Starship Asterisk*

I'm curious as to why 300 1-minute exposures were chosen as the methodology for taking this superb composite. Why not some other combination of longer exposures? Can the photographer explain his/her rationale please?

http://apod.nasa.gov/apod/ap090314.html wrote:
Explanation: Sirius, alpha star of Canis Major, traces the bright arc over the dome at the left.
Astronomer Alexandre Santerne also briefly illuminated a foreground oak tree during the exposure sequence.

The Memoirs of Sherlock Holmes » The Musgrave Ritual

• "'Where was the sun?'
  
  "'Over the oak.'
  
  "'Where was the shadow?'
  
  "'Under the elm.'

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<<The earthly priest-king consort of this Goddess-incarnation was
originally castrated--as mistletoe euphemistically was lopped off the
oak tree bough--and eucharistically eaten. One local survival of this
woodland cult at the sacred grove of Diana on the shores of Lake
Nemi in the Alban Hills just south of Rome originally inspired Sir James
George Frazer to develop the theories for his ground-breaking study of
folklore in The Golden Bough. The Neolithic mother-queen-goddess was
called by many names, such as Diana, Demeter, or Ceres; one of Her
early important names was Alphito: synonymous with pearl barley
and also identified with the star Spica in the constellation Virgo,
that was conjunct with the sun at the Vernal Equinox at the
beginning of Neolithic grain agriculture, around 12,500 BC.>>

- Graves, White Goddess.
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Circular Universal Product Code.

aristarchusinexile wrote:Circular Universal Product Code.

CUPC-ache

I have never seen a photo quite like this one. Why do the stars not follow parallel paths? Is it the focal length of the lens?

bhrobards wrote:I have never seen a photo quite like this one. Why do the stars not follow parallel paths? Is it the focal length of the lens?

I believe the appearance is due to the use of a wide angle lens. The straight star trails follow the equatorial path with those on curved paths being stars both north and south of the equator

bhrobards wrote:I have never seen a photo quite like this one. Why do the stars not follow parallel paths? Is it the focal length of the lens?

1) A simple lens or pinhole camera produces a gnomonic map projection of the celestial sphere.

2) The gnomonic map projection displays all great circles as straight lines.

3) Right ascension lines and the celestial equator are great circles
. and, therefore, always appear as straight lines a gnomonic map projection:



4) All other celestial declination lines are curved in a gnomonic map projection
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BMAONE23 wrote:

bhrobards wrote:I have never seen a photo quite like this one. Why do the stars not follow parallel paths? Is it the focal length of the lens?

I believe the appearance is due to the use of a wide angle lens. The straight star trails follow the equatorial path with those on curved paths being stars both north and south of the equator

Actually, wide angle lens force the stars to follow MORE parallel paths; scientific
"circular fisheye" lens pointing at the celestial equator would, in fact, produce parallel paths:

http://en.wikipedia.org/wiki/Fish-eye_lens wrote:
The first types of fisheye lenses to be developed were "circular fisheyes" — lenses which took in a 180° hemisphere and projected this as a circle within the film frame. Some circular fisheyes were available in orthographic projection models for scientific applications.

lkistler wrote:I'm curious as to why 300 1-minute exposures were chosen as the methodology for taking this superb composite. Why not some other combination of longer exposures? Can the photographer explain his/her rationale please?

There are a few reasons. Digital cameras have pretty high dark current, which starts filling the pixels, until there's no room for signal. So short exposures are used to maintain reasonable dynamic range. (Most DSLRs are limited to a max of about 10 minutes before this becomes a really large problem). Sky glow can also start raising the background unacceptably, especially with a fast, wide angle lens. A short exposure also prevents the brighter stars from saturating, which would result in the loss of their color information and make it impossible to see their relative intensities. Finally, there could be practical reasons, such as 60 seconds being the maximum exposure time without going to bulb mode and requiring more exotic exposure control (that may or may not be the case, depending on the camera used).

The line in the explanation to this APOD that states "Brilliant planet Venus makes the short bright trail at the lower right, while trails of stars in the end near the lower right observatory dome." seems to have something missing!

DavidLeodis wrote:The line in the explanation to this APOD that states "Brilliant planet Venus makes the short bright trail at the lower right, while trails of stars in the end near the lower right observatory dome." seems to have something missing!

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FRANK: But you see my point here?
You only hear of a hen, a rooster and a chicken. Something's missing!

MRS. ROSS: Something's missing all right.
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I'm a photographer who is feeling a bit frustrated.

I'd love to take a picture like the one of the Haute-Provence Star Trails, (http://antwrp.gsfc.nasa.gov/apod/ap090314.html) but so ar despite some effort it continues to elude me.

Know about aiming for the north star so that part I've got, however it's the exposure that I stuggle with.

I created an image with a single exposure of some eight hours at Christmas, but the only streaks were fairly minimal. I'm assuming that the key, as with Alexandre Santerne's picture, (does anyone have contact details for him by the way?) so to do several exposures with small gaps in between. What should these gaps be? 30 Seconds? 1 Minute? More?

Any advice, or contact details for Alexandre Santerne would be appreciated.

cloudwalker_3 wrote:Know about aiming for the north star so that part I've got, however it's the exposure that I stuggle with.

I created an image with a single exposure of some eight hours at Christmas, but the only streaks were fairly minimal. I'm assuming that the key, as with Alexandre Santerne's picture, (does anyone have contact details for him by the way?) so to do several exposures with small gaps in between. What should these gaps be? 30 Seconds? 1 Minute? More?

Your problem was the result of using a single exposure. Even under the darkest skies, the sky background will wash out the stars. Just consider a single star: it was on any given pixel for no more than a minute or two, but the sky background was filling that pixel for eight hours. On top of that, you had dark current filling the pixels. (I'm assuming you were using digital, but there are analogous problems with film).

These problems can be brought under control by stacking many short exposures. Ideally you'd have no time gap between them, since that leads to dashed trails. But there's always going to be some minimum time determined by the camera electronics. If it's short enough (as in this APOD), the gaps in the trails won't be apparent.

Thanks Chris, I really appreciate the advice.

For what it's worth, the image I tried to get was taken in my parents garden. They live in the countryside and there's minimal light polution so the sky was inky black. Having said this, I totally appreciate the point you're making about the sky being there for eight hours. In terms of stacking exposures, how long would you recommend leaving between them? Does the time of year/location on the globe matter?

cloudwalker_3 wrote:For what it's worth, the image I tried to get was taken in my parents garden. They live in the countryside and there's minimal light polution so the sky was inky black. Having said this, I totally appreciate the point you're making about the sky being there for eight hours. In terms of stacking exposures, how long would you recommend leaving between them? Does the time of year/location on the globe matter?

I'd probably start with just what the APOD imager used: one-minute exposures with no more time between them than the camera requires to save the image. Given one-minute exposures, most recent DSLRs can shoot continuously for 8 hours, with less than a second between shots.

It shouldn't matter where or when you are, or what direction you aim the camera. The method is the same.

It certainly gives an interesting perspective on Orion

I don't wish to appear dense but, reading all the comments pertaining to the picture was very enlightening in regard to exposure but confusing with regard to the camera. Without knowing, it would seem a pinhole camera would be a equivalent to a normal lense. This image is not a fisheye but the distorsion of the left dome and leaning tree make it seem wide. I realized the longest parallel lines were in the center, corner to corner.So, wide lense aimed at the celestial equator?

bhrobards wrote:Without knowing, it would seem a pinhole camera would be a equivalent to a normal lense. This image is not a fisheye but the distorsion of the left dome and leaning tree make it seem wide. I realized the longest parallel lines were in the center, corner to corner.So, wide lense aimed at the celestial equator?

Yes...or, at least, aimed close to the celestial equator.

DavidLeodis wrote:The line in the explanation to this APOD that states "Brilliant planet Venus makes the short bright trail at the lower right, while trails of stars in the end near the lower right observatory dome." seems to have something missing!

I've noticed that that line has been amended and now reads "Brilliant planet Venus makes the short bright trail at the lower right, while trails of stars in the constellation Orion end near the lower right observatory dome.". That makes much more sense!