Crisscrossing versus bubbling

[Ben the Ignorant]

Last week Crossway asked this...

...and I wanted to illustrate the answer with images but clouds don't obey me so I postponed this until now.

 

The crisscrossing I talked about has these patterns:

Dark and brighter lines, uneven in width but straight, these are the result of rough industrial polishing, they stop when the glass looks flat and smooth enough to the naked eye, although modern windows are far flatter and smoother than those of decades ago, which often showed some waviness to the eye alone, even when not looking at an angle. Straight-through was good enough but not when the view was magnified as little as 10x with a binoc.

 

Taking pictures through another part of the double-pane window:

See the pattern that looks like a compass needle, it is a complex form of astigmatism, but despite being changed a bit by turbulence, it is mostly the same. The needle is on top of a faint cross that remains visible, the two indentations at the top and bottom of the pattern are preserved, too. The overall rounded diamond shape of the outer ring is almost maintained, too. The color swing is because my phone camera doesn't know what to do in the dark, especially when settings are modified. But we're only talking shapes here, not camera whims.

The star is Procyon shot with a semi-apo 80mm f/7 scope and a 10mm Hyperion, focus was not changed at all during these and the next photos, and they were made over a few minutes, air conditions didn't change. The line of sight was more than 60° away from perpendicular, so the defects in the glass caused more deformation.

 

Now what will turbulence do to Procyon, and is it preferable to what modern industrial glass does? I opened the window and got these shots:

What is different and common between the the two types of images? In all window shots the outer diffraction ring is visible, but in several no-window shots it is too broken up to recognize. No color mixup with the window, but some lime and yellow-white arcs and dots appear in no-window pics, the glass blocked air draft between my kitchen and the outside, having a calming effect on turbulent break-up. I have often noticed, over all seasons and times of day and night, that observing through a good window can be better because glass astigmatism is offset by that calming effect. But only at low to medium power, of course.

Plus, you can chose a better spot in the window by moving the tripod a few centimeters, but you can't choose a better spot in turbulence, it changes all the time. Accepting acrilyc sunglasses, random colored film or plate glass as telescope accessories is out of the question, but when choice is not possible, the modern window can be an acceptable compromise.

Baader produces a non-aluminized version of its Astro Solar film, and the substrate only doesn't cost much, so while it's difficult to shut a house window with that Turbo Film - but some have done it - shutting a newtonian's tube seems to be worth it. Its optical quality matches that of fine glass optics.

Again, the air-only patterns have smaller, more amorphous, and more numerous deformations, but the window patterns show fewer and straighter defects. So it is not always a sin anymore to observe through windows, especially when extreme cold or a flu decides for you. But that is not an excuse for lowering our optical standards, which by the way are good in the scope that took these shots, its diffraction pattern in an indoors test with an artificial star is pictured here:

So you know I'm not suggesting to compromise on that, and the scope is a valid test of other things' defects because it has none.