Flats - Darks - Fits - PNG - Live Stack

[tmyers]

I was going to discuss live stacking but never got to it. Figure I need to get things straightened out with individual files first.

[turfpit]

Tim

Complexity reduction is always a good strategy for problem solving. When you get chance, will you capture a FITS image of an astro object and upload it somewhere it can be downloaded for analaysis.

Dave

[tmyers]

I will do that Dave, just waiting on clear skies, I am assuming you want .fits. How many images do you want to see and do you want me to get some flats together as well?

I did get .fits flats to work a little better with .fits light frames this afternoon.

I also used the guide I think you authored on Gimp Processing and like the result. Will post it later.

I think I have GIGO (Garbage in Garbage out) because I was only using what I could see to adjust exposure and gain. I will use the histogram next night out and play around with the Histogram Brain as well.

[turfpit]

Tim

A single light frame to start with. Some histogram background here https://docs.sharpcap.co.uk/3.2/#The%20Histogram.

Glad you found my GIMP Guide useful.

Dave

[tmyers]

I will work in that, Saturday looks to be promising.

This is M42 using my .fits, light and flat frames, and following your guide to using Gimp

M42 Processed.jpg (379.15 KiB) Viewed 2344 times

[turfpit]

That is very encouraging Tim. What exposure did you use?

If you look at my documents around the forum (especially Tutorials), they were all written as I ran into problems. I was determined to leave something better than I had for the next person along. You should be able to work out what is coming as you progress.

Dave

[tmyers]

My imaging computer is in a backpack in the garage, I will go get it and let you know what settings I used. My attempt to image Sat night were thwarted by cloud cover. had the scope set up from about 5PM till after 1AM Sunday, never did clear off for me.

[tmyers]

Here is the data from the photo

[ZWO ASI1600MC-Cool]
Pan=0
Tilt=0
Output Format=FITS files (*.fits)
Binning=1
Capture Area=4656x3520
Colour Space=RGB24
Hardware Binning=Off
High Speed Mode=Off
Turbo USB=80(Auto)
Flip=Horiz
Frame Rate Limit=Maximum
Gain=355
Exposure=30
Timestamp Frames=On
White Bal (B)=64
White Bal (R)=45
Brightness=62
Temperature=-21
Cooler Power=5
Target Temperature=-21
Cooler=On
Auto Exp Max Gain=300
Auto Exp Max Exp M S=30000
Auto Exp Target Brightness=100
Mono Bin=Off
Apply Flat=None
Subtract Dark=None
#Black Point
Display Black Point=0
#MidTone Point
Display MidTone Point=0.5
#White Point
Display White Point=1

I used 90% of 50 exposures at 30 seconds, and brought in 30 Flats and 30 Darks to stack with DSS

[turfpit]

Tim

If you use a Colour Space of RAW (don't use RAW8 for deep sky objects) the files will be smaller (2 bytes per pixel as opposed to the 3 bytes per pixel of RGB24). This will save a lot of disk space with big mega-pixel cameras.

By saving as FITS, a lot of information is available using software such as FITS Liberator https://www.spacetelescope.org/projects ... load_v301/

I capture a single test frame and analyse in FITS Liberator before committing to an imaging run (too many wasted nights but not anymore). The examples below will demonstrate.

Use of the histogram in SharpCap can help getting the initial exposure setting.

This is an in-process capture of M42 using an Altair 183C with 180s exposure. Note that the M42 core is blown, suggesting exposure (and/or gain) is too high. By using the histogram and selecting Logarithmic, a more repeatable analysis can be performed. The Right Hand Side of the histogram is hitting the axis hard implying exposure needs to be reduced. At the Left Hand Side, there is a clear gap. This means no faint data is lost and indicates that the Black Level setting (Brightness in a ZWO camera) is correct. The gain used here is 200 (out of 5000) which is 4%. M42 having a bright core is an object that will capture better with lower gain but not the lowest gain available (100 in the 183C's case).

183C-M42-capture.JPG (111.13 KiB) Viewed 2034 times

A single FITS frame can be captured and evaluated. Here are some important points in the image below which is a 30s exposure with the gain at 4%.

• The background signal of space without stars is above 1000. I always aim for this. If the figure is less than 1000 then increase the exposure and retest.

• The minimum value is greater than zero indicating the frame is not 'black_level clipped' i.e. no faint data is lost.

• The maximum level is less than 65535 indicating the image is not over-exposed. Think of 0 as black and 65535 as white. At these values, data is lost and no amount of processing can recover it.

• See 2nd item.

• See 3rd item.

FITS-info.JPG (86.78 KiB) Viewed 2034 times

The Image Headers tab will show important capture information such as exposure, gain and black_level/brightness/offset (for Altair/ZWO/QHY cameras).

FITS-image-header.JPG (64.89 KiB) Viewed 2034 times

The following are examples of how exposure affects the histogram. Observe that the histogram gets 'fatter' as the exposure increases. 'fatter' = more data, as long as the exposure does not cause the histogram to bang into the right hand side. The histograms are 'the right shape' for deep sky objects. Fainter objects will require higher exposure (and perhaps higher gain) settings.

10s-exposure.JPG (15.28 KiB) Viewed 2034 times

30s-exposure.JPG (17.07 KiB) Viewed 2034 times

60s-exposure.JPG (16.04 KiB) Viewed 2034 times

120s exposure.JPG (16.25 KiB) Viewed 2034 times

Taking single test frames and understanding the effect of the camera controls will lead to a better image which is repeatable on subsequent nights.

Stacking can lead to better results, see my post viewtopic.php?f=18&t=1808. This compares various stacks up to a 100. That was 100x60s for M27 under a Bortle 6 sky.

M42 is a tricky object because there is a large range of brightness from core to faint nebulosity. Ultimately multiple exposure levels and HDR techniques are used to obtain more respectable images. This can be seen in my image here: https://www.astropixelprocessor.com/com ... -exposure/ which is a combination of 4s, 10s, 30s, 60, 120s expsoures. Click on the image for the high resolution version. Only took 2 years to get to that

Dave

[tmyers]

Dave,

Here is a dropbox link for one of the light frames Wednesday night. Bases on what I looked at with Fits Liberator I have slammed into the right side of the Histogram. Bad night of imaging as the clouds started rolling in on me and objects were moving into the trees.This is single shot of M44.

https://www.dropbox.com/transfer/AAAAAD ... YM_kiw41Qw

I did shoot some flats and darks yesterday and used RAW 16 for the imaging, hence the Histogram Brain is there. However since my lights were RGB24, DSS sis not like the mismatch and I couldn't use them. The mentioned appearance of clouds and trees means my stacked image is not that good but understood.

I am all ears and soaking this all in like a sponge. Thanks for your mentoring.