Magnitude

[+Stu]

Hi folks. Imaging opportunities are few and far between at the moment so I don't want to waste them. Are objects above a certain magnitude out of reach? Or can you capture anything given enough time?

[Clive]

1 hour ago, Stu said:

... can you capture anything given enough time?

In theory, yes but in practice no. The light from faint objects will just add to the sky background so if the background level was zero then the the noise generated by the background will also be zero so given sufficient exposure, enough photons can be captured to make even the faintest object visible on an image.  But the sky background level isn't zero and that means the noise isn't zero either, and on top of that their is the dark current and associated noise created by the camera. So the maximum exposure will be limited to the well depth of the camera as the background photons and camera dark current slowly fills up the sensor pixel.

I seem to remember that an object intensity needs to be about 5 times the standard deviation of the background noise for it to become 'detectable', and obviously needs to emit sufficient photons to do so in the exposure time.

With my DSLR and 400mm F5.6 lens I can reach about 17th magnitude at best from my garden without the use of filters.

[+Stu]

51 minutes ago, Clive said:

In theory, yes but in practice no. The light from faint objects will just add to the sky background so if the background level was zero then the the noise generated by the background will also be zero so given sufficient exposure, enough photons can be captured to make even the faintest object visible on an image.  But the sky background level isn't zero and that means the noise isn't zero either, and on top of that their is the dark current and associated noise created by the camera. So the maximum exposure will be limited to the well depth of the camera as the background photons and camera dark current slowly fills up the sensor pixel.

I seem to remember that an object intensity needs to be about 5 times the standard deviation of the background noise for it to become 'detectable', and obviously needs to emit sufficient photons to do so in the exposure time.

With my DSLR and 400mm F5.6 lens I can reach about 17th magnitude at best from my garden without the use of filters.

I was eyeing something up on stellarium that was just under 13, can't remember what it was now 😆 Might wait until the moon has disappeared to try it, if I can remember what it was. Somewhere near M81. 

[Bob Dobber]

3 hours ago, Stu said:

I was eyeing something up on stellarium that was just under 13, can't remember what it was now 😆 Might wait until the moon has disappeared to try it, if I can remember what it was. Somewhere near M81. 

 

If I'm eyeing something up, I generally do a quick Google to see what people have already done, what sorts of exposures, and what sort of equipment. I am definitely not scientific about it, but I do know that imaging the Iris Nebula with 10 min subs may not fly. Whereas 10 min subs on the Elephant's Trunk nebula would be a damn fine idea. All dependent on equipment to an extent though, and you get to know what your set up needs over time, IME, YMMV as always. 🙂 

[+Stu]

7 minutes ago, Bob Dobber said:

 

If I'm eyeing something up, I generally do a quick Google to see what people have already done, what sorts of exposures, and what sort of equipment. I am definitely not scientific about it, but I do know that imaging the Iris Nebula with 10 min subs may not fly. Whereas 10 min subs on the Elephant's Trunk nebula would be a damn fine idea. All dependent on equipment to an extent though, and you get to know what your set up needs over time, IME, YMMV as always. 🙂 

A bit of research beforehand is the key then. Like tonight I should've searched for how much wind is too much for astrophotography, can't even get focused on a 3 second exposure. Visual time again 🙂

[Bob Dobber]

5 minutes ago, Stu said:

A bit of research beforehand is the key then. Like tonight I should've searched for how much wind is too much for astrophotography, can't even get focused on a 3 second exposure. Visual time again 🙂

 

Ah, but I think that also depends on what sized lump you have on the mount. I have two scopes, a tiny SharpStar 61 and a hulking great MN190. The former doesn't even notice the wind, however breathe on the latter and guiding goes off the chart!!

[+Stu]

35 minutes ago, Bob Dobber said:

 

Ah, but I think that also depends on what sized lump you have on the mount. I have two scopes, a tiny SharpStar 61 and a hulking great MN190. The former doesn't even notice the wind, however breathe on the latter and guiding goes off the chart!!

6" newt with a dewsheild on, might enter it in the Americas Cup