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Many thanks for the article, Jeremy. I’ve scheduled a few instruments in Spain and New Mexico, so hopefully will have some data to chew on.
Grant, many thanks for this suggestion. Simple and effective. I have just implemented this, will appear in version 0.15.
Hi Grant,
Yes, fitting à la DAOPHOT, then deriving differential magnitudes via linear regression fit to an ensemble obtained from a VizieR catalog. You can select the catalog you need. The result shouldn’t be very much different from DAOPHOT, albeit I’m using its implementation in Astropy’s photutils. So, MetroPSF is a convenient GUI to photutils with some add-ons like linear regression fitting, plate solving, source matching, etc.
You click on the source you’d like to measure in the window. Here is a short write-up on functionality, check the Quick Start section here:
http://trafyx.com/wp-content/uploads/2021/04/metropsf-3.pdfNear-saturation are not ignored, but you can choose Gaussian sigma weighting for the ensemble, so PSF fits with high sigmas which generally appear with oversaturated sources, will be assigned less weight in the fit. Would you recommend to introduce “hard cut-off” for oversaturated stars?
Max
P.S. Grant, thinking this over again, an obvious manual solution to avoid oversaturation is to limit source magnitudes in the ensemble, which is possible in MetroPSF already. This can be combined with sigma weighting as well.
Nick, many thanks for the information. With PSF photometry the sky annuli are not used at all, so implementing this approach directly is not possible, unless we introduce a sky annulus setting. But then, if we do this, wouldn’t such uncertainty estimate be fundamentally detached from the photometry algorithm used?
Fortunately, I have found that the iterative PSF photometry algorithm implementation in photutils provides flux uncertainties as output. Here is the link to version 0.14 of the program that reports total error = sqrt(flux_unc^2 + fit_unc^2) instead of relying on an external SNR estimate:
http://trafyx.com/wp-content/uploads/2021/04/metropsf-0.14.zip
I don’t think I have the necessary expertise to develop a new method to estimate uncertainties with PSF fitting photometry. It should be safe to use the flux fit uncertainty provided by the underlying algorithm. I wonder if you agree with this approach and if you could give this version a try.
Maxim
Hi Nick,
Fortunately no additional setting are required – I wanted to keep the program with as little settings as possible to avoid overcomplicating it. Looks like the PSFs are sharper in your image than from the telescopes I typically use, so try setting Lower Bound for Sharpness to 0.2 and redoing photometry. It detects most of the sources in your image this way. If you want to go a little deeper, lower the Star Detection Threshold from 10 to, say, 5 sigmas. This detects pretty much all the sources down to SNR < 1, except for a faint extended source under and to the left of the galaxy.
I will next add a feature to save settings so you wouldn’t have to manually adjust things each time. I think it would be beneficial for user to save their settings once and reuse them.
Please let me know if it works.
Maxim
Nick, do you mind sending me the FITS file with this data? You are likely clicking on sources that didn’t have any photometry fits at all, i.e. appearing without circles surrounding them. This means that the IRAFStarFinder procedure didn’t detect a point source at this location. You should get a full photometry result if you click on a source with circle surrounding it. I initially thought that it could be that some of your sources are below the default 10-sigma detection limit, but I can see that some of the brighter sources weren’t detected as well, so we probably need to introduce another adjustment to the algorithm. Maybe something up with roundness. Would like to play with this FITS data to resolve this.
On the ensemble fit – yes, you are correct, the brightest magnitudes are in the lower left corner. Do you think we should invert the axes?
V = 16.17 +/- 0.09 on April 12.
Telescope 0.5 m f/6.8 Corrected Dall-Kirkham with f/4.5 Focal Reducer
Camera FLI PL11002MNick, looks like it cannot find the magnitudes of comparison stars in the ensemble. I’ve just went all the way – opening a 16-bit FITS file (ugc7513_000.fits), doing Iteratively Subtracted PSF Photometry, then set catalog to Gaia DR2, CCD Filter to G manually, ran Get Comparison Stars, then Find Regression Model and finally was able to get differential magnitudes by mouse-clicking on the sources. Try removing the old .phot file and executing the same steps. I’m going out of town for a couple of days but hopefully will be able to assist with delays.
Max
V = 15.778 ± 0.07 with slightly better data from tonight.
Telescope 0.5 m f/6.8 Corrected Dall-Kirkham with f/4.5 Focal Reducer
Camera FLI PL11002M
Measured it at V = 14.665 ± 0.04.
Telescope 0.43-m f/6.8 reflector with f/4.5 focal reducer
Camera FLI PL6303E
Nick, here is 0.13 with Gaia DR2 and non-linear image stretching for better views:
http://trafyx.com/wp-content/uploads/2021/04/metropsf-0.13.zip
Updated user guide PDF inside. Please let me know if you need anything else.Maxim
For the time being just ignore those. I’ll clean things up soon. As long as the program itself works, it shouldn’t impact on anything.
Nick, great! Please don’t hesitate to submit feature requests. I am trying to feel where to take this next. Perhaps, command-line processing… Definitely need a way to save all the settings.
Sorry, for some reason replied in the wrong thread. This version should open Nick’s FITS fine:
Nick, Michael, thank you very much. This one should open Nick’s FITS fine:
http://trafyx.com/wp-content/uploads/2021/04/metropsf-0.12.zip
Nick, I am using SNR = Peak Flux at the centroid / Median Sky Background Level, where Peak Flux is taken from the raw FITS data, and not estimated from the photometric fit, and the sky background is taken over the entire image. Do we need any adjustments here? I wonder how 168 was derived.
Note I was able to improve the uncertainty by limiting the ensemble range to 13-16 mag – see attachment.