<p>The blind spot is a limitation on tracking an object through zenith, not on where the telescope can move to. An alt-azimuth telescope can rotate around two axes. If an object transits through the zenith then the rate of change of azimuthal angle increases, <em>when tracking it</em>, and becomes very large when the object is near zenith and then decreases again.</p>
<p>The motors on the telescope drives will have a limit as to how fast they can rotate the telescope in azimuth. That means there will be a &quot;no go&quot; region of the sky near zenith where the motors cannot rotate the telescope quickly enough (or accurately enough, e.g., see <a href="http://www.astro.uni.torun.pl/%7Ekb/Papers/ActaA/NearZenith.htm" rel="nofollow noreferrer">Borkowski 1987</a>) to continue to track the object. This leads to a &quot;blind spot&quot;, typically within a degree of zenith (e.g., 0.2 degrees for the <a href="https://www.ing.iac.es//Astronomy/telescopes/wht/index_jma.html" rel="nofollow noreferrer">William Herschel Telescope</a>, where I have been caught out by this, resulting in prematurely having to finish an exposure).</p>

CC BY-SA 4.0
2022-12-29T04:08:48.793Z
Oh, I see. All this time I was only considering an object that exactly crossed the zenith; as long as the altitude drive can pass smoothly through 90° degrees then the azimuth doesn't have to do anything. BUT if you want to keep the axis pointing at a spot that passes 0.5° from the zenith, THEN you need do set the azimuth to "warp speed" to slew by 180°. Of course there's also the issue of rapid field rotation if you slew azimuth by 180° that doesn't happen if you can pass smoothly through the zenith and keep on going.
uhoh
CC BY-SA 4.0
2022-12-28T23:54:42.443Z
2022-12-29T07:00:04.593Z
2022-12-29T07:00:04.593Z
ProfRob
<p>I understand the subject mathematically. Azimuth of the celestrial object under track changes a finite 180 degrees instead of a infinitesimal amount as the body passes zenith. This would require infinite rotational speed from the telescope, which is impossible.<br />
But what I need is really a physical and intuitive picture. I just don't understand why the telescope can't simply 'flip over' around the horizontal axis. And how would the telescope actually move to keep track of the object?<br />
I've been searching for a educational illustration or animation, but the best result google offers is this Youtube video <div class="youtube-embed"><div>
                <iframe width="640px" height="395px" src="https://www.youtube.com/embed/N9BIOL16lhE?start=0"></iframe>
            </div></div> and this picture from Wikipedia.<a href="https://i.sstatic.net/nHHSy.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/nHHSy.png" alt="enter image description here" /></a> They both illustrate the concept of alt mount nicely, but leave me no clue for the concept of blind spot. I've been staring at them for 20 minutes, trying and failing to mentally picture the process.<br />
Any help would be appreciated, especially if one with an illustration. Thanks!</p>

CC BY-SA 4.0
2022-12-28T10:33:43.687Z
*Welcome to Astronomy SE!* Who says there's a blind spot? I'm not saying there isn't, but it would be better to cite a source, it may help answer authors. Of course some mounts don't have to stop their altitude motion at the zenith and can pass right through it and back down to the other horizon, but that may prove inconvenient for some telescopes. Certainly the big boxy mount with the short stubby telescope in the video can do it, but a long telescope in a short fork can't for example.
uhoh
CC BY-SA 4.0
2022-12-28T10:36:13.220Z
this could http://themcdonalds.net/wp-content/uploads/2017/05/Astro-SetAZ-13-SV105-Discmount-IMG_0605.jpg but this couldn't http://themcdonalds.net/wp-content/uploads/2017/05/Astro-SetAZ-1-105-Posed-DSCN0031.jpg (from [here](http://themcdonalds.net/setting-up-an-alt-az-mount/))
uhoh
CC BY-SA 4.0
2022-12-28T16:01:18.103Z
Many large observatories use AltAz mounts, like the 8.2m "Very Large Telescope".  They make no mention of a blind spot.  Amateur Alt-Az telescopes generally do have this blind spot, since they are rarely used for long exposure imaging it doesn't play much role in their function.
Greg Miller
CC BY-SA 4.0
2022-12-29T00:15:08.607Z
@GregMiller AFAIK the VLT blind spot is within about 0.5 degrees of zenith.
ProfRob
<observational-astronomy><telescope><astrometry><azimuth>
Why does the altitude-azimuth mount have a blind spot near zenith?
Splinter_X
# Why does the altitude-azimuth mount have a blind spot near zenith?

(Asked by: username_0 on 2022-12-28)

I understand the subject mathematically. Azimuth of the celestrial object under track changes a finite 180 degrees instead of a infinitesimal amount as the body passes zenith. This would require infinite rotational speed from the telescope, which is impossible.  
But what I need is really a physical and intuitive picture. I just don't understand why the telescope can't simply 'flip over' around the horizontal axis. And how would the telescope actually move to keep track of the object?  
I've been searching for a educational illustration or animation, but the best result google offers is this Youtube videoand this picture from Wikipedia.[![enter image description here](https://i.sstatic.net/nHHSy.png)](https://i.sstatic.net/nHHSy.png) They both illustrate the concept of alt mount nicely, but leave me no clue for the concept of blind spot. I've been staring at them for 20 minutes, trying and failing to mentally picture the process.  
Any help would be appreciated, especially if one with an illustration. Thanks!

**Tags:** observational-astronomy, telescope, astrometry, azimuth
> **Comments:**
> **username_1**: *Welcome to Astronomy SE!* Who says there's a blind spot? I'm not saying there isn't, but it would be better to cite a source, it may help answer authors. Of course some mounts don't have to stop their altitude motion at the zenith and can pass right through it and back down to the other horizon, but that may prove inconvenient for some telescopes. Certainly the big boxy mount with the short stubby telescope in the video can do it, but a long telescope in a short fork can't for example.  
> **username_1**: this could http://themcdonalds.net/wp-content/uploads/2017/05/Astro-SetAZ-13-SV105-Discmount-IMG_0605.jpg but this couldn't http://themcdonalds.net/wp-content/uploads/2017/05/Astro-SetAZ-1-105-Posed-DSCN0031.jpg (from [here](http://themcdonalds.net/setting-up-an-alt-az-mount/))  
> **username_2**: Many large observatories use AltAz mounts, like the 8.2m "Very Large Telescope".  They make no mention of a blind spot.  Amateur Alt-Az telescopes generally do have this blind spot, since they are rarely used for long exposure imaging it doesn't play much role in their function.  
> **username_3**: @GregMiller AFAIK the VLT blind spot is within about 0.5 degrees of zenith.  

---

## 1 answers

### Answer by username_3 on 2022-12-28. Score: 3

The blind spot is a limitation on tracking an object through zenith, not on where the telescope can move to. An alt-azimuth telescope can rotate around two axes. If an object transits through the zenith then the rate of change of azimuthal angle increases, *when tracking it*, and becomes very large when the object is near zenith and then decreases again.

The motors on the telescope drives will have a limit as to how fast they can rotate the telescope in azimuth. That means there will be a "no go" region of the sky near zenith where the motors cannot rotate the telescope quickly enough (or accurately enough, e.g., see [Borkowski 1987](http://www.astro.uni.torun.pl/%7Ekb/Papers/ActaA/NearZenith.htm)) to continue to track the object. This leads to a "blind spot", typically within a degree of zenith (e.g., 0.2 degrees for the [William Herschel Telescope](https://www.ing.iac.es//Astronomy/telescopes/wht/index_jma.html), where I have been caught out by this, resulting in prematurely having to finish an exposure).

> **Comments:**
> **username_1**: Oh, I see. All this time I was only considering an object that exactly crossed the zenith; as long as the altitude drive can pass smoothly through 90° degrees then the azimuth doesn't have to do anything. BUT if you want to keep the axis pointing at a spot that passes 0.5° from the zenith, THEN you need do set the azimuth to "warp speed" to slew by 180°. Of course there's also the issue of rapid field rotation if you slew azimuth by 180° that doesn't happen if you can pass smoothly through the zenith and keep on going.  

---

