http://spacenews.com/cameras-on-nasa-exoplanet-spacecraft-slightly-out-of-focus/QuoteThe TESS team thinks there will be a 10 percent cut in terms of the number of planets that they expect to be able to detect.QuoteDespite the reduction, Boss said TESS scientists believe they will still be able to meet the mission’s primary science requirements, and thus there is no need to fix the cameras.
The TESS team thinks there will be a 10 percent cut in terms of the number of planets that they expect to be able to detect.
Despite the reduction, Boss said TESS scientists believe they will still be able to meet the mission’s primary science requirements, and thus there is no need to fix the cameras.
Jeff Foust @jeff_foustUpdated our article on TESS spacecraft camera focus issue with additional NASA comment playing down the problem:
Chou added July 28 that the out-of-focus area is limited to the outer edges of the image, and that “recent testing shows that the camera focus towards the image center is better than originally designed.”
Well, that makes sense. I am working on scientific instruments, not on TESS though. It is common that the focal surface of some optical system is curved (spherical) but the detector usually flat. There are curved detectors but they are experimental and not used (yet). So imagine a spherical focal surface intersected by a flat detector surface. If you minimize the offset between both surfaces everywhere, both the center and the edges of the flat detector surface are somewhat out of focus. The optimal focus is on a ring somewhere in between. If the optics is such that the edges are more out of focus, by geometry, the center is more in focus, meaning the flat detector surface is closer to the spherical optical focal surface. For this reason, usually the detector or optics can be pistoned just a little bit to adjust the focus for operational conditions. This is done manually in commissioning and not touched later. Unless the conditions change of course. I am surprised that TESS does not have such a system.
Quote from: Semmel on 07/28/2017 07:25 pmWell, that makes sense. I am working on scientific instruments, not on TESS though. It is common that the focal surface of some optical system is curved (spherical) but the detector usually flat. There are curved detectors but they are experimental and not used (yet). So imagine a spherical focal surface intersected by a flat detector surface. If you minimize the offset between both surfaces everywhere, both the center and the edges of the flat detector surface are somewhat out of focus. The optimal focus is on a ring somewhere in between. If the optics is such that the edges are more out of focus, by geometry, the center is more in focus, meaning the flat detector surface is closer to the spherical optical focal surface. For this reason, usually the detector or optics can be pistoned just a little bit to adjust the focus for operational conditions. This is done manually in commissioning and not touched later. Unless the conditions change of course. I am surprised that TESS does not have such a system. There is no curvature of field in the design(The first two authors are Primeau and Chrisp. They are terrific optical designers and optimized several orders beyond field curvature.)That the center is better than expected and the edges worse is probably a complex issue of tolerancing.And "crystallization" may be a simplification of having the bond material pass its glass transition temperature warmer or colder than expected. Beyond that point the coefficient of thermal expansion changes significantly. Passive focus of very cold instruments is really tricky. Even for JPL.
Or the corrector plates aren't up to snuff.
Quote from: jg on 07/30/2017 04:27 pmOr the corrector plates aren't up to snuff.This is an all-refractive lens with seven elements, two aspherical.If you read the report you'll see that the assembly flow is very complex and finicky. If I had been reviewing this design I'd have been very skeptical that a high level of athermalization was a realistic expectation -- the results don't look too bad to me.
There is no curvature of field in the design
Quote from: Comga on 07/30/2017 05:17 amThere is no curvature of field in the designI think a better statement would be "as far as practical, the focal plane has no curvature". If you look at the graphs of lens performance (see TESS status, page 10) you will see that if you get best focus in the middle, best focus at 6o degrees off axis needs a 20 micron shift, then 40 microns at 12 o, then back to 20 microns at 16o. This is pretty typical - best focus typically looks like a low order polynomial in distance from the center. The fundamental problem is that a flat focal surface is a very "unnatural" configuration. Each surface, when varied, produces a smooth radial function of best focus. But the function is different for every surface, so all you can do is get them to cancel approximately. This also induces tough constraints on other surfaces as well.To see how hard optical systems need to work to approximate a flat field is, consider this paper. By allowing a curved focal plane, with otherwise the same specs, the reduce a design with 14 lenses, 2 aspheres, and 10 types of glass, to a design with 9 lenses, no aspheres, and 3 types of glass.Or intuitively, how does your eye get such good results for a single lens made of jelly? A lot of it is because your retina is a strongly curved focal surface.
After @NASA_TESS cameras were assembled, they were carefully covered with thermal blankets.
Gerard van Belle @FringeDoctorRicker notes that the @NASA_TESS orbit is stable and could enable an extended mission for 20+ years.Squee!
Date: JuneMission: TESS (Transiting Exoplanet Survey Satellite)
https://www.nasa.gov/launchschedule/QuoteDate: JuneMission: TESS (Transiting Exoplanet Survey Satellite)
The current launch window is no-earlier-than March 20, 2018 and not-later-than June 2018.