Omege Nebula - what are these dark sopts?

[JohnH]

Here is an attempt at the Omega Nebula (aka Swan). This one is processed with darks and bias but no flats to try and eliminate a problem I have seen im my images recently - I have black spots. Are these DARK as opposed to hot pixels or processing artefacts?

[Geoff45]

Dust on the sensor?

[JohnH]

Thought that would show up as the doughnut shape?

[ballaratdragons]

Yes John, the black marks are from Hot Pixel removal.

I get the exact same thing when I use 'Darks'. They also indicate how good or bad your guiding is by the length of the black line left behind. Any corrections by autoguiding will make the black line longer or travel in a different direction. A single black dot tells you your mount is pretty well spot on, and guiding didn't need to do any corrections (Good P.E.).

To check for yourself, go to your original Raw image and you will see that the black marks correspond to the location of Hot pixels in the original.

[leon]

Dark spots or not, a pretty good image you have there John.

Leon

[citivolus]

If they bother you, these should be fairly easy to process out. If I recall, Images Plus has a tutorial on just such a process. Try doing noise reduction on a small selected area around the spot, or a median blur.

You could make a mask in Photoshop for easily processing these out of the individual frames as an intermediate step after dark adjustment and prior to alignment. Load up a median combine of your darks, do a select by colour range of highlights, expand it by a few pixels, and use that as your selection mask for the median blur. The negatives about doing this would of course be a slight loss of detail in the regions immediately adjacent to a hot pixel, for example slightly dimmed stars, and that it is an extra step in the work flow that probably does not involve Photoshop at this point anyway :) I tried it on a test image, and the artifacts were not noticeable. It would be very noticeable, though, in a globular cluster, for example.

Your best bet at this point may just to manually remove any offenders.

[ballaratdragons]

I just clone them out in PhotoShop in a few seconds :thumbsup:

[JohnH]

Thanks guys...took some new flats anyway and I will re-process when I get some time...

[JohnH]

Re-processed with flats and removed the spots - quite a good result I think...was 24*240s @ iso 800.

[Garyh]

A very nice Swan John! :thumbsup: Nice and sharp, I like it!
cheers

[alan meehan]

nice and sharp a amazing shot well done

[mill]

Wow! that looks good and about 1000 times better than my first attempt.

[Dr Nick]

Great shot! It looks like there are a few pixelated spots but it might just be my speed accelerater... ;)