Tele Vue Dioptrx Astigmatism Corrector

[Grump Martian]

I am considering the purchase of a Televue 5 mm Nagler eyepiece. This would be used primerily on my 102 mm Starfield refractor. One of the reasons to choose a Nagler is that a Tele Vue Dioptrx Astigmatism Corrector can be attached. I would be interested to hear comments from others who use the astigmatism correctors. I have not been able to see the Cassinin division on Saturn the last couple of apparitions and wonder if these will improve the views.

Edited June 11, 2023 by Grump Martian
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[Zermelo]

tagging @Second Time Around

[Second Time Around]

Cheers.  Will post tomorrow when back home.

[Don Pensack]

9 hours ago, Grump Martian said:

I am considering the purchase of a Televue 5 mm Nagler eyepiece. This would be used primerily on my 102 mm Starfield refractor. One of the reasons to choose a Nagler is that a Tele Vue Dioptrx Astigmatism Corrector can be attached. I would be interested to hear comments from others who use the astigmatism correctors. I have not been able to see the Cassinin division on Saturn the last couple of apparitions and wonder if these will improve the views.

Is the 5mm Nagler DioptRx-compatible?

Yes, with the TeleVue adapter that allows it.

Do I recommend the DioptRx on the 5mm?

No, I don't.  The eyepiece has only 12mm of eye relief before the DioptRx is added and it reduces the eye relief to about eyelash length or less.

Either you pull your eye back every time you blink, or you get eyelash oils on the DioptRx every time.

Your scope is f/7.  A 5mm yields a 0.7mm exit pupil in that scope.

For you to need glasses at that focal length of eyepiece means your astigmatism needs to be 3.5 diopters or worse.

If you have that much astigmatism, you do need the correction.

But if you don't, the eyepiece would be usable without glasses (you simply refocus the scope for refractive error, either myopia or hyperopia or presbyopia).

 

The alternative, if you do have that much astigmatism, is to get an eyepiece that is usable with glasses on, like the 4.5mm Baader Morpheus, 5mm Baader Hyperion, 5mm Pentax XW, etc.

[Second Time Around]

Whether or not a Dioptrx will help depends on  the degree of astigmatism you have in your observing eye.  You can find this on your prescription under cylinder, often abbreviated to CYL.  Note that your left eye appears on the right hand side of the prescription and vice versa.  The higher the number, the greater the astigmatism.

It also depends on the exit pupil of the scope with a given eyepiece.  I assume that your Starfield refractor is an f/7, in which case a 5mm eyepiece will give an exit pupil of 5/7, i.e. less than 0.75.

As Don has pointed out, with such a small exit pupil your astigmatism would have to be fairly bad to need correction.  The graph on the link below will quantify this.  From this you'll see that your astigmatism would have to be about 3 dioptres or worse for a Dioptrx to be any use to you with an exit pupil of 0.75.

https://www.televue.com/engine/TV3b_page.asp?id=54&Tab=_Choose

At bigger exit pupils/lower magnifications you may benefit from a Dioptrx even if your astigmatism isn't very bad. Again the above graph will help here.

I'd add that it's not just Televue eyepieces that accept a Dioptrx.  Quite a few others do as well such as the Pentax 70 Deg XW and Baader Morpheus ranges.  However, that really needs a separate topic.

The same applies to whether to observe with glasses or a Dioptrx, for which there are pros and cons.  Don prefers the former while I prefer the latter.  A search (certainly on Cloudy Nights, but possibly here on SGL as well) should show our reasons and that of others.

Edited June 12, 2023 by Second Time Around

[Don Pensack]

3 hours ago, Second Time Around said:

Whether or not a Dioptrx will help depends on  the degree of astigmatism you have in your observing eye.  You can find this on your prescription under spherical, often abbreviated to SPH.  Note that your left eye appears on the right hand side of the prescription and vice versa.  The higher the number, the greater the astigmatism.

It also depends on the exit pupil of the scope with a given eyepiece.  I assume that your Starfield refractor is an f/7, in which case a 5mm eyepiece will give an exit pupil of 5/7, i.e. less than 0.75.

As Don has pointed out, with such a small exit pupil your astigmatism would have to be fairly bad to need correction.  The graph on the link below will quantify this.  From this you'll see that your astigmatism would have to be about 3 dioptres or worse for a Dioptrx to be any use to you with an exit pupil of 0.75.

https://www.televue.com/engine/TV3b_page.asp?id=54&Tab=_Choose

At bigger exit pupils/lower magnifications you may benefit from a Dioptrx even if your astigmatism isn't very bad. Again the above graph will help here.

I'd add that it's not just Televue eyepieces that accept a Dioptrx.  Quite a few others do as well such as the Pentax 70 Deg XW and Baader Morpheus ranges.  However, that really needs a separate topic.

The same applies to whether to observe with glasses or a Dioptrx, for which there are pros and cons.  Don prefers the former while I prefer the latter.  A search (certainly on Cloudy Nights, but possibly here on SGL as well) should show our reasons and that of others.

Spherical, or SPH, refers to distance correction.  This can be adjusted using your focuser.

Cylinder, or CYL is the astigmatism specification on a prescription.  This is what a DioptRx corrects.

[Second Time Around]

Whoops! Sorry, Don is right of course.  I'll correct my post.

[Grump Martian]

Thankyou all for the information. I have decided not to go this route.

[WatcherOfTheNightSkies]

I got one for my 2.0 cyl and have used it with a 32mm TV Plossl. I'm not a fan but perhaps it's "user error".  To get good focus, I have to take it out of focus, play with the rotation to makes the stars round, then focus back in. I think I must be tilting/rotating my head slightly when I look around, because I'm having to fiddle and can't get the feel of it. Perhaps this is more of a problem on an equatorial where the eyepiece orientation changes object to object, and because I'm quickly changing from low to high power and back; it feels like an annoyance to keep tweaking. For the use case of a Dob/AltAz and staying many minutes on an object without changing the eyepiece, I'm sure it's better. 

[Don Pensack]

Every time you turn your head to look through the eyepiece at a different angle, especially in a dob where the angle changes every time the scope moves in altitude, you have to turn the DioptRx to correct the angle.

You should be able to do that with in focus stars as well as out of focus stars.

[Second Time Around]

I think glasses v Dioptrx is very much a matter of personal choice. Don for instance has valid reasons for prefering glasses. Depending on one's eyesight there are pros and cons of each, and what's best for one person may not be the best for another. 

However, I wonder how many have tried a Dioptrx? And of those, how many have actually done a head to head comparison?

I have, and these are the reasons I prefer a Dioptrx:

1) The coatings are better than on even the top of the range Rodenstock lenses on my glasses.

2) The polish is almost certainly better than that of glasses - few people would pay for this on glasses so almost no demand, so no supply.

3) My glasses have plastic lenses that scratch more easily than the glass lens in a Dioptrx.

4) The angle of my astigmatism changes between my annual eye tests so my glasses become less sharp. With a Dioptrx I can adjust this angle at the telescope.

5) I find that, like many, I need an extra 0.25 correction for astigmatism at night.  So I'd need to buy an extra pair of glasses for night time use.

6) I wear my glasses on a cord and have no problems taking them on and off for looking at the sky - it quickly becomes automatic.

7) I have no presbyopia and so don't need glasses at all for looking at close range objects such as screens or maps.

😎 I have a Dioptrx on my finderscope eyepiece, but even without this I don't find astigmatism critical on a finder as it's doing just that - finding.

9) On the rare occasions I share my scope I wear my glasses.

10) And most of all, and this is the clincher, having done head to head tests I found that I can see more with a Dioptrx than with my glasses