How to focus manually a lens for which it was not designed. - Page 4

xilman · 2016-11-28, 07:42

've just got out of bed and am severely hypocaffeinaemic so the idea I had a couple of hours ago might be complete BS.

To reduce sky glow in a wide angle image you could try setting to zero all Fourier components in a (small) circle centred on the origin in frequency space. This will undoubtedly remove the mean and some large scale variation but without experimentation I don't know whether the result will be visually pleasing. AFAIK it should be possible to do this from the command line in ImageMagick without having to write custom code.

xilman · 2016-11-30, 10:35

Quote:

Originally Posted by fivemack

There is nothing terribly interesting in this field; I was using Capella as a point source to focus the camera, and thought I might as well take a longer exposure to see what the limiting magnitude was like - it's greater than 10, and I don't have a good enough star chart to figure out how *much* greater than ten.

After further mucking around, with dcraw this time, I noticed something else which might not be boring. At (2096,1095), in co-ordinates where Capella is ~(2380,1710), is a fuzzy patch of about mag 10. Nothing is shown there in Uranometria 2000.0, which generally shows such things to at least that limit. Neither can I find anything in Sky Catalogue 2000.0. Careful use of the acetate overlays suggest that the sky co-ordinates are about 5^h14^m45^s +47°7' to about 2' accuracy in each direction. No bright comets were in that part of the sky on that date.

Nothing appears at http://aladin.u-strasbg.fr/AladinLite/ either. It's a mystery to me right now. About the only thing I can think of is that it might be a ghost image of Capella.

fivemack · 2016-11-30, 11:56

I think that's a ghost image produced by the lens-and-camera combination, because of the greenish colour; that's an easy hypothesis to test with a bright star at two different points in the field, I will do that at the next convenient opportunity.

(this particular lens has a nearly-flat rear element near enough to the CCD that I would not be amazed for the light to have reflected off the CCD and the rear element)

xilman · 2016-12-01, 21:18

Quote:

Originally Posted by fivemack

Would you like some more images? I cannot guarantee consistency of point spread function, but it was a nice clear night on Thursday and I can make available any of:

I'm getting interested in discovering the limiting magnitude of your set up. The canonical area for determining such things is the North Polar Sequence. I don't know if the north celestial pole is visible from your location (it might be behind a building or be excessively light polluted for instance) but you might want to consider taking a few images around there.

I also have some supernova patrol charts for M33 with good magnitudes going down quite faint so that also might be appropriate. Most any field containing an eruptive variable such as SS Cyg would also be good (back in the day I used to observe AB Draconis which is in a less crowded field and where companions run from mag 9.4 to 15.7 with a good selection in the range 11.1 to 12.9, the brighter end of which should be visible), as would RCrB. Unfortunately the latter is particularly badly placed at the moment.

Whether you're interested in photometry is another matter entirely...

fivemack · 2016-12-02, 10:15

I think M33 is the answer, because I have sets of photos of that region both with the 200/2.8 and the 400/3.5 lenses - I have not been able to produce a satisfying stack that shows the galaxy nicely, because it's quite a lot fainter per unit area than the light pollution, though your background-subtraction software ought to be able to help out well for that.

I am terribly busy this weekend, but will try to get something sorted out on Monday - I suspect just starting up Apache again on the world-visible machine and putting a few gigabytes of .NEF files there would give you something to chew on.

xilman · 2016-12-02, 19:00

Quote:

Originally Posted by fivemack

I think M33 is the answer, because I have sets of photos of that region both with the 200/2.8 and the 400/3.5 lenses - I have not been able to produce a satisfying stack that shows the galaxy nicely, because it's quite a lot fainter per unit area than the light pollution, though your background-subtraction software ought to be able to help out well for that.

I am terribly busy this weekend, but will try to get something sorted out on Monday - I suspect just starting up Apache again on the world-visible machine and putting a few gigabytes of .NEF files there would give you something to chew on.

That would work well, thanks.

xilman · 2016-12-03, 10:34

More fun. The PSF in the green channel has now been estimated by co-adding 230 unsaturated star images (each centred on the local maximum pixel) and then setting to zero all pixels with <1% intensity. The result is displayed. It may not pure coma but looks pretty close to me. The FWHM is 6 pixels in each dimension, corresponding to around 0.5 arcmin on the sky. Not sure how much deconvolution will improve on that resolution but it's certainly worth a try. More later.

fivemack · 2016-12-03, 12:59

I have knocked a relevant-shaped hole in my firewall, and you will find

http://fivemack.dynamic.greenend.org.uk/astrophoto/M33

hopefully of interest.

/200 and /400 refer to the focal length of the lens used (with this camera, the image scale is about 6 arc seconds per pixel for 200, about 3 for 400). /400 is rotated about 45 degrees counter-clockwise with respect to /200, and was taken three nights later and I think rather later in the night. Camera details are in the EXIF data in the jpg files: about 9.7 seconds at ISO 2000 for /400, 20 seconds at ISO 2000 for /200.

You will find the whole battery of astrophotography problems present - I can certainly discern light-pollution, vignetting, tracking error, and something that looks like slight differential focus across the field of the /400 - but I'm hoping that's where the fun part lies :)

xilman · 2016-12-03, 16:12

Quote:

Originally Posted by fivemack

I have knocked a relevant-shaped hole in my firewall, and you will find

http://fivemack.dynamic.greenend.org.uk/astrophoto/M33

hopefully of interest.

/200 and /400 refer to the focal length of the lens used (with this camera, the image scale is about 6 arc seconds per pixel for 200, about 3 for 400). /400 is rotated about 45 degrees counter-clockwise with respect to /200, and was taken three nights later and I think rather later in the night. Camera details are in the EXIF data in the jpg files: about 9.7 seconds at ISO 2000 for /400, 20 seconds at ISO 2000 for /200.

You will find the whole battery of astrophotography problems present - I can certainly discern light-pollution, vignetting, tracking error, and something that looks like slight differential focus across the field of the /400 - but I'm hoping that's where the fun part lies :)

Thanks. I grabbed four NEF files from each lens which should be enough to get me started.

Paul

Batalov · 2016-12-03, 17:09

Lovely sputnik photobomb in DSC0606 !

xilman · 2016-12-03, 18:01

You might like to look at this version of DSC_0617.NEF --- it's a quick hack and I'm sure I can do much better. Gently raising the gamma will bring out more detail but raising it to more than 1.2 or so will show the artefacts which a slow hack would try to avoid. Nonetheless it's a marked improvement on the original IMAO

2016-12-03, 10:34	#40
xilman Bamboozled! "𒉺𒌌𒇷𒆷𒀭" May 2003 Down not across 2⁹·3·7 Posts	More fun. The PSF in the green channel has now been estimated by co-adding 230 unsaturated star images (each centred on the local maximum pixel) and then setting to zero all pixels with <1% intensity. The result is displayed. It may not pure coma but looks pretty close to me. The FWHM is 6 pixels in each dimension, corresponding to around 0.5 arcmin on the sky. Not sure how much deconvolution will improve on that resolution but it's certainly worth a try. More later. Attached Images

2016-12-03, 12:59	#41
fivemack (loop (#_fork)) Feb 2006 Cambridge, England 14423₈ Posts	I have knocked a relevant-shaped hole in my firewall, and you will find http://fivemack.dynamic.greenend.org.uk/astrophoto/M33 hopefully of interest. /200 and /400 refer to the focal length of the lens used (with this camera, the image scale is about 6 arc seconds per pixel for 200, about 3 for 400). /400 is rotated about 45 degrees counter-clockwise with respect to /200, and was taken three nights later and I think rather later in the night. Camera details are in the EXIF data in the jpg files: about 9.7 seconds at ISO 2000 for /400, 20 seconds at ISO 2000 for /200. You will find the whole battery of astrophotography problems present - I can certainly discern light-pollution, vignetting, tracking error, and something that looks like slight differential focus across the field of the /400 - but I'm hoping that's where the fun part lies :) Last fiddled with by fivemack on 2016-12-03 at 13:00

2016-12-03, 18:01	#44
xilman Bamboozled! "𒉺𒌌𒇷𒆷𒀭" May 2003 Down not across 2⁹×3×7 Posts	You might like to look at this version of DSC_0617.NEF --- it's a quick hack and I'm sure I can do much better. Gently raising the gamma will bring out more detail but raising it to more than 1.2 or so will show the artefacts which a slow hack would try to avoid. Nonetheless it's a marked improvement on the original IMAO Attached Thumbnails

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2016-11-28, 07:42	#34
xilman Bamboozled! "𒉺𒌌𒇷𒆷𒀭" May 2003 Down not across 2A00₁₆ Posts	've just got out of bed and am severely hypocaffeinaemic so the idea I had a couple of hours ago might be complete BS. To reduce sky glow in a wide angle image you could try setting to zero all Fourier components in a (small) circle centred on the origin in frequency space. This will undoubtedly remove the mean and some large scale variation but without experimentation I don't know whether the result will be visually pleasing. AFAIK it should be possible to do this from the command line in ImageMagick without having to write custom code.

2016-11-30, 11:56	#36
fivemack (loop (#_fork)) Feb 2006 Cambridge, England 7²·131 Posts	I think that's a ghost image produced by the lens-and-camera combination, because of the greenish colour; that's an easy hypothesis to test with a bright star at two different points in the field, I will do that at the next convenient opportunity. (this particular lens has a nearly-flat rear element near enough to the CCD that I would not be amazed for the light to have reflected off the CCD and the rear element)

2016-12-02, 10:15	#38
fivemack (loop (#_fork)) Feb 2006 Cambridge, England 7²×131 Posts	I think M33 is the answer, because I have sets of photos of that region both with the 200/2.8 and the 400/3.5 lenses - I have not been able to produce a satisfying stack that shows the galaxy nicely, because it's quite a lot fainter per unit area than the light pollution, though your background-subtraction software ought to be able to help out well for that. I am terribly busy this weekend, but will try to get something sorted out on Monday - I suspect just starting up Apache again on the world-visible machine and putting a few gigabytes of .NEF files there would give you something to chew on.

2016-12-03, 17:09	#43
Batalov "Serge" Mar 2008 Phi(4,2^7658614+1)/2 2²·23·103 Posts	Lovely sputnik photobomb in DSC0606 !