New Video: Astronomical Seeing

[theskyhound]

I have released a new tutorial video for both SkyTools 4 Visual & Imaging. It explores how the Seeing selection in SkyTools affects the calculations.

https://youtu.be/xB9IjwRD0Lg

I have been struggling to find a way to make my tutorials both better and less time consuming. This one came together very quickly, and it is my hope that I have found a good formula that will allow me to make many more. I need to remake some of the basic tutorial videos, but in addition to that, I intend to create one relatively short video every week or two that will focus on on aspect of SkyTools and how it relates to observation planning. This video covers both Visual and Imaging, but I'll probably trade off for each of them in the future.

Please let me know what you think of it, and if you have suggestions for videos that you would like to see.

[PMSchu]

Hi Greg,

The video looks very good. I noticed 2 things though. When the video was talking about viewing the moon, the sun was shown on the video at 1:02. Also there are 5 different Seeing settings in ST4v & 7 in ST4i. I guess that the 6 mentioned in the video is nitpicking.

Phil S.

[theskyhound]

Phil, thanks, but I was really asking about the style, not the details.

[obrazell]

Interesting as a very basic guide to seeing but my experiences are that telescope aperture also has an effect on seeing as you are potentially looking through more air cells and that impacts the view as well.

[theskyhound]

Interesting as a very basic guide to seeing but my experiences are that telescope aperture also has an effect on seeing as you are potentially looking through more air cells and that impacts the view as well.

Hi Owen,

I've heard that repeated all my life, but I never thought it made sense, if for no other reason than these simplified "cells" likely vary in size throughout the atmospheric column. Yet people do report that a larger aperture telescope appears to be more affected by seeing, including in side-by-side tests.

An alternative explanation that has also been around for a long time is more subtle: the resolution limit of a smaller aperture is less than that of a larger aperture, so when viewing stars in telescopes of identical design, but different apertures, there is a tendency for the larger aperture to be employing a higher effective magnification. As a result, the turbulence is more apparent in the larger aperture.

But there is another factor, and I hazily recall this being explored in an old article in Sky & Telescope: when we compare large and small apertures we are seldom comparing telescopes of the same optical design. Rather, we are typically comparing a small refactor to a larger reflecting telescope. In this scenario, it is internal thermal currents in the reflector that may account for the apparent difference in seeing. There is also the added time it takes for to reach thermal equilibrium, including the thick mirror. This difference is something that may also show up when comparing large and small reflectors. 

All that being said, ultimately astronomical seeing is subjective. For visual observing we aren't usually putting that fine of a point on it. An experienced observer can usually estimate the relative quality of the seeing by looking at at bright stars via the naked eye. For a more precise estimate, one would use the Pickering scale on a star nearly overhead.

The same applies to imaging, except that the FWHM can often be measured for a star near the zenith.

In the end, when we make our own estimate it doesn't really matter if the person next to us may have a somewhat different experience.

[PMSchu]

Hi Greg,

During my observing days with the 13" Coulter Odyssey newtonian, Coulter recommended using a 5" off-axis mask for planetary & lunar observing. In my experience, the image with the mask was much clearer & exhibited smaller star images while the full aperture stars were rather bloated. This was especially true earlier in the night as the atmosphere was cooling. With the mask, the stars shifted their positions in the eyepiece, but were sharp, while at full aperture the stars didn't move as much, but appeared larger.

The effect was attributed to the seeing cells being ~6" in diameter such that the 5" aperture's light path passed through a single set of cells & would shift with the cells, but give sharp images. At 13" the light passed through several cells & the image got fuzzier, but was more stable in the eyepiece.

Several experienced observers in the astro club said that the view of Saturn with the masked aperture was among the best that they'd seen.

The mask may have also reduced the tube currents.

Phil S.

[theskyhound]

Hi Phil,

I think what the mask really does is reduce optical aberrations. A mirror, like our eyes, is figured much better closer to the center. Optical aberrations mostly occur near the outer edges, so an aperture mask can reduce them, improving the quality of the image.

I could well be wrong, but I think this seeing cell idea is bunk. I did some digging and could not find any actual science that supported the seeing cell claim. Its all just hand waving as far as I can tell.

I'd love to see actual research on this, if anyone can dig it up.

[PMSchu]

The 5" mask was an off-axis mask that used an unobstructed portion of the mirror - no secondary mirror involved. The scope functioned as an achromatic optical system since the image was formed by reflective optics. I also had an 8" on-axis mask, but didn't use it much. It effectively increased the effects of the central obstruction on the image, reducing contrast & resolution. I can't recall if it helped with the size of the star images. It did help to reduce the brightness of the moon.

All I have is the observed effect on the behavior of the star images. The off-axis mask helped reduce the size of the stellar images in the eyepiece, but they shifted around more. This suggests that seeing cells were causing the image through the 5" aperture to show the effects of "twinkling" - shifting the image. At full aperture the stellar image expanded. I interpreted that to be caused by an integration of the several possible 5" apertures produced by the 13" aperture simultaneously.

I've never looked for any papers on the subject.

I did find this S&T article from 2006:
https://skyandtelescope.org/astronomy-eq...he-seeing/

Phil S.

[theskyhound]

"This suggests that seeing cells were causing the image through the 5" aperture to show the effects of "twinkling" - shifting the image. At full aperture the stellar image expanded." -- I think that's quite consistent with the reduction of aberrations, not to mention removing the central obstruction, which is also going to make a big difference. You could see more detail, but the movement of the star indicated that seeing wasn't affected that much.

That's a pretty good article! The best part is at the end with the advice to be patient when observing visually.

Here's my line of thinking: central obstructions, optical aberrations, tube currents, longer times to reach thermal equilibrium--these things are all measurable and documented. The seeing cell idea has no direct evidence as far as I can tell. Its just what someone thought made sense, and historically, that has often proven to be a red flag.

[obrazell]

I thought Coulter telescopes had notoriously poor optics so stopping it down would have been a good thing.