Re: Possibility of JWST flares and antiflares?

From: John A. Dormer 2 via Seesat-l <>
Date: Thu, 6 Jan 2022 17:23:54 -0600
wkitty42 has it right. While I understood the orbit, I didn't fully 
understand the relationship between the bus and the telescope.

So, any flare prediction will need to know where the -V3 axis is 
pointing. If it's within a degree of half the angle between -V3 and the 
barycenter of Earth, there's a good chance for flares.

The orbit is shown in detail here, Figure 1:

The bus axes are shown here, Figure 1:

How the vehicle will tilt while on orbit around L2 is shown here, 
specifically Figure 3:

So, flare prediction will need to take into account the orbital position 
of the vehicle plus its angle to the sun, which is a lot easier than all 
of the physics which I first thought would be involved. Perhaps there's 
a way to get the project office to publish when -V3 will match those 
conditions: there's still a lot of time-sensitive data and motion 
planning which aren't readily accessible.


On 1/6/2022 16:28, Marshall Eubanks via Seesat-l wrote:
> Here is a movie from the University of Hertfordshire Observatory
> _at_BayfordburyObs
> and a plot of magnitude changes.
> Note around 23:30 there is a > 1 magnitude change up and down in about 
> 9 minutes. I'd guess that this is a spot (glint) moving under the 
> rotation of the spacecraft.
> Regards
> Marshall
> On 2022-01-06 14:31, wkitty42--- via Seesat-l wrote:
>> On 1/6/22 1:00 PM, John A. Dormer 2 via Seesat-l wrote:
>>> I think it necessary to know the approximate total mass of JWST and 
>>> either the mass of the sunshield side, below the rotation points 
>>> used in a particular pointing, or the mass of the telescope above 
>>> the same rotation points.
>>> The ratio between the masses will affect how much the sunshield 
>>> moves when the telescope points. Only the moving mass will affect 
>>> the orientation of the sunshield. Think "conservation of angular 
>>> momentum." Lever moments will be important, too. High precision 
>>> would also contemplate the history of movements up to the time that 
>>> a new calculation is performed.
>>> The mass of the sunshield side will change with time as fuel is 
>>> used, and knowing the percentage of sunshield mass this makes up 
>>> will determine how important this is in any calculations. There is 
>>> also a great deal of importance to how the burns will be used to 
>>> correct the sunshield's orientation over time.
>> it took some hunting and quite a few attempts to formulate search
>> terms that would return what i was looking for... i was finally able
>> to find the following...
>> [quote]
>> The telescope axis is fixed relative to the shield. However the shield
>> is sufficiently big that the entire JWST can tilt forward 5º and back
>> 45º, and 5º side to side before the angle to the sun is too close. It
>> can of course rotate a full 360º around its z axis. Overall it can see
>> about 35% of the sky at any one time, and over the course of the year
>> the full extent. There are two tiny patches of sky it can see all year
>> around without issue.
>> [/quote]
>> from this and other information, it seems to me that the sun shield
>> will always be between the telescope and the sun/earth/moon... i've
>> not found anything, in layman's terms, that says how much the shield's
>> perpendicular angle may change with regard to the direct line from the
>> sun to L2... it may change some but my understanding is that the
>> alignment system (reaction control wheels?) will keep it within a few
>> degrees of perpendicular and will move to return it to the optimum
>> positioning for best protection of the scope and instruments...
>> if we see flares, i expect they will be fairly steady for long periods
>> of time and not like the fast brightening and dimming flares we see
>> from ISS and other satellites... granted, it takes only a little bit
>> of movement (fractional degrees at this range?) to redirect the "beam"
>> of reflected light slightly off of the earth and those of us looking
>> at the scope... i am but a layman, though... this topic is very
>> interesting to me and i shall keep reading and learning with intrigue
>> and much interest...
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Received on Thu Jan 06 2022 - 17:24:46 UTC

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