Limiting Magnitude of CCD Imaging

Drakkith

Hey all. Looking at the following link http://starizona.com/acb/ccd/projectsfaint.aspx , the page says that limiting magnitude is mostly a function of aperture.(First paragraph in the Project Ideas section) I'm wondering why that is. Doesn't skyglow increase at the same rate as the light from your target as your aperture increases? Can't you just expose for longer or take more exposures to get deeper? Other than thermal effects on the CCD, what else might limit the magnitude?

Chronos

It takes a certain number of photons to 'trip' a ccd pixel. Increasing aperature increases the number of photon available per second. Increasing exposure time also increases the number of photons available for capture. An object too faint to 'trip' a pixel with a one second second exposure will not register no matter how many one second exposures you stack.

Drakkith

Sure, but you can just take a longer exposure then, correct? Obviously your maximum exposure time is limited to the amount of time it's visible in the sky and maximum pixel well count. Also, skyglow itself will be much higher than the faintest objects, so all the object is doing is adding a small amount to that pixel on top of the skyglow. IE if an object gives me 10 adu counts over 10 minutes, the skyglow will most likely be much higher than that depending on your sky brightness, so if the skyglow gives you 200 adu counts over the same time then you end up with 210.

So, unless I am mistaken, then you're limited by the maximum exposure time you can take and the faintness of the object. Obviously if you can only take 15 minute exposures maximum, and your object only adds 1-2 photons over that period of time, you're probably never going to see it in the picture. Does that sound about right?

Chronos

Yes, you can increase exposure duration of single frames to compensate for lesser aperature. The software used to process ccd images is surprisingly capable of nursing faint fuzzies out of noisy images. It works to the extent pixel saturation is not an issue.

Drakkith

Thanks Chronos!

vociferous

I think it depends on what you mean by that. CCD pixels used in astronomical cameras actually can count individual photons (usually). The issue is collecting enough of them to register a statistically significant increase versus the various noises present in the image, or the signal to noise ratio.

A prominent issue with the aperture and exposure time is not collecting too little photons but too many. If a pixel "well" fills up, it will "overflow" and form lines. That makes it more difficult to use the image because the pixel is generating more flux than you can measure with the CCD.

Creator

True, but don't the new sensors have anti-blooming protecton to prevent bleed over from overexposure.....(or does that reduce the sensitivity substantially) ...and can't it be twarted by proper software ?

Creator

Drakkith

I believe anti-blooming can cause the photo sites to no longer be linear above a certain value.

Creator

Yes, I believe you are right, Drakkith; the anti-blooming grids block part of the pixel area and the QE goes down. Apparently the better solution is to "stack" frames of shorter duration using non-anti-blooming ccd. From what I've read, that seems to be most common technique to prevent blooming and still get deeper imaging.

BTW, interesting software you linked to get precise faint magnitudes from images. I always wonder what the price is for these software packages, not usually stated.

Creator

Drakkith

Yes, stacking increases the S/N ratio, allowing faint objects to show up above the noise.

My link wasn't to software, just to a site dedicated to astronomy. My own software is AIP4WIN, which is around 100 dollars for the book and software. (The book includes the software) MaximDL is around 300 I think, but includes the ability to control your camera, which AIP4WIN doesn't.

Creator

Thx;
AIP4WIN allows you to get the SNR and the FWHM ?

These programs seem to be perfect for variable star measurements, and the like.

Chronos

To achieve a sufficient signal to noise ratio would have been a better technical term than 'trip', but, 'trip' worked for me. Another problem with lengthy CCD exposures is thermal effects. The best CCD cameras are supercooled for this reason. Most of these are simply unaffordable to all but the most dedicated [ie, unmarried] amateurs.

Drakkith

Shh! Stop telling people I'm unmarried!

vociferous

Most of the commercial CCDs have solid-state cooling. The professional ones usually use some kind of liquified gas to keep them cool (obviously at a significant expense and hazard that makes them unsuitable for most amateur applications). If you get the CCD cool enough, you really only have to worry about electrical noise and background radiation from outside the CCD.

Drakkith

Hmmm...I gotta get me some liquid nitrogen...