Photographic Magnitude

[koputai]

Folks,
Pleas bear with me, I'm only just starting to take my first tentative steps down the slippery slope of astro imaging.
When you look at the specs for a scope, you often see it's 'limiting magnitude' which I gather is its Visual limiting magnitude under perfect skies. Now, surely when the same scope is used for CCD astrophotography, the magnitude of the stars it can image would be significantly more. Is there any way of working this out? I realise that if you just take longer images, then fainter stars will be imaged, but where does the practical limit lie?

Cheers,
Jason.

[citivolus]

If I recall, it is around 3-4 magnitude in a 60 second exposure, depending on CCD sensitivity. In a Canon DSLR I get to about magnitude 17 in 60 seconds on the FLT98, with a Canon 40D, vs the manufacturer claim of 14.5 visual, for a magnitude gain of around 2.5. The sensor is 37% QE on green light, with an RGGB bayer matrix, so it is 18% efficient on green. A good CCD would be up to 4x that efficiency for purely luminance exposures, so I would be pushing magnitude 18.5 in that same time, or a gain of 4. Filtered, of course, I would still end up with QE loss and lower net gain.

One magnitude is 2.512x brightness, so doubling your exposure time will yield slightly less than one magnitude gain. A 6 minute exposure should go about 2 mag deeper than a 1 minute exposure, if I have done my math right. 10x the exposure will give you 2.5 mag gain. As you can see, it is a diminishing return.

Regards,
Eric

[Merlin66]

When imaging the limiting magnitude is restricted by the noise and pollution in the image.
Getting down to +18 mag is very doable with DSLR's

[CometGuy]

Jason,

Practical limits are difficult to quantify but CCD magnitude limits can easily be 5 or more magnitudes fainter than what can be seen visually.

Interestingly the C8 I have has an advertised "limiting stellar magnitude" of 14. In practice a good visual observer could see stars of mag 15 under good conditions with this scope.

As a quick test I tried the C8 under good suburban skies tonight with a cooled monochrome CCD camera. Because I wasn't guiding, I had to limit exposures to 45 seconds, and just took a series of photos and stacked them in IRIS. With 15 exposures, or about 11 minutes total exposure the stellar limiting magnitude was 19.5!

If I were to attempt this more seriously, I would use an autoguider make longer single exposures and with 2-3 hours exposure mag 21 would then be achievable. With an adaptive optics unit, a multi-night exposure, true dark skies you can go deeper still...

Terry

[koputai]

Fantastic, thanks Terry. So minor planets aren't totally impossible then!

Cheers,
Jason.

[Dennis]

Hi Jason

I’ve recorded stars at mag 21.8 from the mag 4 skies of my suburban Brisbane location with a C9.25 and an ST7 ccd camera.

http://www.iceinspace.com.au/forum/showthread.php?t=36627

Cheers

Dennis