Resolved Photometry

Discussion in 'Demo User Tech Support' started by Martin J Smallen, Apr 25, 2020.

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  1. Martin J Smallen

    Martin J Smallen Standard User

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    I am having a problem with Photometry. The software works Ok until I get to these steps.

    "Find the variable star in your image and right-click on it, and select Tag New Object. Type an ID for it (usually the variable name); this is the name that will appear on the graph and the reports.

    Next right-click on your comparison star and click Tag New Reference Star. If you have more than one filter set, then a dialog box will pop up to allow you to enter the reference magnitude in each plane. Otherwise you can simply enter it into the table."

    When I right-click on the star or the ref. star, a dialog box opens but there is no "Tag New Object" category. Then I tried to do it in the "Identify" tab but nothing happens.

    My files are color JPEG. What am I doing wrong?
     
    Last edited: Apr 25, 2020
  2. Colin Haig

    Colin Haig Staff Member

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    Here's a suggestion, you might want to play with this.
    Try clicking the Tag Mode button, and it will toggle between modes.
    Try doubleclicking on the star of interest.

    Regarding color JPEG, that's not a good choice for photometry.
    Why? Two reasons.
    JPEG uses lossy compression to make them a convenient size, at the expense of missing information. It's fine for the human eye, but not good enough for good science.
    Photometry is usually done with precisely calibrated filters (from the Johnson-Cousins set, or Sloan Digital Sky Survey variant). The micro-lenses and dyed colors in front of each pixel mean you are getting a different amount of light passing through. This is very hard to calibrate for. Binning 2x2 will combine all RGGB pixels into a single combined pixel, which might help approximate.
    The AAVSO has some great material on working with the limitations of DSLRs.
     
  3. Martin J Smallen

    Martin J Smallen Standard User

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    Thanks for your help. I was able to get it to work by converting from color to mono in the program. However, the star alignments were off in my images and the auto-align didn't work on all for the images. I will try to collect better aligned images tonight and then try photometry again. BTY, I have a Unistellar scope with a built-in Sony IMX 224 LQR camera, so I am stuck with JPEG images.
     
  4. Colin Haig

    Colin Haig Staff Member

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    You could try using Analyse... PinPoint Astrometry to "plate solve" all your images.
    It needs a bit of work to set up - you'll have to refer to the manual.
    You'll need to set up your Site and Optics, and you will need the GSC1.1 catalog installed properly.
    Plate solving will match the observed stars to an official catalog, and then that information will be put in the saves images. You will have to save the JPGS to FITS.
    This is probably a bit more involved than you want to tackle, but figured I should mention it.
     
  5. Martin J Smallen

    Martin J Smallen Standard User

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    I took some new images last night and they are all aligned. So the software was able to find both the target and reference star on each image using auto star matching.
    When I tried graphing the images, all the points lined up on the y-axis. With Auto selected on the time axis, the axis label read "Hours since -4712-01-01 12h UT (JD 0."). So, I right clicked on the time axis and selected "Base Time". I entered my start time, which was 2020-04027T04:50:00 UT. But then there were no points on the graph when I clicked OK. Also, the time said 5:50 instead of 4:50. In addition how does the software know the time between images? Please help. Thanks.
     
  6. Colin Haig

    Colin Haig Staff Member

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    This is probably because you are working with JPEGs that are missing astronomical data.
    JD = Julian Date, which is derived from an astronomically corrected version of terrestrial time. It is saying "0" which is invalid. Probably a problem with the images not having the necessary data.
    The image itself needs to have correct time data - if it doesn't then there is no way for MaxIm to know the time for each image.
    This is probably a limitation of whatever software Unistellar has.

    Can you send us 3 JPG files (taken one after another) to have a quick look at? Use the [Upload a File] button at the lower right.
     
  7. Martin J Smallen

    Martin J Smallen Standard User

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    Here are the files.
     

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  8. Colin Haig

    Colin Haig Staff Member

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    Martin,
    I'm sorry to report that those files have very little information in them, and are basically useless for astronomy.
    The files have minimal information in them that is computer-readable, and the file dates reflect the time saved on my PC here, nothing in the file as to when they were captured.

    No offense intended, but after a review of the Unistellar website, it looks like this is a fun but expensive toy, maybe for education, but it's not currently capable of getting scientifically usable data without the manufacturer upgrading their product.

    Most hobbyist level equipment for astronomy supports ASCOM Standards to control the telescope and accessories like the camera or focuser.
    Astronomical cameras (even cheap ones under $1000) makes astronomical FITS format files including the SBIG FITS Extensions for the data.
    Some people use a consumer digital camera like a Canon, Nikon, or Pentax DSLR that includes some EXIF or other data in it's RAW files and those can be used.

    The astronomical FITS file format includes data including accurate time and position information as well as optical information about the telescope and camera.
    The image sensor is not actually circular in that scope, and the data has to be calibrated (or at least you need to be able to get uncompressed raw data and calibrate it.

    So there's really nothing we can do for you - I suggest you contact the seller/manufacturer and ask them what their plans are regarding ASCOM, FITS, and producing calibrated data.
     
  9. Martin J Smallen

    Martin J Smallen Standard User

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    Thanks for all your help.

    I did participate in a project with them to collect images of a WASP 85 transit for a graduate student. But I was using a different section of the software. In that section, I wasn't able to see the images once they were saved but could only send them to their server. The student is supposed to send us the curves later.

    I will contact Unistellar today and see what they have to say.
     
  10. Colin Haig

    Colin Haig Staff Member

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    If they need some "pointers" we can send them a few links etc.
     
  11. Martin J Smallen

    Martin J Smallen Standard User

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    I did contact Unistellar and they are working on getting FITS files but they are very busy and they didn't give me a delivery date. Apparently other EVscope owners have also been asking about FITS files. So, I will buy Maxim DL after the FITS files are ready.
     
  12. Colin Haig

    Colin Haig Staff Member

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    Ok, very good.
     

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