No air-mass calculation

Madrid Astronomical Association M1 (Spain) (AAM)
Sun, 06/24/2012 - 11:39

When uploaded and processed, in my last set of images, air-mass appears to be 0.000, which means that vphot is not computing air-mass from altitude.

I've checked fits header and I've seen that altitude appears correctly.


American Association of Variable Star Observers (AAVSO)
Please share your VPHOT images to KTC

Please share your VPHOT images to KTC

Madrid Astronomical Association M1 (Spain) (AAM)
Airmas calculation goes fine in images uploaded today

Today it worked fine. But problem persists with the same images, they are the problem, whatever the bug is.

I hope airmass computing would keep working.

Thanks for your interest KTC, I've shared images with you, if it would help.




American Association of Variable Star Observers (AAVSO)
FITS image feedback for RMU

FITS image feedback for RMU

Based on a review of your images/FITS header, etc. here are some comments and suggestions and questions:

TT Boo image (and SN2011FE, and maybe others): 

- CCD-TEMP -26.2C   but   SET-TEMP -35C  ....cooling system unable to maintain setpoint...can make dark frame calibration very difficult.

- LMAXADU = 40000   ....but sky background is 49000.  (Wow!  How bright is your sky?)  You have exceeded the linear limit for every pixel.  This entire image is not usable for photometry.

Why no calculation of air mass?  You have in the FITS header:



[ISO 8601] UTC date/time of exposure start



Julian Date at start of exposure



40 28 05



-03 42 24



14 58 11.30



+40 40 19.6



Nominal Azimuth of center of image in deg



Nominal Altitude of center of image in deg

...this should be enough to determine air mass...and the values appear reasonable...Bootes is up in your western sky at this time of night.

You may have to contact Geir about this issue.  I do not know if VPHOT is unable to read the formatted data in your FITS header.


It appears your sky is very bright.  If that's the case, and with a 20cm aperture telescope...I recommend you work on brighter targets that require shorter exposures.  (When I examine the MK421 image and call up the AAVSO comparison stars....many of the brighter ones are unusable because they are above the non-linear limit.  Therefore you can only use fainter comparison stars.  And the sky is very bright...which means you have a smaller range of stars that you can measure...and they are at lower SNR because the bright sky adds much noise to the measure of the stars.  Find targets that are brighter by at least two magnitudes, and use exposures that are shorter.

Norwegian Astronomical Society, Variable Star Section (NAS)

Hi Miguel,

By default VPHOT uses AIRMASS from the FITS header. If that is not present it looks for OBJALT or OBJCTALT and does a simplified airmass calculations.

For some reason neither of these fields are present in your problem images. But they are in your most recent ones.