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I want to model an image of a galaxy from Hubble. For this, I need the PSF image as well.

I already tried to use TinyTim, but I don't have half of the parameters and cant be sure about the quality of output.

Is there any way for me to get it from the .fits image of a galaxy?

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  • $\begingroup$ Which parameters for TinyTim do you not have? $\endgroup$ – student Aug 1 at 9:40
  • $\begingroup$ I am not sure about chip but suppose its 2; also not sure about the position on detector, but think it could be 400 400 (stsci.edu/ftp/instrument_news/WFPC2/Wfpc2_psf/PLOTS/555w2.html); passband is fine, f555w; absolutely have no idea about the spectrum, and don't know what to put in focus, but I think its 0. $\endgroup$ – Igor Kolesnikov Aug 1 at 11:28
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    $\begingroup$ The spectrum is necessary because the PSF at the red end of a wide bandpass might not be the same as the PSF at the blue end. You can select one of the provided colour templates that matches the colour of your object. $\endgroup$ – student Aug 1 at 12:30
  • $\begingroup$ How can I get the color of my object? Can't find it in the header or on the library.. $\endgroup$ – Igor Kolesnikov Aug 1 at 18:44
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    $\begingroup$ You estimate the flux in different bands (or look it up). But even simpler, and perhaps also good enough, is if you use the monochromatic filter option and specify the wavelength of the F555W band (around 5550A). $\endgroup$ – student Aug 1 at 19:10
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To answer the question: Yes, there is a way. Use a bright but unsaturated star. That might already be good enough, or you might want to do any number of corrections to this PSF model.

A better way is to figure out the parameters to use with TinyTim, and generate an approximately right PSF from those. Most parameters you know, or they are easy to look up, such as camera, chip, filter. For the pixel position, you need to look up the original exposures that went in to your (presumably) drizzled image; the object will be more or less in the same place in all of them, so you can use the average position.

With these, you can generate an overresolved PSF image. If your (presumably) drizzled image is north-aligned, you will need to rotate the PSF accordingly; ideally using drizzling, but a simple image rotation might work well enough if your PSF resolution is much higher than the HST resolution. The final result is a PSF that might not be useful for precision photometry, but should be good enough for modelling a galaxy. In any case, compare the PSF to the image of a star to make you it is roughly right (east-of-north angles in HST images have a way of being wrong).

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  • $\begingroup$ Thank you so much for your tips. Just to make sure that I understand. I trying to get PSF for NGC6166 (hst_07265_03_wfpc2_f555w_wf on this link hla.stsci.edu/hlaview.html). I tried to look for the parameters there or in the header of the image but without success. Is there another place to look for it? $\endgroup$ – Igor Kolesnikov Aug 1 at 18:36
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    $\begingroup$ Well there you go. Instrument is WFPC2 with the Wide Field camera. If you click on the proposal number (7265) in the HLA results list, you are lead to the proposal page. At the bottom you can find all raw data products, including the 4 exposures that went into your FITS. If you click on any one of those, and then go to the "interactive display", you can figure out the pixel position of your object on the actual chip. $\endgroup$ – student Aug 1 at 19:27
  • $\begingroup$ Okay, I got the part about position, instrument, and spectrum. Now I'm only lacking behind on Focus and chip. On TinyTim I got this question for chip - Enter chip (2,3,4); and this - Secondary mirror despace is scaled by 0.011 and added to Z4. Focus, secondary mirror despace? [microns] for the focus. $\endgroup$ – Igor Kolesnikov Aug 2 at 0:22
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    $\begingroup$ If you look at the WFPC2 handbook, you can find Figure 1.1 to see the chip layout, and you get which chip your object is on from the individual exposures. I am not sure if position refers to the overall exposure or position on each 800x800 pixel chip, so you might want to try a position outside that range to see if TinyTim complains. $\endgroup$ – student Aug 2 at 0:34
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    $\begingroup$ Focus is due to the mirrors shifting a little over time, you can probably keep that to zero. $\endgroup$ – student Aug 2 at 0:35
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What instrument was used to take the data? This page has PSFs for WFC3.

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