one more stupid newbie question

[jimmyh1555]

Hi Guys! Your answers to my earlier questions regarding F numbers and shutter speeds and ISO's for DSLR's are great! However there is one last query I can't seem to work out too well, and that is, that with my super fast lens on my Pentax of 1.4, I can get an almost passable picture of the crooked J in Scorpious at 40 seconds and F1.4. My little camera lens is a mere 2.5 inches diameter. Now I realise that most telescopes are lucky if they have F4 numbers, so an equivalent exposure to my 40 sec at F1.4 will be about 320 sec (over 5 minutes), BUT on the other hand, the telescope will have a much larger diameter lens and so will be able to concentrate all that light from its large aperture, onto the CCD chip. So how come some folk take humungous numbers of shots maybe totalling 1 hour or more. Why doesn't all that light overexpose the photos and turn them all white? Is it because the scope is looking at a much dimmer location?? (But...M42 isn't that dim though?) I guess that must be it then! Am I right??:shrug: Answers please on a postcard;)

[xelasnave]

M42 , well as I recall over expose rather easily around the Trapezium before you bring out fainter detail so you need to integrate a capture of the outer regions with a shorter exposure showing the blown out region.

And there are no stupid questions.

Good luck with your journey.

Having a great time wish you were here...in the interest of postcard speak.
Alex

[cometcatcher]

The "brightness" of light reaching the CCD chip is the same at any given F ratio. Your camera lens set to F4 and my 10 inch F4 have the same brightness on the CCD. For both systems, the same exposure at the same ISO will give the same brightness. The difference is focal length and consequently field of view. The camera lens will take in a large part of the sky, the telescope field of view will be much narrower. When using long focal length (more magnification) it takes a big mirror or lens to match the brightness of a wide angle lens. It's pretty much the same rules ( focal ratio) that exist for daylight photography - if you were to take a single shot.

Focal length, aperture and focal ratio. It's a bit like ohms law. You can calculate the third by knowing any of the other two. If something is F4 then the focal length must be 4 times the aperture.

So how do we get hours and hours without overexposing? We take short sub frames and stack them to simulate a long exposure, otherwise we would overexpose. You can always wind the gain or ISO down also to prevent overexposing.

Stacking lots of short sub frames works on the principle of signal to noise ratio. Cameras are noisy. Fortunately a lot of it is random so with multiple sub frames the random noise gets cancelled out, but the signal which is constant remains.

[rcheshire]

The image stack is more often the averaged sum of all the subs. Sometimes the median for noise reduction, at the expense of SNR. Were the subs just added, then you are correct, the stacked image would be over exposed.

Lenses telescopes and aperture influence sub exposure time (signal and noise). M42 usually requires a few shorter subs to reveal the core in more detail. The shorter subs are noisier, but this is averaged in the total time of the entire stack.

[Shano592]

Something like M42, you would take shorter subs and then stack them. That would help to prevent blowout of the Trapezium.

The attached image of 47 Tuc is a stack of 10x 5-second subs at iso1600. I probably could have brought it down to iso1000 or 800 for less noise, but you can see that the centre isn't blown out too badly at all. I can zoom in on the original sized image, and pick out a good number specific stars.

For me, the bigger issue is getting good focus with the DSLR, initially.