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The video Earth's Rotation Visualized in a Timelapse of the Milky Way Galaxy - 4K (linked below) and discussion below this answer to Why does a timelapse video of a stationary Milky Way make the horizon appear to move from horizontal to vertical? about field rotators has got me thinking.

In addition to the benefit of (essentially) single axis constant speed tracking for distant celestial objects, an equatorial mount also rotates the telescope tube about it's axis such that there is no rotation of the image on the focal plane during long exposures.

This was probably essential for hours-long exposures of single emulsion plates.

Asked separately: Did astronomers ever use photographic plate rotation along with alt-az mounts?

But for the very largest and heaviest reflecting telescopes equatorial mounts are massive and unwieldily and require huge counterbalances compared to alt-az mounts which can have the azimuth bearing right on the ground and the altitude bearing straight through the telescope's center of mass.

Question: What are the technological advances that made it possible for modern large telescopes to work with alt-az mounts instead of equatorial?

I'm expecting most of the answer to be related to electronic advancements of one kind or another, but there may be mechanical and optical and even electro-optical (optoelectronic?) advancements as well.

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  • $\begingroup$ what kind of tech advancements you would like to know ? $\endgroup$ Dec 26, 2021 at 12:42
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    $\begingroup$ @KavinIshwaran the ones "that made it possible for modern large telescopes to work with alt-az mounts instead of equatorial mounts. I'm expecting most of the answer to be related to electronic advancements of one kind or another, but there may be mechanical and optical and even electro-optical (optoelectronic?) advancements as well." That kind. $\endgroup$
    – uhoh
    Dec 26, 2021 at 13:01
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    $\begingroup$ @KavinIshwaran "precise computerized navigation in mounts" may have several aspects, what makes these so precise? Which technologies have been developed to correct for field rotation (for emulsion?, for CCD?) $\endgroup$
    – uhoh
    Dec 26, 2021 at 13:27
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    $\begingroup$ in large optical telescopes the orientation of sensor array itself rotates as the earth rotates to avoid field rotation. in any kind of radio telescopes, which are mostly synthetic aperture type, don't always take field rotation into concern $\endgroup$ Dec 26, 2021 at 13:40
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    $\begingroup$ I doubt there were any real advancements other than a simple computer. It's more that at some point it's cheaper to have three high precision drive axis, rather than one which is tilted and rigid enough to hold the scope's weight + counterweights. Amateur astrophotographers still prefer eq mounts simply because they're cheaper, not due to some technological shortcomming. $\endgroup$ Feb 2 at 5:09

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I would say the field derotator was a great advancement. The field of view of an altazimuthally mounted instrument will rotate, just like for the naked eye—for example, the “Lady in the Moon” seems to be tilted left at moonrise, straight at transit, and tilted right at moonset. If one is to take long-exposure images (more than about a minute, for example, on my 10″ ƒ/4.7 Dobsonian), one need to “derotate” the image so it stays straight on the image.

Derotating an image seems easy in practice, but the exact speed at which the field rotates varies with the azimuth and altitude of the target. Some computing power is thus needed “live” to drive the motors at the proper rate depending on where the telescope is pointing.

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    $\begingroup$ Then computers and stepper motors were probably critical enabling technical advances as well, no? Also, are you sure the rate of rotation is not constant? Doesnt every field rotate 360° in 23h 56m 4s? Can you add a citation (or the math) that shows why a plate rotator can't a simple, steady clock mechanism, independent of where one is looking or when? $\endgroup$
    – uhoh
    Feb 3 at 1:26
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    $\begingroup$ Not right now, but I’ll try to find time for that soon. I know that due East or due West, field rotation is zero, which means that rotation can not be constant. $\endgroup$ Feb 3 at 3:02
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    $\begingroup$ Why don't I ask that as a new question instead, that will give more time to research, and make the information more findable if that's the question of the title. $\endgroup$
    – uhoh
    Feb 3 at 5:00
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    $\begingroup$ @uhoh Stepper motors are pretty simple tech that had been around a long time. But a stepper doesn't tell you where you're pointing. For that you need an encoder. We managed to point a low resolution telescope open loop for the observations in ui.adsabs.harvard.edu/abs/1975ApJ...197L..83R/abstract, but it wasn't a method we would have used if our downlink was working right. Not trustworthy. $\endgroup$
    – John Doty
    Feb 3 at 12:31
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    $\begingroup$ @uhoh We had a star camera that recorded where we were pointing on photographic film for later analysis. We were developing a CCD camera for use in operation, but it wasn't ready. But that development led us into other CCD applications, ultimately into space with ASCA, Suzaku, HETE-2, Chandra, and TESS. $\endgroup$
    – John Doty
    Feb 4 at 2:06
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The equatorial mount is an analog computer that performs the transformation from local terrestrial coordinates to celestial coordinates. Building it into the structure of the telescope is necessary for the analog approach: while you can make a smaller model, there is no analog technology that can transfer the result to a larger structure with the required accuracy.

So, the obvious innovations are the digital computer along with digital angle encoders to allow it to tell where the telescope is pointing. Sounds simple, but getting computers to operate reliably on mountaintops required a fair amount of finesse: shaky power, lightning-induced currents, ESD, temperature fluctuations and other hazards (Mauna Kea ash is nastier than ordinary dirt) needed addressing. The mount, of course, was not the only thing people were interested in automating.

The question notes that “the very largest and heaviest reflecting telescopes equatorial mounts are massive and unwieldily and require huge counterbalances compared to alt-az mounts which can have the azimuth bearing right on the ground and the altitude bearing straight through the telescope's center of mass.” This is true, but it also reflects old misunderstandings.

The first big alt-az telescope was the Soviet BTA-6. It indeed mitigates the structural problems associated with balancing its massive mirror. It is, however, a mediocre telescope, plagued by poor "seeing".

The next one was Smithsonians's MMT. In its orginal form, it used six lightweight mirrors supported by a lightweight structure in a boxy building of minimal size. The whole building rotates in azimuth, while the telescope within rotates in elevation.

Unlike the BTA-6, the MMT was a very good telescope. It had remarkably good seeing. The contrast here changed the way that astronomers thought about telescope architecture.

Why the difference? The BTA-6 has enormous thermal inertia in its heavy mirror, heavy structure, and oversized dome. That means that it never really comes close to thermal equilibrium with the air around it. It's always surrounded by turbulent convection currents. Atmospheric turbulence blurs images, an effect astronomers call "seeing". On the other hand, the lightweight MMT operates much closer to thermal equilibrium, so there is less turbulent convection.

Before the MMT, seeing was thought to be mainly a property of the site and not the telescope. The MMT demonstrated that for big telescopes, much of the seeing originated in the telescope and observatory structure.

So, it wasn't simply that technical innovation enabled alt-az mounts for huge telescopes. It was necessary to find the correct motivation to use them. As it turned out, dealing with huge mass was not good motivation. Alt-az mounts became part of a strategy to make lighter telescopes with better thermal properties.

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  • $\begingroup$ I really like your answer, but I think it also deserves a new + better question, so that people looking for this perspective will more likely be able to read it. Perhaps something like "technologies that allowed big telescopes to be effective (besides adaptive optics)?" Adaptive optics has its own series of questions. I'll ping back here when I ask it. $\endgroup$
    – uhoh
    Feb 4 at 1:35

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