Ha Help

[Gary Beal]

OK, while I am still learning, I could use some help and advice with the following image.
I am on a bit of an Ha kick at present, mostly as the moon is interfering with "normal" DSO imaging.
Last night I shot a sequence of nine 600 second images with the E130/ST8/Baader 7Nm filter. Subject was the Horse Head.
A quick stack etc this morning in Maxim DL, and I find I have a fair bit of "black speckles" across and throughout the image. Any ideas. The attached image is a low res JPEG, merely to show the artifacts. I recall seeing a stunning shot from Bud, where the image was smooth and "creamy".
If it helps, I am more than happy to upload the FITs file for all to pull to pieces.
Any advice or suggestions appreciated.
Gary

[budguinn]

Gary, this is almost always caused by bad darks/flats...and mostly darks.
You're using an ST8 aren't you?....so you are taking darks? I think they might have been off somehow.

bud

[Gary Beal]

Originally Posted by budguinn

Gary, this is almost always caused by bad darks/flats...and mostly darks.
You're using an ST8 aren't you?....so you are taking darks? I think they might have been off somehow.

bud

Cheers Bud, I was sort of hoping you more knowledgeable guys would help.
I do use darks, and on this occasion it was one from a week or so earlier. Maybe I need to shoot them at the same time, as opposed to the "library" concept?
Maybe I will try another, but it will have to be again at a different time to the original imaging session.
Thanks
Gary

[BT Technologies]

Yup, Bud beat me to it.. Definitely take some darks at the same time and temperature as when you are imaging Gary. Subtract the darks from your lights and these artifacts should disappear.

Originally Posted by Gary Beal

Cheers Bud, I was sort of hoping you more knowledgeable guys would help.
I do use darks, and on this occasion it was one from a week or so earlier. Maybe I need to shoot them at the same time, as opposed to the "library" concept?
Maybe I will try another, but it will have to be again at a different time to the original imaging session.
Thanks
Gary

[Gary Beal]

So, given two knowledgeable folk saying "take darks", is this image doomed, or can I use a "library" like is promoted by the camera manufacturer? This is what happened here, and to be fair, I will crank the gear up tonight, as it is cloudy/raining, and try.
Thanks guys.

[E2Pilot]

Chad,

Were the darks you shot a week earlier shot at the same exposure length and temperature as the images you captured? If so, I would be surprised that you ended up wiht the holed seen in your image. I can generally use a library of darks for some time before I reshoot them and I really only end up reshooting them when I think that something has changed that would affect the images. But for the most part, a library can work. To be sure your darks, flats, etc match your lights, your best bet is to take them every time but especially for darks, you really shouldn't have to.

Steve Hamilton

[budguinn]

Hi Gary,

I don't use darks with any of my SX cameras, so my information is only from observation and reading many posts. I understand that many use a library of darks taken at different temperatures and that this usually works well for them.
I wouldn't think the information is lost, however.....I'd try some median filters on the originals before combining....and then maybe some outlier combine methods...sigma or median. Also, does the program that you use....Maxim...allow you to scale the darks? This will usually allow you to take darks that were taken at a different temperature be used.
I find these rainy days the best times to fool with this stuff and figure out different ways of fixin' things. It's guaranteed that if something can go wrong it will....such as the darks not working...or bad flats...etc.

EDIT:

Gary, I took the picture...into PS....make a duplicate layer....then used median and dust and scratches to remove the majority of the black speckles.
This will also lose the stars.....just change the blend on the top layer to lighten and it'll bring back the stars.
I then did a mask to bring back the detail to the flame and horse head that was lost with the median and dust filters.
then a quick deep space noise reduction...from Noel's actions.....then a quick curves to bring it out a tad more (this is to taste..could have left it alone).



bud

[KenMiller]

Originally Posted by Gary Beal

Maybe I need to shoot them at the same time, as opposed to the "library" concept?

Gary,

My understanding is that to use darks shot a different temperature and/or exposure duration you must use bias frames to adjust for those differences.

Also, the characteristic noise of the camera changes over time so it is necessary to reshoot darks periodically even if you do use bias frames.

I believe I understand the theory behind these concepts well enough to be confident in recommending you explore the use of bias frames further. Unfortunately, I don't have the empirical experience to make specific recommendations.

[Gary Beal]

Thanks Guys, and especially Bud. I can see that with your processing skills Bud, you could open a niche business fixing folks pictures. You have simply worked magic on this one. (and that from a small 155kb image).
I have all but decided to simply shoot darks at the time I image, OK, it takes a bit longer, but seems the best option. Even though regulated, I think the temps are different before during and after the sequence.
More practice needed.

[EricCCD]

Hi Gary,

Library darks work well for me (ST-8 and now STL-6303).

Always use darks shot at the same time (and temperature!) as the light frames you are reducing.

You can scale darks shot at different times, but not different temperatures, with bias frames. You can scale down in time, but not up (i.e., a 15-minute dark at -20C can be used to reduce a 10-minute light frame, as long as the light frame is also shot at -20C).

Darks should also be shot with the same binning mode as the lights they are reducing.

I add to my library when it's cloudy, though I have not done so in a while.

Here's one more tip: early on in my narrow-band imaging days, I got exactly the same result as you. How did you combine your darks (you need more than one for sure!)?

One possible cause of the dark spots is bright spots (either noise or cosmic-ray hits) on the chip during dark acquisition. If you use a single dark (or an averaged master dark), those bright spots could potentially be retained in the master dark. The net effect after dark reduction: the dark spots you see!

Try median-combining your darks (you need at least three in order to do this effectively), then applying the new master dark to your raw light frames. In my case, the "pepper spots" vanished when I did that!

HTH,
Eric