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My husband and I are complete amateurs. Tonight was only our second night using our new Orion XT6 Dobsonian telescope. Betelgeuse looked very clear, bright red. However, when we pointed the telescope at it, it appeared in a pulsing bright rainbow of colors.

What did we see? Our telescope is a reflecting telescope, so we didn't expect chromatic abberation. Did we manage to see the rainbow nebula around Betelgeuse? Or, did this have something to do with light pollution, which was pretty bad in that part of the sky?

Location: the Netherlands

Telescope: 6" Dobsonian, 25 mm eye piece

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  • $\begingroup$ Re "Tonight": What was the exact time? And the approximate latitude and longitude (within 1° or so). (From this, the altitude of Betelgeuse can be computed.) The highest possible altitude of Betelgeuse is approx. 45° ((90° - 52°) + ). $\endgroup$ Nov 22 at 16:52
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screenshot from YouTube video "Stars through a telescope 1" https://youtu.be/LdigmkmEjUo screenshot from YouTube video "Stars through a telescope 1" https://youtu.be/LdigmkmEjUo screenshot from YouTube video "Stars through a telescope 1" https://youtu.be/LdigmkmEjUo

...pulsing bright rainbow of colors...

Sounds like very bad astronomical seeing effects. Even a steady atmosphere acts like a weak prism and in fact all dielectrics tend to bend blue light more than red.

Astronomical seeing is basically disordered refraction due to turbulence and it has a chromatic component as well. A bright point-like object seen through thick turbulent air will definitely appear to "twinkle" not only in intensity but in color, since the intensity noise will be different for blue than green or red.

Your Dobsonian has quite a large light collection area so even a random star, not otherwise considered bright, will become extremely bright when viewed through it.

So there's probably nothing amiss with your telescope based on this observation. If you'd like to see the effect reduced, you can move to extended objects (as opposed to unresolved, point-like stars) such as planets and nebulae and even the disk of the moon at very high magnification when it won't be so overwhelmingly bright. Try the terminator; the line between the bright and dark areas of the moon where shadows are long and the 3D nature of the surface is more dramatic.

You can also try:

  • waiting until much later at night when the Earth's surface and atmosphere has cooled a bit and there's less turbulence
  • looking closer to the zenith where you light path traverses less atmosphere than it would looking closer to the horizon
  • looking in directions away from any hotter areas like urban centers
  • if possible, moving your scope to a substantially higher elevation (helps only if the source of the turbulence is local)

enter image description here source


cued at 12:23 watch for about 30 seconds to see the color scintillations.

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    $\begingroup$ Moving the scope to a substantially higher elevation isn't easy in the Netherlands. "The Vaalserberg is a mountain with a height of 322.4 metres (1,058 ft) above NAP and is the highest point in the European part of the Netherlands". $\endgroup$
    – PM 2Ring
    Nov 22 at 1:24
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    $\begingroup$ @PM2Ring indeed! While the question addresses the OP directly it is also meant to be helpful to future readers as well ${}^1$; and for some, a vehicle and some mountains might be available. I've been thinking about assembling a DIY Dobsonian for myself; I don't have a vehicle but if it's transportable enough (disassembled) that I can bring it on public transportation there are plenty of mountains within reach here. $${}^1 \text{SE is both a floor wax and a desert topping}$$ $\endgroup$
    – uhoh
    Nov 22 at 2:14
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    $\begingroup$ In the right conditions, one can see our lovely Sun in the same fashion without any optical equipment : en.wikipedia.org/wiki/Green_flash $\endgroup$
    – fraxinus
    Nov 22 at 12:12
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    $\begingroup$ Also, collimation could help, as well as simply letting the air column inside the telescope cool down, far away from hot air flowing out of indoor rooms. $\endgroup$ Nov 22 at 14:21

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