Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

  • Announcements

    SGL 2017 SP

Recommended Posts

Hellstorm    0

I'm looking for a simple algorithm to compare astronomical images (of the same sky region) against each other, compute their movement and rotation, to finally stack them.

At the moment I'm already having a more or less working algorithm. First I extract all the stars out of an image (including information like brightness and FWHM), and then I walk trough all the resulting "points" and create triangles out of the current point and those two other stars that have the shortest distance to this star.

This list of triangles is created for every image. After this I take one image as reference and then I walk through the list of triangles in the reference image to find a triangle in the other image with the same length of each side of the triangle (I also "allow" some tolerance due minimal relative differences of the star positions in each image). For this matches I calculate movement and rotation relative to the reference image. Last step is to find the matched triangles that have to same relative movement and rotation like the other matches. This is done by calculating the standard deviation, sorting out triangles that are not within 1 or 2 sigma and repeat this process until I have a very small standard deviation.

The last part, finding "valid" triangles with the same movement/rotation, is working fine. The problem is that sometimes I have only like 2 or 3 "valid" triangles out of 300 initial triangles. All other triangles have side lengths different to those of the reference image.

So I assume it's the way I generate my initial triangles which causes the problem. Sortings stars by their brightness and using this data to generate the triangles also doesn't work. So is there a better way to create the initial triangles in all the images?

Clear Skies,

David

Share this post


Link to post
Share on other sites
wimvb    1,827

Aren't you reinventing the wheel?

This is what stacking/alignment algorithms already do.

Have you tried searching for scientific articles on the matter? (Not necessarily astro related). Juan Conejero if Pleiades Astrophoto wrote about this. His article is burried somewhere in the PixInsight web site / forum. Most likely the latter, in the section about new updates and features.

Due to minor geometrical defects (field curvature) of the optical system, stacking involves geometrical stretching of the images, to get the triangles to fit.

This is one reason why dss users sometimes complain of 'weird behaviour' in the corners of an image.

Good luck.

Edited by wimvb

Share this post


Link to post
Share on other sites
Hellstorm    0

Yeah I know that there are already programs. I just want to write my own one as some "small" free time project. I know that there is some distortion on the edge of the images. That's the reason why a create triangles out of nearby stars and allow some tolerance in the side length of those triangles.

Pixinsight uses the RANSAC algorithm, however I use a different approach. In general my version also achieves good results - see the attached image - but it has some flaws building the initial "lookup structure". And I guess that's only a questions of how to select the stars per triangle. There are a lot of papers out there, but most of them are not within my free time schedule. :D - Like it said, I guess I just need some hint on which stars I should create my triangle. I have the following data of each star: position, FWHM on x and y axis and maximum brightness (but I could also calculate average brightness over occluded pixels). The question is how to use this wisely.

2017-03-25 22_22_42-.jpg

Edited by Hellstorm

Share this post


Link to post
Share on other sites
wimvb    1,827

Sorry, can't help you any further. (Other than this:)

PixInsight has a tool to calculate the point spread function of stars. When using this tool it is recommended to exclude too bright (obviously) and too dim stars. Do you do something similar in your process? Probably FWHM and brightness is a way to achieve the same.

(and THAT's really all I can think of)

Good luck with your project, and keep us informed on your results.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Recently Browsing   0 members

    No registered users viewing this page.

  • Similar Content

    • By jdmgray
      Hi everyone.
      I'm currently running Linux on my laptop and just wanted to know the best stacking software is for an amateur to use.
      I tried installing registax with wine but it didn't work.
      I've tried out PIPP and didn't really get on with it.
      So far the only one I've gotten my head around is Siril by free astro.
      Would like to know what other software there is that works well on linux that is good for amateurs who have just started playing around with this. 
    • By michaelmorris
      I had a pretty successful session last night capturing 390 x 10 sec subs of asteroid Florence through patchy thin cloud with my Atik 460EX camera
      I want to now turn these into an animation.  My thought is to convert the FITS files into TIFF files, align them, batch process them in Photoshop or Lightroom, then convert the resulting aligned, processed TIFF files into an animation.
      Is there software that will automatically align (but not stack) FITS files and save the aligned subs as TIFF files?  If not, what is the best way of batch converting 390 x FITS files to TIFFs?
      Thanks
       
       
    • By mike005
      ( Edit 20 Aug: adjusted to increase brightness )
      ...
      The Fighting Dragons of Ara ( NGC 6188 )

      ( please click/tap on image to see larger and sharper )
       
      ......................
      Original:
      The Fighting Dragons of Ara ( NGC 6188 )

      ( please click/tap on image to see larger and sharper )
      Bright Nebula NGC 6188 and open cluster NGC 6193 are embedded 4,300 light years away in the Sagittarius arm of our Milky Way galaxy and can be seen with the naked eye south of Scorpius in the constellation of Ara.
      With powerful stellar winds and energetic ultra-violet radiation, massive stars sculpt the interstellar gas and dust of the nebula into wonderful shapes and cause the interstellar gas to brightly fluoresce.  
      Closer to the hot young stars of the cluster, bright blue “sunlight” reflects off the clouds of gas and dust to produce the blue reflection nebulae seenin the image.
      Magnitude +5.19, RA 16h 41m 42s, Dec -48deg 48' 46".
      Approx. 3800 light years away.
      Image details:
      Plate Solution:
      Resolution .......0.586 arcsec/px ( original full size image ).
      Rotation .......... 89.764 deg.
      Pixel size ........ 3.90 um.
      Field of view ..... 58' 41.6" x 39' 9.5".
      Image center ...... RA: 16 40 09.856  Dec: -48 41 22.50.
      Image bounds:.   
      top-left ....... RA: 16 42 10.059  Dec: -49 10 30.54.   
      top-right ...... RA: 16 42 06.489  Dec: -48 11 57.14.   
      bottom-left .... RA: 16 38 11.010  Dec: -49 10 39.74.   
      bottom-right ... RA: 16 38 11.897  Dec: -48 12 05.58.
      Telescope: 
      Orion Optics CT12 Newtonian ( mirror 300mm, fl 1200mm, f4 ).
      Corrector: ASA 2" Coma Corrector Quattro 1.175x.
      Effective Focal Length / Aperture : 1410mm f4.7.
      Mount: Skywatcher AZ Eq6 GT.
      Guiding: TSOAG9 Off-Axis-Guider, Starlight Xpress Lodestar X2, PHD2 .
      Camera:
      Nikon D5300 (unmodified) (sensor 23.5 x 15.6mm, 6016x4016 3.9um pixels).
      Location:
      Blue Mountains, Australia 
      Moderate light pollution ( pale green zone on darksitefinder.com map ).
      Capture ( 24 June 2017 ).
      12 sets of sub-images with exposure duration for each set doubling ( 1/8s to 240s ) all at ISO800.
      34 x 240s + 10 each @ 1/8s to 120s.
      Processing ( Pixinsight - 19 Aug 2017  ).
      Calibration: master bias, master flat and no darks.
      Integration in 12 sets.
      HDR combination
      PhotometricColorCalibration.
       
    • By jjosefsen
      Hi,
      Hoping for some help here..
      Im trying to learn some basic processing in PixInsight, including the PreProcessing (calibration, registrering, stacking, etc.)
      Im using the only data I have, from my very first lights of M13, calibrated with a SUPER BIAS, no darks and flats yet.
      When doing it in DSS I get 7 images stacked and the stacked file looks OK.
       
      Switching to PixInsight it goes horribly wrong after stacking, and the stacked picture looks like the attached.
      I have done the whole process in two different ways, but get the same basic result.
      1. Doing all the steps manually: Calibration, Debayering, SubFrameSelection, StarAlignment, Stacking.
      2. Doing everyting with the BatchPreprocessing script
      I yield the same result, its like the subs aren't properly aligned, even though the process has eliminated A LOT of bad frames (from 26 to 5).
       
      Link to zip with the 5 registered light: https://www.dropbox.com/s/sj2ytaa00u5qlap/GREAT CLUSTER_LIGHT_45s_400iso.zip?dl=0
       
      Hope someone can give me some pointers, none of the tutorials I have been following explains why this can happen.
       
      Thanks in advance stargazers!
       


×