| FAQ last modified | Sat Jun 9 16:35:31 2007 GMT |
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| Current time | Fri Nov 21 19:45:50 2008 GMT |
1.0. General
1.1. What is this site all about?
1.2. What level of astronomical knowledge does this FAQ assume?
1.3. What quality are the photographs?
1.4. Why are there so many trees in some of the photographs?
2.0. Accuracy and Completeness
2.1. How accurate are the annotations?
2.2. Are planets annotated?
2.3. Are Messier and other deep-sky objects annotated?
2.4. How complete are the annotations for stars?
2.5. How complete are the annotations for constellations?
3.0. Aesthetics
3.1. Why do the labels for right ascension and declination overlap in the middle of the image?
3.2. The annotations appear in an odd color. Why?
3.3. When using the chart annotation, some of the chart dots look like stars.
3.4. Why are the Greek letters abbreviated "improperly" in English, instead of being written in Greek?
3.5. How are the stars named?
3.6. Why are some fairly bright stars not labelled?
3.7. Why do the constellation lines sometimes "miss" stars?
3.8. Why are constellations only partially drawn sometimes?
3.9. Why do I sometimes see a spurious constellation line?
3.10. Why don't the constellations look like the ones in my star atlas/book?
3.11. How closely spaced are the grid lines in the chart annotation?
4.0. Errata
4.1. Why do some annotations appear below the horizon or behind land objects?
4.2. Why are some photographs not annotated?
5.0. Miscellaneous
This is the place where I post my astrophotography (photographs of stars in the night sky). You can view the "raw" image (which is a scan of the photograph exactly the way I took it), or you can view the image with annotations. You can see the names of bright stars, view a grid of right ascension/declination (the celestial version of longitude and latitude), or see constellation lines. These options may be combined.
1.2. What level of astronomical knowledge does this FAQ assume?
The general section (this one) does not assume any prior knowledge of astronomy. The remaining sections assume a basic knowledge of astronomy.
1.3. What quality are the photographs?
The photographs are of low quality. I submitted some of them to Corel for inclusion on of their Photo CDs, and they were rejected as being of "too low quality". So don't get your hopes up <G>.
1.4. Why are there so many trees in some of the photographs?
The photographs, especially those taken in California, were taken from my backyard, and there were quite a few trees there. For the photographs taken in Albuquerque, I try to avoid trees, but some of them do manage to get into the photographs.
Not very. The annotations are based on an approximate mathematical model. Since the model is not exact, some stars are annotated several pixels away from their actual location. So far, 25 pixels is about the maximum error I've spotted, but it could be larger in some cases. The exact error depends on how the original mathematical model was created (it's created a little differently for each photograph). I've tried to be careful to avoid very large errors, but some error is inherent.
2.2. Are planets annotated?
No. Although it's possible to compute the position of a planet at a given time, it would require knowing when the photograph was taken. While I do have this information for some photographs, I don't have it for all of them. It's also somewhat computationally intensive to compute planetary positions, so I've decided to leave them out for now (if a planet appears in a photograph, it's often mentioned in the description anyway).
2.3. Are Messier and other deep-sky objects annotated?
No. I plan to eventually annotate Messier objects (though probably not other deep-sky objects, which are normally too faint to show up on my photographs anyway), but these are not currently annotated.
2.4. How complete are the annotations for stars?
The annotations for star names come from a catalog which lists the brightest 1,628 stars; these annotations should thus be fairly complete.
2.5. How complete are the annotations for constellations?
Not very. I manually enter constellation data, and while I've entered several constellations, there are still many I have not yet entered.
Roughly speaking, the declination lines are labelled where they intersect the "median" line of right ascension (ie, the line of right ascension closest to the center of the photograph), and the right ascension lines are labelled where they intersect the median line of declination. Unfortunately, at the point where the two median lines intersect, both labels appear, overlapping. I hope to fix this at some point.
3.2. The annotations appear in an odd color. Why?
Annotations are created using PHP's JPEG image creation features. I haven't found a way to get these to be of a consistent color (there's supposedly a way to do it, I just haven't gotten it to work), so I just tell PHP to use "the color closest to white".
3.3. When using the chart annotation, some of the chart dots look like stars.
I've considered several ways of avoiding this (changing the dots to "x"s, changing the color of the dots, removing the dots entirely, etc), but haven't really found a good way around this. If you're not sure whether you're seeing a dot or a star, try viewing the raw image.
3.4. Why are the Greek letters abbreviated "improperly" in English, instead of being written in Greek?
I'm sure there's a way to get PHP's JPEG image creation feature to print out Greek letters, but I haven't found it yet. As for the improper abbreviations (eg, "alf" for "alpha"), these abbreviations come directly from the star catalog I use.
3.5. How are the stars named?
The 21 brightest stars (those with magnitudes less than 1.5) are labelled with their proper names. For stars with magnitudes greater than 1.5, I use the Bayer name (Greek letters) where available, and the Flamsteed number otherwise. If neither of these is available, I use the HR bright star number.
3.6. Why are some fairly bright stars not labelled?
All stars with visual magnitude less than 5.01 should be labelled. However, photographic film is more sensitive to blue light than human eyesight. Thus, a blue star that appears fairly bright might in fact have a magnitude greater than 5.01 and thus not be labelled. There are some cases where a faint red star is labelled, but a nearby brighter blue star is not. If these stars were viewed in the sky by a human observer, the blue star would actually appear duller.
3.7. Why do the constellation lines sometimes "miss" stars?
The constellation lines, like the stellar name annotations, are calculated using an approximate mathematical formula. Since the formual is not exact, the mathematical position of a constellation line might be different from its true position.
3.8. Why are constellations only partially drawn sometimes?
A constellation line is only drawn if the stars on both ends of line are visible in the photograph. If either star is not visible, the line is not drawn. This can lead to some "broken" looking constellations, especially when a portion of the constellation is not in the photograph.
3.9. Why do I sometimes see a spurious constellation line?
Some of the photographs have a fairly small field of view. Of the many lines in a given constellation, it's possible that only one of them was actually visible in the photograph.
3.10. Why don't the constellations look like the ones in my star atlas/book?
I use H.A. Rey's excellent book, "The Stars: A New Way To See Them", to draw constellation lines. The lines in this book may be different from other star atlases.
3.11. How closely spaced are the grid lines in the chart annotation?
The "heavier" dotted lines are spaced 5 degrees apart (which is 20 minutes of right ascension); the lighter lines are spaced 1 degree apart (4 minutes of right ascension).
The annotations are created using a mathematical model which doesn't take account of the horizon (or objects near the horizon, such as mountains, buildings, trees, etc), so some stars are annotated even though they are not visible in the photograph.
4.2. Why are some photographs not annotated?
In many cases, it's because I'm lazy and haven't gotten around to annotating several photographs, though I hope to do this soon. In other cases, I took a photograph of something and wasn't able to identify it later on. These "unknown" photographs are here. If you recognize something in these photographs, let me know.
It varies per photograph, but 7th magnitude stars are visible in most of the better photographs, and 8th magnitude stars are visible in some of the photographs. I believe I've seen blue stars with visual magnitudes as high as 9.0 in one or two of the photographs-- but it's quite possible that I'm just imagining this.
5.2. Where did you get the data for stellar positions?
From "The catalogue of the brightest stars machine-readable version prepared by F. Ochsenbein and J.L. Halbwachs Stellar Data Center (Strasbourg Observatory)".
5.3. What equipment did you use to take these photographs?
I used a regular 35mm camera (sometimes with a 50mm attachable lens), and a tripod. I did not use a clock drive, telescope, binoculars, or optical aid of any sort.
5.4. Where did you take these photographs?
Many of these photographs were taken from a dark location near where I live in Albuquerque, New Mexico, USA. Several of the older photographs were taken from my backyard in Cupertino, California, USA.
5.5. When did you take these photographs?
Many of these photographs were taken on January 14, 1996, when I attempted to photograph as much of the visible sky as possible. The older photographs were taken at various times from 1990 through 1992.
5.6. In this FAQ, when you say you'll do something "soon", how soon is that?
The photographs I took on January 14, 1996 weren't uploaded to my website for over 4 1/2 years. That should give you some idea ;)
5.7. Can you help me annotate my own astrophotography?
I can't promise to do this, but if you have some astrophotography on the web, and can identify at least three "bright" stars (magnitude 5.01 or less), send me an email and I'll see what I can do.
5.8. Is Plett Keng a real person?
No, it's an extremely subtle (in the sense of stupid) joke.
5.1. What is the absolute limiting magnitude on these photographs?
5.2. Where did you get the data for stellar positions?
5.3. What equipment did you use to take these photographs?
5.4. Where did you take these photographs?
5.5. When did you take these photographs?
5.6. In this FAQ, when you say you'll do something "soon", how soon is that?
5.7. Can you help me annotate my own astrophotography?
5.8. Is Plett Keng a real person? 1. General
2. Accuracy and Completeness
3. Aesthetics
4. Errata
5. Miscellaneous