Will LSST start with a reduced area for the first year?

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#1

Hello, first of all if you know somebody else that would answer to this question , please give me his name and email address!

I’m very interested in the LSST. And I need to know if the Pisces/Cetus sector will be surveyed at the beginning, which is scheduled for November.

Or if we will have to wait until July-August 2026, when this sector will appear again in the night sky and the Survey will cover the entire planned area, not just a reduced area from which the Pisces/Cetus sector would have been excluded!

Because, I found an article from the University of Arizona undated, so perhaps obsolete, (besides, they are the only ones to talk about it on the entire internet, while the Vera Rubin website only talks about a beginning that will be only limited during the first 6 months by 50% of image quality and speed, but does not mention a reduced area explored! : Ref: “Criteria to start the legacy survey”: https://rtn-093.lsst.io ), so I was talking about the article from Arizona University which announces that in the first year (“Year 1 data”) there will only be 15,000 deg^2 covered… …while the complete territory is 18,000 deg^2 for the main survey (southern sky) + 4,000 deg^2 (according to my estimate) of specific regions like the northern ecliptic, the galactic plane, the Magellanic Clouds, etc. = total 22,000 deg^2! 7,000 deg^2 are missing!

https://cosmolab.arizona.edu/missionsexperiments/vera-c-rubin-observatory-legacy-survey-space-and-time-lsst

“Rubin Observatory’s Legacy Survey of Space and Time (LSST) will commence science operations for the main survey in 2024, imaging more than 18,000 square degrees in 6 optical bands to an unprecendented depth over the following 10 years. Already the LSST Year 1 data will cover ~15,000 deg^2 to i-mag 24.3, a fantastic data set for cosmological discoveries.”

Note that they use the future tense to talk about events taking place in 2024! (“LSST will commence science operations in 2024…”). So this article must be out of date by now… But, who knows?

Because, as I said, on the Vera Rubin Telescope website (and anywhere else), there’s no information about a possible reduction in the observation area in the first year!

So I ask you. Is this correct or not? And if so, in that case will the Pisces/Cetus area be excluded, even partially, at the beginning of the survey?

Cetus and Southern Pisces are in the southern sky, and therefore will be part of the Main Survey (in orange on the sky map), but Northern Pisces is outside it (Northern Hemisphere Sky, northern ecliptic [“special region”] in blue):

The sector Cetus/Pisces is shown as a green square :

It would be great if you had this information, as I have high expectations for this survey in that sector, and I really wish to know if we have to wait 5 months or 14 months !!

With all my gratitude!

PS : and also, I didn’t understand well if the first 6 months it’s absolutely sure that the image quality will be reduced of 50% (0.70" instead of 0.35"), from what I read in "Criteria to start the legacy survey ": https://rtn-093.lsst.io section 3 “Criteria to begin the LSST” page 5 :

“Beginning the scheduled observations no sooner that the system can run at 50% effective speed and with image quality (system contribution) below 0.7” is accpetable as long as a clear path to improved performance exists. We assume the path to improvment will arrive at nominal effectiveness (speed 1.0, Image Quality 0.35") within 6 months of the start of survey. Indeed, the ComCam campaign in Q1 FY24 showed the system contribution to image quality is likely as good 0.4" already without full control over the in-dome environment (See On behalf of Rubin Observatory Project, SITCOMTN-149, section 2.2)."

What does it mean ? That they will absolutely respect a protocol where the 6 first months the IQ (Image Quality) is maintained at only 0.70" or is it that 0.70" is only the limit to reach to start the survey, but that if it’s possible they will use a better IQ from the beginning ??

That’s also something very important for me as I wish to know what kind of size of possible dwarf planets they could detect in this sector. Typically, some dwarf planets of a similar size than Pluto or Eris (~2000km), but at a farther distance (130-180 AU, instead of 30-70AU for Pluto and 130AU for Eris). An object of around 21-22 apparent magnitude, at a distance of around 140-170AU.

So, if it’s not possible with an IQ of 0.70" and that we will have to wait obligatory 6 months for the IQ to increase, then that means that I’ll have, also in that case, to wait until summer 2026…

So any opinion on that too is very welcome !!:pray:

Thank you so much to have given me some of your precious time !

Sincerely

Raoul

Will the Pisces/Cetus area be included at the beginning of the survey?
#2

Hi @Rabu , thank you for your questions! As you noted, the Rubin Observatory website and technical notes will have the most up to date information. Section 3.8 of PSTN-056 may be of interest to you, since it includes recommendations for the early survey in year 1. In the text from RTN-093 that you included in your second question, the image quality below 0.70” would only refer to the acceptable conditions for the survey to start, as you mentioned.

For the specific question of “will the Pisces/Cetus area be excluded, even partially, at the beginning of the survey?”, perhaps @ljones or other survey strategy experts can provide more details.

#3

Thank you for your answer !

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#4

Hi Rabu,

We don’t plan on running with a reduced area in year one … I think the quote you found on the University of Arizona page is referring to only the area which reaches a particular coadded depth, as not all of our footprint reaches the same depths - about 15,000 sq degrees is the area corresponding to the “WFD” area that is deeper in this plot (which comes from our latest baseline simulation, which starts in November 2025).

Screenshot 2025-06-05 at 1.15.39 PM
Screenshot 2025-06-05 at 1.15.39 PM1920×1382 223 KB

You noted that the area you are interested in crosses the boundary between the WFD and the North Ecliptic Spur, but we do view both in the first year.

While it’s not certain yet, it’s also quite likely that we will actually observe the area you’re interested in earlier, during commissioning this fall – so more information may be available even sooner!

#5

Regarding meeting the criteria for starting the survey
(system contribution to IQ of 0.7" and 50% efficiency for observing)
– these are reasonably generous conditions to meet for starting the survey, and I believe we’re either close or meeting these in commissioning, over limited areas of the sky, and are working to show that we could meet these over the entire sky included in the survey footprint.

Note that 0.7" is the system (hardware) contribution to IQ – the atmosphere also contributes, so the final delivered IQ is always worse than this (which is why the system requirements are really 0.35"). We will, of course, deliver the best IQ we can.

#6

And to address this question:

This is a question that can be addressed by looking at the single visit limiting magnitude for each visit … and while the seeing contribution does impact that, I do believe that we will be doing close to the expected (simulated) IQ distribution by the start of the survey.

We have some simulations that explore survey strategy, which would include per-visit limiting magnitudes in the area of the sky that you are interested in. So, for example, you could download one of these simulations (https://usdf-maf.slac.stanford.edu/sims_featureScheduler_runs4.3/baseline/baseline_v4.3.5_10yrs.db) and look at the simulated “telescope logs” in the sqlite database file. We run a lot of analysis of these files ourselves (see baseline_v4.3.5_10yrs, for example) but you could look at the file and look at the visits within the RA/Dec region that you are interested in to see how many times and when we would observe those regions, along with the limiting magnitude that we would expect (given the airmass, seeing, filter, and skybrightness background) for each observation. While the simulations do not include degraded performance of the observatory, I do think we’ll be doing as-expected in delivered IQ, so I would also expect the per-visit limiting magnitudes in this region to stay close to 23-24th magnitude (as shown in the simulations).

#7

Thank you so much lynne for your detailed answer !!! :hugs:

Waow, that’s too complicated for me as I am no astronomer. (This simulation thing)

So to make it simple (if I understood well) :

_Yes, the survey will cover all the surface from the beginning in November, And with a magnitude in single visits (23-24) higher than the apparent magnitude of this hypothetical dwarf planet (21-22) also from the beginning, so the LSST should find it already in November !
(Btw, do you know how it works to detect an object ? There’s a software of mobile objets tracking used IMMEDIATELY (automatically) (or maybe during the next day [day time job, when there is no shooting]) (or a few days later) ? And then the localisation of that object would be announced quickly to the minor planet center (or at least kept in your database) and we will be able to get that information as soon as few days or weeks after detection ?) (maybe I am going too fast and making too many questions that are not of your field ! Sorry !)

_And you are saying that it could be even before the beginning of the survey in November (in fall, so in September/october) thanks to the commissioning previous study that could be done on the sector Pisces/cetus… (what is called “Previous Data”) ?

Isn’t it ?

Then it’s just great !! I probably won’t have to wait until August 2026 or November 2026 ! (From my understanding)

Tell me please if my interpretation is correct !

Thx again !

PS : sorry I don’t understand anything in optics with the difference between IQ, Magnitude limit, depth, diameter of mirror, exposure time, aperture, power of resolution, zoom, etc ! So are you saying that the only interesting thing to know is the apparent magnitude ? Or is the power of resolution important too ? For example, this hypothetical object could have a dimension (in seconds of arc) as small as 0.014" (2000 km diameter at a distance of 200 AU Maximum). Is it interesting ? (Of course I imagine apparent magnitude is the only important thing, because the visibility of the object depends on the light it reflects which depends on the distance, the diameter but also on the reflectibility of the object (albedo). And apparent magnitude takes all of that in consideration ! So it must be that I don’t understand really what power of resolution is about…)

#8

Thanks Lynne, for answering these questions.

As a Forum moderator, I’ve marked the last of Lynne’s questions as the solution for this topic, and will answer the further questions raised below in this topic thread.

Do you know how it works to detect an object?
Yes, Rubin Observatory runs software that detects moving objects automatically and immediately after new images are acquired. The current best resources to learn more about Rubin’s moving object detection and the related data products that are made available to users are The Solar System Processing Pipeline page in the Data Preview 0.3 documentation and Section 3.2.2 of the Data Products Definitions Document.

And you are saying that it could be even before the beginning of the survey in November?
Based on Lynne’s response, yes it sounded possible that your area of interest might be covered this fall. Keep in mind that these plans are very tentative.

So are you saying that the only interesting thing to know is the apparent magnitude? Or is the power of resolution important too?
Yes, with the Rubin Observatory, an object must have a brightness above the detection threshold to be detected, whether it is point-like or extended.

As this topic thread has a solution, it’s best to ask new questions in new topics. Thanks @Rabu.

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