by alter-ego » Mon Mar 08, 2021 2:01 am
Chris Peterson wrote: ↑Sun Mar 07, 2021 6:21 pm
johnnydeep wrote: ↑Sun Mar 07, 2021 5:40 pm
This is a very cool (or warm!) image. I see there's a detailed page link I still need to read all about Hubble imaging (namely,
https://asd.gsfc.nasa.gov/blueshift/ind ... lse-color/), but can someone give the the executive summary of what has to change in Hubble for it to capture infra-red as opposed to visible light? Is it just swapping a different filter in place, or is there also a different sensor that is more sensitive to infrared? Obviously, the mirror itself can't change!
It is unfortunate that the filter details aren't provided. That said, "infrared" covers a very broad range. The IR that the HST can capture is pretty much the same optically as visible light. Its wavelengths are what can be detected by an ordinary silicon detector, which falls off rapidly above 1.5 micrometers. That is shorter than thermal IR. So yes... it's just a question of choosing an IR-pass filter in place of a visible light filter. The camera and the optics are the same. This is quite different than space telescopes designed for IR imaging, which typically use special optics and detectors to image out to many micrometers, which means they are imaging thermal emissions from cool objects, not the hot sources that the HST camera is designed for.
The NIR image was capture by the WFC3 in 2014. This camera has a relatively high efficiency over a wide spectral range: 200nm → 1700nm, where 200nm to 1000 is covered by the Si CCD sensor, and IR channel detector is optimized for ~800nm → 1700nm. Much over 1.2um, Si detectors become transparent and not enough is absorbed for useful photon detection. Using an Si array, I've profiled laser beams out to ~1.2um maximum where sensitivity is exceedingly low. When I saw the listed WFC3 spectral range, I suspected there was either a different or additional sensor used.
HST Documentation - Optical Design and Detectors
WFC3 uses two different types of detectors. The UVIS channel uses two butted 4096 × 2051 thinned, back-illuminated e2v Ltd. (formerly Marconi) CCD detectors to support imaging between 200 and 1000 nm. The IR channel uses a 1024 × 1024 Teledyne (formerly Rockwell Scientific) HgCdTe detector array, with the central 1014 × 1014 pixels useful for imaging, and covering the near-infrared between 800 and 1700 nm.
The primary characteristics of the two channels are summarized in Table 2.1.
[quote="Chris Peterson" post_id=311427 time=1615141309 user_id=117706]
[quote=johnnydeep post_id=311424 time=1615138808 user_id=132061]
This is a very cool (or warm!) image. I see there's a detailed page link I still need to read all about Hubble imaging (namely, https://asd.gsfc.nasa.gov/blueshift/index.php/2016/09/13/hubble-false-color/), but can someone give the the executive summary of what has to change in Hubble for it to capture infra-red as opposed to visible light? Is it just swapping a different filter in place, or is there also a different sensor that is more sensitive to infrared? Obviously, the mirror itself can't change!
[/quote]
It is unfortunate that the filter details aren't provided. That said, "infrared" covers a very broad range. The IR that the HST can capture is pretty much the same optically as visible light. Its wavelengths are what can be detected by an ordinary silicon detector, which falls off rapidly above 1.5 micrometers. That is shorter than thermal IR. So yes... it's just a question of choosing an IR-pass filter in place of a visible light filter. The camera and the optics are the same. This is quite different than space telescopes designed for IR imaging, which typically use special optics and detectors to image out to many micrometers, which means they are imaging thermal emissions from cool objects, not the hot sources that the HST camera is designed for.
[/quote]
The NIR image was capture by the WFC3 in 2014. This camera has a relatively high efficiency over a wide spectral range: 200nm → 1700nm, where 200nm to 1000 is covered by the Si CCD sensor, and IR channel detector is optimized for ~800nm → 1700nm. Much over 1.2um, Si detectors become transparent and not enough is absorbed for useful photon detection. Using an Si array, I've profiled laser beams out to ~1.2um maximum where sensitivity is exceedingly low. When I saw the listed WFC3 spectral range, I suspected there was either a different or additional sensor used.
[quote][url=https://hst-docs.stsci.edu/wfc3ihb/chapter-2-wfc3-instrument-description/2-1-optical-design-and-detectors][b]HST Documentation - Optical Design and Detectors[/b][/url]
WFC3 uses two different types of detectors. The UVIS channel uses two butted 4096 × 2051 thinned, back-illuminated e2v Ltd. (formerly Marconi) CCD detectors to support imaging between 200 and 1000 nm. The IR channel uses a 1024 × 1024 Teledyne (formerly Rockwell Scientific) HgCdTe detector array, with the central 1014 × 1014 pixels useful for imaging, and covering the near-infrared between 800 and 1700 nm.
The primary characteristics of the two channels are summarized in Table 2.1.
[attachment=0]WFC3 Detector .jpg[/attachment][/quote]