How can 60mm be better than 80mm (help needed)? (1 Viewer)

[4th_point]

I was wondering if someone would be able to help me understand the statements quoted below, which are at BVD, where a 60/65mm scope can outperform an 80mm in "bad" conditions. I think I understand how lower magnification may help regarding the appearance of the view, but don't understand the science behind the smaller aperture. If I placed a piece of cardboard with 60mm opening on the objective of the 80mm scope, would it help in "bad" conditions?

Thanks in advance for any insights.

Jason

I remain convinced that in 95% of birding situations, an exceptional 60 mm scope will show you all there is that you need to see. In a very few situations (dirty air, unstable air, or fog) the 60 mm scope will actually outperform the the bigger scopes at equivalent powers.

Nikon Fieldscope III ED & 20-60x Zoom | Better View Desired

"My feeling is that 65mm is a kind of threshold; that, in fact, a 65mm scope may provide all the detail and brightness the human eye can use in all but the most extreme conditions. It would have to be nearly dark for the 80mm scope to show a clear advantage. Then too, in terrestrial viewing where some combination of heat haze, fog, and dust...dirty, wet, unstable air...is too often the real limiting factor in the view you are able to get, going from 65mm to 80mm is not going to help and may, in fact, hurt. A smaller scope, as I have pointed out before, can actually outperform a larger one in bad “seeing” conditions."

The Ultimate Birding Scope! (2 of them) | Better View Desired

 

[Binastro]

Yes.

Stopping down a lens may help in poor conditions.

The focal ratio improves, aberrations improve and the view may improve.

Usually, for astronomy if the conditions are poor one simply gives up on planetary viewing, but might carry on looking at lower magnification objects like galaxies.

But bird watchers observe in such poor conditions I wouldn't bother to observe.
I suppose out of necessity.

It may help in poor conditions to observe from an elevated position or even from inside through window glass, but not through an open window.

In bright sunshine at low magnification the small pupils of ones eyes may stop down the aperture of the telescope.

Regards,
B.

 

[Vespobuteo]

I was wondering if someone would be able to help me understand the statements quoted below, which are at BVD, where a 60/65mm scope can outperform an 80mm in "bad" conditions. I think I understand how lower magnification may help regarding the appearance of the view, but don't understand the science behind the smaller aperture. If I placed a piece of cardboard with 60mm opening on the objective of the 80mm scope, would it help in "bad" conditions?

Thanks in advance for any insights.

Jason


Nikon Fieldscope III ED & 20-60x Zoom | Better View Desired


The Ultimate Birding Scope! (2 of them) | Better View Desired

See this thread, same question (and reference to the BVD/Steve Ingram claims) in the thread start:

Big Scopes

I have a Nikon Fieldscope 82 EDa which I think is really excellent in terms of optics and handling. However, there are many times when I feel scopes in the 77-85 mm range are at their limits. So I have four questions: 1) Steve Ingram has suggested in some situations (heat shimmer, fog) a...

www.birdforum.net

 

[Binastro]

In astronomy at night the columns of air are reckoned to be 4 inches across..

In turbulent air these columns interfere with each other and a large telescope say 12 inches or more may appear to show less than say a 4 inch scope.
Usually this isn't true.

Because of our atmosphere, not much is gained for planetary views with telescopes bigger than 16 inches except in very favourable locations, usually above 8,000 ft.
But magnifications are usually above 200x.

In the daytime, I have seen it quoted that air columns are 2 inches across, although I am not sure of this.

So something similar may occur.

But spotting scope magnifications are often so low, I don't know how much size affects viewing.

I think the effects of poor Seeing are less with low magnifications with spotting scopes.

Certainly try masks of various sizes to see if it helps.
A high quality telescope is less affected than a medium quality one.

B.

 

[kabsetz]

Jason,

The old thread linked by Vespobuteo gives a good discussion on the subject. I can only add that since that thread, new and even bigger scopes have been introduced, and the best, low-aberration samples of those are even better than the 82-85 millimetre Nikons and Zeisses talked about there. My current ATX95 shows a sharper, better contrast image both in clear air and in heat haze than did the excellent Nikon Fieldscope 82 A I had before it.

- Kimmo

 

[Binastro]

Apodization.

I don't know if this has been discussed before on Birdforum.

In the 1960s I was given instructions on how to make an Apodization screen for astro telescopes.

I never actually got round to making one, so I have no personal experience.

This is a complex subject, involving the modification of the diffraction rings of star images.
It has drawbacks, but apparently does work in poor Seeing.
In good Seeing full clear apertures work best.

An 82mm scope seems too small to me to bother, but it may work.

There is discussion of this in Suiter's book Star Testing Astronomical Telescopes, 2003. pages 160 to 166.

Also much earlier in astronomical Journals.

Regards,
B

 

[Binastro]

Boosting Performance by Apodization.

Pfannenschmidt, E.

Journal of the Royal Astronomical Society of Canada.

Volume 97 Issue 2 p.103.

Regards,
B.

 

[tenex]

"All there is that you need to see", "all the detail the human eye can use"... I recognize these phrases instantly. Who else has ever written like this, and without even knowing what "science" was relevant?

I read some BVD pieces years ago myself and wish I had made a list of claims like this. One I do recall was that larger apertures collect more "light information" and will therefore show color better, and more detail in shadow areas, etc (forgetting daylight pupil size). I should have been reading BirdForum.

 

[4th_point]

Thanks for the replies. The various aberrations in the environment, instruments, and observer are really interesting to me. And how it all stacks up.

 

[4th_point]

I spent several hours over the past few days comparing two scopes. A 60mm EDIII-A and an 82mm ED82-A, with one MCII zoom and one fixed 30x/38x. We have reduced air quality in the PNW of the US right now due to forest fires, so I thought I'd try to see if there were any differences observed.

On day one, there was smoke in the air but little heat haze that I could see. With the 60mm and fixed 30x I saw little to any difference compared to the 82mm using the zoom set to what appeared to be ~30x. I viewed the tops of very tall conifers, mainly the needles and their spacing, pine cones, bark, etc. And birds when possible, but that wasn't very often. Out to several hundred yards, thinking there would be more particles to look through.

On a side note, the image from the 60mm with fixed 30x looks remarkably similar in terms of brightness to my eyes compared to the 82mm with zoom piece. It seems like a great combination. When I use the zoom eyepiece in the 60mm there's a noticeable drop in brightness compared to the 82mm, as one would expect. I really like the 60mm with fixed 30x!

On day two, there was a little less smoke but some heat haze. There were definite moments where I had a more steady and clear view with the 60mm vs the 82mm. Not always, but one time in particular I had the 60mm set at what appeared to be ~38x with the zoom, and the 82mm at 38x fixed. The 60mm was obviously better. I then zoomed to 60x on the 60mm and it was still better than 82mm at 38x. I was really shocked by the difference.

I had each scope within inches of each other, mounted on separate tripods at standing height, viewing the same objects (tops of tall conifers). I also made sure that I accounted for the different FOV (i.e. the wider view would show possibly more haze, tricking me into thinking it was worse). Both scopes appear to be good based on star tests, but I am not an expert at that. I don't see any things that stand out on either side of focus. They both have crisp images at their max zoom, with the 82mm being absolutely stunning to my eyes.

I continued to compare the two scopes later on day two and most of the time they seemed about equal but the smoke and heat haze had diminished. There were a few times where the 60mm seemed marginally better but I think that the mild conditions were changing too rapidly. Even with both eyepieces so close together, I couldn't move my head fast enough to catch the same scene. It was only earlier in the day where the smoke was thicker and the heat haze more heavy and constant.

I would not say definitively that a 60mm is better under those conditions. Perhaps I overlooked something, or there is a simple explanation for what I thought I saw? Would the eyepieces have an effect, or is the claim simply related to the scope bodies?

I just thought I would share this observation. I hope to continue this sort of comparison, although I don't like the idea of more forest fire smoke!

Jason

 

[RobMorane]

I spent several hours over the past few days comparing two scopes. A 60mm EDIII-A and an 82mm ED82-A, with one MCII zoom and one fixed 30x/38x. We have reduced air quality in the PNW of the US right now due to forest fires, so I thought I'd try to see if there were any differences observed.

On day one, there was smoke in the air but little heat haze that I could see. With the 60mm and fixed 30x I saw little to any difference compared to the 82mm using the zoom set to what appeared to be ~30x. I viewed the tops of very tall conifers, mainly the needles and their spacing, pine cones, bark, etc. And birds when possible, but that wasn't very often. Out to several hundred yards, thinking there would be more particles to look through.

On a side note, the image from the 60mm with fixed 30x looks remarkably similar in terms of brightness to my eyes compared to the 82mm with zoom piece. It seems like a great combination. When I use the zoom eyepiece in the 60mm there's a noticeable drop in brightness compared to the 82mm, as one would expect. I really like the 60mm with fixed 30x!

On day two, there was a little less smoke but some heat haze. There were definite moments where I had a more steady and clear view with the 60mm vs the 82mm. Not always, but one time in particular I had the 60mm set at what appeared to be ~38x with the zoom, and the 82mm at 38x fixed. The 60mm was obviously better. I then zoomed to 60x on the 60mm and it was still better than 82mm at 38x. I was really shocked by the difference.

I had each scope within inches of each other, mounted on separate tripods at standing height, viewing the same objects (tops of tall conifers). I also made sure that I accounted for the different FOV (i.e. the wider view would show possibly more haze, tricking me into thinking it was worse). Both scopes appear to be good based on star tests, but I am not an expert at that. I don't see any things that stand out on either side of focus. They both have crisp images at their max zoom, with the 82mm being absolutely stunning to my eyes.

I continued to compare the two scopes later on day two and most of the time they seemed about equal but the smoke and heat haze had diminished. There were a few times where the 60mm seemed marginally better but I think that the mild conditions were changing too rapidly. Even with both eyepieces so close together, I couldn't move my head fast enough to catch the same scene. It was only earlier in the day where the smoke was thicker and the heat haze more heavy and constant.

I would not say definitively that a 60mm is better under those conditions. Perhaps I overlooked something, or there is a simple explanation for what I thought I saw? Would the eyepieces have an effect, or is the claim simply related to the scope bodies?

I just thought I would share this observation. I hope to continue this sort of comparison, although I don't like the idea of more forest fire smoke!

Jason

Jason,

I think your observations are correct.

Not the first time I read that a 60-ish mm scope is performing "better" than a 80+ mm in heat haze conditions .

On one opportunity, I had a Kowa 77mm showing a better image than a Kowa 88mm when heat haze was high, and with the same zoom eye piece on (despite of the 88mm having clearly more light in the picture).

Some say that having a scope under 80mm helps with heat haze conditions, and that 80mm is the "limit".

I don't know the reason behind it (more photons comming in the larger diameter, so more chance of having the atmospheric turbulences showing up on the image or something linked to the depth of field ?) but when I read the "less than good" reviews of the larger diameter scopes (99+mm), I do wonder if larger is always better.

 

[Binastro]

An 11 inch aperture long focal length refractor used to be parked on a truck in New York city for views of the top of the Empire State building.

A small fee was charged.

Certainly better than an 80mm spotting scope.

My 150mm Maksutov was certainly better than smaller scopes at distances up to 5 miles during the daytime, especially viewing over water.

The secret is to choose good Seeing conditions.

If one views through turbulent air one just sees poor images.

At night 16 inch, 24 inch and larger scopes such as the Pic Du Midi 1 metre scope are incredible, as is the 1 metre scope near Nice in France.
On a particularly superb night the Pic Du Midi scope was used at 1500x on Mars to provide some of the best views ever seen from ground based scopes.

Regards,
B.

 

[Tringa45]

An 11 inch aperture long focal length refractor used to be parked on a truck in New York city for views of the top of the Empire State building.

A small fee was charged.

Certainly better than an 80mm spotting scope.

My 150mm Maksutov was certainly better than smaller scopes at distances up to 5 miles during the daytime, especially viewing over water.

The secret is to choose good Seeing conditions.

If one views through turbulent air one just sees poor images.

At night 16 inch, 24 inch and larger scopes such as the Pic Du Midi 1 metre scope are incredible, as is the 1 metre scope near Nice in France.
On a particularly superb night the Pic Du Midi scope was used at 1500x on Mars to provide some of the best views ever seen from ground based scopes.

Regards,
B.

A big reflector or big achromat will certainly allow more magnification than a good spotter but terrestrially the reflector will lack contrast and the achromat will show false colour.
There was a thread on CN recently, asserting that smaller faster apochromats could outperform long achromats.
Some years ago at a bird festival I looked through a 25x150 Fujinon binocular (achromatic version) as used by Hyakutake. Terrestrially it was a big disappointment and I believe many who have had the chance to look through large refractors of the late 19th century at observatories have been underwhemed.
One of the CN moderators, who presumably has experience with astronomical refractors, had high praise for the Nikon ED82.
It would be interesting if Henry or Joachim (jring) could provide us with some comparisons.

Regards,
John

 

[Binastro]

Hi John,

They were underwhelmed because they saw CA.

But many great discoveries were made with these large refractors, and some, such as the great Meudon 32 inch are still used on Mars.

There is a great difference between the optical folk looking for perfection, who are armchair astronomers, and observers who actually make discoveries despite imperfect scopes.

I think the Yerkes 40 inch has 5 inches between red and blue focus.

Also the 1.5mm exit pupil preference is I think a personal thing, as I have no problem with 0.7mm or 0.5mm exit pupils when needed.

We probably agree to differ, but I much prefer an excellent long focus doublet or triplet to a fast ED scope.

Also my reflectors have plenty enough contrast for me.

Regards,
B.

 

[Binastro]

When it comes to the military, who are amongst the most demanding of users, it is usually mirror optics that are used, both visually and for automatic electronic detection.

I have seen Italian police use Swarovski spotting scopes and unknown night vision devices.

Police use regular binoculars, as do servicemen, but for high magnification and detection at tens of kilometres, it is mirror optics.

There are of course the Safran laser/ thermal devices also.

I think the Zeiss 20x60S is used on the ISS.

But I understand that for birdwatching spotting scopes are preferred.

Regards,
B.

 

[Tringa45]

When it comes to the military, who are amongst the most demanding of users, it is usually mirror optics that are used, both visually and for automatic electronic detection.

I read recently that US and British military use a 12-40x60 Leupold scope for a certain application.
Would have thought that if they needed a milrad reticle, a Swarovski STR-80 weould have been the better choice.
Surely not a case of 60 mm being better than 80 mm?

Regards,
John

 

[RobMorane]

Binastro,

For info, HUMINT sections from the French army (13RDP and GCM) are using Swarovski STS 80 Scopes for more than a decade (together with a Nikon DSLR Pro camera behind).

 

[jring]

It would be interesting if Henry or Joachim (jring) could provide us with some comparisons.

Hi,

it seems I was called... so I complied and put the astro refractors out on the balcony for some close up grey herons...

ED 120/900 with Baader Zoom at 8mm for 113x and ED80/600 with Nagler 5mm for 120x - both images are quite sharp today at 25 degree centrigrade over water, gentle breeze and partially cloudy. The ED120 is brighter and shows a bit more detail but I can count feathers or read a ring easily with both at 150m or so.

But yes, I remember having the same combo on the moon in bad seeing after a hot day with the ED80 showing a nice and sharp view for a second at the same magnification while the ED120 did not show those lucky views...

Joachim, who cannot really use the 8" dobsonian for terrestial quickly as the balcony railing is in the way...

 

[Binastro]

The British police used the Vivitar 600mm f/8 solid cat with probably Nikon cameras.
At £605 new about 1980 they were expensive.

I got mine from a professional photographer who used it for one job. Cost £120. Easily splits epsilon Lyrae double at about 2.3 arcseconds at 180x visually.
He charged £2,000 a day for his work.
His 10x8 inch transparencies of beautiful women were mind blowing.
Reduced in size for magazines.

The Questars came from the army ex gov auction. There were five, I got two. Complete outfits with all accessories in cases.

The Den Oude Delft military 140mm Maksutov was also ex gov. Absolutely superb on Saturn visually.
Unfortunately the buyer wouldn't sell it to me.

I am told that similar lenses can lock onto car door handles at about 90 km from aerial platforms.

The 8 inch f/1.4 Taylor Hobson lens also came from a professional photographer who used it for train locomotive photographs on a large format camera.

The TTH 6 inch f/1.5 lenses came from the Baird television system. Three were used.

The Schneider T.V. lenses are also of the highest quality.

Regards,
B.

 

[Tringa45]

Thanks, Joachim, but I was thinking more in terms of astro vs. birding, i.e. the ED80 vs. your TSN.

John