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refractor terrestrial viewing to test primary optic quality

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#1 emflocater

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Posted 29 November 2019 - 03:39 PM

Hi Folks. Just curious question...with Black Friday and Christmas sales here, many Folks may purchase a small or high quality refractor telescope. Most Folks being anxious to try the scope out ASAP and running into days if not weeks of cloudy night skies, will try some terrestrial viewing. With that said and many of us refrator owners have done this...what advice can be offered of what to look for when doing some terrestrial viewing that can be good or bad indications of the quality of the scopes primary optic lens, until a star test or clear night sky viewing can be had..

 

True part of this will be the quality of eyepiece used as well as checking for CA (Chromatic aberration) around terrestrial objects such as tree branches, leaves, roof tops and towers, but what other things should one check for visually that are good or bad initial signs of the scopes primary glass lens quality?

 

Cheers

Don


Edited by emflocater, 29 November 2019 - 03:41 PM.

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#2 lylver

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Posted 29 November 2019 - 03:57 PM

Hi Don, astronomical refractors haven't the same requirement of spotting scopes, you know it.

 

My personal opinion.

 

Moon : easier, green-yellow-orange features (525-600nm), 546-589 (e to D ray). Moon is reflecting the sun light and eye choose often yellow as "white point"

Stars : go up to sky and look at clouds features

Planets : more difficult, look for colored fruits/flower with red and many color range.

 

violet/purple as a halo color is around 420nm in terrestrial vision : this is not to be considered with night or dim vision. Distracting be forget it, at night the intensity would be very decreased. (S cônes 447nm are less sensitive than rods that are near 500nm). There is a color perception range that changes.

bezold-brucke-Hurvich1997.JPG

Orange curve at night : daylight violet cut.

luminosite-discrimination-couleur.JPG

 

Color saturation appears with high correction in the color viewed. (well focused)

Color saturation enhances brightness sensation.

This is named the psycho-visual Helmholtz-Kohlrausch effect

(translated)

 

 

Helmholtz-Kohlrausch This is an effect that shows the dependence of purity on luminance and chromaticity (hue). "Color objects appear brighter than achromatic objects with the same luminance. The most saturated colors appear the brightest "(33)


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#3 emflocater

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Posted 29 November 2019 - 04:00 PM

Hi Don, astronomical refractors haven't the same requirement of spotting scopes, you know it.

 

Moon : easier, green-yellow-orange features (525-600nm), 546-589 (e to D ray). Moon is reflecting the sun light and eye choose often yellow as "white point"

Stars : go up to sky and look at clouds features

Planets : more difficult, look for colored fruits/flower with red and many color range.

Very kool ideas!

Cheers

Don



#4 Jon Isaacs

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Posted 29 November 2019 - 04:28 PM

Color correction:

 

A dark power line against a bright sky at high magnifications.  It brings out the CA. 

 

Sharpness: The bright sun on an power line insulator, they'll be a bright point that can serve as an artificial star. 

 

Both require clear skies..which means it might well be clear at night.

 

Otherwise. Make sure you evaluate the optic at high magnifications. At low magnifications, the small diameter of your pupil can mask the aperture and a fast achromat can look quite sharp and color free.

 

Jon


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#5 Steve Allison

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Posted 29 November 2019 - 04:47 PM

"Snap-focusing" is usually indicative of good optics.


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#6 lylver

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Posted 29 November 2019 - 05:10 PM

"Snap-focusing" is usually indicative of good optics.

easy finding of the best focus point : yes

very small range : no, for me it means that this optic system is not stable in front of turbulence.

 

Topmost optic are stable in keeping the best image in view, they are resilient


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#7 j.gardavsky

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Posted 29 November 2019 - 05:25 PM

Hello Don,

 

I am using my 6" F/5 achro refractor both on the astronomy objects, and casual terrestrial viewings.

From our terrace, we have a good view on the golf course across the valley. This requires very good seeing conditions, of course.

 

When I want to compare the performance of the eyepieces, I watch some fine textures, like the lichens on the roofs, as my test targets.

Watching the trees against bright skies will revela how good the contrast performance in the view is.

 

You can also document your tests mounting a camera behind the eyepiece (afocal photography),

 

Delos 17,3mm Zeiss 16,7mm.jpg

 

Delos 17.3mm at left, and Zeiss B WW 30x 16,7mm at right, village 1300m down in the valley at late afternoon.

 

Best,

 


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#8 j.gardavsky

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Posted 29 November 2019 - 05:27 PM

Hello Don,

 

I am using my 6" F/5 achro refractor both on the astronomy objects, and casual terrestrial viewings.

From our terrace, we have a good view on the golf course across the valley. This requires very good seeing conditions, of course.

 

When I want to compare the performance of the eyepieces, I watch some fine textures, like the lichens on the roofs, as my test targets.

Watching the trees against bright skies will revela how good the contrast performance in the view is.

 

You can also document your tests mounting a camera behind the eyepiece (afocal photography),

 

Delos 17,3mm Zeiss 16,7mm.jpg

 

Delos 17.3mm at left, and Zeiss B WW 30x 16,7mm at right, village 1300m in the valley at late afternoon.

 

Best,

JG


Edited by j.gardavsky, 29 November 2019 - 05:28 PM.


#9 emflocater

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Posted 30 November 2019 - 12:54 AM

Very nice suggestions for those Folks checking out their new scope and things to notice when viewing through the eyepiece!

Cheers

Don


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#10 Erik Bakker

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Posted 30 November 2019 - 04:36 AM

Adding to the wonderful recommendations already mentioned, I also use the light in my iPhone to create an artificial star; masking the led light with a few pieces of black tape with a small whole pushed through with a needle.

Always there, quite useful for startesting small scopes at distances from about 15m to 50m, either in house or in a backyard.

 

Just remember that telescope objectives should reach their finest correction at infinity smile.gif


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#11 punk35

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Posted 30 November 2019 - 08:58 AM

At this time of year lots of people are putting up Christmas lights.  Many are led’s. You could look at them in both daytime and on a cloudy night. Plus, you can choose any color you prefer as well. 


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#12 emflocater

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Posted 30 November 2019 - 09:03 AM

At this time of year lots of people are putting up Christmas lights.  Many are led’s. You could look at them in both daytime and on a cloudy night. Plus, you can choose any color you prefer as well. 

Another great idea!!

Cheers

Don



#13 gwlee

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Posted 30 November 2019 - 01:40 PM

"Snap-focusing" is usually indicative of good optics.

In using good quality or better instruments, I haven’t really noticed the “snap focusing” that people talk about. I have looked through poor instruments and poorly adjusted instruments that didn’t seem come to sharp focus though. 
 

The last few years, I have been doing a fair amount of daytime terrestrial viewing with small astronomical refractors, but seeing usually limits magnification to 16-23x, so it’s difficult to tell much about instrument quality from these views. 
 

At night, I star test frequently to access seeing conditions and to confirm telescope adjustments, but seeing is seldom good enough here for me to definitely determine much about optical quality unless it’s a poor instrument. 

 

To tease out the differences between a good quality instrument and an instrument that approaches perfection, I have an artificial star, which works well and can be used indoors with small refractors to get around outdoor seeing problems. In using the artificial star, I have noticed good correlation between price and optical quality. 



#14 barbie

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Posted 30 November 2019 - 02:42 PM

I agree with Steve. When optics snap to focus under even average seeing and the image is very sharp, that's usually a pretty good indicator that the optics have been figured well. In my many years of building telescopes of different types, a well made optical system exhibits this as well as other properties (good stray light suppression, spherical correction), etc. In all of the telescopes I've purchased from vendors, my Takahashi's have been the best in these regards, right up there with my former Astro-physics 5" F12 apochromat. My former Synta made ED100's were excellent as well. In viewing with today's mass produced scopes, stray reflections have really not been much of an issue except for a C90 that I had to do the flocking of the primary baffle tube.
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#15 gwlee

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Posted 30 November 2019 - 04:57 PM

I agree with Steve. When optics snap to focus under even average seeing and the image is very sharp, that's usually a pretty good indicator that the optics have been figured well. In my many years of building telescopes of different types, a well made optical system exhibits this as well as other properties (good stray light suppression, spherical correction), etc. In all of the telescopes I've purchased from vendors, my Takahashi's have been the best in these regards, right up there with my former Astro-physics 5" F12 apochromat. My former Synta made ED100's were excellent as well. In viewing with today's mass produced scopes, stray reflections have really not been much of an issue except for a C90 that I had to do the flocking of the primary baffle tube.

I see the difference in the sharpness of the in-focus views between a good quality scope and a sensibly perfect scope, but don’t see the “snap” that people talk about. Perhaps because one person’s snap is another person’s ooze. 



#16 barbie

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Posted 30 November 2019 - 04:59 PM

Well then I guess you're oozing.lol.gif


Edited by barbie, 30 November 2019 - 05:00 PM.


#17 gwlee

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Posted 30 November 2019 - 06:06 PM

Well then I guess you're oozing.lol.gif

For sure, what I am seeing is a bit less dramatic than “snap” suggests. 



#18 punk35

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Posted 30 November 2019 - 06:08 PM

My current refractor has what I’ll call the “snappiest” focus I’ve seen between a reflector, sct and refractor. It’s not like Pow! Instantly in perfect focus, but more like suddenly it’s almost at best focus, and a small tweak on the fine focus knob makes it perfect. 



#19 25585

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Posted 30 November 2019 - 06:55 PM

Color correction:

 

A dark power line against a bright sky at high magnifications.  It brings out the CA. 

 

Sharpness: The bright sun on an power line insulator, they'll be a bright point that can serve as an artificial star. 

 

Both require clear skies..which means it might well be clear at night.

 

Otherwise. Make sure you evaluate the optic at high magnifications. At low magnifications, the small diameter of your pupil can mask the aperture and a fast achromat can look quite sharp and color free.

 

Jon

Any specific lower & higher limit Jon? Also any extra test for Petzvals?  



#20 gwlee

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Posted 30 November 2019 - 08:14 PM

My current refractor has what I’ll call the “snappiest” focus I’ve seen between a reflector, sct and refractor. It’s not like Pow! Instantly in perfect focus, but more like suddenly it’s almost at best focus, and a small tweak on the fine focus knob makes it perfect. 

 

Yes, some scopes provide sharper images than others, so are easier to focus. “Snap to focus” is a cliche that’s often used to describe this characteristic, but like many cliches, there’s a touch of hyperbole in it. 



#21 barbie

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Posted 30 November 2019 - 09:00 PM

For sure, what I am seeing is a bit less dramatic than “snap” suggests. 

Sounds like your optics may be deficient in some way. After 50 years of making and observing with many telescopes, I know an excellent optic by it's snap to focus and the caliber of its spherical aberration correction. Since I'm only surmising about your experiences and optical quality, the argument is basically now one of subjectivity on your part versus decades of experience and observation on my part. My experience is contrary to yours and I've seen my fair share of optics that didn't snap to focus or provide a sharp image, both homemade and mass produced. I also know when an optic has excellent correction for spherical aberration and when it doesn't. Your seeing conditions that you've described elsewhere in this thread seem to suggest that this may be one of many factors to consider.


Edited by barbie, 30 November 2019 - 10:04 PM.

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#22 Jon Isaacs

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Posted 01 December 2019 - 04:18 AM

Yes, some scopes provide sharper images than others, so are easier to focus. “Snap to focus” is a cliche that’s often used to describe this characteristic, but like many cliches, there’s a touch of hyperbole in it. 

 

:waytogo:

 

I think it's clear that the scope itself, cannot "snap to focus."  What can snap to focus is the observer's eye. Once the scope is close enough to focus that the accommodation of the observer's eye can bring the image to focus, the image could "snap" to focus.  This would seem to be dependent on the accommodation of the observer's eye as well as a number of other factors.

 

Jon


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#23 j.gardavsky

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Posted 01 December 2019 - 06:04 AM

Sounds like your optics may be deficient in some way. After 50 years of making and observing with many telescopes, I know an excellent optic by it's snap to focus and the caliber of its spherical aberration correction. Since I'm only surmising about your experiences and optical quality, the argument is basically now one of subjectivity on your part versus decades of experience and observation on my part. My experience is contrary to yours and I've seen my fair share of optics that didn't snap to focus or provide a sharp image, both homemade and mass produced. I also know when an optic has excellent correction for spherical aberration and when it doesn't. Your seeing conditions that you've described elsewhere in this thread seem to suggest that this may be one of many factors to consider.

This is as well my experience.

It is the quality/performance of the individual optics parts, and how they match together. It applies both to the telescopes (binoculars), and to the microscopes. Some call it the Harmonic Concept of the optics design.

 

Best,

JG


Edited by j.gardavsky, 01 December 2019 - 06:05 AM.


#24 emflocater

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Posted 01 December 2019 - 10:59 AM

So if you notice "snap focus" testing via terrestrial viewing than is it safe to say this would transpose to night sky/planet observing as well?

Cheers

Don 



#25 STEEL

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Posted 01 December 2019 - 12:14 PM

I have done a lot of tests on high magnification eyepieces at night, on advertising or aeronautical panels, better when they are old or dirty because they simulate very well what I can see on the planets, like bands or surfaces.
I also recommend pylons or steel constructions that support illuminated panels, often with old or discolored paint and sometimes with rust.
These tests are very reliable because they take away the doubt of seeing when I compare the eyepieces.

 

Examples:

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