Software for photometry (image calibration)

[Kevin West]

Helo Grant,
Thanks for your replies.
Where do you see reference to WCS in the file?
Kevin

[Kevin West]

Thanks Ian,
I don’t know what stretching is.
I have attached two images. The left is one of the original raw images 30s exp. Right is the calibrated output file of the stacked (only two lights) after application of darks, flats and flat darks in ASTAP.
The calibrated file looks rather washed out compared to the nice dark sky original which was a surprise.
Kevin

Attachments:

1. 

   NGC7790-comp-raw-and-calibrated.jpg
   

[Kevin West]

Thanks Mark,
What does Calibrate Photometry and MZero actually do?
Kevin

[Kevin West]

Thanks Grant,
Yes I see the CALSTAT entry
I didn’t see much vignetting on the raw although flats show it all too clearly.
Have a look at the images I uploaded comparing raw and calibrated

[Grant Privett]

Is MZERO the magnitude scale Zp?

[Mr Ian David Sharp]

Kevin West wrote:

I don’t know what stretching is.

Ah! then you have to learn about this!

Astronomical deep-sky images contain mostly black (or near black) sky. When you view an image un-stretched astronomical image you see virtually nothing – just a few pin pricks of light from the brightest stars. This is because most of the data is residing near the black-point of the ‘histogram’. But applying a non-linear stretch to the image, the data is spread more evenly across the histogram from the black point to the white point.

See my attached screen shot. The image on the right is a clone of the one on the left but I have applied a histogram stretch to the one on the right. Without this skill you will not be able to see your data properly, nor be able to check the calibration properly.

I feel you have learning curves all over the place! If you would like a Skype or Zoom (or similar) call I could share my screen and take you through a few things? Otherwise there will be a lot of back and forth with messages etc.

You can PM me here if you want to set something up…

Cheers
Ian.

• This reply was modified 1 year ago by Mr Ian David Sharp.
• This reply was modified 1 year ago by Mr Ian David Sharp.

Attachments:

1. 

   stretch.png
   

[Grant Privett]

I’m no expert, but I don’t recall seeing A_ORDER, AP_ORDER, CTYPE1/2 or CRPIX1/2 in any FITS file that I had not plate solved. I think they all relate to parameters defining how the x/y pixel location is related to the RA/Dec value associated.

Its a polynomial, usually, so the comments about polynomial order are suggestive. 🙂

• This reply was modified 1 year ago by Grant Privett.

[Mark Phillips]

Kevin West wrote:

Thanks Mark,
What does Calibrate Photometry and MZero actually do?
Kevin

ASTAP compares the flux levels of the stars in the image against a catalog of stars which allows you to measure the brightness of any objects (photometry) in the image easily. MZERO is the magnitude zeropoint of the image and this can be used by comphot to measure the brightness of a comet for example.

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