FITS Header: AirMass question

Discussion in 'General' started by CraigNZ, Aug 22, 2019.

  1. CraigNZ

    CraigNZ Cyanogen Customer

    Joined:
    Dec 29, 2014
    Messages:
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    Location:
    Ngutunui, New Zealand
    Examining a series of images I acquired with MaximDL I noticed something odd with the AirMass value in the FITS header. Here are the keywords from some images before and after a jump in the airmass:

    DATE-OBS JD OBJCTRA OBJCTDEC OBJCTALT OBJCTAZ AIRMASS
    2019-07-22T06.42.03 2458686.77920139 16 01 00 -29 59 54 68 06 58 75 50 45 1.112454
    2019-07-22T06.44.24 2458686.78083333 16 01 01 -29 59 51 68 33 45 75 11 32 1.112457
    2019-07-22T06.46.39 2458686.78239583 16 01 01 -29 59 49 68 59 28 74 32 35 1.112460
    2019-07-22T06.48.54 2458686.78395833 16 01 01 -29 59 46 69 25 20 73 52 08 1.112462
    2019-07-22T06.51.16 2458686.78560185 16 01 01 -29 59 44 69 51 49 73 09 15 1.034274
    2019-07-22T06.53.31 2458686.78716435 16 01 01 -29 59 41 70 17 10 72 26 37 1.034276
    2019-07-22T06.55.47 2458686.78873843 16 01 01 -29 59 39 70 42 50 71 41 56 1.034278
    2019-07-22T06.58.08 2458686.79037037 16 01 01 -29 59 37 71 09 09 70 54 15 1.034279
    2019-07-22T07.00.24 2458686.79194444 16 01 02 -29 59 35 71 34 31 70 06 25 1.034281
    2019-07-22T07.02.41 2458686.79353009 16 01 02 -29 59 33 71 59 45 69 16 49 1.034283

    The images were taken consecutively, each image 120 seconds in duration so the airmass should be changing slowly. Notice the 5th image the airmass jumps from 1.112 to 1.034. Prior to this the airmass changed slowly, then it jumped and then changed slowly again after this. Is this due to some sort of rounding issue before airmass is calculated?

    Craig
     
  2. CraigNZ

    CraigNZ Cyanogen Customer

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    Location:
    Ngutunui, New Zealand
    Also notice that on the first four images the altitude of the telescope is increasing (68 06 58 to 69 25 20) which means less air mass, but the airmass is actually increasing (1.112454 to 1.112462). Then it drops on the fifth image and continues increasing. I ran SkyX Pro and pointed the simulated telescope at the same object at the same date and time. SkyX shows the airmass changing 0.01 approximately every 3 images, or a change of 0.0033 in airmass every 3 images. This is not the case in the data above where it is changing 0.000008 per 3 images.
     
  3. Colin Haig

    Colin Haig Staff Member

    Joined:
    Oct 27, 2014
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    3,431
    Craig,
    I justr tried a rough simulation, with southern hemisphere coordinates, and I get what is expected (airmass about 1.0 near zenith, and about 37 near horizon).
    Are you using MaxIm 6.20 ?

    What are your scope's coords (lat, lon, elevation) from Site and Optics ? Do they match what your mount has?
    Have you incorporated any weather data (eg barometric pressure, temperature) ?

    A quick check would be to point scope at the zenith (alt 90), and look at the Observatory Control Status tab, should show an airmass of 1; and then point near horizon and should be in the high 30's eg 37 or 38.

    Did you plate solve the images? am just trying to figure out whether the images airmass are as acquired or had post processig that may have impacted the airmass values.
     
  4. CraigNZ

    CraigNZ Cyanogen Customer

    Joined:
    Dec 29, 2014
    Messages:
    86
    Location:
    Ngutunui, New Zealand
    Hi Colin,
    I did a day time test using only MaximDL and find that the airmass value is changing correctly. I think what may be happening is Voyager is remotely accessing MaximDL and may be changing the FITS header airmass value. I will check on this and come back with what I find. So for now I believe MaximDL is working okay .. thank you for your quick assistance on this.
    Craig
     

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