Image of the day

From the
ATWB Customer Gallery

NGC 6960, Veil Nebula

My Account

New to Astromart?

Register an account...

Need Help?

I got a new 20Da camera

Started by phil426, 09/06/2007 08:19AM
Posted 09/06/2007 08:19AM Opening Post
First try with 20Da. M33 2 sets of images 24 and 25 8 min ISO800 subs median combined with IP. The 2 sets were then added. Question? Is there a preferred method of combining all these images? It seems there may be a limiting factor and then there is no more improvement to the image.

Attached Image:

phil426's attachment for post 38555

Phil
Kennewick, WA.
AP 152(old blue)on CI700 w/80mm f/11 guider, C-8 on forks w/80mm f/5 guider, 20x80 binos, ST5C, 201XT, Ricoh 35mm, 20Da.
Posted 09/08/2007 07:43AM #1
I am no expert but after comparing some of mine, if grouping into batches and then recombining Average method worked better than Median. For a single group Median seems to be good enough! I noticed it makes the image background little darker but again I loose some of the very finer details.
I was earliar using Mx516 and after receiving birthday gift from my wife the cd: A Guide to Astrophotography With Digital SLR Camares” by Jerry Lodriguss, turned to 20D. I am not sure one has to follow blindly but it gives a starting point.


FluffyDuck observatory
Celestron 8N/CG5 NON GOTO
Canon 20D, Neximage,MX516
Losmandy G11 GOTO
Posted 09/09/2007 08:04PM #2
Frankly, with as many images as you are combining, you won't see much difference with most of the different methods.

But what method one should use depends on a lot of factors.

If you have done things right (and you have), you have lots of images to use. I think I see 24 images in one set, and 25 in another set. (Why is it that you don't just consider it 49 total images?)

With that many images, I would be using a sigma combined in IP. (Assuming you have sufficient exposure, and with 8 minutes at ISO you should have.) Sigma combined runs a statistical analysis of all the values for that pixel in the array, and then rejects all the values that are not within a given standard deviation of the mean, while combining the rest. That means that all values are considered, and the center is weighted.

Median combine instead uses only the median value. One value out of the twenty four possible values. Now, with twenty four exposures, this has got to be pretty close to a true middle of the pack. However, let's say you only had three exposures, and two of them had a hot pixel, or a satellite going through, or a cloud passing by-- your "median" would not be truly representative.

A simple average combine would overcome some of these problems, but let's say you had three exposures, and one of them was bad. That one bad value would have one third the data you used. It would be averaged out, but still have lots of effect on the end result. Again, if you have twenty four exposures, it is not that big a deal, since the bad pixel value would represent only one twenty fourth of the data.

In short, You should always try to have at least eight exposures, and then use a sigma combined.

If you have fewer, you can try one of the other methods. If it is particularly dim, you can either straight add (and not divide by the number of exposures) or try the funny add (adaptive add???--I cannot remember the name) where you change the divisor after you add.

However, at 24, you reach the point of diminishing returns.

Your mileage may vary.

Pretty picture, though.

Alex
Posted 09/11/2007 09:23AM #3
Thank you Alex and Ansuman for your input. What value should I use for sigma combine. The default is 1 -and where did you find an explanation of what the different combine functions do? It makes a lot of sense to use sigma combine on this many images. I have collected 73 8 min exposures and would like to combine them all. Just trying to see what the capability of the camera is and M33 was at a dark place in the sky.

Also is there a magic formula for flat frame exposures? It seems that I need to run up the background in digital development to as high as 8000 on the flats prior to building a master to get the desired effect. I'm thinking the exp times are to short on the flats.

Also here is a pic of the Helix -not that great but its something new to look at. 8 8min ISO 800 shooting through a smoky horizon next to my neighbors house yard light - its atrocious.

Attached Image:

phil426's attachment for post 116752

Phil
Kennewick, WA.
AP 152(old blue)on CI700 w/80mm f/11 guider, C-8 on forks w/80mm f/5 guider, 20x80 binos, ST5C, 201XT, Ricoh 35mm, 20Da.