Why do really expensive scopes sell and what attracts us to them ?

[jetstream]

37 minutes ago, neil phillips said:

If the depth of focus vs F ratio is real for imaging. Then it would also be real visually. A camera only sees what the eye would see. But is actually enhanced. Quite lost on that. Perhaps you didnt mean how i interpreted what you meant.  Can you explain what you mean by that please. 

I dont know much about this, Glenn on CN does however

https://www.cloudynights.com/topic/455942-what-factors-affect-depth-of-focus/

 

"I take it we're discissing depth of focus for visual instruments. If so, objective focal ratio has nothing to do with it (but it does for prime focus imaging, and that only.) If we assume good quality, at given object distance depth of focus scales inversely with both magnification and exit pupil diameter (or the eye's iris diameter if the smaller.)

Double the power, dof is halved. Double the exit pupil diameter, dof is halved.

These two factors compete in any one instrument as eyepiece focal length is changed. Halving the eyepiece focal length doubles magnification and halves the exit pupil diameter. And so the depth of focus remains the same.

Note: The foregoing assumes an inability of the eye itself to focus (which applies to those having artificial lens implants), which is necessary for understanding how the optics work. Considering eye accommodation adds an additional element, wherein eye accommodation varies inversely with exit pupil/iris diameter. But for any one individual accommodation is a fixed quantity.

Fundamentally, depth of focus depends on the focal ratio of our eye's own lens. This is why the exit pupil is important in this respect for telescopes in the afocal configuration (where it directs essentially parallel light to the eye as from a great distance.) Note the implication; the telescope objective's f/ratio is of no import, while the eye's own f/ratio is. "

[neil phillips]

4 minutes ago, jetstream said:

I dont know much about this, Glenn on CN does however

https://www.cloudynights.com/topic/455942-what-factors-affect-depth-of-focus/

 

"I take it we're discissing depth of focus for visual instruments. If so, objective focal ratio has nothing to do with it (but it does for prime focus imaging, and that only.) If we assume good quality, at given object distance depth of focus scales inversely with both magnification and exit pupil diameter (or the eye's iris diameter if the smaller.)

Double the power, dof is halved. Double the exit pupil diameter, dof is halved.

These two factors compete in any one instrument as eyepiece focal length is changed. Halving the eyepiece focal length doubles magnification and halves the exit pupil diameter. And so the depth of focus remains the same.

Note: The foregoing assumes an inability of the eye itself to focus (which applies to those having artificial lens implants), which is necessary for understanding how the optics work. Considering eye accommodation adds an additional element, wherein eye accommodation varies inversely with exit pupil/iris diameter. But for any one individual accommodation is a fixed quantity.

Fundamentally, depth of focus depends on the focal ratio of our eye's own lens. This is why the exit pupil is important in this respect for telescopes in the afocal configuration (where it directs essentially parallel light to the eye as from a great distance.) Note the implication; the telescope objective's f/ratio is of no import, while the eye's own f/ratio is. "

Well i am not really clued up on the theory to be honest but if that is correct. its certainly news to me. You got me wanting to go in the feild and test that now. Interesting theory. So you can see a long depth of focus on a laptop screen with a high focal ratio instrument. But not at the eyepeice. Have i got that correct ?  

[andrew s]

4 hours ago, neil phillips said:

If the depth of focus vs F ratio is real for imaging. Then it would also be real visually. A camera only sees what the eye would see. But is actually enhanced. Quite lost on that. Perhaps you didnt mean how i interpreted what you meant.  Can you explain what you mean by that please. 

Not sure this is totally correct. A camera is fixed focus and integrates over the exposure.  The eye/brain is adaptive and has a short integration time and the eye can "auto" focus.

Having said that the outcome in any given circumstances is not at all obvious to me.

Regards Andrew 

[ollypenrice]

4 minutes ago, andrew s said:

Not sure this is totally correct. A camera is fixed focus and integrates over the exposure.  The eye/brain is adaptive and has a short integration time and the eye can "auto" focus.

Having said that the outcome in any given circumstances is not at all obvious to me.

Regards Andrew 

This is an interesting conundrum because, although the eye can indeed focus continuously, we still need to focus an eyepiece carefully. They eye clearly has its limits. In nature it is not required to handle steep light cones, which is why I now need specs to see anything less than a hundred yards away . I'm not up on EP ray diagrams but the light cone must be very steep, so how far can the eye adjust its focus? A question for an eye expert. Fortunately I will be able to ask the affable Jim next time I see him: he was an eye surgeon and will doubtless know the answer.

Olly

[andrew s]

@ollypenrice as I understand it when using a telescope you relax your eye to focus on infinity and adjust the focus to that. If turbulence causes focus shift the eye can compensate to a degree but causes fatigue.  

My eyes focal range is from infinity to 1m or so  now without glasse

Looking forward to your surgeon insight. 

Regards Andrew 

[Earl]

Something like a FSQ865 (much better than some 106 versions) is a dream to image with and that low F, colour correction and ease of use comes with a price tag.

Edited December 1, 2021 by Earl

[Stu]

7 hours ago, ONIKKINEN said:

My experience so far in astronomy is either climbing up or falling down Mt. Stupid.

Brilliant, I love that.

I often say that I have fallen off the cliff of my own ignorance which is along similar lines 🤣🤣🤣

[Stu]

7 hours ago, jetstream said:

One of the biggest myths is that OIII filters will not work with small apertures, it is simply not true. Another struggle many have or resist, sometimes vigorously is the concept of using exit pupil and not magnification. IMHO.

👍👍👍

[Stu]

5 hours ago, jetstream said:

I dont know much about this, Glenn on CN does however

https://www.cloudynights.com/topic/455942-what-factors-affect-depth-of-focus/

 

"I take it we're discissing depth of focus for visual instruments. If so, objective focal ratio has nothing to do with it (but it does for prime focus imaging, and that only.) If we assume good quality, at given object distance depth of focus scales inversely with both magnification and exit pupil diameter (or the eye's iris diameter if the smaller.)

Double the power, dof is halved. Double the exit pupil diameter, dof is halved.

These two factors compete in any one instrument as eyepiece focal length is changed. Halving the eyepiece focal length doubles magnification and halves the exit pupil diameter. And so the depth of focus remains the same.

Note: The foregoing assumes an inability of the eye itself to focus (which applies to those having artificial lens implants), which is necessary for understanding how the optics work. Considering eye accommodation adds an additional element, wherein eye accommodation varies inversely with exit pupil/iris diameter. But for any one individual accommodation is a fixed quantity.

Fundamentally, depth of focus depends on the focal ratio of our eye's own lens. This is why the exit pupil is important in this respect for telescopes in the afocal configuration (where it directs essentially parallel light to the eye as from a great distance.) Note the implication; the telescope objective's f/ratio is of no import, while the eye's own f/ratio is. "

Interesting stuff. So, the fact that a scope is a triplet has no impact on its depth of focus? Correct? That would be my expectation at least. For a triplet f6 apo I would expect it to snap to focus quickly, not have a long focus range.

[ollypenrice]

15 minutes ago, Earl said:

Something like a FSQ865 (much better than some 106 versions) is a dream to image with and that low F, colour correction and ease of use comes with a price tag.

I agree, but there have been a lot of quality control issues afflicting both types of FSQ in the last couple of years.

Olly

[Stu]

8 hours ago, johninderby said:

I did a head to head between my 100mm f/13 Carton and a WO FLT98 for planetary observing and the greater depth of field of the Carton made a real difference on Jupiter for example as the entire disk was in sharp focus whereas with the FLT98 it wasn’t. The Carton was simply the better planetary scope. The FLT98 was however a better all rounder even though it was beaten for planetary. 

I haven’t experienced that effect, Jupiter has either been in or on of focus for me, without variation across it. I sometimes feel that centre to edge of the solar disk needs slightly different focus but I’m not certain what causes that effect. I’m often tweaking focus to maintain best views when solar observing anyway.

[neil phillips]

34 minutes ago, Stu said:

I haven’t experienced that effect, Jupiter has either been in or on of focus for me, without variation across it. I sometimes feel that centre to edge of the solar disk needs slightly different focus but I’m not certain what causes that effect. I’m often tweaking focus to maintain best views when solar observing anyway.

What John describes could just as well be a effect of the long focus scope being generally sharper. And holding focus somewhat better. I dont doubt he saw the effect. 😃

[neil phillips]

1 hour ago, andrew s said:

Not sure this is totally correct. A camera is fixed focus and integrates over the exposure.  The eye/brain is adaptive and has a short integration time and the eye can "auto" focus.

Having said that the outcome in any given circumstances is not at all obvious to me.

Regards Andrew 

Me neither as i said its news to me. I assume if true the effect is variable depending on the power of the eyepeice in question ? 

[Stu]

7 minutes ago, neil phillips said:

What John describes could just as well be a effect of the long focus scope being generally sharper. And holding focus somewhat better. I dont doubt he saw the effect. 😃

Perhaps, although I would have expected an FLT98 to be none too shabby….

[neil phillips]

5 minutes ago, Stu said:

Perhaps, although I would have expected an FLT98 to be none too shabby….

In these cases i dont belive its because of the wavelengths being brought to focus at the same point, unlike shorter f ratio achromats. But more likely less spherical aberration and a flatter feild the longer focal ratio lens might be showing. Is my best guess

Though of course i could be wrong ? 

Edited December 1, 2021 by neil phillips

[Earl]

59 minutes ago, ollypenrice said:

I agree, but there have been a lot of quality control issues afflicting both types of FSQ in the last couple of years.

Olly

That is disappointing to hear

[John]

Thanks for the continued and quality input folks. I've been following the discussion with interest but some of the optical theory stuff has me teetering on the brink of @Stu's cliff so I've not added thoughts of my own to avoid the possibility of falling off !.

I recall that for quite a while, the author / reviewer Neil English was a great proponent of long, slow refractors although he was never a fan of the expensive, shorter apochromats or "poodles" as he termed them. Lately however Neil has had a change of heart and favours newtonians.

http://neilenglish.net/optimising-an-8-inch-newtonian-for-visual-use/

 

 

 

 

 

 

Edited December 1, 2021 by John

[Stu]

51 minutes ago, neil phillips said:

In these cases i dont belive its because of the wavelengths being brought to focus at the same point, unlike shorter f ratio achromats. But more likely less spherical aberration and a flatter feild the longer focal ratio lens might be showing. Is my best guess

Though of course i could be wrong ? 

But a top notch triplet apo should be very well corrected in SA surely?

[Stu]

15 hours ago, neil phillips said:

So i am glad your not saying you can see more than the camera. Because after doing visual for many years. and then imaging. I became aware infact that a lot more can actually be seen with a camera involved.

Certainly not saying that I can see more than a camera. My experience is purely with smartphone cameras and I can certainly see more than in any single frame or even live view, but stacked images obviously show more.

I’ve no desire to get into an imaging vs visual debate but personally the enjoyment is about pulling out the maximum detail I can visually; I love the delicacy of faint stars and lunar/planetary detail which can be seen with effort and perseverance.

[neil phillips]

42 minutes ago, Stu said:

Certainly not saying that I can see more than a camera. My experience is purely with smartphone cameras and I can certainly see more than in any single frame or even live view, but stacked images obviously show more.

I’ve no desire to get into an imaging call visual debate but personally the enjoyment is about pulling out the maximum detail I can visually; I love the delicacy of faint stars and lunar/planetary detail which can be seen with effort and perseverance.

Dont disagree at all with the reasons you enjoy observing. Heck ive been enjoying more visual myself lately. In the summer i plan to put a 2" diagonal on my bresser 127l and using mostly as a visual instrument. I have other better imaging telescopes.

to be working with.

Much enjoyment to be had at a eyepeice for sure.

Was just pointing out after years of visual observing. That often i have seen live on screen. More detail. Than i have been able to observe at the eyepeice. Some high magnification live views of jupiter on a laptop screen have certianly showed me more fine detail.  So not debating either. Just making a observation of my own experiances regarding eyepeice versus live camera views regarding planets. Over many years

[neil phillips]

1 hour ago, Stu said:

But a top notch triplet apo should be very well corrected in SA surely?

Most likely they are. As i said its only a theory to explain Johns experiance. I agree i could be wrong. Was just pondering if the longer focal ratio of the Carton was producing a flatter feild compared to the WO refractor. Which often have very short focal ratios. But being a triplet. I agree i may be wrong about this. 

[Stu]

3 hours ago, neil phillips said:

Most likely they are. As i said its only a theory to explain Johns experiance. I agree i could be wrong. Was just pondering if the longer focal ratio of the Carton was producing a flatter feild compared to the WO refractor. Which often have very short focal ratios. But being a triplet. I agree i may be wrong about this. 

No, you may be right about the field curvature. I don’t think a triplet is any flatter than a doublet, and they tend to be shorter focal length so field curvature could be a factor, although if on axis on a driven mount maybe less so. Very hard to be sure!

[neil phillips]

3 minutes ago, Stu said:

No, you may be right about the field curvature. I don’t think a triplet is any flatter than a doublet, and they tend to be shorter focal length so field curvature could be a factor, although if on axis on a driven mount maybe less si. Very hard to be sure!

Yes i am uncertain too. tbh Stu

Edited December 1, 2021 by neil phillips

[jetstream]

2 hours ago, Stu said:

But a top notch triplet apo should be very well corrected in SA surely?

👍

Optical quality can effect whether the focuser needs to be moved a lot to find best focus, possibly, IMHO.

[jetstream]

9 hours ago, neil phillips said:

Well i am not really clued up on the theory to be honest but if that is correct. its certainly news to me. You got me wanting to go in the feild and test that now. Interesting theory. So you can see a long depth of focus on a laptop screen with a high focal ratio instrument. But not at the eyepeice. Have i got that correct ?  

You need 2 scope of the same focal length and aperture- one that has a fast snap focus and one that has a longer focus range. This will eliminate f ratio as the factor and confirm or deny this idea.

Eagerly waiting test!