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Varifocals and Observing


Jasonb

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Hi everyone,

I got into the Hobby a couple of years ago, but time is taking its toll on me, and now I wear Varifocal Glasses. I have found this quite awkward when viewing, and sometimes my glasses hit against the eyepiece, and therefore causes vibrations etc. It also feels like if I pull back to stop the glasses hitting the eyepiece, then I'm not seeing the full field of view.

The eyepieces I used are BST Starguiders, which have what I think are twist in/out eyecups. I usually leave these twisted in completely, would that be correct? I don't know if my glasses being varifocal suit the hobby either, and I wonder if I should try without glasses, or maybe just get a cheap fixed distance pair or something? I've read about astigmatism being key, and you can get away with glasses if you don't have an astigmatism, and just change the focus. I need to check my prescription to see if I do or not!

Any advice appreciated, thanks!

Jason.

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Hi Jason. It is not so much the varifocals (I use them too) but glasses by themselves combined with eyepieces with comparatively short eye relief (the optima distance between rear of eyepiece and eye). If you do not have problems like (cylindrical) astigmatism, observing without glasses is fine. However, if you do have such astigmatism (like I do) then the best option is to get eyepieces with long eye relief (like the Vixen SLV series and many others). These tend to be more expensive, however. Varifocal glasses can cause issues with only part of the field of view being in focus at any given setting. This depends largely on how strong your reading part of the glasses is, and the width of the field of view. I tend to get along quite well with my varifocals, others prefer to keep a separate pair of glasses without varifocal lenses for observing.

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This has been discussed quite few times in other threads. The brief answer above by @michael.h.f.wilkinson is a good summary.

I would add (an experienced user) that varifocal lenses are absolutely full of compromises.
With my prescription the 'normal' uses like reading, driving, TV and the like are fine.
As for odd ball activity like a telescope - that is something else.
Try this test......
One eye on a distance light source like a street light at hundreds of metres.
Keep looking at the light and move your head around so the light enters from all corners of the lens.
You will see the light go from near point, to lozenge, to oval, to blob.
Now which bit of the lens will you use for viewing?

My choice has been contact lens with fixed focus.
This allows you to use any eyepiece (short or long eye relief) and provides a god view - assuming a well matched and fitted cnotact lens.

HTH, David.

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Although my visual astronomy is only casual (I'm 99.99% imager) I did have a pair of distance glasses made up by my optician as an alternative to my varifocals and find them a huge improvement both for just eyeballing the sky and using a pair of bins.

I have quite a complex prescription with short sight, astigmatism, and prism.

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Don't do what I did, and think that getting a pair of single vision spectacles for astronomy purposes would be a good idea (one of our leading chains usually does 'second pair free' deals). 

It might help for the first year or so, but after a couple more years of further deterioration in your "close up" focus ability, you'll find that you can't read the buttons on the handset/phone with your single vision glasses, and your problem is reversed!

A lot of people use spectacle chains, so that they can just remove their glasses when going to the eyepiece.  Mrs G does this with her binoculars - of course, it means that if I'm looking through her binoculars, or she's looking through my telescope, we need to re-focus. 

ETA - ooops, I see DaveS says the opposite - all I can say is, it was only a short term measure for me :) 

Edited by Gfamily
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I’m very short sighted and a couple of yrs ago I was recommended varifocals. Nothing to do with astronomy but I just couldn’t get on with them. They gave me unpleasant headaches even after several months. So I went back to my standard glasses and have not had any problems. 

For observing at the telescope, sometimes I wear my glasses, sometimes contact lens, sometimes I’ll take my glasses off when looking in the eyepiece. Doesn’t bother me too much but I have a slight preference to NOT wearing glasses.

However, my daughter prefers to wear her glasses at the eyepiece so often I’ll wear mine so that I can see in the same/similar way that she does.

I also have BST StarGuiders. In order to get closer-in while wearing glasses I sometimes totally remove the eyecups. Very easy to do even in the dark. Then use a hood to keep out any stray light. 

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I have shockingly bad astigmatism, an astronomers worst nightmare in optical terms as well as needing varifocals and here are my solutions. 

Exit pupil of greater than 3.5mm, I use a either a televue dioptrix or wear my glasses or use astigmatism correcting contact lens. I prefer contact lens but the other two options are acceptable depending upon what I am viewing.

An exit pupil of <3.5mm I put up with the astigmatism. I find it is generally not too severe in the smaller exit pupils and barely noticeable once I get down below 2mm.

When using my large binoculars nearly always use contact lens. I can use glasses too but CL are preferred. 

Hope this helps.

Steve

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Thanks for all your replies, much appreciated!

I checked my prescription, and I am neither short- nor far-sighted (as I thought). I have an astigmatism of 1 in one eye and 0.75 in the other. From what I've read online, this would be considered mild, with the 1 just hitting the bottom of the moderate scale.

My Sky-Watcher Explorer 150P is an F5 scope, and my BSTs are 25mm, 12mm and 8mm, giving an exit pupil size (if I'm correct) of 5mm, 2.4mm and 1.6mm. From what I can tell the BSTs all have an Eye Relief of 16mm.

So, that's the data. What I really need to do now is try some observing and see how it works. The first step seems to be do I feel like I can get away without the glasses at all, especially with the 8mm and 12mm eyepieces.

Thanks for all your advice...

 

Edited by Jasonb
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46 minutes ago, Jasonb said:

Thanks for all your replies, much appreciated!

I checked my prescription, and I am neither short- nor far-sighted (as I thought). I have an astigmatism of 1 in one eye and 0.75 in the other. From what I've read online, this would be considered mild, with the 1 just hitting the bottom of the moderate scale.

My Sky-Watcher Explorer 150P is an F5 scope, and my BSTs are 25mm, 12mm and 8mm, giving an exit pupil size (if I'm correct) of 5mm, 2.4mm and 1.6mm. From what I can tell the BSTs all have an Eye Relief of 16mm.

So, that's the data. What I really need to do now is try some observing and see how it works. The first step seems to be do I feel like I can get away without the glasses at all, especially with the 8mm and 12mm eyepieces.

Thanks for all your advice...

 

I would go without glasses. Once you get used to the feel of not wearing them whilst viewing (it can seem strange at first), you should be fine imho.

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I wear distance specs, but the problem is compounded by having strong double-vision.

I’ve solved this problem when using the binoviewer or binoculars by dispensing with the specs and instead using the lenses from three of my old pairs of specs that I’ve sawn and filed down to fit into the eyepiece cups.

Works fine. I get the full field and no double-vision.

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28 minutes ago, Merlin said:

I wear distance specs, but the problem is compounded by having strong double-vision.

I’ve solved this problem when using the binoviewer or binoculars by dispensing with the specs and instead using the lenses from three of my old pairs of specs that I’ve sawn and filed down to fit into the eyepiece cups.

Works fine. I get the full field and no double-vision.

Three, please do explain?

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Dont think lined trifocals will help either.  I wear them daily and using them for astro doesnt work for me at all.  So i just take the glasses off and focus.  It works for me  

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21 minutes ago, michael.h.f.wilkinson said:

Incidentally, much depends on the type of varifocals. The usable field of view varies a lot. Cheaper ones have the narrower usable FOV.

Agree absolutely.

Another factor is how rapidly they change from far to near vertically.
If you choose 'Harry Potter' round lenses in a 40mm diamter frame, it is not a worry.
But if you choose letter box shape frames that are only 20mm high, it is a different matter.
If you want to use a screen, or read a lot, you don't want a spot 'near' section. Though a large near section may compromise middle/distance results.

In the back, or under the counter, the optician has a lens catalogue. It shows the lens characteristics, rather like contour lines on a map.
Imposing this over your desired frame will be very useful.
Ask to see this if the 'front of house' staff can't (as if often the case) answer your questions. Or go elsewhere.

HTH, David.

 

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Thanks again for your replies...

To clarify, I already have the Varifocals, have had them a while, but I'm struggling with observing with them the few times I've been out. They do have a fairly wide FOV, as I made sure I picked the ones that do.

Once I get a clear night I'm going to try to observe without them, with different eyepieces, and see how it goes...

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I wear a pair of distance-only eyeglasses at the eyepiece to correct my 2 diopters of astigmatism.  I bought them online for about $20.  Make sure to get the lowest index plastic lenses to minimize chromatic aberrations at the edge when looking off axis through them with wide field eyepieces.

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Just a quick update on this. I got enough of a clear spell last night to bring out my scope, and had a look at Mars, the Orion Nebula and the Pleiades, with my different eyepieces. I could not notice any reduction in clarity with my glasses off compared to on, and it was a lot more comfortable with them off, not hitting off eyepieces etc. so I reckon I'm just not gonna wear them while observing etc. for the moment, until my eyesight gets worse! :)

Thanks for all your help and advice, I can see contact lenses in my future at some point!

Now, the step, finding somewhere darker to observe from!

Edited by Jasonb
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this is from Televue:

image.png.fe9f9fd65927c88bfb2335caea85eff3.png

Exit pupil = eyepiece FL / scope focal ratio (which is 5 in your 150p)

So you might suffer visible astigmatism in your 1 dioptre eye  with an exit pupil of 2mm or above. With your F5 scope, that'll be with 10mm eyepieces and above. With your 0.75 eye, it'll be a 3mm exit pupil - and therefore 15mm eyepieces and above.

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Although I normally wear Varifocals, I have a pair of single-vision distance glasses (from a cheap online supplier) for astro use.  In practice, they get used for coarse aiming through the Telrad; then I take them off for fine aiming and viewing.  I have TV Dioptrix, which work well but are fiddly to set up and get exactly right.

Because my left eye is weaker than my right, I made up a 1.25" "washer" out of 2mm thick printed circuit board to raise the left eyepiece farther out of the binoviewer than the right eyepiece.  Then each eye can be adjusted within the range of the binoviewer (a Maxbright II, which has less adjustment range than the William Optics BV it replaces).

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On 21/02/2023 at 17:04, bryand2 said:

Although I normally wear Varifocals, I have a pair of single-vision distance glasses (from a cheap online supplier) for astro use.  In practice, they get used for coarse aiming through the Telrad; then I take them off for fine aiming and viewing.  I have TV Dioptrix, which work well but are fiddly to set up and get exactly right.

Because my left eye is weaker than my right, I made up a 1.25" "washer" out of 2mm thick printed circuit board to raise the left eyepiece farther out of the binoviewer than the right eyepiece.  Then each eye can be adjusted within the range of the binoviewer (a Maxbright II, which has less adjustment range than the William Optics BV it replaces).

It's great that you found a solution that works for you, even if it involves using multiple pairs of glasses and a custom-made "washer". I totally get what you mean about the TV Dioptrix being fiddly to set up, but it's all worth it for that perfect view, am I right? It's really cool that you're using a Telrad too - it's definitely a handy tool to have for aiming.

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  • 2 weeks later...

I wear varifocals. Whilst observing I have them attached on a glasses string. So when looking through the ep I'll take the glasses off and whilst looking into the sky or through my rigel qf I'll put them on. Not much hassle doing it this, way. Maybe a monocle like the great Sir Patrick Moore could be in order for me 🧐

Lee

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