Astronomy amateurs myths – part I

Deep sky (extended) objects seen with telescope have greater surface brightness than objects observed with naked eye – FALSE

It’s quite unintuitive, but it is so 🙂 The human eye in connection with the brain is a very complex sense, the perception of weak light sources is influenced by several factors – their brightness, background brightness and angular size of the object, and also personal predispositions. The minimum magnification of the telescope, when the exit pupil of the instrument has the same diameter as the pupil of our eye, will give us exactly the same surface brightness as observations with the naked eye. Increasing the magnification above the minimum will reduce the surface brightness of nebular objects. But at the same time their observed angular size will increase and the background brightness will decrease. The combined effect of all these factors means that we start to see nebulae or galaxies in the telescope. Optimal magnification for observation of different object varies and depends on many factors – object apparent size, surface brightness, sky background brightness.
Based on “Visual Astronomy of the Deep Sky” – Roger N. Clark

These astronomy images are really amazing – I will buy a telescope and see it all with my eyes – FALSE

Astronomy becomes more and more popular among people (that is good) , but in the same time more and more people believe that photos reflects real views precisely. This is not true neither in astronomy nor in many other areas. Go outside of the city and look at the stars. Every second retina is literally bombarded with millions of photons that were born in the objects placed many light years away. All these light pass through two small, seven millimetres holes in your eyes. Then your brain needs to process these information in real time to create the image of the stars. On the other hand – astrophotographies contains data collected over many hours of integration with telescopes that have apertures of hundreds or thousands millimetres and detectors that are capable to record almost each photon (human retina can also record single photon, but neural filters only allow a signal to pass to the brain to trigger a conscious response when at least about five to nine arrive within less than 100 ms). After many hours of integration image is processed by many hours to reveal the best, and present it to you. It is not possible to see the space gems in the same way they are presented in photos. Even using the largest telescopes. But looking through telescope at distant nebulae, clusters or galaxies brings its own thrill.
See also “Human Vision and The Night Sky” – Michael Borgia

Abell 21 planetary nebula

In a fast (short focal ratio) telescope, nebular objects are brighter than in a slow one – FALSE.

If we take two telescopes, e.g. 8 “f / 5 and 8” f / 7, and put both an eyepiece with the same focal length in both, then we will achieve different exit pupils (at f / 7 smaller) and the images will actually have both different apparent dimensions and different surface brightness.
But if we use eyepieces with different focal lengths, so that the magnification in both tubes is the same, then we achieve the same exit pupils, and the image will also be the same! This is due to the facts, that both telescopes gather the same amount of light (both have the same aperture), and the apparent image size is the same, so it must also have the same brightness.

Expensive, sophisticated eyepieces correct coma in newtonians – FALSE

It is not truth at all. Expensive and sophisticated eyepieces works like listening quality monitors in the sound engineer’s cabin. They can’t fix anything if it’s broken, they perfectly reproduce the material they get. In other words, they will show to our eyes exactly what they “see” in the image plane in the main focus of the telescope. If the telescope (newtonian parabolic mirror), creates images of stars affected by coma, a perfectly corrected eyepiece will show an excellent “book” coma of a parabolic mirror. The only method to remove it is coma corrector. In cheaper glasses, the coma is masked by the eyepiece’s own aberrations, i.e. when the eyepiece “adds something from itself” to the image, that already has been degraded by the mirror. Then, instead coma commas, we can see patches of astigmatism, blur and other aberrations, with the mirror coma stitched “underneath”.

Each eye adapts to the dark separately – TRUE

The adaptation of the eye to the dark is mainly a chemical reaction – the accumulation of rhodopsin pigment in the retina of the eye, which is responsible for night vision, i.e. scotopic vision. In each eye, rhodopsin regenerates independently, and even in each rod regardless of neighbours. So we can observe with our “better” eye, then put on the band and read the maps with the other eye. This however requires some attention, because a mistake with the eyes will cost us a lot of time to re-adapt 🙂

These beautiful red nebulae will look great in the telescope’s eyepiece – FALSE

The human eye only uses rods for night (scotopic) vision. The rods have their maximum sensitivity for the green-blue colour for a wavelength of about 500 nm. In the red range rods are completely blind (hence we use red flashlights that do not spoil the adaptation of the eye). Therefore, while observing the emission nebulae we do not really see the light coming from the red H alpha hydrogen emission line (about 656nm). We actually see the light from the H beta line and Oiii and they shape the image of emission nebulae observed by the eye. And there are no visual H alpha filters either.

There are no green stars – FALSE

Although this statement is not very precise. It is the surface temperature of the star that determines the wavelength at which it radiates the most. The hotter the star, the shorter the wavelength. When surface temperature is about 5500K, the star will radiate the most in green, but at the same time a large part of the energy will be emitted in the other colours. Then our eye, in conjunction with the brain, will interpret this colour mixture as white, not green. Hence, the stars shines most strongly in the green spectrum are recorded by us as white.
The green shade of the star can sometimes be seen in binary systems, where the hot, blue star has a bright but cool companion glowing orange or red. Optical illusion caused by the proximity of a bright red object can make the blue companion appear green to us (Antares, Almach).
Zubeneschamali (β Lib) in turn is a bright star (2.6m), which a large group of observers perceive as green or pale emerald. Its surface temperature is 11,000K.

Planckian locus with star color range

Clear skies!

Based on the original thread
Cover photo by Jaredd Craig on Unsplash

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