Alnitak issue

[Peter Reader]

Hi all,

Tried to image horse head and lame last night. Got nearly 100 60s subs and the horsehead shows but alnitak is blumming huge! It rather ruins the image as it draws the eye away from the horse head and covers part of the the flame.

I'm using ISO 1000 and 800

DSS and GIMP

Any thoughts?

[dugpatrick]

Maybe a picture would help, but I'll speculate...

If you're talking about diffraction spikes, then it should be possible to rotate the camera a little, several times during the session, and then the algorithm in DSS (kappa-sigma clipping) should remove the spikes. I haven't tried this method yet, but it should work.

Or, if the problem is with the size of the Alnitak blob, then maybe a HDR merge would help?

Just a couple of thoughts. Let us know what works!

Doug

[Peter Reader]

Thanks for the rotation idea!

What is HDR merging?

[dugpatrick]

"HDR" refers to high dynamic range. More detail here: High dynamic range imaging - Wikipedia, the free encyclopedia

Basically you would collect a set of images with short exposures, so that Alnitak looks correct (not bloated), and then merge the images. The merge could be accomplished with cut/paste and feathering, or you could try the "HDR merge" feature available in many photo editing applications.

I don't have a lot of experience with HDR merge myself. Perhaps other people could recommend good software. I've used HDR merge in Paint Shop Pro with limited success. Often it's easier, I think, to cut/past a section of the image.

Doug

[Peter Reader]

Thanks very much

[Sp@ce_d]

I've been thinking about this issue too as this is a target I tried for the first time last weekend. I tried 180s subs which certainly brings out detail in the flame & horsehead with the modded 1000D. But like you say Alnitak is well blown.

I was wondering about using masks as in this guide about combining different exposures Compositing 2 Different Exposures via Layer Masks

I'm also going to try getting some Ha subs to add to it.

I must say it's one of the most beautiful & perfect group of targets to fit onto the APS size sensor with the ED80

[ollypenrice]

Alnitak is a rogue and short exposures don't help much because they collect no faint data from close the star, so even if you replace the huge blob with a smaller blob there is nothing to fill in the bit in between!

What I would suggest is multiple stretches of the normal data. Make a short stretch of Alnitak and paste the full stretch onto it. Use a feathered marquee around it and partially erase the top image. Repeat till it's rather better. Then place an even shorter stretch underneath and repeat. There's a lot of trial end error and, above all, don't expect any one iteration to solve the problem. Lots of little steps.

You might not like it but my method gave this;

http://ollypenrice.smugmug.com/Photography/Widefield-images-including/SWORDOFORIONdec09/740529927_ogEv5-X2.jpg

Olly

[dugpatrick]

You might not like it but my method gave this;

http://ollypenrice.smugmug.com/Photography/Widefield-images-including/SWORDOFORIONdec09/740529927_ogEv5-X2.jpg

Olly

Wow, that's a very nice photo Olly! I think your technique worked well.

Doug