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How do you identify stars in an image?


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Hi All

Might sound like a daft question. When you have a dissapointing hazy night and you get probably the worst image of Andromeda ever stacked (11 x 1min, ISO 3200, EOS 500D, Deepsky Stacker), I started to think maybe I could try identifying all the stars in the image as an excercise before consigning it to the photo dump folder (or deleting it). Theres quite a lot of faint items - I wana know who they all are!

Well, I've spent probably a couple of hours comparing this image to Stellarium and I cant find any decent matches - its driving me mad, now. Surely it should be a case of rotate / scale / there it is...?

I think the image is about a degree high (which I know is too small, Andromeda is about 2 degrees across yes?) I had a vague idea about doing a mosaic:P

So, Any advice on how to quickly pin down who's who in the picture?

Cheers

Steve

post-11101-0-00524800-1351769500_thumb.p

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A good place to start is to work out the image scale and field of view of your image. Then mark this up on a transparent sheet to the scale of your star chart and use this to help find where the stars are. Also remember your image maybe flipped & or rotated with respect to the star chart and that depends on what sort of scope you have and the orientation of the camera.

Hope this helps Andrew

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A good place to start is to work out the image scale and field of view of your image. Then mark this up on a transparent sheet to the scale of your star chart and use this to help find where the stars are. Also remember your image maybe flipped & or rotated with respect to the star chart and that depends on what sort of scope you have and the orientation of the camera.

Hope this helps Andrew

Doh! I hadn't thought of that - flipped! I have to go drop the wife at her moms then I'll have another look.

Cheers

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Astrometry is the method of determining the identification of the celestial objects.

I've submitted your pic to an online astrometry engine and it's failing to identify but it certainly 'looks' like a portion of M31 at the bottom of the picture.

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Fuzzy blob bottom centre is M32 and the bright star right and slightly below it is HIP3333. The core of M31 is off past the top right corner.

Wow, I totally see it now. There is at least 10 other points which confirm where I was looking. So that sums up why it was the worst Andromeda picture ever - wasnt actually pointing at it, in fact!

Was that just from personal knowledge, James, or did you use an unmentionned tool?

Thanks

Steve

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I used astrometry.net software, running under Windows+cygwin. No Henry Draper catalog stars found.



$ solve-field.exe -O --sigma 20 foo.png
Reading input file 1 of 1: "foo.png"...
Read file stdin: 1130 x 710 pixels x 1 color(s); maxval 255
Using 8-bit output
Extracting sources...
simplexy: found 19 sources.
Solving...
Reading file "./foo.axy"...
Field 1 did not solve (index index-219.fits, field objects 1-10).
Field 1 did not solve (index index-218.fits, field objects 1-10).
Field 1 did not solve (index index-217.fits, field objects 1-10).
Field 1 did not solve (index index-216.fits, field objects 1-10).
Field 1 did not solve (index index-215.fits, field objects 1-10).
Field 1 did not solve (index index-214.fits, field objects 1-10).
Field 1 did not solve (index index-213.fits, field objects 1-10).
Field 1 did not solve (index index-212.fits, field objects 1-10).
Field 1 did not solve (index index-211.fits, field objects 1-10).
Field 1 did not solve (index index-210.fits, field objects 1-10).
Field 1 did not solve (index index-209.fits, field objects 1-10).
Field 1 did not solve (index index-208.fits, field objects 1-10).
Field 1 did not solve (index index-207.fits, field objects 1-10).
Field 1 did not solve (index index-206.fits, field objects 1-10).
log-odds ratio 68.3472 (4.81735e+29), 13 match, 0 conflict, 2 distractors, 43 index.
RA,Dec = (10.7985,40.8505), pixel scale 1.65159 arcsec/pix.
Hit/miss: +++++++++-+-+++(best)++++
Field 1: solved with index index-205.fits.
Field 1 solved: writing to file ./foo.solved to indicate this.
Field: foo.png
Field center: (RA,Dec) = (10.8, 40.85) deg.
Field center: (RA H:M:S, Dec D:M:S) = (00:43:11.829, +40:51:03.181).
Field size: 31.0553 x 19.5405 arcminutes
Creating new FITS file "./foo.new"...
Creating index object overlay plot...
Creating annotation plot...
Your field contains:
NGC 221 / M 32



$ plot-constellations.exe -w foo.wcs -i foo.pnm -G 2 -N -o foo-an.png -J
NGC 221 / M 32
{
"status": "solved",
"svn-revision": 16745,
"svn-date": "2010-11-19 20:47:53 +0000 (Fri, 19 Nov 2010)",
"annotations": [
{ "type" : "ngc", "names" : [ "NGC 221", "M 32" ], "pixelx" : 633.228, "pixely" : 551.518, "radius" : 138.264 }
]
}



post-774-0-95231700-1351783279_thumb.png

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Cheers all

Found it!

Although I have to admit this is the first try out for our new NEQ5 Pro mount (same Skywatcher 150).

OMG Goto is SO worth it! With the NEQ5 and allowing for not knowing what we were doing, and we ate dinner part way through setup, we were still ready in about an hour. Then we spent 2 hours doing photos of M31, M33, M52, M103, and NGC869. Managed 20 exposures of 30 secs each (short I know, because of being overlooked by 3 sodium lamps here).

The Met Office said it would be clear from 6 till 9 and they were right to the minute. WOW, so clear, what a night!

Many thanks to Bernard at Modern Astronomy for great advice and fast service around the new mount! Thx to whoever it was here as well who said, "spend money on a mount, not a scope", cause they were right.

I posted this here cause it kinda follows on the topic.

Cheers

Steve

post-11101-0-37156500-1351810122_thumb.j

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Well that's a big improvement :)

If you crop off the edges where you appear to have some stacking artefacts (particularly obvious at the top and bottom) you may find that there's more room for manoeuvre with levels, too.

James

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Couldn´t agree more. But why do I feel so guilty relying on it so heavily?

Probably because we're not finding our own way across the sky. However, I feel a little less guilty if I research the objects so I know roughly where they are before I use the GOTO.

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Couldn´t agree more. But why do I feel so guilty relying on it so heavily?

Probably because we're not finding our own way across the sky. However, I feel a little less guilty if I research the objects so I know roughly where they are before I use the GOTO.

Hmm, Guilt... U know what? Ive retyped this tons of times - I think in an attempt to justify using goto over manual searching. Well, hang it, I got a real rush out of getting so many photo's in 2 hours last night. So, huh, I'm on the side of goto.

There, I said it!

Steve

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Can't see any reason for guilt myself. it's not a competition. Do whatever makes you happy. I enjoy the "thrill of the chase" myself and the feat of finding a tricky target adds to the enjoyment of seeing it, but if I were in a hurry or wanted my children to be able to find things I'd happily use GOTO.

James

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At the moment the Cambridge Photographic Star Atlas is a free gift with a subscription to S@N mag, I have a copy and found it most useful in identifying star fields/asterisms around objects of interest. On one page digital photo of the night sky at a scale of 1cm per deg on the opposite page the same image is converted to black and white with all important star information, constellation boundaries and other items of interest. What is more, you can still see the night sky on a cloudy night, you do not need a telescope, just a large magnifying glass :)

John.

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