Flame Nebula

[IDM]

Hi,

I took this last night. The equipment was a Skywatcher Equinox with a Zwo ASI294 MC Pro mounted on an EQ6R. 180 60 second subs taken with a gain of 164 and cooled to -10. Calibration subs were take the following day during daylight. After stacking in DSS the final image was stretched using Gimp, stars were slightly reduced in intensity. I am not sure if I am pleased with the image or not. I would really appreciate some feedback on what I might try to improve my data acquisition as well as processing. Thanks for looking.

Ian

[vlaiv]

You quite killed the background / black point.

Look at histogram:

It is glued to left side and it looks like part of it is missing "outside" of histogram. That is clipping of the black point - not good thing to do in image processing.

Try reprocessing your image but this time having nice bell shaped curve in histogram that is almost completely to the left but not touching left side.

[Budgie1]

You may also want to drop the gain back a bit. I normally use a gain of 120 with my ASI294MC Pro.

[IDM]

3 hours ago, vlaiv said:

Try reprocessing your image but this time having nice bell shaped curve in histogram that is almost completely to the left but not touching left side.

I tried reprocessing with the histogram as described and  it seems to result in a very light background (certainly nowhere near black) and there is much less detail in the nebula. Is there a way to extract the detail and yet make the image more natural looking?

[vlaiv]

1 minute ago, IDM said:

I tried reprocessing with the histogram as described and  it seems to result in a very light background (certainly nowhere near black) and there is much less detail in the nebula. Is there a way to extract the detail and yet make the image more natural looking?

If you post your linear data, I'll be happy to do basic stretch in Gimp and document the steps for you.

[IDM]

34 minutes ago, vlaiv said:

If you post your linear data, I'll be happy to do basic stretch in Gimp and document the steps for you.

That is so kind! I have attached the TIF and really look forward to learning more.

Thanks,

Ian

Autosave.tif

[vlaiv]

Ok, so here is result of quick / simple process in Gimp (just levels / curves and rotation and crop):

And here are the steps.

I first loaded image in Gimp and saw that background is a bit bright as is, so my first step was to raise black point a bit:

Be careful not to move left (black) slider too much - bring it to the foot of histogram but leave a bit space. We will deal with any residual space later.

Second step was to do non linear stretch using levels again. Take middle slide and move it down towards bulk of histogram like this:

This non linear part makes that small gap between black point and foot of histogram rather large, so next step was to remove it again:

Same thing - we bring black point just short of histogram foot.

Now we see that we still don't have enough of signal visible and we again use middle slider and pull it down, after that we again adjust black point:

Then I did vertical and horizontal flip to orient the image the way I like it orientated, and did one final curves stretch:

When doing curves I create one anchor point below histogram peak. I don't move that point - it serves to leave background as is. I then take another point in the middle of the curve and raise it up until it brightens up detail in the image.

That is it.

If you look at histogram:

It ends before left side - black point is not clipping any of the data - and it shows in the image.

I'm now going to try a bit more with this data - namely to remove that background gradient and to bin data a bit since it is not very sharp and it does not need such a large sampling rate.

Will post results again.

[vlaiv]

Here is the result after background gradient removal and binning

Data is rather clean except for the issues with stars - Alnitak is wreaking havoc as usual and some of the stars are a bit out of shape. There is also issue with flat calibration of the data - it did not work quite well and interesting thing is that it failed differently depending on channel - it's different for red, green and blue - which you can see in the image - red channel dominates with issues in the corners (over correction of vignetting).

Here is little animation that I made that highlights how issue with flat calibration changes between channels:

Red channels shows over correction while green and blue show under correction of vignetting - blue more than green.

This causes red rim and slight glow in the middle of the finished image.

[IDM]

19 hours ago, vlaiv said:

There is also issue with flat calibration of the data - it did not work quite well and interesting thing is that it failed differently depending on channel - it's different for red, green and blue - which you can see in the image - red channel dominates with issues in the corners (over correction of vignetting).

I wasn't very happy with the flat calibration frames, I noticed that the histogram for RGB weren't very well lined up (I presume that they should). I used a tablet PC for the white light source and I wondered if there was some unevenness. I will retake the calibration and restack to see what if any difference that makes.

Many thanks for the pointers you have provided so far.

Kind regards,

Ian  

[IDM]

19 hours ago, vlaiv said:

I'm now going to try a bit more with this data - namely to remove that background gradient and to bin data a bit since it is not very sharp and it does not need such a large sampling rate.

How do you bin data post acquisition? I had a Google search but couldn't find anything obvious with regard to changing bin or sample rate.

Thanks

Ian

[vlaiv]

3 minutes ago, IDM said:

How do you bin data post acquisition? I had a Google search but couldn't find anything obvious with regard to changing bin or sample rate.

Thanks

Ian

I use ImageJ - free / open source software that is java based (so will work on most platforms / operating systems), though there are other software packages that offer this functionality.

It is as simple as load then Image / transform / bin and selecting bin factor and method (average or sum).