asteroid photometry

Calibration tools, stacking, aligning,..
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Aldebaran1011
Posts: 6
Joined: Fri Aug 04, 2017 4:55 am

Fri Oct 20, 2017 6:44 am

Hello,
I use Prism often for asteroid photometry, and I like the programm.
While using it I encountered 3 items over which I have a question:

1-Co-ordinates:
When an image is solved, the exact co-ordinates of the center of the image are saved. These co-ordinates are visible in the image header, row 038 and 039 (in prism-1.jpg).
If you continue the next day, and you open the same image again, the RA co-ordinates in the image information (Prism-1.jpg) are the old co-ordinates (before solving). However the DEC coordinates are the new co-orinates as found after solving.
In the Fits header also the correct co-ordinates are visible in row 038 and 038.
If you want to solve the image again, also the wrong RA coordinates are visible(prism-3.jpg)
It seems that the RA coordinates in the image header (Prism-2.jpg) and the solving tab do not come from row 038 from the header, and are not updated after solving.

2-Time of image:
If Batch photometry is done on asteroids the software reads the time of the image, and calculates the position of the asteroid in pixels (x,y) on the image.
In the header of the image the time is written at the beginning of the exposure.
If you look at the by Prism calculated pixel position (X,Y) of the asteroid, it is always some pixels from the real center of the asteroid. As a result of that, the asteroid is not centered in the flux measuring circle, and the brightness accuracy is sometimes lesser or the value wrong, depending on the choosen circle diameter.
If I change the time in row 12 in the Fits header (Prism-4.jpg) in end of exposure time, Prism calculates the pixel position of the asteroid exactly centered in the asteroid center.

If I change the time in Edit -- Edit image header, only the time on row 13 is adjusted. Appearantly Prism looks only after the time in row 12, and the center is still calculated wrong.
So, I would think that not the right time is used for the position calculation of the asteroid on the image.

3-Asteroid Photometry:
It seems to me that the brightness of the reference stars and asteroid are relative. The brightness of all objects change a little when the asteroid passes the meridiaan, and the mount has to be switched from E to W.
You can see that in an image of 22 Kalliope.jpg.
Should the brightness not be calculated to absolute values, by comparing the measured V- magnitude of the reference stars with their actual V-magnitude from the star catalogue?

Best Regards
Henk de Groot
Attachments
prism-3.jpg
prism-1.jpg
22 Kalliope.jpg
Cyril Cavadore
Site Admin
Posts: 74
Joined: Sat May 20, 2017 6:15 pm

Mon Oct 23, 2017 4:48 am

Hello

-1>what you find in the FITS header does not prevail, what prevail is what you can get after a measurement on the image. the WCS coefficient you see on the image header are taking into account image distortion like cdx-y terms.

-2> Here is what I have, what date is wrong ?
fits_ent.JPG
-3>
It seems to me that the brightness of the reference stars and asteroid are relative. The brightness of all objects change a little when the asteroid passes the meridiaan, and the mount has to be switched from E to...

This is a classical issue with GEM mounts, there are a lot scientific papers on that, I think the image rotates 180° another pixels are involved, and thus there is such a shift.
Cyril Cavadore
Principal Prism Author,
Alcor-System
www.alcor-system.com
Aldebaran1011
Posts: 6
Joined: Fri Aug 04, 2017 4:55 am

Tue Oct 24, 2017 8:32 am

Hello Cyril,
Thanks for your answers.
1. OK

2. The date is not wrong. The point is that sometimes the by Prism calculated x,y position of the asteroid on the image can be 1-3 pixels from the real center of the asteroid, more or less for the complete series of images. If the displacement is in the X-as (RA axis) you can adjust it by changing the image time of all images a little, but this is not the way to do it.
At first I thought the time used for the calculation by Prism was wrong, but that cannot be the reason, as mostly it works fine.
I do think now that poor solving, and for some images registering, may be the cause. This can happen when the seeing is very poor or turbulence in the telescope and the stars are not pinpoint and circular, or if there are not so many stars on the image. By using a larger radius for measuring the flux in such circumstances you can still get good results, but the accuracy is lesser.
If the software would search for the center of the asteroid on the image, near the calculated position, would also help.

My remark about changing the time of the image in the -Edit Image Header- is that the image header apparently only changes the time in row 13 of the FITS header, and Prism still uses the unchanged time in row 12 for calculations, what can be confusing.

3. Yes, I know.
But if I report the measurements with the -Measurement report Header- the brightness of the asteroid will jump if I switch the mount from E to W. Would it not be better to correct the brightness of the asteroid with the reference stars measured brightness and catalogue V-magnitude before reporting?
Best Regards
Henk
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