creation of comp star list

[Astro_Roe]

Hello,
using SIRIL 1.4.0 beta2 on Ubuntu 20.04 and getting a strange behaviour when creating a list of comp star for a variable placed in the outer area of an image. Doing it for variable star RS UMa which lies in the upper left of the image. The image itself is centered ( due to result of plate solving ) to 12h22min54s and 57°14’25". If choosing the restrictions for the compstar creation very wide - only for getting a visual impression ( narrow field, mag range: 5, index range: 0.7, mag error: 0.1, APASS catalogue ) - then I get results which look like a conesearch in APASS which is centered at the image center but not at the coordinates of RS UMa ( 12 38 57.55 +58 29 00.2 ).

See several screenshots attached.

If original fits file is needed let me know how to share it, as it is 100MB in size.

Mario

PS: output of last comp star creation added partialy
09:41:50: Comparison stars: processing…
09:41:50: Object RS UMa (exact match: V* RS UMa) was found in the user-DSO catalogue at: 12h38m57.54s, +58°29’0.19"
09:41:50: Using already downloaded catalogue APASS
09:41:50: → 2553 comparison stars found within the image from APASS
09:41:50: Target star is off the center of field of view by 148.2 arcmin, photometry results may be impacted
09:41:50: Using already downloaded catalogue Gaia DR3 Variability
09:41:50: → 109 variable stars found within the image from Gaia DR3 Variability
09:41:50: Filtering parameters: delta_Vmag: 5.00, delta_BV: 0.70, max e_Vmag: 0.100
09:41:50: Target star: Vmag = 11.15, B-V = +0.77
09:41:50: → 50 stars matching those criteria were found in Gaia DR3 Variability and discarded
09:41:50: Stars sorted by increasing distance (in arcmin) wrt. image center
09:41:50: d_mag and d_BV are discrepancies from Vmag and B-V of the target star
09:41:50: e_Vmag and e_Bmag are photometric errors supplied from APASS
09:41:50: Index_nbr V B-V d_mag d_BV e_Vmag e_Bmag Dist
09:41:50: Comp star 1: 15.86, +0.77, +4.709, -0.001, 0.008, 0.068, 2.01
09:41:50: Comp star 2: 14.37, +0.62, +3.220, -0.151, 0.041, 0.101, 2.48
09:41:50: Comp star 3: 12.54, +0.66, +1.392, -0.111, 0.046, 0.106, 2.79

09:41:50: Comp star 1589: 14.56, +0.94, +3.407, +0.168, 0.070, 0.127, 97.74
09:41:50: Comp star 1590: 15.69, +0.71, +4.541, -0.058, 0.010, 0.021, 97.76
09:41:50: Comp star 1591: 11.43, +0.51, +0.275, -0.263, 0.006, 0.138, 97.79
09:41:50: 1591 comparison stars after sort.
09:41:50: Creating csv output file V*_RS_UMa_SirilstarList_5.00_0.70_0.10_APASS.csv

[vinvin (Vincent Hourdin)]

Hello and welcome,
it is indeed using the image centre for this request, not the star location.
But the goal of the narrow field is to avoid the image corners, not to stay close to the target star. The idea is that sometimes optical or chromatic aberration, possibly imperfect flat calibration too, can make stars in the corners unreliable.

[Astro_Roe]

Thanks for the quick reply!
I also tried the comp star search without the narrow field option checked and got results distributed in a rectangle without the image borders.
So the ( correct ) solution here would be just not to create lightcurves for variables at the image edge?
Mario

[vinvin (Vincent Hourdin)]

using reference stars from anywhere in the image may not pose a problem, especially if the flat calibration is good, give it a try, the target doesn’t have to be in the centre.

Another concern is that if tracking is not good, the star may move too much in the sequence and some reference stars could not be available all the time.

[Batbihirulau (Fred)]

Hi,
There is no strange behaviour here, and it’s all perfect i’d say.
-The rectangle shape you observed is an internal limit (10% of the width/height margins) to avoid “too-close-to-the-edges” stars. This avoids distorted stars and also potential background annulus overlapping the border.

-when you ckeck “Narrow field”, another mask is applied: a cercle centered on the center of the image as said, with a diameter being the height of the picture. This is to avoid stars in non-flat regions (with gradient).

-Regarding the moving stars, a good practice is to register the sequence with Global Star/2-pass but only with the first pass (no apply). In the end, what’s important is that the star (whatever is its roudness) is entirely enclosed in the signal aperture.

In all cases, if you want to get a good light curve, it’s not recommanded to have a target star too far from the center.

One last thing: it’s useless having more than 19 compstars (internally limited).

You can have a look at the video I posted related to the exoplanet workflow with siril 1.4.0.

In addition, if you want to go further, here is another one with enhanced capabilities in photometry.

Photometry is fun!!!

Fred