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Glasses, eye relief, astigmatism ...


Froglord

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I'm new to astronomy - just bought a Skymax 127 and I only have the included Super MA eyepieces (25mm and 10mm) which I'm looking to upgrade.

So far, I have been observing without my (varifocal) glasses because it seemed more natural. Then I remembered I have astigmatism in both eyes, so perhaps I should be putting on the specs after all! (I might try a slightly older, non-varifocal pair).

I think I wear my glasses slightly further away than many people do (comfort, eyelash issues), so what sort of eye relief will I need? 20mm? Even more? I was looking at the much-recommended BST Starguiders (I'm not looking to pay Tele Vue money) but will I find the 16mm eye relief too tight?

Also, will I really get the benefit if I splash out on a wide-field 68 degree eyepiece, or will my glasses restrict my FOV?

Any moderately-priced eyepiece recommendations gratefully received ...

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Good questions! (and welcome!)

Using spectacles will always reduce the apparent field of view (AFOV) so ideally you won't want to wear them, but....

Its annoying to keep removing them!
You may need them to correct astigmatism.

Whether you need them for astigmatism depends on how bad yours is. I would say up to 1 diopter cylinder is OK without correction, but others will differ. Also the exit pupil size is critical. At high power, with say an exit pupil of 1 or 2 mm, correction probably isn't needed.

Varifocals are difficult to use. You must make sure you are looking completely through the distance section, even then it may not be good. Plus they are often made from plastics that can produce significant chromatic aberration.

Best if you can try a wide AFOV eyepiece before you 'splash' out. Its very personal. Without my spectacles I can use a 100 deg AFOV with 15mm eye relief. With the spectacles I reckon I can see only half the field. I take my specs off as I really want the full field.

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40 minutes ago, AstroKeith said:

At high power, with say an exit pupil of 1 or 2 mm, correction probably isn't needed.

That's very useful info, as the Mak 127 can't go much higher than 2mm. That may be why I haven't particularly noticed bad views.

My astigmatism is 0.75 in one eye and 1.00 in the other, so right on the borderline.

Edited by Froglord
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Although not an observer as such, so can't advise on specific eyepieces, I do just enough visual with bins to have my optician provide me with a set of non-varifocal distance glasses, since everything I want to look at is at infinity. I have quite a complex prescription and find that even for casual "just looking around" they provide a sharper view than my varifocals.

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I bought a pair of distance only eyeglasses online from EyeBuyDirect for $20 5 years ago.  They rarely need cleaning, so they don't get much in the way of microscratches that my daily wear bifocals get.  That, and the entire field of view is in focus at the same time with no intrusive line.  I've never tried varifocals, but I can't imagine how they can show a 100 degree AFOV all in focus at once.  I bought the lowest index plastic lenses because they introduce the least chromatic aberration at the edges of the view when not looking directly on-axis.

5 hours ago, AstroKeith said:

Using spectacles will always reduce the apparent field of view (AFOV) so ideally you won't want to wear them, but....

Not true with long eye relief (>18mm usable ER) eyepieces for most folks.  I have a tall nose bridge that also limits how wide of an eyepiece top I can tolerate.  It also prevents me from using many fat eyepieces in my binoviewer even if they are narrower than my IPD.

5 hours ago, Froglord said:

My astigmatism is 0.75 in one eye and 1.00 in the other, so right on the borderline.

Here is Tele Vue's Dioptrx chart that is a good guideline for most folks:

DioptrixAstigmatismVis.gif

Since I'm a very picky observer and notice the least bit of astigmatism in the view, I would shift the line downward by 0.5mm EP for folks like me with 2.0 diopters of astigmatism or more.

6 hours ago, Froglord said:

I think I wear my glasses slightly further away than many people do (comfort, eyelash issues), so what sort of eye relief will I need? 20mm? Even more? I was looking at the much-recommended BST Starguiders (I'm not looking to pay Tele Vue money) but will I find the 16mm eye relief too tight?

You will probably need a minimum of 17mm to 18mm of usable eye relief in my experience.  Ask on here before trusting even Tele Vue's ER specs since they are quoting design ER from the center of the eye lens.  That's not helpful for eyeglass wearers.

Most of the BSTs have 12mm of usable eye relief due to eye lens recession.  The 25mm is the lone exception with 17mm ER and is quite usable with eyeglasses.  The others will require tipping your head a bit to see the edges.

5 hours ago, Froglord said:

That's very useful info, as the Mak 127 can't go much higher than 2mm. That may be why I haven't particularly noticed bad views.

Probably true.  You should be good without eyeglasses using a 25mm eyepiece, although just.

You can use 2" eyepieces with a Synta 127 Mak with the proper Mak to SCT thread adapter, SCT visual back, and 2" diagonal to get to larger exit pupils.  There will be some illumination falloff (vignetting) with eyepieces having a larger than 27mm field stop.  I also see halo reflections off the inside of the rear baffle tube as bright stars pass the edge of it.  Below is an image showing the difference in TFOV between a max TFOV 1.25" and 2" eyepiece and demonstrating the associated edge darkening.

220226258_Max127MakTFOVComparison.thumb.jpg.fa1c73bddd25963f5af583532ef1f858.jpg

Edited by Louis D
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Yes varifocal lenses are full of compromises and generally give poor or mediocre results in a scope type of setup.
I speak here from experience.

An alternative is to try contact lenses.
Get a lens to correct for distance and astig. It should give really good results. You can also look at the sky and generally get around
For fiddling with small bits on the scope, reading a display or chart, then add cheap off the peg reading specs.
This scheme works for me.

There are various schemes around for lens trials and subscriptions. If you look around you can probably avoid a big spend.

HTH, David.

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4 hours ago, Carbon Brush said:

An alternative is to try contact lenses.

I've never tried them, but I can't stand the idea of sticking things in my eyes!

Tonight (sky willing) I'll try my old non-varifocals and see if that's better than just eyes alone. It's possible that my astigmatism isn't strong enough to notice.

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If you are trying to correct for long or short sight using specs, don't bother. Just wind the focusser in/out instead.
Specs only come into play if you want to correct for astigmatism.
Compare the views with non-vari specs and uncorrected eye to get a feel for whether the astig is an issue.

When I first tried contact lenses, there were two big incentives. First being the astronomy issues. The second being scuba diving without a corrective mask, or lens inserts.
Yes there was an 'aargh! things in my eyes' element to be thought about.
The optician stood behind me an inserted the lenses before I had time to worry. The results were eye opening. OK I will stick with the day job.

When it came to practicing lens in/out for myself, yes there was a learning curve.
I think the worst was 20 minutes to get them in and ending up looking like an advert for hay fever symptoms.
But I soon got the hang of things and now they usually go in first try, a couple of seconds. Removal is similar.

HTH, David.

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On 24/04/2022 at 07:07, Froglord said:

I'm new to astronomy - just bought a Skymax 127 and I only have the included Super MA eyepieces (25mm and 10mm) which I'm looking to upgrade.

So far, I have been observing without my (varifocal) glasses because it seemed more natural. Then I remembered I have astigmatism in both eyes, so perhaps I should be putting on the specs after all! (I might try a slightly older, non-varifocal pair).

I think I wear my glasses slightly further away than many people do (comfort, eyelash issues), so what sort of eye relief will I need? 20mm? Even more? I was looking at the much-recommended BST Starguiders (I'm not looking to pay Tele Vue money) but will I find the 16mm eye relief too tight?

Also, will I really get the benefit if I splash out on a wide-field 68 degree eyepiece, or will my glasses restrict my FOV?

Any moderately-priced eyepiece recommendations gratefully received ...

Owned the same scope for several years.  Per the S&T review, and my own measurements, this is actually a 121mm scope.

With a 90° mirror 1.25" diagonal, the focal length is 1540mm (f/12.7).

A 1.25" prism can reduce that to 1518mm.

The Visual back has a thread on the outside that can be cut off/removed and doing so reduces the focal length to 1510mm with a 1.25" mirror diagonal and 1488mm (f/12.3) with a 1.25" prism diagonal (assuming BK-4).

The claimed 1500mm focal length is only approximate and only applies to a 1.25" diagonal.

 

If your telescope came with the 2" mirror diagonal, this increases the focal length to about 1615mm (about f/13.3)

 

As for astigmatism, the f/ratio of this scope is so long it will take a 25mm eyepiece to yield close to a 2mm exit pupil, the cutoff point for 1 diopter of astigmatism.

So, for the most part, except, perhaps, for a 32mm Plössl (the lowest power I recommend), you won't need glasses.

A typical run of eyepieces for the scope will run from 32mm to 10mm.  It will be a rare circumstance when the atmosphere, cooling of the scope, and dimness of image in the eyepiece will allow for more than that, so I wouldn't plan on it, except for, perhaps, close double stars.

A 32mm Plössl or 24mm 68° eyepiece will yield about a 1° field with the scope, which is wide enough for all but the very largest objects.  Yes, it's a narrow field of view telescope, but it is also quite good optically and very compact.

 

Progressive glasses will only allow a small strip of the field to be in focus at one time.  If you can afford to do so, get a pair of reading glasses, let them hang around your neck and use them for charts and notes,

and simply observe without glasses.  Your astigmatism will only affect the lowest power.  If you need glasses to see the sky sharply as well as read, a pair of bifocals will work as long as you have the lab make the bifocal section at the bottom as small as possible.

That will help keep the reader section out of the field of view.  If you do use glasses at the eyepiece, look for 20mm of eye relief unless you wear the lenses very close to the eye.

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23 minutes ago, Don Pensack said:

If you can afford to do so, get a pair of reading glasses, let them hang around your neck and use them for charts and notes,

and simply observe without glasses.  Your astigmatism will only affect the lowest power.  If you need glasses to see the sky sharply as well as read, a pair of bifocals will work as long as you have the lab make the bifocal section at the bottom as small as possible.

This depends heavily on what distance your eyes are fixed focus at.  In my case, my eyes focus somewhere around 8 to 12 inches without correction; thus, I just look under my distance-only eyeglasses to read things while at the scope.  My eyes have yet to change from highly myopic to somewhat more farsighted as I age.  The lower part of my bifocals has almost zero distance power, just astigmatism correction.

Surprisingly, my 2.0 diopters of astigmatism don't bother me much at all when reading for short periods of time.  I actually have my optometrist prescribe the reading part of my bifocals for focus at 18 to 24 inches so I can read things laying on a table without having to lift it.  That also makes them handy for reading things on shelves in stores without having to get so close.  However, I despise the near/far line intruding into my field of view with wide field eyepieces; so I have the distance-only pair for astro.

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Cheers, all (and thanks for the info, @Don Pensack - very useful).

I went out last night and had a good gander at the stars, with and without astigmatism-correcting glasses. I stared at a bright star (Regulus) and I really couldn't see any issues using my widest EP (25mm, 2mm-ish exit pupil) and my bare eyeballs. In fact, I found it far more comfortable (and stable) viewing without glasses. (I realise now, after a bit more reading, that I maybe should have been paying attention to out of focus star shapes as well, but in-focus looked pretty good to me.)

Tonight I'll try the same star test using binoculars, and see if the glasses make a difference with the higher exit pupil (4.2mm).

I also hunted for a few new Messiers. Using that accursed 6x30 straight-through finder (which is going to be replaced very soon!) I managed to find M65 and M66 in Leo, plus M81 and M82 in Ursa Major.

 

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On 25/04/2022 at 11:44, Louis D said:

This depends heavily on what distance your eyes are fixed focus at.  In my case, my eyes focus somewhere around 8 to 12 inches without correction; thus, I just look under my distance-only eyeglasses to read things while at the scope.  My eyes have yet to change from highly myopic to somewhat more farsighted as I age.  The lower part of my bifocals has almost zero distance power, just astigmatism correction.

Surprisingly, my 2.0 diopters of astigmatism don't bother me much at all when reading for short periods of time.  I actually have my optometrist prescribe the reading part of my bifocals for focus at 18 to 24 inches so I can read things laying on a table without having to lift it.  That also makes them handy for reading things on shelves in stores without having to get so close.  However, I despise the near/far line intruding into my field of view with wide field eyepieces; so I have the distance-only pair for astro.

Yeah, I'm not nearsighted, just presbyopic with astigmatism.  My vision "froze" at about the 20' distance, so I need correction near and far and every other distance except 20'.

So I am stuck with bifocals at the scope.

I tried large lenses (50mm round) with only 8mm of bifocal section at the bottom, and that worked, but I eventually went to single vision glasses for infinity and another pair of glasses at the table where I record notes and look at charts.

The distance glasses hang around my neck during the table visits.

Presbyopia is a pain, because my prescription is different at 6", 12", 18", 2' 6', 12', 20' etc.

Were it not for progressives, I'd have 10 different pairs of glasses.

 

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I'm really not seeing the effects of astigmatism, even with binoculars (although, to be fair, the hand-held wobble factor might be hiding it). So, I'm going to observe with bare eyeballs for the time being. Well, I might put a telescope between me and the stars, but you know what I mean ...

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