Imaging and lower Magnitude

[adamsp123]

Hi All,

I wonder is there a basic rule/guide on limiting magnitude versus telescope size for imaging. Clearly there must be a point where high magnitude (low output of light) limits the usefulness of imaging the target, or could it be very dim objects are going to be so small in apparent size and lack resolution it is not worth bothering.

Any thoughts anyone?

Cheers Pete

[brianb]

Modern CCD cameras do not have reciprocity failure, so in principle there is no limit to the faintness of objects that can be imaged ... just expose for long enough. In practice the diffuse sky light (even in the absence of pollution) will eventually contribute so much light that very faint objects cannot be distinguished from the background without increasing the image scale, which means more focal length, which means a bigger f-number and so longer exposures still .... unless the aperture is increased.

Mag 15 stars can be shown quite easily with a 300mm f/4 lens on a DSLR, and amateur scopes in the 20-30 cm class can image objects down to mag. 20 when equipped with a good quality CCD camera.

In practice the quailty of the mount (freedom from vibration & accuracy of drive / guiding) is the limiting factor rather than the "light bucket", especially as focal lengths start to get over one metre.

[dph1nm]

Actually what kills you from the ground is confusion - eventually you pick up so many faint images that they merge together and you cannot distinguish one from another. This is made worse by seeing limiting all objects to a minimum size of ~1arcsec - from space it is much easier. The practical limit from the ground is probably around 30th magnitude, maybe a little fainter ...

NigelM

[Merlin66]

I've definately seen star images below +18 mag in a DSLR image.

Comparing with USNO database in Carte du Ciel...

[adamsp123]

Thanks for the replies guys I suppose for now I need to get my act together on some brighter objects anyway then move on to harder and fainter subjects.