Aperture Photometry Script - when you can't plate solve?

[mcgillca]

Hi - I'm trying to use the Aperture Photometry Script to calibrate my system.

Firstly, congratulations - this is an impressive script!

I took images of a blue/red pair over several hours to look at second order extinction coefficients. Since the stars are quite bright, I'm limited to about 20s exposure time to keep these in a linear range. Unfortunately, this is in the part of the sky with very few stars and the script is unable to plate solve.

Is there any way to get the script to carry out the photometry anyway? I have several hundred images and can't do this by hand .. .

Thank you,

Colin

[Andres.Pozo]

The script is designed around astrometrically solved images so you can not use it if you can not solve the images. In any case, the ImageSolver should be able to solve images with very few stars. I have solved many 1 or 2 second images at 1"/pixel. Can you show me one of your 20s images? I need the image, approximate center and resolution.

[vicent_peris]

Hi Colin,

Thank you, I'm happy to hear that this tool is useful for you!

The script locates objects based on the astrometric solution and a star catalog, so unfortunately you cannot do the photometry if you don't have the astrometry.

I would like to take a look to some of your images. Could you upload some JPEG files? I guess there should be at least some stars... Try decreasing the star sensitivity of ImageSolver to the minimum value (-3) and you'll probably need to enable some noise reduction.

Could you please upload a process icon set containing the settings of the ImageSolver and AperturePhotometry scripts?

I've been capable of solving wide field images (40º FOV) of only 15 seconds exposures with a 10 nm filter. In these images, you have very few stars and they are terribly noisy. So I'm pretty sure we'll find a solution for yours with the right settings.

Best regards,
Vicent.

[mcgillca]

Thank you both for your replies - enclosed is a link to a file.

The RA and DEC of the object is in the file header, and the scale is 0.58"/pixel.

https://www.dropbox.com/s/uo2e4kdz8b0b8ol/1x1_20s_B_HIP%20739311x1B20.000secs00062553.fit?dl=0

Enclosed are the platesolver settings.

It really is an empty field though - not sure I've ever seen quite so few stars in a 20s exposure .

If you can solve it, that would be great.

Colin

[Andres.Pozo]

Hi Colin,

I have solved your image using ManualImageSolver and I've found that the coordinates in the OBJCRA/OBJCDEC keys ('15 07 13.0',  '+28 55 02.0') are quite far from the real center of the image ('15 06 23.919' '+28 59 39.56'). At 0.58 sec/pix that is nearly outside the image.

Using a better starting point and the PPXML catalog at 15th magnitude ImageSolver can solve the image.

Code: [Select]

Image Plate Solver script version 4.2.2
===============================================================================
Referentiation Matrix (Gnomonic projection = Matrix * Coords[x,y]):
           -0.000160237       +1.72533e-007           +0.220097
          -2.18228e-007        -0.000160253           +0.176527
                     +0                  +0                  +1
Projection origin.. [1374.758151 1099.682597]pix -> [RA:+15 06 23.91 Dec:+28 59 39.48]
Resolution ........ 0.577 arcsec/pix
Rotation .......... -0.073 deg
Focal ............. 1787.76 mm
Pixel size ........ 5.00 um
Field of view ..... 26' 25.8" x 21' 8.6"
Image center ...... RA: 15 06 23.922  Dec: +28 59 39.59
Image bounds:
   top-left ....... RA: 15 07 24.406  Dec: +29 10 14.13
   top-right ...... RA: 15 05 23.334  Dec: +29 10 11.97
   bottom-left .... RA: 15 07 24.304  Dec: +28 49 05.52
   bottom-right ... RA: 15 05 23.644  Dec: +28 49 03.36
===============================================================================

A few reminders:

• The coordinates should be J200
• The FITS file must be loaded the same way it was written (up-down or down-up). In case of doubt, you can set the "Align algorithm" "Triangles" instead of "Polygons".

[mcgillca]

Thank you very much, Andres.

I'll try it with the parameters you suggest. Not sure why the RA/DEC is off since it gets recorded by The Sky X - perhaps difference between J2000 and JNOW. I will investigate.

I'm also impressed it can solve this at all - TSX could not solve it, and its normally very robust.

Thank you,

Colin