Starship Asterisk*

owlice wrote:

pim zethoven wrote:Nice picture , but why are the stars nor circling around during the 2,5 hours aperture?

series of exposures

Yes, a series of short exposures over the 2.5 hrs would prevent star trailing. This could be done in one of two ways. The first would be to take many separate photos and then align them into a composite image. The second way would be to take many exposures on the same frame. This would require an alt-azimuth mounted, polar-aligned, electric drive system, allowing the camera to track the movement of the stars, so that they would be in the same position relative to the frame of the photo. (Also, at least occasional two-dimensional corrections would probably have to be made between photos to keep the tracking sufficiently accurate.) BUT, under these conditions, due to the earth's rotation over 2.5 hours, the stars would be in different positions relative to the terrestrial objects (the volcano peak) in the photo. Over that length of time, this relative motion would be apparent, even with a wide-angle lens. So, with either of the two approaches, we would see multiple "ghost" images of the terrestrial objects in the photographs that were composited to make the final image. Since we don't see this, I can think of only one explanation. The terrestrial objects were not in the same images which recorded the stars and meteors. The terrestrial objects were recorded in a single separate photo which did not capture any stars. The terrestrial photo was superimposed onto the final composite of the many photos of the star & meteor images. This being the case, the image of the volcano could have been replaced with a night-time photo of any terrestrial object: NASA Headquarters, Yankee Stadium, or a Taco Bell ! Anyone got a better explanation ?

Mike D:
The landscape probably was in the frame. Either the sky or the landscape will appear to rotate depending on how the camera was mounted just as you noted. So what you are seeing are a bunch of photos of *just* meteors with the rest of the sky and landscape masked off overlaid on a backdrop. And yes, you could put the meteors at Yankee Stadium but the photographer was at Tenerife, not New York.

owlice wrote:

pim zethoven wrote:Nice picture , but why are the stars nor circling around during the 2,5 hours aperture?

series of exposures

Yes, a series of short exposures over the 2.5 hrs would prevent star trailing. This could be done in one of two ways. The first would be to take many separate photos and then align them into a composite image. The second way would be to take many exposures on the same frame. This would require an alt-azimuth mounted, polar-aligned, electric drive system, allowing the camera to track the movement of the stars, so that they would be in the same position relative to the frame of the photo. (Also, at least occasional two-dimensional corrections would probably have to be made between photos to keep the tracking sufficiently accurate.) BUT, under these conditions, due to the earth's rotation over 2.5 hours, the stars would be in different positions relative to the terrestrial objects (the volcano peak) in the photo. Over that length of time, this relative motion would be apparent in the photograph, even with a wide-angle lens. So, with either of the two approaches, we would see multiple "ghost" images of the terrestrial objects in the final image. Since we don't see this, I can think of only one explanation. The terrestrial objects were not recorded in the same images as the stars and meteors. The terrestrial objects were recorded in a single separate photo, which was then superimposed onto the final composite of the many photos of stars & meteors. Anyone got a better explanation ? If not, the image of the volcano could have been replaced with a night-time photo of any terrestrial object: NASA Headquarters, Yankee Stadium, or a Taco Bell ! I think the volcano was a tasteful choice.

geckzilla wrote:Mike D:
The landscape probably was in the frame. Either the sky or the landscape will appear to rotate depending on how the camera was mounted just as you noted. So what you are seeing are a bunch of photos of *just* meteors with the rest of the sky and landscape masked off overlaid on a backdrop. And yes, you could put the meteors at Yankee Stadium but the photographer was at Tenerife, not New York.

First, sorry for the double-post - that was inadvertant. I think I understand your explanation that it is the meteor images that are separate. Thanks. Is it possible for the images of the meteors to be imposed on the background without blocking any of that background ? And how are the meteor-only images aligned in the correct position relative to the stars ?

Mike D wrote:And how are the meteor-only images aligned in the correct position relative to the stars ?

I can't say how the photographer did it, but personally I'd probably just line the stars up for each frame as best as I could to the backdrop image and then mask off everything but the meteors. To me that would be the most accurate way to depict a meteor shower in a still image like this.

The exif data in this APOD image file, lists the date taken as 2013-12-14, 06:04+00. At that time in the Canary Islands, Rigel was ~5° above the western horizon. So that part makes perfect sense. This time is also about 20 minutes before astronomical twilight. The slight glow of dawn above the horizon suggests the 2.3 hours photo shoot may have extended slightly into astronomical twilight.

Because of the consistent direction of all the meteor streaks emanating from Gemini, I suspect that the skyscape of stars and meteors was achieved by stacking multiple sub-exposures centred and tracking on Alnilam (camera may have been mounted to track equatorially or horizontally, as the field doesn't rotate too much at the eastern and western horizons). Each one of these sub-exposures would have needed to have any landscape masked out (probably only the last few subs).

The final stage would have been to superimpose the landscape, probably achieved separately using a fixed tripod, and the photographer has chosen (beautifully) to align the top of the mountain with Rigel (as closely as possible without making it geometrically impossible).

I am perfectly willing to give the photographer the benefit of the doubt that the skyscape was recorded from the same location as the landscape. But even if it wasn't, it would still be okay in my books as long as the alignment was physically possible. But breaking things down like this reminds me of the reasons why one should also avoid watching sausages and laws being made. It can be distasteful.

53 streaks by my count. Amazing photo, well done

Nitpicker wrote:... I am perfectly willing to give the photographer the benefit of the doubt that the skyscape was recorded from the same location as the landscape.

It was.

Brian douglas wrote:53 streaks by my count. Amazing photo, well done

I agree on all counts.

Alright, all you counting more than 51, show me which ones I missed.

alter-ego wrote:

Nitpicker wrote:... I am perfectly willing to give the photographer the benefit of the doubt that the skyscape was recorded from the same location as the landscape.

It was.

We agree again. I may have misunderstood your reason for responding to my comment. But I'd probably be right in assuming you are not a cricket fan, so maybe the phrase "benefit of the doubt" has a slightly different meaning to you. I was responding to the comments of Mike D.

PS: I got 51 streaks too, when I stopped subitizing.

Nitpicker wrote:The exif data in this APOD image file, lists the date taken as 2013-12-14, 06:04+00. At that time in the Canary Islands, Rigel was ~5° above the western horizon. So that part makes perfect sense. This time is also about 20 minutes before astronomical twilight. The slight glow of dawn above the horizon suggests the 2.3 hours photo shoot may have extended slightly into astronomical twilight.

Because of the consistent direction of all the meteor streaks emanating from Gemini, I suspect that the skyscape of stars and meteors was achieved by stacking multiple sub-exposures centred and tracking on Alnilam (camera may have been mounted to track equatorially or horizontally, as the field doesn't rotate too much at the eastern and western horizons). Each one of these sub-exposures would have needed to have any landscape masked out (probably only the last few subs).

The final stage would have been to superimpose the landscape, probably achieved separately using a fixed tripod, and the photographer has chosen (beautifully) to align the top of the mountain with Rigel (as closely as possible without making it geometrically impossible).

I am perfectly willing to give the photographer the benefit of the doubt that the skyscape was recorded from the same location as the landscape. But even if it wasn't, it would still be okay in my books as long as the alignment was physically possible. But breaking things down like this reminds me of the reasons why one should also avoid watching sausages and laws being made. It can be distasteful.

Agreed. I think the key point is that the landscape and skyscape have to be photographed separately for the photo to look like this. It is a beautiful piece of work, but having done some astrophotography, I’m always keen to sort out the techniques. For me, having an understanding of the technical components enhances, rather than detracts from, the appreciation of the work.

Mike D wrote:I think the key point is that the landscape and skyscape have to be photographed separately for the photo to look like this

I don't think the land has to be separated from the sky for the backdrop. Just the meteors.

geckzilla wrote:

Mike D wrote:I think the key point is that the landscape and skyscape have to be photographed separately for the photo to look like this

I don't think the land has to be separated from the sky for the backdrop. Just the meteors.

I don't get that. Are you suggesting that the meteors from each of the up to ~50 sub-exposures are each painstaking cut from the sub and pasted into their correct place into the "backdrop" frame, where the "backdrop" is a single exposure of stars and land but no meteors? That sounds like an enormous effort.

Alternatively, if the meteors were recorded from sub-exposures on a fixed tripod pointed at the mountain, some would be coming down from the left, some would be coming straight down and some would be coming down from the right, in keeping with the motion of Gemini over the period of the photo shoot. This would also mean that the meteors could not be placed correctly in relation to the stars.

Just track the sky while taking all the exposures. Then stack those, get all the meteors, then align that stack to the backdrop and mask off everything but the meteors?

geckzilla wrote:Just track the sky while taking all the exposures. Then stack those, get all the meteors, then align that stack to the backdrop and mask off everything but the meteors?

What are you calling the backdrop in this instance? Some of the sky subs (the later ones) would include land, which needs to be masked before stacking. Why mask off everything but the meteors? I would have thought the stars and meteors need to be kept together.

53

Nitpicker wrote:

geckzilla wrote:Just track the sky while taking all the exposures. Then stack those, get all the meteors, then align that stack to the backdrop and mask off everything but the meteors?

What are you calling the backdrop in this instance? Some of the sky subs (the later ones) would include land, which needs to be masked before stacking. Why mask off everything but the meteors? I would have thought the stars and meteors need to be kept together.

Because the pictures of the meteors might not be a good landscape composition. I'd mask everything but the meteors because it would probably not match up exactly because of perspective distortion and make the overall image look weird.

geckzilla wrote:

Nitpicker wrote:

geckzilla wrote:Just track the sky while taking all the exposures. Then stack those, get all the meteors, then align that stack to the backdrop and mask off everything but the meteors?

What are you calling the backdrop in this instance? Some of the sky subs (the later ones) would include land, which needs to be masked before stacking. Why mask off everything but the meteors? I would have thought the stars and meteors need to be kept together.

Because the pictures of the meteors might not be a good landscape composition. I'd mask everything but the meteors because it would probably not match up exactly because of perspective distortion and make the overall image look weird.

Sorry, you've lost me. I think we are defining our terms differently for one thing. I've used the terms landscape and skyscape to differentiate between land and sky. If the tracking for the sky subs has been accurate, there is no reason to think that the stars will not match up when stacked, as any slight distortions will appear the same way in each sky sub. And if the meteors are kept with the stars on the sky subs, they will be oriented correctly relative to the celestial sphere (which is what we see in this APOD) and the amount of masking required will be minimised (to mask just the land from those few late sky subs which include a portion of land at the bottom).

geckzilla wrote:Alright, all you counting more than 51, show me which ones I missed.

Last night, for about the 4th time now, I've been unable to access asterisk. I went back to respond to your post and nada - page could not be displayed So my response had to wait until tonight.

The only other streak that I can find (#53) is a faint one, but it has characteristics which suggest it is a meteor: Length, straightness & orientation. It does not appear to be an asterism. Except for one oddity, I have no problem accepting it as a bona fide meteor trail - it is the only faint streak I can find. Given the number of meteors, I'd expect to see more than just one faint one. There are several diffraction artifacts but they show up as horizontal or vertical streaks. These are easy to ignore. The streak I identify below does puzzle me. Back in the day, a scratch on the negative would be a good suspect here. In any case, this is the streak I found and it may be due to a meteor. Although you put a question mark on 52, I think that's a meteor streak without question.

Nitpicker wrote: Sorry, you've lost me.

I do not know how to explain it any better. I'd have to actually go out and do it and have you come look at my computer and watch it get put together.

alter-ego wrote:Although you put a question mark on 52, I think that's a meteor streak without question.

Because it could have easily become slightly shifted during post processing.

geckzilla wrote:Because the pictures of the meteors might not be a good landscape composition. I'd mask everything but the meteors because it would probably not match up exactly because of perspective distortion and make the overall image look weird.

I've made images like this before, and I've done it three ways.

One way is to use a tracking mount, and take a series of images. Exposure is tricky: you want as short an exposure as possible to maximize meteor sensitivity, but you don't want it so short that the dead time between exposures represents a significant percentage of the total. Fortunately, newer DSLRs can take images very quickly. I like 10-30 second exposures for catching meteors. The landscape is masked out of all the exposures except one, and the images stacked.

Another way is to use a fixed mount, and align the images before stacking. Otherwise, processing is the same as above, with the landscape masked out.

Finally, each image (except for a star and landscape reference) can have everything masked out except the meteors. The problem with that is you don't get such a good star field, since you have a lot less data. It's also quite a bit of work.

FWIW, in a two hour or so exposure, you don't really need to track at all to see the radiant. It doesn't move that much.

Thanks Chris.

Chris Peterson wrote:Another way is to use a fixed mount, and align the images before stacking. Otherwise, processing is the same as above, with the landscape masked out.

That would require a FOV about double the size, for an image like today's APOD, with the final result being cropped down. Or do you mean a fixed tripod with regular manual adjustments to roughly follow the sky?

Chris Peterson wrote:FWIW, in a two hour or so exposure, you don't really need to track at all to see the radiant. It doesn't move that much.

For this APOD, the centre of Gemini would have moved in azimuth about 20 degrees, which could arguably spoil the perspective.

...

By the way, I have probably sounded a tad combative on this issue, geckzilla. I'm sorry for that, it was not my intention, and it is certainly unjustified. I'm just thinking about how I might have a first attempt at such a composite (one day).

No worries. I'm just making educated guesses, anyway. You could be right.

(You didn't seem combative to me.)

Nitpicker wrote:That would require a FOV about double the size, for an image like today's APOD, with the final result being cropped down. Or do you mean a fixed tripod with regular manual adjustments to roughly follow the sky?

No adjustments. Just a fixed camera, followed by alignment and stacking, then cropping.

For this APOD, the centre of Gemini would have moved in azimuth about 20 degrees, which could arguably spoil the perspective.

Very likely not. The actual shift in radiant position over 2.3 hours was 27°, which isn't all that much for a radiant well outside the FOV. The effect on apparent vanishing point from that drift is probably no more than lens aberration itself creates.

I just went to the photographer's website (http://www.starryearth.com) - pretty amazing. After seeing more of his work, I'm more convinced that my first guess (way back when) might be right. The many meteor images were recorded in separate exposures but on the same frame, and not in a large number of separate frames that would have to be synchronized which strikes me as a pretty greulling task. I've used the technique of capturing multiple phases of a lunar eclipse on one frame as does the photographer on his site. This still requires tracking over the 2.5 hrs, and the landscape would have to be recorded separately, or as Chris suggests, only in the last image captured. The other thing that occurred to me is that any time that I have observed meteor showers, I always see a few sporadics. It's interesting that in fifty-some exposures, the photographer appears not to have captured a single sporadic meteor. Too bad - I think that would have made the photo even more interesting.