Re: [Utah-astronomy] TYLER'S TALK
Im sure I missed a great presentation, hope to hear all about it. hopefully I will get another opportunity to meet and hear all about his imaging processes another time. Mike
The central point of Tyler's technique was step in image processing at which limited stretching is applied. The more typical method after image calibration is to stack first and then apply stretch or deconvolution filters. Tyler pointed out that that stretching is often applied improperly so that detail is in the image is erased into either uniform dark (0 value) or uniform bright (65536 for a 16 bit camera). This improper use of stretching destroys detail. The histogram of a source image might look like this: Galaxy Stars * ** * *** * **** * **************** 0 65536 B G W The pixel spike at the right are the pixels related to Milky Way stars in the foreground of the image; the pixel group to the right consists of the pixels related to a galaxy in the background of the image. "B", "G" and "W" concern the black point, the grey point and the white point of the image. The more typical, but errorneous, way to stretch an image, erases some detail: Galaxy Stars * * * ** * *** * **************** 0 65536 B G W In this stretch, the absolute black and white points are moved to stretch the pixel values. This destroys detail in the galaxy pixels at the left side of the histogram. His processing recommendation centered on grey point stretching. The White and Black points are left alone - the grey point (often the "Contrast" slider in some terresterial photo processing programs) is stretched, thus: Galaxy Stars ***** ****** * ******* * ********* * **************** 0 65536 B G W This grey point stretch enhances detail in the each calibrated fits image. It also makes for an odd looking counter-intutitive image. A grey point stretched image will look washed out - like an image that has the contrast setting in it wrong. He does this stretch for each captured source frame. _Then_ he stacks his individually pre-grey-point stretched frames, yielding: Galaxy Stars * * * ** * *** * **** * ***** * ****** * ******* * ******** * **************** 0 65536 B G W Again, this yields an image that looks washed out - the background sky will look grey instead of black. Next, to make the sky black again - but while retaining the now enhanced details of the original galaxy, he does a white and black point stretch: Galaxy Stars *** **** **** ***** * ****** * ******* * ******** * ********* * ********** * *********** * **************** 0 65536 B G W This method retains the detail in the galaxy pixels collected in the original images while enhancing them. Tyler noted that the incorrect stretch process also generates the dreaded "fat stars", while his process does not. In the incorrect process, unstretched images are stacked from this: Galaxy Stars * ** * *** * **** * **************** 0 65536 B G W to this - Galaxy Stars * ** ** * *** ** *** ** **** *** **** *** **** *** ***** *** **************** 0 65536 B G W and then get white-black point stretched to this - Galaxy Stars * * * * ** ** ** ** *** ** *** ** *** ** **** ** **************** 0 65536 B G W - thus making the "bloated" or "fat" stars. The remainder of his tips went to the need for a good tracking mount and black art tips specific to PhotoShop. The PhotoShop tips were dependent, however, on first mastering the basic skills of proper grey-point stretching and stacking. Without that first step, there are no details in the stacked image that PhotoShop can manipulate. Hope that helps. - Kurt P.S. - It is uncertain whether the old DOS style asterik "*" graphs will make it through modern listserv and email and/or web client processing.
Kurt, thank you for relaying that info, the hardest part of imaging is definately processing the images. (not to take away from proper alignment, clear steady skies, and lots of patients) I have given it an attempt, and have nothing yet to be proud of. I am always looking for tips and tricks to improve, and learn from. I will put this info to good use, and will apply it to some of my future efforts. thanks again Mike On Wed, Feb 18, 2009 at 4:58 PM, Canopus56 <canopus56@yahoo.com> wrote:
Re: [Utah-astronomy] TYLER'S TALK
Im sure I missed a great presentation, hope to hear all about it. hopefully I will get another opportunity to meet and hear all about his imaging processes another time. Mike
The central point of Tyler's technique was step in image processing at which limited stretching is applied. The more typical method after image calibration is to stack first and then apply stretch or deconvolution filters. Tyler pointed out that that stretching is often applied improperly so that detail is in the image is erased into either uniform dark (0 value) or uniform bright (65536 for a 16 bit camera). This improper use of stretching destroys detail.
The histogram of a source image might look like this:
Galaxy Stars * ** * *** * **** * **************** 0 65536 B G W
The pixel spike at the right are the pixels related to Milky Way stars in the foreground of the image; the pixel group to the right consists of the pixels related to a galaxy in the background of the image. "B", "G" and "W" concern the black point, the grey point and the white point of the image.
The more typical, but errorneous, way to stretch an image, erases some detail:
Galaxy Stars * * * ** * *** * **************** 0 65536 B G W
In this stretch, the absolute black and white points are moved to stretch the pixel values. This destroys detail in the galaxy pixels at the left side of the histogram.
His processing recommendation centered on grey point stretching. The White and Black points are left alone - the grey point (often the "Contrast" slider in some terresterial photo processing programs) is stretched, thus:
Galaxy Stars ***** ****** * ******* * ********* * **************** 0 65536 B G W
This grey point stretch enhances detail in the each calibrated fits image. It also makes for an odd looking counter-intutitive image. A grey point stretched image will look washed out - like an image that has the contrast setting in it wrong.
He does this stretch for each captured source frame.
_Then_ he stacks his individually pre-grey-point stretched frames, yielding:
Galaxy Stars * * * ** * *** * **** * ***** * ****** * ******* * ******** * **************** 0 65536 B G W
Again, this yields an image that looks washed out - the background sky will look grey instead of black.
Next, to make the sky black again - but while retaining the now enhanced details of the original galaxy, he does a white and black point stretch:
Galaxy Stars *** **** **** ***** * ****** * ******* * ******** * ********* * ********** * *********** * **************** 0 65536 B G W
This method retains the detail in the galaxy pixels collected in the original images while enhancing them.
Tyler noted that the incorrect stretch process also generates the dreaded "fat stars", while his process does not. In the incorrect process, unstretched images are stacked from this:
Galaxy Stars * ** * *** * **** * **************** 0 65536 B G W
to this -
Galaxy Stars * ** ** * *** ** *** ** **** *** **** *** **** *** ***** *** **************** 0 65536 B G W
and then get white-black point stretched to this -
Galaxy Stars * * * * ** ** ** ** *** ** *** ** *** ** **** ** **************** 0 65536 B G W
- thus making the "bloated" or "fat" stars.
The remainder of his tips went to the need for a good tracking mount and black art tips specific to PhotoShop. The PhotoShop tips were dependent, however, on first mastering the basic skills of proper grey-point stretching and stacking. Without that first step, there are no details in the stacked image that PhotoShop can manipulate.
Hope that helps.
- Kurt
P.S. - It is uncertain whether the old DOS style asterik "*" graphs will make it through modern listserv and email and/or web client processing.
_______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://gallery.utahastronomy.com Visit the Wiki: http://www.utahastronomy.com
-- "Astronomy compels the soul to look upwards and leads us from this world to another" ~Plato~
The thing that gives me fits is focus. Not like the old days when you could just look through the viewfinder and focus. Now it's a matter of taking a picture, adjust focus, take picture, adjust focus, take picture, adjust focus and so on and on. And then when focus is reached one must keep an eye on the thermometer as falling temperatures cause the metal in the telescope's tube to shrink thereby lessening the distance between the mirror and the camera. So (at least with my set up) every time the temp changes 1 degree C it's time to adjust focus. This, BTW, if the reason some of the newer scopes use carbon fibre tubes. patrick On 18 Feb 2009, at 21:23, Mike Nelson wrote:
thank you for relaying that info, the hardest part of imaging is definately processing the images.
Kurt, You obviously have a young mind. I found it difficult taking notes and keeping up with Tyler’s very interesting presentation all at the same time. Of course I was chewing gum which really slowed me down, but thanks for taking such good notes. Jim --- On Wed, 2/18/09, Canopus56 <canopus56@yahoo.com> wrote: From: Canopus56 <canopus56@yahoo.com> Subject: [Utah-astronomy] (no subject) To: "Utah Astronomy List Serv" <utah-astronomy@mailman.xmission.com> Date: Wednesday, February 18, 2009, 4:58 PM Re: [Utah-astronomy] TYLER'S TALK
Im sure I missed a great presentation, hope to hear all about it. hopefully I will get another opportunity to meet and hear all about his imaging processes another time. Mike
The central point of Tyler's technique was step in image processing at which limited stretching is applied. The more typical method after image calibration is to stack first and then apply stretch or deconvolution filters. Tyler pointed out that that stretching is often applied improperly so that detail is in the image is erased into either uniform dark (0 value) or uniform bright (65536 for a 16 bit camera). This improper use of stretching destroys detail. The histogram of a source image might look like this: Galaxy Stars * ** * *** * **** * **************** 0 65536 B G W The pixel spike at the right are the pixels related to Milky Way stars in the foreground of the image; the pixel group to the right consists of the pixels related to a galaxy in the background of the image. "B", "G" and "W" concern the black point, the grey point and the white point of the image. The more typical, but errorneous, way to stretch an image, erases some detail: Galaxy Stars * * * ** * *** * **************** 0 65536 B G W In this stretch, the absolute black and white points are moved to stretch the pixel values. This destroys detail in the galaxy pixels at the left side of the histogram. His processing recommendation centered on grey point stretching. The White and Black points are left alone - the grey point (often the "Contrast" slider in some terresterial photo processing programs) is stretched, thus: Galaxy Stars ***** ****** * ******* * ********* * **************** 0 65536 B G W This grey point stretch enhances detail in the each calibrated fits image. It also makes for an odd looking counter-intutitive image. A grey point stretched image will look washed out - like an image that has the contrast setting in it wrong. He does this stretch for each captured source frame. _Then_ he stacks his individually pre-grey-point stretched frames, yielding: Galaxy Stars * * * ** * *** * **** * ***** * ****** * ******* * ******** * **************** 0 65536 B G W Again, this yields an image that looks washed out - the background sky will look grey instead of black. Next, to make the sky black again - but while retaining the now enhanced details of the original galaxy, he does a white and black point stretch: Galaxy Stars *** **** **** ***** * ****** * ******* * ******** * ********* * ********** * *********** * **************** 0 65536 B G W This method retains the detail in the galaxy pixels collected in the original images while enhancing them. Tyler noted that the incorrect stretch process also generates the dreaded "fat stars", while his process does not. In the incorrect process, unstretched images are stacked from this: Galaxy Stars * ** * *** * **** * **************** 0 65536 B G W to this - Galaxy Stars * ** ** * *** ** *** ** **** *** **** *** **** *** ***** *** **************** 0 65536 B G W and then get white-black point stretched to this - Galaxy Stars * * * * ** ** ** ** *** ** *** ** *** ** **** ** **************** 0 65536 B G W - thus making the "bloated" or "fat" stars. The remainder of his tips went to the need for a good tracking mount and black art tips specific to PhotoShop. The PhotoShop tips were dependent, however, on first mastering the basic skills of proper grey-point stretching and stacking. Without that first step, there are no details in the stacked image that PhotoShop can manipulate. Hope that helps. - Kurt P.S. - It is uncertain whether the old DOS style asterik "*" graphs will make it through modern listserv and email and/or web client processing. _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://gallery.utahastronomy.com Visit the Wiki: http://www.utahastronomy.com
participants (4)
-
Canopus56 -
Jim Gibson -
Mike Nelson -
Patrick Wiggins