Planning for M31

[rmaddy]

After some struggles, I think I have my acquisition process down.  The nuts and bolts are a QSI683 and an FSQ85, and I am getting much better subs using an autofocus.  

My next big target is going to be M31, of which I've never gotten a satisfactory image.  Shooting LRGB (I can add Ha if I get enough clear sky time) what sub length should I be thinking?  And, if I need to shoot at different exposure lengths for core detail, can someone please direct me to resources on how to blend the images?

I am using Pixinsight, but still a novice with it.

[wimvb]

Pixinsight has several tools that allow you to deal with high dynamic range, so no immediate need to do multiple exposure times. Just make sure you don't overexpose.

But if you decude to do different exposures, pixinsight also has a tool for that. It's called HDR composition. You can google this to find several tutorials.

[rmaddy]

I've found it.  I ran it on my last image, and it made no difference because the dynamic range was really quite low.  I think I got a sense of how to use it though.  This next image will only be my second, so I'll still be leaning on Harry, LV astronomy, and a few others at every step of the way.

By overexpose, I assume that I shouldn't let the core go to 64K.  Any guesses on how long of exposures I'll be able to manage if M31 itself and not my local sky fog is the limiting factor?

[wimvb]

Can't help you with exact figures, since I don't know the scope/camera/filters you're using. You could check on astrobin.com; search for the same camera/scope that you have and note which exposure times for subs were used.

You're correct about the core at 64K. I would also avoid overexposing any but the very brightest stars in the frame. When you start overexposing, you lose colour control, and stars will grow.

Good luck

[ollypenrice]

Your sky conditions and processing techniques will have a big say in the story. If you limit yourself to 64K at the upper end I doubt you'll get the fainter stuff. Let's ignore that for now and think about the faint outer glow. Firstly this will require a 2 panel mosaic with your rig. This galaxy gets larger and larger the more data you collect. Processing will make it bigger still, of course. Even framing it on the diagonal will crop it. I have a slightly longer FL than you (Tak FSQ106N) but a considerably bigger chip (KAF 11meg.) For me it was a 2 panel.

From a dark site I used 30 minute luminance subs which allowed me to capture, for the first time, the upturned end on the left which I'd seen in an image on the net. Now, surprisingly, these subs did not result in an impossibly saturted core. However, I did shoot a set of 3 minute subs for that. The problem with M31's core is that it contains faint details which ultra short subs don't record, so you are between a rock and a hard place. It is remarkable how much faint data is really registered in a core which looks saturated, so dragging it out is the trick. I can't help on PI because I did mine in Ps.

Olly

[rmaddy]

Thanks Olly.  Knowing that you once used the BabyQ, I had hoped you'd respond.  I intend to add mosaics to my process eventually, but I'm not there yet, unfortunately.

Good to know I can reach a little deeper without boxing the core., although I'm sure my semi-rural backyard obs isn't a match for your dark sky site.

Fantastic shot.  One of the best I've seen!