Catching the Moon in LWIR before it can be caught in Vis/Nir... - Page 1
Forum section: electronics / general

author: Lambda
author_rank: Regular Contributor
timestamp: July 20, 2021, 08:26:02 pm
content: Hello.Just an additional little experiment wherein i tried to "see" the Moon before it can be seen...It is not spectacular, i have to find a better lens, among plenty of other things to improveThe description of the experiment is in the description of the video:First video (part 1) wherein the Moon is not yet visible for the Vis/Nir camera, but only for the LWIR camera:Second video (part 2) wherein the Moon become also visible for the Vis/Nir camera:I will do better the next time, but it is encouraging...Best regards.Stéphane

author: Vipitis
author_rank: Frequent Contributor
timestamp: July 20, 2021, 10:11:53 pm
content: Aiming for the moon is something I want to achieve as well, I got my first signal but not a great image yet.With the title here I wonder: does it not show up in visible due to the brighter sky? It surely isn't hidden hiding the planet and atmospherically lensed into place earlier in LWIR. We have about 92% moon illumination tonight, so the moon will be really bright, you should even see it during the day (it's not up during the day much). But this made me wonder - would you be able to see the new moon in LWIR any better than visible (using earthshine for example). And if one has the weather, time and effort for it. A side by side month long timelapse of a lunar day, seen from earth. I have a feeling the terminator line isn't as extreme as you see it in the visible since lunar regolith might be a tiny bit emissive as well (it's visual albedo is like .11). Or do you only see the moon against the coldness of deep space?Is there any other object (past the sun and the moon), that is bright enough in the N-band (+/-2μm around 10.3) that might show up on a uncooled microbolometer? A question I have asked for years and haven't found any answers yet. Perhaps M42

author: Lambda
author_rank: Regular Contributor
timestamp: July 20, 2021, 11:32:05 pm
content: Hello Vipiitis.As long as the angular distance between the sun, during the day, and the Moon, is sufficient, the brightness of the moon is not completely overwhelmed by the light coming from the sun. And in this condition, as long as its surfacic magnitude of the Moon is higher than the one of the sky, at the moment of the observation, the Moon stays visible during the day, as it is can be the case also for Venus, for instance.Observing the new Moon in LWIR would be great and, it has be done, in a certain manner, by a french astronome, Thierry Midavaine. More precisely, it was during a lunar eclipse, which appears during the new Moon phase by principle.https://fr-fr.facebook.com/cieletespace/videos/la-lune-en-infrarouge-film%C3%A9e-par-thierry-midavaine/10156244091440346/But also by other people:https://www.flir.eu/discover/rd-science/lunar-eclipse-when-viewed-in-thermal/I guess, with the proper set-up, such experiment may be replicated during a normal new Moon, and by the way, during the whole set of phases of the Moon cycle.It would be very interesting to see that!What seems to be critical, in order to see some details, is to have access to an LWIR optics having sufficient focal length for giving a proper sampling for the image and reaching therefore a proper resolution, in accordance with the pixel size of the FPA of the thermal sensor (i humbly guess).

author: Ultrapurple
author_rank: Super Contributor
timestamp: July 21, 2021, 09:37:31 am
content: Just to add a little to the conversation.An inexpensive (<US$100) visible light reflector telescope can readily be adapted for LWIR use and is 'just good enough' to see a bright target like the Moon. As long as there are no lenses in the system you'll be just fine.You'll need to use a 'bare' (no lens) thermal sensor. I used a Therm-App but there are many other suitable types.Mount the sensor into the telescope focusing mechanism, point the telescope at the Moon. Adjust focus. Admire image.The secret is that the mirrors in reflecting telescopes are very broad band - they'll usually work from high-GHz frequencies upwards - and although the efficiency may not be as good as in the visible range, when you're looking at something as bright (hot) as the Moon, that's not so much of a problem. Likewise the terrible effective aperture, which is probably f/6 or worse.

author: Lambda
author_rank: Regular Contributor
timestamp: July 21, 2021, 11:27:42 am
content: Indeed, it would be a very cheap appreciable alternative!Just perhaps to be sure that the mirror is not coated by any protective layer like SiO2, which is currently the case for now-day telescopes in the public market...Or can we considered such coating sufficiently reflective in the range of [8um-14um] ?I have a old mirror D=200mm, f/d=4, which would worth to be extracted from my junk box...One possibility to render the effective aperture faster would be to use a conventional thermal lens as a focal reducer. for the mirror i have, and 15mm lens like the one i got from Bill, it should be doable to obtain an effective aperture around 2. Likewise what is currently done in standard astronomy in the visible domain.Just thoughts for food.

author: Bill W
author_rank: Super Contributor
timestamp: July 21, 2021, 09:08:45 pm
content: Quote from: Vipitis on July 20, 2021, 10:11:53 pmI have noted before that:The phase in LWIR does seem greater than in visible. Odd as for a 'growing' phase the surface is the bit that has not seen sun for the longest.In hazy skies, yes the daytime moon is visible in LWIR but not in visible to the eye. However as LWIR needs some form of 'instument' this may not be an equal test.

author: Lambda
author_rank: Regular Contributor
timestamp: July 23, 2021, 09:27:29 am
content: Yes Bill, you right.My small comparison is just qualitative and for giving a simple example for illustrating the different capabilities in such specific context between the Vis and LWIR "imagers".Optics are not the same, sensors also, with different pixel pitch,....graphique interfaces also....It has to be seen as a comparison between two "systems" to be considered in their whole, not between two sensors (for instance)...One example is that, by playing with the histogram, i could have seen perhaps indeed the moon in Vis domain earlier, in the first video....And with a better optics for the LWIR camera, i could have also obtained better results, also earlier....This simple test has to be seen as relative comparison between two systems working "at their best" for a compromise between dynamic range and detection.....Some further comparative sessions will be done, especially during the period around New Moon:When the Moon will be really closer from the Sun (on angular point of view), and tests will be performed roughly in the middle of the day, such new context will give less chance to the Vis camera to be able to make the moon visible again the super high bright sky background, while normally i hope not preventing the LWIR camera do its job (just to be sure for safety to have the Sun out of the field of view for the two cameras!).Regards.Stéphane

author: Ultrapurple
author_rank: Super Contributor
timestamp: July 23, 2021, 09:58:52 am
content: I'm sure I've posted this image before but even so it does serve to illustrate how I (crudely!) mounted my Therm-App on a reflector telescope.

author: Lambda
author_rank: Regular Contributor
timestamp: July 23, 2021, 10:23:26 am
content: Thank you Ultrapurple for this talkative photo!Please, some questions:- Is your telescope equipped with standardcoatedmirrors (primary and secondary), or is it one with anuncoatedversion (primary and secondary)?- Do you have some photos obtained with your setup?Thank you in advance!Best regards.Stephane

author: Vipitis
author_rank: Frequent Contributor
timestamp: July 23, 2021, 10:53:36 am
content: Click on the image and you get to the Flickr page whee it was posted. The moon pics should be adjacent.

author: Lambda
author_rank: Regular Contributor
timestamp: July 23, 2021, 11:22:43 am
content: Ok Vipitis!Excact, i have now my answers...

author: Bill W
author_rank: Super Contributor
timestamp: July 25, 2021, 10:22:58 am
content: Had a try last night - a full moon just rising.There was some low cloud / mist very low down.The moon was visible by eye (in a red sort of sunrise effect) but not visible to LWIR.LWIR soon caught up as the moon rose.Bill

author: Ultrapurple
author_rank: Super Contributor
timestamp: July 25, 2021, 01:49:52 pm
content: @LambdaThank you for your kind words.I have tried a couple or three telescopes, all of which were consumer-grade, so I expect they were standard (cheaply!) coated mirrors. Coated just enough to last beyond the warranty period...In recent years there have been some suspiciously cheap 500mm focal length 6" aperture telescopes around (think GBP99 / US$99 new) that are, frankly, awful for visible light observations but which are actually reasonably good when pressed into service at LWIR. The large aperture and (relatively) short focal length are a better match to an uncooled LWIR microbolometer than longer focal length and/or smaller aperture.Do not expect to see anything smaller or dimmer than the Moon. You are extremely unlikely to be able to detect our neighbouring planets. And there is only one star close enough - but it's a bit too close to want to point anything optical in its direction. There's no doubt your LWIR bolometer could detect it, briefly, before it melted...