M81

Hi

Heres an image I took of M81 a while back.

This is the longest exposure I have been able to get at 2 mins. Even with this length of exposure you can just barely seems to pick up the spiral arms. Methinks a wedge might be my next purchase:unsure:

Hi Dave,

You are heading in the right direction.But you are not going to get any sort of an detail with a 2 min sub exposure. What you need to do is take 100x 30sec exposure and stack them all together to get an image. If I was you I would forget about a wedge for deep sky astrophotography the reason is you get noting but egg shaped stars. Save your money and get yourself a good equatorial mount. I know some people will gripe about the equatorial flip but that is no big issue at all. Then next on the list is guiding I am surprised you are even getting 2 min exposure with an altazimuth mount try keeping it to 30 sec of an exposure that way you stars will stay rounder.I find a lot of things has to come together while out imaging. like you have to put a lot of time into an image and do not try get all the data in one night. Just spend one night getting the luminance then the other nights getting the the RGB.One thing you also need is a good monitor it is just as important as a telescope/mount or any camera.

Carl.

Dave,

Look into non-linear stretching (ex. DDP, arcsinh, gamma, log etc) so as adjust the histogram, for bright galaxies have an overwhelming core as well as dim detail around it and the only means to getting a good final result is with non-linear stretching.

Also, keep an eye on your histogram to ensure that you are not burning out the core. I cannot tell if this is the case or not with your current effort but you can easily check by looking at your original and its histogram.

M81 is one of my favourite galaxies and I always enjoy seeing photos of this stunning spiral galaxy.

Anthony.

Dave - good advice from both Carl and Anthony.

I'd modify Carl's advice a little - take as long an image as you can without any trailing (perhaps 60 or 90 seconds based on your image above) then just repeat as many times as you can. As Carl says, it doesn't need to be all on the same night.

Echoing Anthony's advice, you'll only get core detail or spiral arm detail with linear processing. To get detail on both, you'll need the non-linear processing tools.

If you could provide more detail on your kit and processing software, perhaps we could guide you better (no pun...)

Dave

Hi

I'm imaging at the moment in Altaz using an lx90 8' with an Atic 16ic camera, unguided.

I stack in DSS and further process in The Gimp, Nebulosity and a variety of other packages.

I have tried m81 before with shorter exposures but I didnt really get any detail no matter how many subs I got. Other then bright objects I am beginning to think there is no substitute for longer exposures for some objects

DB

DeeBee,

I'd also echo what Carl/Anthony/DaveMc said, I would however be inclined to keep the exposures shorter as Carl suggested. Lert me explain why.

Imagine you take a (fictitious) sequence of exposures like

10 x 30
10 x 60
10 x 120

Out of the 120sec stack you might only get one or two useable, out of the 60 secs you might only get 2-4 but you will get a lot more useable frames out of the 30 second lot. As DaveMc said, getting the longest you can get away with is good, but only if it is reproducable every time.

Also you must try to centre the object as much as possible, I know its time consuming, but your image would have been a gazillion times better if it were more centred. I know its not easy. Even a 1 second exposure will show the core of M81, so let the camera take repeated 1 second exposures while you centre it up.

OK now heres the brutal truth, imaging with an SCT at f/10 is a challenge for even the high end mounts. Even autoguiding can be problematic at those sort of focal lenghts. So don't get too bogged down with the tracking. Why not consider mounting a camera lens on top and hooking up your CCD to it. I can show you how you can do that really cheap. That way you wouldn;t always be fighting the mount.

Anyway, I don;t mean to be too critical. I think you've done a fine job of even getting a two minute exposure in altaz mode. So lotsa kudos for that!