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Equipment Discussions >> Eyepieces

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nanjason
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Dioptrix needed? new
      #5837981 - 05/03/13 10:14 AM

Looking for some help from you experts!

I'm thinking I may need one of those dioptrix astigmatism correcting things for my new 31 Nagler. From what I'm reading, if a person has astigmatism it is more pronounced with long focal lengths and larger exit pupils. I did notice some slight star blobing towards the edge of the eyepiece the other night, which I don't see with my shorter focal length eyepieces. I am thinking that I don't notice the problem with the shorter FL's because of the smaller exit pupil. (this was all with my NP101). So my thought was to get a dioptrix for the 31 Nagler.

I wear contact lenses normally, but I usually do not have them in when I'm at the telescope. I do this simply because it's late in the day, I'm tired, and they just feel dried out at that point. Here is my prescription for my right (dominant) eye:

sph: -0.75
cyl: -0.75
ax: 130
BC: 8.7
dia: 14.5

So I'm guessing I need a 0.75 power dioptrix, correct? Do any of you use them, and would you recommend it?

Also, if I have the dioptrix on the EP, and someone comes along and wants to take a look and they don't have astigmatism, would it affect their view? Would I need to remove it for them?

Thanks!

Edited by nanjason (05/03/13 10:15 AM)


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GlennLeDrew
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Re: Dioptrix needed? new [Re: nanjason]
      #5838055 - 05/03/13 10:42 AM

In my dominant eye I also have 0.75 diopters of astigmatism (0.5 in the other). It does affect sharpness at larger exit pupils, but I don't bother with correcting for it. Actually, as I mostly use binoculars, I direct my gaze a small distance above or below the field center, using the eyepiece's own astigmatism to cancel out mine; works very well!

Edge-of-field 'blobbing' which is not apparent at the field center is instrumental in origin, and not due to your eyes. When assessing your peepers, use the field center ONLY, for that is where the image is best. Off axis you will find telescope- and eyepiece-induced aberrations.

A person suffering no astigmatism will have 0.75 diopters worth imposed when peering through your Dioptrix-fitted eyepiece.


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Jim Romanski
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Re: Dioptrix needed? new [Re: nanjason]
      #5838087 - 05/03/13 10:55 AM

I have 2.00 diopters of astigmatism in my eyes so I've used both the Dioptrx and Eyeglasses to correct for it. Optically, I see the best results with the Dioptrx. But I don't use my Dioptrx much anymore. Instead I have glasses that I use strictly for astronomy. They sit close to my eye so that I can see the entire FOV with an Ethos. I prefer observing this way because it's a hassle taking glasses on and off to between looking up at the sky and looking through the eyepiece.

You will notice a difference with the Dioptrx but not as much as I do since I have so much astigmatism.


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Jim Romanski
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Re: Dioptrix needed? new [Re: GlennLeDrew]
      #5838101 - 05/03/13 11:02 AM

Quote:

Edge-of-field 'blobbing' which is not apparent at the field center is instrumental in origin, and not due to your eyes. When assessing your peepers, use the field center ONLY, for that is where the image is best. Off axis you will find telescope- and eyepiece-induced aberrations.



I'm not sure I agree with this. Any aberrations will be multiplied by your own astigmatism. As good as the Nagler 31mm and NP101 are (I've owned both myself) they're not perfect and off axis is where you'll see a tad of "non-perfection". Your astigmatism will make this look worse. Also, wouldn't the light rays coming from the center of the field go through the center of your eye lens which is where you have less astigmatism?

Quote:

A person suffering no astigmatism will have 0.75 diopters worth imposed when peering through your Dioptrix-fitted eyepiece.



You don't have to mount the Dioptrx on top of the eyepiece. You can hold it in place. But it is a bit of a hassle when viewing with other people to take it on and off.


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SteveG
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Re: Dioptrix needed? new [Re: Jim Romanski]
      #5838403 - 05/03/13 01:31 PM

I use Dioptrx with any eyepiece that produces an exit pupil over 2, 2-1/2 mm. That is where I will see astigmatism - it is dependent on your astigmatism and exit pupil of any given eyepiece/scope combo. For me it is typically anything above about 20 mm.

It is really amazing that when I twist the Dioptrx, the sharp stars appear at a very precise point (and repeats at 180 deg from that point). I find it to be a perfect solution as opposed to wearing glasses.

The Dioptrx formula is as follows:
Your astigmatism correction (left column) vs the exit pupil (right column). Example: with 1.0 astigmatism in your eye, you would see it at exit pupil of 2mm and above.

Astigmatism -- Exit Pupil
.5 ------------- 4mm
.75 ------------ 3mm
1.0 ------------ 2mm
1.5 ---------- 1.5mm
2.0 ------------ 1mm

Edited by SteveG (05/03/13 01:33 PM)


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Starman81
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Re: Dioptrix needed? new [Re: SteveG]
      #5838427 - 05/03/13 01:48 PM

Quote:


It is really amazing that when I twist the Dioptrx, the sharp stars appear at a very precise point (and repeats at 180 deg from that point). I find it to be a perfect solution as opposed to wearing glasses.





+1, I really love it. It has allowed me to keep the 13 Ethos in the stable even with the insufficient (for me) eye relief. With practice, I manage to not even put any eyelash marks on the DIOPTRX during a session. Also, with the DIOPTRX option, I can dream of a 21 Ethos!


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GlennLeDrew
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Re: Dioptrix needed? new [Re: SteveG]
      #5838430 - 05/03/13 01:50 PM

Jim,
Eye-induced aberrations (like astigmatism) are not multiplicative. One's astigmatism merely adds to the aberrations introduced by the instrument. At some point, the other aberrations dominate, rendering the eye's contribution as of no import. If the astigmatism on-axis is of a certain magnitude, it will be the same over the full field. If the off-axis aberrations coma, eyepiece astigmatism and defocus due to field curvature are present, the eye's own astigmatism can be rendered invisible toward the field edge.


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GlennLeDrew
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Re: Dioptrix needed? new [Re: GlennLeDrew]
      #5838677 - 05/03/13 04:13 PM

I should be more explicit. Suppose one's astigmatism elongates a point into a 2 arcminute 'line.' If the optics are perfect, the same elongation would be observed over the full field. But in the real world there is often worsening blur/ aberration as one looks farther off axis. It's not uncommon for this to amount to 10, 20 or more (!) arcminutes near the field edge. In such cases, a 2 (or 5) arcminute contribution from one's eye is not significant.

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faackanders2
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Re: Dioptrix needed? new [Re: Starman81]
      #5848824 - 05/08/13 09:43 PM

Syed, Next time we observe together I'd like to look through your dioptrix. I have +75 in one eye and +50 in the other. I assume if you are within 25-50 (of my eyes) I should see better with it(yours).

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Starman81
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Re: Dioptrix needed? [Re: faackanders2]
      #5850357 - 05/09/13 04:04 PM Attachment (7 downloads)

Quote:

Syed, Next time we observe together I'd like to look through your dioptrix. I have +75 in one eye and +50 in the other. I assume if you are within 25-50 (of my eyes) I should see better with it(yours).




No dice, Ken. My astigmatism is 3.00 cylinders (!). You are good up to 3 or 4mm exit pupil (see graph below) depending on which eye you use to observe with. I found this graph to be be reasonably accurate for myself personally. So maybe you would want to try it on your ES82 30 or other longer focal lengths only.


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SteveG
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Re: Dioptrix needed? new [Re: Starman81]
      #5852026 - 05/10/13 01:24 PM

That graph is where I derived my list posted above. Funny, I'm finding the numbers don't quite work for me. With 1.0 diopters, I don't see it at exit pupil of 1.5 mm and below - that's good as I should see it at 2 mm and below !

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Starman81
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Re: Dioptrix needed? new [Re: SteveG]
      #5852084 - 05/10/13 01:58 PM

Quote:

That graph is where I derived my list posted above. Funny, I'm finding the numbers don't quite work for me. With 1.0 diopters, I don't see it at exit pupil of 1.5 mm and below - that's good as I should see it at 2 mm and below !




Steve, perhaps I am not undertanding you correctly. With 1.0 diopters, you should only see the effects of your astigmatism at exit pupils of 2.0mm or greater. So not seeing it at 1.5mm is expected. If you have an eyepiece that is DIOPTRX capable closer to the 2.0mm exit pupil, try it and see what results.


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russell23
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Re: Dioptrix needed? new [Re: Starman81]
      #5852489 - 05/10/13 06:36 PM

The graph seems to work for my eyes. I've recently discovered that I can accomodate a larger exit pupil than I realized before astigmatism becomes problematic. At 80x with my 140mm scope (1.75mm exit pupil) the view is sharp without my glasses. That seems to fall right on the line for my 1.25 diopters of astigmatism. At 70x (2.0mm) the view is no longer as sharp.

The odd thing is that the 14mm ES100 is sharp at 57x in the same scope (2.5mm exit pupil). I've been trying to understand why that would be the case.

Dave


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Cotts
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Reged: 10/10/05

Loc: Toronto, Ontario
Re: Dioptrix needed? new [Re: russell23]
      #5854020 - 05/11/13 03:32 PM

When using my 16" f/5 Teeter/Zambuto I use a dioptrix of 0.75 diopters on my 31 Nagler (Exit Pupil 6mm) and my 17 Ethos (Exit Pupil 3.4mm) with excellent results. In combination with the Paracorr 1 my views with these two eyepieces are stunning - sharp, pinpoint stars from one edge of the field to another.

CAVEAT: The dioptrix covers a bit of the outside edge of the 100 degree field of the Ethos - the Nagler's 82 degree field is not affected.

Dave


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Starman81
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Re: Dioptrix needed? new [Re: Cotts]
      #5854194 - 05/11/13 05:52 PM

Quote:

When using my 16" f/5 Teeter/Zambuto I use a dioptrix of 0.75 diopters on my 31 Nagler (Exit Pupil 6mm) and my 17 Ethos (Exit Pupil 3.4mm) with excellent results. In combination with the Paracorr 1 my views with these two eyepieces are stunning - sharp, pinpoint stars from one edge of the field to another.

CAVEAT: The dioptrix covers a bit of the outside edge of the 100 degree field of the Ethos - the Nagler's 82 degree field is not affected.

Dave




Dave, I tried using the DIOPTRX with the Ethos while using it as a finder and it didn't work for me. The reason was, as you are scanning the sky, moving my dob in alt (up/down) changes the angle that you interface with the DIOPTRX. That causes it to lose its proper alignment, if you are scanning large swathes of sky. Since your astigmatism in much more minor than mine, this might not be a bother to you.

About the second point, the DIOPTRX covering the edge of the Ethos, what do you mean? Are you seeing vignetting of the field? I am pretty sure I can see the field stop unimpeded, when I do try to look that far.


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Cotts
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Re: Dioptrix needed? new [Re: Starman81]
      #5854376 - 05/11/13 08:43 PM

As for adjusting the Dioptrix when the elevation changes it is a very minor tweak of the rotation to get it perfect. I don't tend to zoom all over the sky anyway.

The vignetting occurs with the 17mm Ethos. I can't get my eye in close enough to see the field stop with the dioptrix in place. I would estimate the eyepiece becomes a 95 degree field....

The fact that this affects me but not you is probably down to anatomical differences between our noses, orbital bones and eye depth.....

Dave


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SteveG
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Re: Dioptrix needed? new [Re: Starman81]
      #5855863 - 05/12/13 04:38 PM

Quote:

Quote:

That graph is where I derived my list posted above. Funny, I'm finding the numbers don't quite work for me. With 1.0 diopters, I don't see it at exit pupil of 1.5 mm and below - that's good as I should see it at 2 mm and below !




Steve, perhaps I am not undertanding you correctly. With 1.0 diopters, you should only see the effects of your astigmatism at exit pupils of 2.0mm or greater. So not seeing it at 1.5mm is expected. If you have an eyepiece that is DIOPTRX capable closer to the 2.0mm exit pupil, try it and see what results.




Wow I really got my terms and numbers turned around!

What I meant was that I seem to only need correction at about 2.5 mm exit pupil and up. I use a tuneable top Paracorr at focal lengths of about 20mm and up.


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Starman81
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Re: Dioptrix needed? new [Re: Cotts]
      #5856009 - 05/12/13 05:57 PM

Quote:

As for adjusting the Dioptrix when the elevation changes it is a very minor tweak of the rotation to get it perfect. I don't tend to zoom all over the sky anyway.

The vignetting occurs with the 17mm Ethos. I can't get my eye in close enough to see the field stop with the dioptrix in place. I would estimate the eyepiece becomes a 95 degree field....

The fact that this affects me but not you is probably down to anatomical differences between our noses, orbital bones and eye depth.....

Dave




Yes, definitely physiological differences at play here. Things are getting a little "chippy" in a few threads here on CN lately so thank you for your very civil answer.


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azure1961p
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Re: Dioptrix needed? new [Re: Starman81]
      #5856097 - 05/12/13 06:49 PM

Lol so true.

Pete


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Cotts
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Re: Dioptrix needed? new [Re: Starman81]
      #5858855 - 05/13/13 11:08 PM

Quote:

Quote:

As for adjusting the Dioptrix when the elevation changes it is a very minor tweak of the rotation to get it perfect. I don't tend to zoom all over the sky anyway.

The vignetting occurs with the 17mm Ethos. I can't get my eye in close enough to see the field stop with the dioptrix in place. I would estimate the eyepiece becomes a 95 degree field....

The fact that this affects me but not you is probably down to anatomical differences between our noses, orbital bones and eye depth.....

Dave




Yes, definitely physiological differences at play here. Things are getting a little "chippy" in a few threads here on CN lately so thank you for your very civil answer.




Are you sure? I may have just said you have a big nose!!!!!

Dave


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