there is not much dust in there but I did blow it with a rocket blower.

What I did notice however was some hazing inside the wedge below the top piece of glass. So I do not have access to it. I had a thread in the solar forum about my wedge producing smoke when I tried it for the 1st time. It no longer smokes. I guess something was burned off but I wonder if this hazing is a result of the smoke. I will try to find out on the solar forum if anyone sees a haze in their wedge. It doesn't look like I could clean it myself so I will have to decide what to do if the haze is a problem.

Thanks.


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GaryJCarter
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Reged: 06/06/09

Loc: Fairview, Texas
Re: Dark Spot in Pentax XO 5mm new [Re: Kent10]
      #5551337 - 12/03/12 12:37 AM

I believe what you are experiencing is the perception of the blind spot and rod-free area of the fovea when using higher magnification eyepieces on very bright objects that fill the majority of the FOV. I have experienced this with certain eyepieces on white light solar, Ha solar, and lunar viewing.

http://www.cis.rit.edu/people/faculty/montag/vandplite/pages/chap_9/ch9p1.html

Edited by GaryJCarter (12/03/12 12:37 AM)


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Kent10
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Re: Dark Spot in Pentax XO 5mm new [Re: GaryJCarter]
      #5551850 - 12/03/12 11:10 AM

Gary:

Interesting. Thanks for the link.


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