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Image scale for CCD photometry


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I do not think there is a "specific" answer to this. Similar question came up about a year back.

On the SBIG site they say 1 arc second per pixel, however I find that many people use 3, 4 or 5 arcsec per pixel.

Then SBIG threw in that if planetary images then you could go to 0.5 or less arcsec per pixel.

Overall I have read values from 1 arcsec to 5 arcsec.

Going to suggest that 2, 3 or 4 arc sec per pixel is likely going to get you by, I would drop 2 arcsec and the scope focal length into the equation then see what came out for the pixel size and start from there. The SBIG suggestion seemed to be the most "extreme" value. You will not get an exact match on the pixel size anyway. However also consider the chip size as you likely will want to fit the whole object on the chip and a few nebula are big, M42 is a degree and a bit more across as is M45.

Makes you wonder what the 20 most imaged objects by amateurs are, that way you get an idea of chip size required.

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Hi James,

You want to match your CCD with the scope you'll be using. Also bear in mind the quality of sky you genrally get too.

You want to aim for around 1.5 arcsec/pixel.

Another decision is weather you get a ABG or NABG CCD (anti blooming or non anti blooming) NABG CCDs have no 'drains' 

so their QE 'quantum efficiency) is higher.

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  • 3 weeks later...
  • 2 weeks later...

Hi James,

Getting image scale right is crucial for doing good photometry. The image scale to choose depends on your local seeing conditions and you should aim for between 2 and 4 pixels to sample the FWHM seeing. As an example, using my old LX200 at home in the UK to do asteroid and exoplanet photometry, I typically had seeing of 3-4 arcseconds. Thus I set my image scale to ~1 arcsec per pixel. In contrast, recent photometry that i've done using the wide field camera on the INT on La Palma required an image scale of 0.33 arcsec/pixel, to capitalise on the often sub-arcsec seeing. It is of course a bit of a trade off as you ideally want a field of view of at least 15 arcmin or so to get a reasonable number of good comparison stars. 

I'm not sure where you are but a low altitude site (such as all of the UK) will typically exhibit seeing of 3-4 arcsec, I've rarely seen much better. 

The 2-4 pix per FWHM is important for PSF fitting to get good photometry in crowded fields, but it is also crucial when using aperture photometry too. This is because to do good aperture photometry you need to be able to calculate where the centroid of the star is (so you can accurately place the aperture) and also because spreading the light over a few pixels will reduce the systematic errors that you can suffer from pixel to pixel sensitivity variability (flat fields only fix this up to a point - hence the exoplanet people defocussing their images for photometry).

A great reference book for doing good photometry is this: http://www.minorplanetobserver.com/pgbook/PracticalGuide.htm

Also Bruce Gary's book is available free here: http://brucegary.net/book_EOA/EOA.pdfand is well worth a read.

Cheers,

Darryl

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