Different focus points.

[Marvin Jenkins]

I was observing with a friend who doesn't have a scope of his own.

Taking turns at the EP I couldn't help but notice that each time observed he gave a slight tweek of the 10:1 fine focuser.

My turn to look and I had to re tweek to regain focus. All the tweeking started to annoy me as I thought once an EP was in focus, it is simply at the sweet spot.

I don't know how to explain my question, but do we see EP focus point differently for each individual.

This has also made me wonder about spectacle wearers. I always assumed that specs corrected long site or short site which I would have thought would be made redundant at the EP by simply focusing.

What is going on here?

Marvin

[vlaiv]

It really depends on two things.

First is if person is near / far sighted or has "correct" eyesight. This changes focus position and focus must be tweaked.

But even if you take two people that have exactly the same eyeballs - there is still chance that they will tweak the focus. Muscles are responsible for focusing the eye and in ideal conditions one should fully relax muscles responsible for this to have focus at infinity, but there are number of reasons why muscles can't be fully relaxed.

If we are tired, not having had enough sleep or any number of conditions like dry eyes or whatever can influence our muscles not being able to fully relax - we may feel that they are relaxed but there is still some residual tension.

Muscles get tired and once tired - they stiffen up a bit not having full range of motion. We can also be mentally tense - which stops us from fully relaxing our muscles and so on ...

So even a single person might need to tweak focus between two viewing sessions because of this.

[John]

Over the weekend I took part in an outreach solar observing session at a festival in Bristol. I must have shown the white light solar views through my scope to hundreds of people over the 2 days. 

Most of them seemed happy with the focus point that I had set. Most of the glasses wearers found the views easier / clearer when they took their glasses off to view. A few of those needed a small focuser adjustment, usually inwards a few mm, to sharpen things up for their eye. Those who got a better view with their glasses on didn't seem to need to adjust the focuser. 

Of course a good sized cross section of the above had not viewed through a scope before so would probably have not noticed if the view was not quite sharp for them, thinking that was normal !

Not scientific of course, but interesting anyway and all seemed to enjoy seeing the sun spot activity "for real" 🙂 

[Marvin Jenkins]

Interesting  to have your experience on hand. I live the life of a hermit so I have very little on hand to base observations/comparisons. 

I also would have thought that spectacle wearers were at a dis advantage having to look through an extra layer of glass.

Marv

[John]

Just now, Marvin Jenkins said:

....I also would have thought that spectacle wearers were at a dis advantage having to look through an extra layer of glass.

 

It's not a normal activity for me (I usually observe alone) but my astro society wanted volunteers and I stuck my hand up !

I don't envy those who wear glasses having that additional layer of glass but they seemed to do OK and I was using an eyepiece that had a decent amount of eye relief and not too wide a field of view. 

 

[glafnazur]

I've done quite a bit of outreach with my local society and I focus whilst wearing my specs (I don't wear them when observing on my own) and I've found that in the main most people don't refocus. As noted above most are inexperienced and probably wouldn't notice if the focus was slightly off. People who take their specs off have to make slight adjustments depending on their eyes and I always get one or two that are a bit more interested and they usually have a little twiddle to get best focus. I find that when we have observing sessions at the society's observatory that most people do tend to adjust the focus, even if it just a slight touch.

[vlaiv]

Forgot to mention this as well. This also works the other way around.

There is some "room" for accommodation and mostly younger are actually able to adjust the sharpness of the view using their eyes (eye muscles) after someone else without the need to change the focus.

[Marvin Jenkins]

Next clear night with some moon I am going force my wife to partake in a visual experiment on account of of her terrible eyesight. Corrected with glasses she has no issue but without I have referred to her vision as molevision.

I propose an experiment. I am going to achieve pin sharp focus for my eyeball and mark the focus tube, then ask the subject to observe with and without specs.

For both of these observations I will ask the subject to adjust focus to attain what they think is perfect focus, then mark the draw tube.

My wife is short sighted. I am going to use a 130 newt collimated correctly. With a Plossle EP around 24mm for decent eye relief giving around 27x magnification.

Has anyone else tried anything like this?

Marv

[vlaiv]

8 minutes ago, Marvin Jenkins said:

Has anyone else tried anything like this?

Not as "controlled" experiment.

I have observing buddy that also wears glasses. I do too when I work. I'm far sighted and while I was OK to be without glasses in everyday life for quite a bit of time (eyes adjusted) - now as I'm getting older, things are naturally moving to far sighted side of things (maybe a bit sooner for me than most because I was already far sighted as a child).

In any case, when observing we noticed - both of us are quite far off in focus position from young people. We don't have exact same focus position and usually need to adjust after we swap at the telescope. However - this adjustment is not "fixed" and can change quite a bit between observing sessions.

Sometimes it just takes a small tweak and sometimes it takes quite a bit of adjusting. I think it has to do with how rested we are when observing versus being a bit tired.

Edited June 4 by vlaiv
typo

[John]

2 minutes ago, vlaiv said:

......both of us are quite far off in focus position from young people......

That could be a statement that applies outside of astronomy as well for me 😁

[Marvin Jenkins]

23 minutes ago, John said:

That could be a statement that applies outside of astronomy as well for me 😁

I think you may be onto something there.

[Marvin Jenkins]

I just thought of something from my original post observing with my friend.

He is 15 years younger than me lucky #*×. However he is a smoker which I am not. I noticed during the night the odd aromatic pungent smell.

I am wondering if my observing guests eyes were a little 'more relaxed' than mine. Probably more dilated giving a bigger aperture possibly, but effecting focus point.

Marvin

 

[ollypenrice]

I run an astronomy guest house (or now an astronomy flat) and, though we are mostly photographic these days, I once did a lot of visual observing with guests using a shared instrument. The focal point varies enormously from person to person.  If I had a guest who didn't want to refocus from my own critical focus position I was instantly suspicious that they were tolerating an imperfect focus and asked them to follow a procedure to verify their focal position. It is exceptional to share a critical focus position with another person.

If observing with a beginner it is best to focus on a field of only stars. If the field contains nebulosity it distracts and deceives the beginner. Ask them simply to make the stars as small as possible, emphasizing the need to make tiny adjustments.

Olly

[Stu]

What I find frustrating is when you KNOW someone will need to refocus but they look through and say ‘no, that’s fine’ 🤬. An example would be if they do not wear glasses, and I have focused the scope using my short sighted eyes. To counter this, I sometimes try to focus wearing my glasses in the hope that this results in a focused view for someone with good vision.

[John]

The challenge is, at a busy outreach event, each person only gets a very short time at the eyepiece. I had queues up to 20 folks long at times this weekend so fine tuning the views for each observer, most of whom had never looked through a scope before, just wasn't practical.

I did start to do what @Stu suggests above and anticipate the next persons needs by racking in a few mm if they were wearing glasses then encouraging them to try without the glasses, initially at least. Quite often that seemed to work but of course I couldn't see what their eye was seeing !

 

[Marvin Jenkins]

24 minutes ago, Stu said:

What I find frustrating is when you KNOW someone will need to refocus but they look through and say ‘no, that’s fine’ 🤬. An example would be if they do not wear glasses, and I have focused the scope using my short sighted eyes. To counter this, I sometimes try to focus wearing my glasses in the hope that this results in a focused view for someone with good vision.

How does this work with a bino viewer! That's two eyes potentially not the same in the same skull 💀 

[Marvin Jenkins]

23 minutes ago, John said:

The challenge is, at a busy outreach event, each person only gets a very short time at the eyepiece. I had queues up to 20 folks long at times this weekend so fine tuning the views for each observer, most of whom had never looked through a scope before, just wasn't practical.

I did start to do what @Stu suggests above and anticipate the next persons needs by racking in a few mm if they were wearing glasses then encouraging them to try without the glasses, initially at least. Quite often that seemed to work but of course I couldn't see what their eye was seeing !

 

More power to you with the out reach. I have tried it at a limited level and found the response baffling.

[ollypenrice]

20 minutes ago, Marvin Jenkins said:

More power to you with the out reach. I have tried it at a limited level and found the response baffling.

It is baffling, sometimes. Someone apparently happy with what they have seen will hand you back the scope so far out of focus that it can only have given them pure visual mush.

What we underestimate is that some people don't take naturally to looking through optical aid, just as some people don't take naturally to driving a car or changing gear on a bicycle.  I stopped waiting for night with beginner guests and got them used to looking at a distant electricity board sign half way up a nearby hillside. The text was readable if you were in focus. If not, it wasn't. This was a simple self-test for the guests and a simple third person test for me.

At astro outreach events it is simply not viable to hand an instrument over to to a new observer in the expectation that they will see what is there. Some will and some won't. This seems unreasonable to us but that's how it is.

Olly

[saac]

23 hours ago, Marvin Jenkins said:

Interesting  to have your experience on hand. I live the life of a hermit so I have very little on hand to base observations/comparisons. 

I also would have thought that spectacle wearers were at a dis advantage having to look through an extra layer of glass.

Marv

I tend to remove my glasses when using any eyepiece be it telescope or microscope. I think it is entirely just a matter of preference as to what people do, what feels most comfortable. 

Jim 

[DaveS]

I have a fair bit of astigmatism, so if I don't wear my glasses then stars appear as crosses rather than points.

[Marvin Jenkins]

1 minute ago, ollypenrice said:

It is baffling, sometimes. Someone apparently happy with what they have seen will hand you back the scope so far out of focus that it can only have given them pure visual mush.

What we underestimate is that some people don't take naturally to looking through optical aid, just as some people don't take naturally to driving a car or changing gear on a bicycle.  I stopped waiting for night with beginner guests and got them used to looking at a distant electricity board sign half way up a nearby hillside. The text was readable if you were in focus. If not, it wasn't. This was a simple self-test for the guests and a simple third person test for me.

At astro outreach events it is simply not viable to hand an instrument over to to a new observer in the expectation that they will see what is there. Some will and some won't. This seems unreasonable to us but that's how it is.

Olly

At least you had a chance, so well done. My experience has been small and different.

A work friend of mine has a six year old daughter who is fascinated by books of the solar system and space stuff in general. I offered a late evening into summer night use of my 150pds EQ5 as a personal outreach as the planets were very good back then.

Instead a local astro group had a night of the stars and the poor young girl was asleep waiting in a line.

My next door neighbour is a summer holiday visitor. He has two teenage sons very interested in science of all kinds. After seeing me observing he asked if his sons could come round and observe which I was more than accommodating.

The next evening they went out to dinner, then to bed. A few nights later they went out and returned at 3am.

The following day they explained that they had paid to have a private tour of an historic observatory at Fumel and looked at Saturn through a large SCT.

They then said they highly recommend it, I should try it some time!!!!

At that point I thought better off on your own son.

[Xilman]

23 hours ago, John said:

It's not a normal activity for me (I usually observe alone) but my astro society wanted volunteers and I stuck my hand up !

Didn't they teach you anything in the army?

[Xilman]

I invariably have to refocus.  I am myopic.

[saac]

51 minutes ago, Marvin Jenkins said:

How does this work with a bino viewer! That's two eyes potentially not the same in the same skull 💀 

There tends to be a dioptre adjustment for each eye on a bino, this narrows the divergence between the eyes. Remember too that the image is interpreted in the brain and it will accommodate to form a single image. There also is usually a more dominant eye which the brain will preference. 

Jim 

[saac]

2 minutes ago, Xilman said:

I invariably have to refocus.  I am myopic.

When I wear glasses then my daughter, non spectacle wearer, will not need to refocus or only minimal adjustment. If I remove my glasses and focus to suit my eyesight then on handing over my daughter will need to make a pretty noticeable focus adjustment. This makes sense as the point of glasses, corrective lenses, is in effective to provide the wearer close to 20:20 vision. 

Jim