Pleiades and a novice....

[Ande]

Just a spot of direction needed please. I’m in the process of getting a few things together for having a crack at imaging. I haven’t yet reached the stage where I’m ready to start stacking, so am just taking single shots, and experimenting a bit with ISO, aperture etc.

So, I pointed the camera at the Pleiades, and was a bit dismayed that all I was getting for longer exposures was more and more stars.  Now, that would be fine, desirable even, if the main stars, the Seven Sisters were prominent in any way. But all I ended up with was a jumbled star cluster, to the point it was impossible to even work out which stars represent the constellation. 

When I start stacking will the above change? Or are all of the wonderful Pleiades images I’ve seen on these pages the result of huge amounts of artistic license during post-processing?  Does anyone get pleasing results from a single image, or am I wasting my time attempting them?

Apologies for the dumbness of the question, but fear not, I have many, many more

[david_taurus83]

Hi Ande, what kit are you using? You would typically use a tracking mount and take long exposures.

[Ande]

16 minutes ago, david_taurus83 said:

Hi Ande, what kit are you using? You would typically use a tracking mount and take long exposures.

I am using a Skyguider Pro, coupled with a DSLR, and a Samyang 135mm lens.  I’ve taken quite a few pictures at max 30 seconds. I haven’t figured out how to use Bulb Mode yet without an intervalometer, which is on it’s way.  I was able to get the Orion Nebula to show some nebulosity and colour at 30 seconds. Just not the Pleiades. To be honest, I wasn’t expecting miracles, but was surprised at how many stars turned up, 😂. 

[Budgie1]

If it's any help then I think you'll need more than a single 30 second exposure to capture nebulosity with M45.

Lots of 30 second exposures stacked will get the nebulosity and the same goes for other targets, like M31. A single 30 second frame of M31 will show a fuzzy blob, put 20 or 30 frames into a stack and you're getting the detail of the target (once processed).  

Here's my last go at M45 with my Skywatcher Evostar 100ED on an EQ5 mount using an unmodded Canon EOS 2000D camera. This is a single guided frame at 300 second (5 minute) exposure at ISO 1600 and the nebulosity is coming through.

This is an old image I took back in 2011 using the same mount but unguided and only 20 second exposures at ISO 800:

 

[rickwayne]

The nebulosity around the Pleiades is so much dimmer than the stars themselves that it takes a lot more integration time to bring it out. You could do it with single exposures, but you'd run into trailing problems and really severely blown-out and bloated stars by then. All our deep-sky work is trickery with dynamic range and contrast, compressing the tonal range at the high end and vastly expanding it at the low.

As an exercise in learning the ropes for acquisition, single frames are a fine idea. But I wouldn't spend a lot of time trying to get good-looking results of much that way. (Maybe you could do  star clusters if you got the exposure just right.)

This is one of the topics very ably covered by Bracken, Lodriguss, Richards, and those folks in their books.  Basically a single image is one sample from a rather low-level, noisy signal. Say two adjacent pixels are slightly different in tone "in actuality" (i.e., the "real" values we never know, but can only approximate). But your sensor, over one minute, happens to  record 83  photons for one and 83 photons for the other during your one exposure. Now forever after there's no way even in principle to discriminate those shades, and when you dramatically increase the contrast by e.g. 10X, those pixels stay the same shade even though they should get more and more different as you stretch (increase the low-end contrast).

But if you take a hundred  samples (83, 85, 83...), you might discover  when you  average them that  Pixel A is really 83.6 photons per minute while Pixel B was 82.4. Now the contrast-enhancement has something to get its teeth into, and some detail emerges that the single exposure could never show.

Of course, multiplying the exposure time by a hundred has the same effect on the math. But exposures that long are usually impractical for the reasons noted above.

[Ande]

10 hours ago, Budgie1 said:

If it's any help then I think you'll need more than a single 30 second exposure to capture nebulosity with M45.

Lots of 30 second exposures stacked will get the nebulosity and the same goes for other targets, like M31. A single 30 second frame of M31 will show a fuzzy blob, put 20 or 30 frames into a stack and you're getting the detail of the target (once processed).  

Here's my last go at M45 with my Skywatcher Evostar 100ED on an EQ5 mount using an unmodded Canon EOS 2000D camera. This is a single guided frame at 300 second (5 minute) exposure at ISO 1600 and the nebulosity is coming through.

This is an old image I took back in 2011 using the same mount but unguided and only 20 second exposures at ISO 800:

 

Thank you. That goes a long way towards helping me understand. My images are probably similar to your second one, but without the extra luminosity of the main stars.

[Ande]

@rickwayne Thank you for that very detailed reply. I think you have given me a greater understanding of the process than anything I have read thus far. The accurate averaging out of what the pixels should be displaying, rather than what a single snapshot captured has sunk in beautifully.

I shall be starting to stack images very shortly, so hopefully results will show a marked improvement.  I have been lucky enough to acquire a couple of books on the subject as Xmas presents. So, between those, and you fine people, I shall forge ahead.

Much appreciated folks

[rickwayne]

Which books, if I might ask? Always looking for things to put on my gift lists 🙂

 

[Ande]

Making Every Photon Count, and Dark Art or Magic Bullet, both by Steven Richards. The former seems to get a lot of love and recommendation on here, so I figured the follow up should be a worthy addition too.