Issue when loading images


Yoann DEGOT LONGHI
 

Hello
I have an issue with astrometrica: when I load master dark, master flat or calibrated images, they all appear black (second picture) ...
When I load the same images in Prism, it's ok (1st image)
note: these are all fits images.
Does anyone have an explanation?…


gajowy
 

Hi Yoann,

I guess it's related to internal format of the FITS files. Each monochrome (eg. no color) pixel is stored as 8 bit integer, 16 bit integer, 32 bit integer or 32 bit float. It's coded in FITS in BITPIX header, something like this:

BITPIX  = 16 / Number of bits per pixel

On PRISM screenshot one can notice "32-bit" in a window title, what suggests you use 32-bit Master files.

For me Astrometrica works well for BITPIX = 16, however has problems with other BITPIX values. Maybe you can test it generating Master files or calibrated files as 16-bit FITS.

Kind Regards,

Gajowy


Yoann DEGOT LONGHI
 

Thanks for your answer

When I load uncalibrated 32 fits single images (dark, flat or light) it works normaly....

----- Mail original -----
De: "gajowy" <gajowy@agzeta.pl>
À: astrometrica@groups.io
Envoyé: Dimanche 25 Avril 2021 16:29:25
Objet: Re: [astrometrica] Issue when loading images



Hi Yoann,

I guess it's related to internal format of the FITS files. Each monochrome (eg. no color) pixel is stored as 8 bit integer, 16 bit integer, 32 bit integer or 32 bit float. It's coded in FITS in BITPIX header, something like this:

BITPIX = 16 / Number of bits per pixel

On PRISM screenshot one can notice "32-bit" in a window title, what suggests you use 32-bit Master files.

For me Astrometrica works well for BITPIX = 16, however has problems with other BITPIX values. Maybe you can test it generating Master files or calibrated files as 16-bit FITS.

Kind Regards,

Gajowy


--
Yoann DEGOT LONGHI
UAI Obs code G20


gajowy
 

Again, I can only guess what's going:

Pure 32-bit pixel, coming directly from 16-bit camera, has exactly the same representation as 16-bit pixel (maybe with 2 additional leading "0" bytes-?). Then operations on pixels (calibration, mean or median calculations) introduce extra information that astrometrica interpretes incorrectly.

 

KR,

Gajowy


Herbert Raab
 


I would recommend to store the image data in 16 bit Integer FITS files, because 16 bit Integer pixel representation is the native format of Astrometrica.
Try to avoid floating point or 32 bit images. Conversion to 16 bit might work for some (many?) cases, but not necessarily for all.

 - Herbert


Yoann DEGOT LONGHI
 

Thanks Herbert