Starship Asterisk*

Chris Peterson wrote:

neufer wrote:
In any event, so far as the telescope is concerned, the target is dead center for both the four separate vanishing points as well as the wider square of the four sodium "artificial guidestars." However, I estimate that the APOD picture was taken a good 400m to the side of the telescope causing the sodium "artificial guidestars" to shift by about a quarter of a degree (from the vanishing point square).

Assuming that the earlier post describing the image was accurate, the distance of the imager was more like 50 meters from the telescope, which would create a 2 arcmin parallax shift, which can be compared with the 10 arcmin diagonal length of the square.

The earlier post describing the distance of the imager was clearly INACCUATE

The distance of the imager was at least 400 meters from the telescope,

• ... and even further than 400 meters if:
  
  • 1) M43 was not near zenith
    
    2) the sodium layer was thinner than 15 km.

(It requires a considerable removal to so easily distinguish between the top & the bottom of the sodium laser beams.)

Chris Peterson wrote:

neufer wrote:

Chris Peterson wrote:
BTW, it is common these days to use a green laser pointer instead of a finder scope in aiming small telescopes. The laser is mounted to the telescope tube, coaxial with its optical axis. Where you see the laser beam end in the sky is where the scope is pointed, and that doesn't change if you move away from the scope.

Try moving 40m away.

People 100 meters away can easily tell where such a telescope is pointing.

And I can easily tell that the APOD telescope was pointing at M43 not M42.

neufer wrote:

Chris Peterson wrote:

neufer wrote:
In any event, so far as the telescope is concerned, the target is dead center for both the four separate vanishing points as well as the wider square of the four sodium "artificial guidestars." However, I estimate that the APOD picture was taken a good 400m to the side of the telescope causing the sodium "artificial guidestars" to shift by about a quarter of a degree (from the vanishing point square).

Assuming that the earlier post describing the image was accurate, the distance of the imager was more like 50 meters from the telescope, which would create a 2 arcmin parallax shift, which can be compared with the 10 arcmin diagonal length of the square.

The earlier post describing the distance of the imager was clearly INACCUATE

Well, the commenter there can provide details if he wishes. For the moment I have no reason to believe it wasn't accurate.

Chris Peterson wrote:

neufer wrote:

Chris Peterson wrote:
Assuming that the earlier post describing the image was accurate, the distance of the imager was more like 50 meters from the telescope, which would create a 2 arcmin parallax shift, which can be compared with the 10 arcmin diagonal length of the square.

The earlier post describing the distance of the imager was clearly INACCUATE

Well, the commenter there can provide details if he wishes.

For the moment I have no reason to believe it wasn't accurate.

• Yes you have:

You can clearly see that width of the sodium "artificial guidestars" have stretched to a width of ~2.5 arcmin.

But you yourself assume that they are ~5km long at ~90km distant ... in which case, from just 50m away

• they should have stretched to a width of only ~0.1 arcmin [= tan^-1(50/87,500) - tan^-1(50/92,500) ]

neufer wrote:

Chris Peterson wrote:

neufer wrote: The earlier post describing the distance of the imager was clearly INACCUATE :!:

Well, the commenter there can provide details if he wishes.

For the moment I have no reason to believe it wasn't accurate.

• Yes you have:

You can clearly see that width of the sodium "artificial guidestars" have stretched to a width of ~2.5 arcmin.

No, I don't see that at all. I don't know if I'm seeing tracking error, I don't know if the lasers were stationary during the exposure, I don't know if the image is a composite. I choose not to make the assumption that I'm looking at a snapshot of the lasers interacting throughout the width of the sodium layer.

In the information brought up through the 'Four laser beams' link http://sguisard.astrosurf.com/Pagim/M42-4LGS.html it states the image was taken using "LRVB filters". I regularly read of LRGB (Luminance Red Green Blue) filters being used in astrophotography imaging but I am having difficulty finding what LRVB filters are. I assume that LRVB is Luminance Red Violet Blue but I would be grateful for confirmation or otherwise.

Chris Peterson wrote:

neufer wrote:
You can clearly see that width of the sodium "artificial guidestars" have stretched to a width of ~2.5 arcmin. But you yourself assume that they are ~5km long at ~90km distant ... in which case, from just 50m away they should have stretched to a width of only ~0.1 arcmin [= tan^-1(50/87,500) - tan^-1(50/92,500) ]

No, I don't see that at all. I don't know if I'm seeing tracking error, I don't know if the lasers were stationary during the exposure, I don't know if the image is a composite. I choose not to make the assumption that I'm looking at a snapshot of the lasers interacting throughout the width of the sodium layer.

• Why would they move the lasers in different directions much less at all during a publicity photo

http://sguisard.astrosurf.com/Pagim/M42-4LGS.html wrote:

[b][color=#0000FF]50 meters away?[/color][/b]

---

During technical tests of the 4 newly installed Laser at Paranal Observatory, UT4 and its Lasers were pointed towards M42, the Orion Nebula. An amateur telescope installed next to UT4 dome was used to take the image below. It shows the 4 laser lines from UT4 pointed at the Orion nebula. These lines are visible because part of their powerful light is diffused by molecules, humidity or dust of the lower atmosphere. These lines are visible up to a few kilometres of altitude on this image, we can actually see them faint out progressively while they propagate up in the atmosphere.

In the direction of these lines we can then observe 4 small "light segments"..
.. the tips of these small segments delimites the boundaries of the Sodium layer.

neufer wrote:Why would they move the lasers in different directions much less at all during a publicity photo :?:

I don't take this to be a publicity photo. And there are all kinds of reasons why they might steer the lasers around during a test. And if they were moved, they were all moving the same way, not in different directions.

Chris Peterson wrote:

neufer wrote:
Why would they move the lasers in different directions much less at all during a publicity photo

I don't take this to be a publicity photo. And there are all kinds of reasons why they might steer the lasers around during a test. And if they were moved, they were all moving the same way, not in different directions.

• Each line segment is perfectly aligned with its matching tropospheric laser beam which point different directions.
  
  The only motion is the slight rotation of the laser beams around their object of interest:

http://sguisard.astrosurf.com/Pagim/Paranal_and_The_Laser_Guide_Star.html wrote:

Click to play embedded YouTube video.

Time Lapse Sky Shows Earth [and its laser beams] Rotating Instead of Stars [Video by Stephane Guisard and Jose Francisco Salgado.]

<<This is a time lapse movie made from individual images taken with a Canon 20Da camera and a 8mm lens. This accelerated movie shows a complete night at Paranal Observatory starting at sunset and finishing at dawn. That night, the Laser Guide Star Facility was in use and its yellow sodium Laser beam left its footprint on our movie. The laser beam creates a Laser Guide Star in the high atmosphere, 90 km above us. This 'bright' artificial star helps the adaptive optics system located in the main telescope, to measure and correct the distorsions of the images produced by the atmosphere, in real time and several hundreds of times per second.>>

I think the answer is that the average sodium layer can be a lot thicker and dynamic than assumed (as suggested by Art).
The Four-Laser Guide Star Facility: Design Considerations and System Implementation

A comprehensive study has been carried out using geophysical lidar data, albeit collected at much lower time resolution, over 30 years by the Brazilian National Space Research Institute in São José dos Compos, São Paulo and analyzed by Moussaoui et al. [Moussaoui 2010]. The latitude difference between São Paulo and Paranal Observatory is only one degree; therefore the statistical results are highly relevant for telescopes in northern Chile. The study found an average centroid height of 91.8 km with a standard deviation of 1.2 km and the average sodium layer width for this long term data set was determined to be 11.2 km with a standard deviation of 1.8 km.

Whereas at the Univerisity of British Columbia:

A study of 3-year statistics from 2008 to 2010 finds the mean sodium centroid height above sea level to be 90.8 ± 0.1 km [Pfrommer 2014]. It extends on average over a width of 13.1 ± 0.3 km for the range containing 95% of the photons. These values represent a mean value averaging over seasonal and nightly variations. Standard errors are indicated as error bars for this study.

... Sometimes, the layer thickness grows to as much as 21 km. The lower sodium edge was detected to lie on average at 81.7 km, and the upper edge, defined to be at five standard deviations above the local background, was found at 104.9 km.

The plume pictured above corresponds to a sodium layer thickness ~15km.

In today's APOD, I measured sodium plume lengths ~ 0.5 arcminutes. Assuming the height and thickness over Paranal Obs. stated above, I calculate the observers distance from UT4 ~100m with a parallax of 3.8 arcminutes. Given, as Chris mentioned, many details are unknown, I'm content with a modest 4' parallax (< 1/2 the guide-star asterism diagonal). Maybe Trapezium wasn't the centered target, but it looks like M42 was.

alter-ego wrote:
In today's APOD, I measured sodium plume lengths ~ 0.5 arcminutes.

• And how, pray tell, did you do that

neufer wrote:

alter-ego wrote:
In today's APOD, I measured sodium plume lengths ~ 0.5 arcminutes.

• And how, pray tell, did you do that

Carefully, but not precisely
I chose a streak length roughly 2/3 the tip-to-tip length and ratioed that to a careful star separation measurement.
I typically don't include the faint wings of images in size estimations. Similar to laser beam profiles, their sizes are defined differently depending on how energy fraction you want contained in that dimension. In this case I did not digitize the streak to determine the streak intensity profile. I eyeballed a reasonable looking dimension - at least I considered it so for this discussion.

alter-ego wrote:

neufer wrote:

alter-ego wrote:
In today's APOD, I measured sodium plume lengths ~ 0.5 arcminutes.

• And how, pray tell, did you do that

Carefully, but not precisely

I chose a streak length roughly 2/3 the tip-to-tip length and ratioed that to a careful star separation measurement.

Which "careful star separation measurement"

[I, myself, took the tip of my finger to guesstimate the full 15 km width of the sodium plume lengths and extended the plume line 6 (= 90 km/15 km) times to produce a box surrounding M43... which I took to be ~15 arcminutes away. That defines a sodium plume lengths ~ 2.5 arcminutes (= 15'/6).]

The width of the laser plumes is about ~40 arcsec, by eye, as they're roughly twice as large as the separation between stars B and D in the trapezium. The telescope used to take this picture is on the main telescope platform, at the very end towards the north, about 80 m or so from UT4. Assuming that the sodium layer is about 80 km high, then this gives a thickness of a few km, which is about right

Cheers!
JC

vendetta wrote:
The width of the laser plumes is about ~40 arcsec, by eye, as they're roughly twice as large as the separation between stars B and D in the trapezium. The telescope used to take this picture is on the main telescope platform, at the very end towards the north, about 80 m or so from UT4. Assuming that the sodium layer is about 80 km high, then this gives a thickness of a few km, which is about right

OK. I may have over estimated the distance between the trapezium and M43 by a factor of 2 or 3

...but I still maintain that UT4 is focused on M43 not M42
(in accord with my simple rule of thumb); ergo, the APOD is wrong in that regard.

In the great laser debate two titans of APOD fame duke it out for intellectual dominance. Incognito alter-ego fingers the problem, and vendetta kills it.

BDanielMayfield wrote:In the great laser debate two titans of APOD fame duke it out for intellectual dominance. Incognito alter-ego fingers the problem, and vendetta kills it.

Well, not quite - vendetta's explanation does not stand up to a little scrutiny. First, the numbers don't work out and, second, the VLT Survey Telescope, which houses a 2.6m mirror and located on the main platform ~80m north of UT4, was not the telescope used. Per Guisard's own description:

... An amateur telescope installed next to UT4 dome was used to take the [APOD] below. ... This image was taken with a 50cm diameter telescope and a Sbig STL-11000 Camera through LRVB filters.

So there are some small buildings at the perimeter of the observatory, roughly 90 meters northward from UT4. One of those smaller buildings could contain a 1/2 meter scope, and Guisard does say it is "installed" so it is likely a permanent facility. In fact it does make sense the scope in question is located in one of the smaller buildings, but it is not the larger dome visible on observatory maps at the northern end of the main platform

Next. regarding numbers, vendetta's conclusion of a few km is underestimated significantly. A 40" plume length and 90-m observer separation requires a sodium layer thickness = 14km at 80km altitude. Even assuming a low 30°-elevation for the guide-star asterism, a uniformly thick sodium layer will only be 50% longer at most. Thus the calculated 14km thick plume in the image would reduce to ~9km at the zenith. A few kilometers thick it is not.

alter-ego wrote:Well, not quite - vendetta's explanation does not stand up to a little scrutiny. First, the numbers don't work out and, second, the VLT Survey Telescope, which houses a 2.6m mirror and located on the main platform ~80m north of UT4, was not the telescope used. Per Guisard's own description:

... An amateur telescope installed next to UT4 dome was used to take the [APOD] below. ... This image was taken with a 50cm diameter telescope and a Sbig STL-11000 Camera through LRVB filters.

So there are some small buildings at the perimeter of the observatory, roughly 90 meters northward from UT4. One of those smaller buildings could contain a 1/2 meter scope, and Guisard does say it is "installed" so it is likely a permanent facility. In fact it does make sense the scope in question is located in one of the smaller buildings, but it is not the larger dome visible on observatory maps at the northern end of the main platform

Next. regarding numbers, vendetta's conclusion of a few km is underestimated significantly. A 40" plume length and 90-m observer separation requires a sodium layer thickness = 14km at 80km altitude. Even assuming a low 30°-elevation for the guide-star asterism, a uniformly thick sodium layer will only be 50% longer at most. Thus the calculated 14km thick plume in the image would reduce to ~9km at the zenith. A few kilometers thick it is not.

I yield to your logical sleuthing. I liked your use of finger as measurement tool too. Very handy. But remain under cover dude, vendetta sounds dangerous.

Bruce

I never said that the picture was taken from VST. I work at Paranal Observatory, I know that Guisard's telescope is permanently installed at the north end of the platform. Hey, I was even there when that picture was taken

The lasers were aimed at M42, not M43. As others have explained, with a 80-100m baseline between UT4 and the other telescope, the parallax error at 80-90km is only 2-3 arcmins, and M43 about 10 arcmins away from the trapezium.

Just to be clear, I wasn't involved in the actual taking of the picture, nor operating the lasers, etc

vendetta wrote:I never said that the picture was taken from VST. I work at Paranal Observatory, I know that Guisard's telescope is permanently installed at the north end of the platform. Hey, I was even there when that picture was taken

The lasers were aimed at M42, not M43. As others have explained, with a 80-100m baseline between UT4 and the other telescope, the parallax error at 80-90km is only 2-3 arcmins, and M43 about 10 arcmins away from the trapezium.

Very nice! What an opportunity and experience! I sincerely hope you get other opportunities like that.
And thanks for the clarifying info. I was actually hoping , even expecting, you'd respond like you did; that ace-up-up-your-sleeve so to speak
A perfect forum ending to a provocative image

Oh, I wasn't trying to hide my job as an ace-up-my-sleeve, I promise! In fact a few posts above I did describe a bit how the lasers are setup depending on the instrument you want to use; only realized now that I never mentioned explicitly that I work there!

I was working on another telescope that night, and I knew they were going to take this PR image. Ever since then I've been really looking forward to seeing it, because Stephane always takes impressive shots, and this one didn't disappoint!

JC

vendetta wrote:
I never said that the picture was taken from VST. I work at Paranal Observatory, I know that Guisard's telescope is permanently installed at the north end of the platform. Hey, I was even there when that picture was taken

The lasers were aimed at M42, not M43. As others have explained, with a 80-100m baseline between UT4 and the other telescope, the parallax error at 80-90km is only 2-3 arcmins, and M43 about 10 arcmins away from the trapezium.

With all that access why can't you be a little more specific about how far Guisard's telescope was from the UT4 telescope?

Originally you stated:

The picture was taken with a small telescope several dozen meters away from the UT4 telescope where the lasers are mounted.

• And now it is 80-100 meters away

In any event, for a 80-100m baseline between UT4 and the other telescope:

• 1) the parallax error at 80-90km is ~4 arcmins [ = tan^-1(90/85,000)]
  
  2) and M43 is just 8 arcmins away from the trapezium.

I was just making a quick back-of-the-envelope estimate. I wasn't concerned about the precise distance because other variables like the actual altitude and thickness of the sodium layer at any given time also carry some uncertainty. I just wanted to show that Guisard's telescope was not 400m from UT4 as stated before. "Several dozen meters" was just a rough estime off the top of my head.

vendetta wrote:
I was just making a quick back-of-the-envelope estimate. I wasn't concerned about the precise distance because other variables like the actual altitude and thickness of the sodium layer at any given time also carry some uncertainty. I just wanted to show that Guisard's telescope was not 400m from UT4 as stated before. "Several dozen meters" was just a rough estime off the top of my head.

But if the distance was 100m or more then the object of interest was more likely M43 than the trapezium.

Can you be more specific about the telescope to telescope distance

This is a PR image to show the lasers around the trapezium. Indeed due to the small parallax you have to shift the pointing a bit so that the plumes nicely encircle the trapezium.

I don't know off the top of my head the exact distance between UT4 and Guisard's telescope, but the edge of the platform is ~80m from UT4, not more.