Hi Robin,
I am having some real difficulty imaging the fireworks galaxy (NGC6946) and was wondering if you could assist. I took about 4 hrs of 2-min subs last night under what appeared to be ideal conditions, but there was excessive noise in the image which I couldn't remove when processing it (using startools). I am not sure if the noise issues were with startools or on the capture (sharpcap) side. When the raw stack (using dss) included lights, darks, and flats, stars were present but the image was not visible at all when I went to process it. When I removed the darks from the raw stack the image appeared when processing, which tells me there was something wrong with my darks, so I removed them from the raw stack. I then tried to process it using only lights and flats, and the image appeared, but there was excessive noise. The noise was so severe I couldn't process it any further. Can you assist in identifying the cause of the noise? Is it on the sharpcap or startools side?
Note (possibly important) - I noticed that I left the dark and flat subtraction function links active in sharpcap by accident (from an earlier livestacking session). Could this have affected anything? The darks and flats I stored in sharpcap had very different parameters that what I used while imaging - different gain, exposure etc.
The autosave (raw stack) link is below if you can try processing it to diagnose the noise problem or other issues. I also included the sharpcap capture file below if that helps. Any help will really be appreciated! Thanks!
Equipment: Celestron C8 with 0.7 reducer, ZWO asi 294 camera, EQ6 mount. Sharpcap pro software used for imaging.
Link to dss autosave file (with light and flat frames only):
https://drive.google.com/file/d/1S5R8DC ... sp=sharing
Sharpcap capture file:
[ZWO ASI294MC Pro]
FrameType=Light
Debayer Preview=On
Pan=528
Tilt=0
Output Format=FITS files (*.fits)
Binning=1
Capture Area=3104x2822
Colour Space=RAW16
High Speed Mode=Off
Turbo USB=40
Flip=None
Frame Rate Limit=Maximum
Gain=121
Exposure=118.892s
Timestamp Frames=Off
White Bal (B)=67
White Bal (R)=58
Brightness=23
Cooler Power=54
Temperature=-10
Target Temperature=-10
Cooler=On
Auto Exp Max Gain=0
Auto Exp Max Exp M S=30000
Auto Exp Target Brightness=100
Mono Bin=Off
Trail Width=3
Minimum Trail Length=100
Trail Detection Sensitivity=9
Remove Satellite Trails=Off
Background Subtraction=Off
Planet/Disk Stabilization=Off
Banding Threshold=10
Banding Suppression=0
Apply Flat=C:\Users\jsnat\OneDrive\Desktop\SharpCap Captures\flats\C8-400gainMasterFlat_2023-03-20-1122_9.fits
Hot Pixel Sensitivity=5
Subtract Dark=C:\Users\jsnat\OneDrive\Desktop\SharpCap Captures\darks\C8 30-sec400gain MasterDark_10_frames_-10.0C_2023-03-20T11_20_03.fits
NegativeDisplay=0
Display Black Point=0.03125
Display MidTone Point=0.123579833702411
Display White Point=0.998046875
Notes=
EQMOD ASCOM HEQ5/6=RA=20:35:14,Dec=+60:14:10 (JNOW)
TimeStamp=2023-07-11T01:42:33.4311038Z
SharpCapVersion=4.0.9538.0
StartCapture=2023-07-11T01:42:12.2916309Z
MidCapture=2023-07-11T04:18:13.9906309Z
EndCapture=2023-07-11T06:54:15.6896674Z
Duration=18723.398s
FrameCount=157
ActualFrameRate=0.0084fps
TimeZone=-4.00
Help needed with fireworks galaxy (NGC6946) - image capture issues?
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Moonstruck
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Re: Help needed with fireworks galaxy (NGC6946) - image capture issues?
Hi,
if you used incorrect darks/flats during the stacking then that will definitely have a negative effect on the saved stack images, but if you have also been saving the raw frames while stacking then those should be unaffected (since they are saved *without* the dark/flat correction applied).
Looking at your stacked image, I can see a lot of raining/walking noise in it, which is probably a result of a normal amount of drift combined with the incorrect dark frame selection. I don't suppose that much can be done to improve that stacked image.
If you can go back to the raw frames, then it is worth checking that your dark frames that you were intending to use for SDD/Startools stacking match properly - in particular, check gain, exposure, brightness and the R/B colour balance controls are all the same. The fact that you saw nothing makes me wonder if the dark frames were too bright and were over-subtracting.
A good way to double check is to look at the FITS frames (dark and raw) in FITSLiberator - that reads out the mean frame values, and if the dark frame is higher than the light then something is amiss!
One more thing to be aware of is that you are running the 294 camera in the slightly dodgy gain range. I've seen cases where 294 based cameras fail to saturate properly with gains of between 4.5x and 9x (about 120 and 185 in ZWO terms). If this is happening then it is possible to get flats that look fine but are actually useless. The test is to set the gain you want to use and let *LOTS* of light onto the sensor - for instance 5s exposure with a torch shining into the telescope. If everything is fine then the histogram should just be a single spike at the far right. If you don't see a single spike at the far right and instead see a broad peak from about 60% to 100% then do not use the gain you have just tested - turn up the gain and try testing again until you find one that does give a histogram of everything at the 100% level.
So, unfortunately I can't point you directly at the cause, but maybe the above will give you a few directions to investigate to see if anything else of interest is revealed.
cheers,
Robin
if you used incorrect darks/flats during the stacking then that will definitely have a negative effect on the saved stack images, but if you have also been saving the raw frames while stacking then those should be unaffected (since they are saved *without* the dark/flat correction applied).
Looking at your stacked image, I can see a lot of raining/walking noise in it, which is probably a result of a normal amount of drift combined with the incorrect dark frame selection. I don't suppose that much can be done to improve that stacked image.
If you can go back to the raw frames, then it is worth checking that your dark frames that you were intending to use for SDD/Startools stacking match properly - in particular, check gain, exposure, brightness and the R/B colour balance controls are all the same. The fact that you saw nothing makes me wonder if the dark frames were too bright and were over-subtracting.
A good way to double check is to look at the FITS frames (dark and raw) in FITSLiberator - that reads out the mean frame values, and if the dark frame is higher than the light then something is amiss!
One more thing to be aware of is that you are running the 294 camera in the slightly dodgy gain range. I've seen cases where 294 based cameras fail to saturate properly with gains of between 4.5x and 9x (about 120 and 185 in ZWO terms). If this is happening then it is possible to get flats that look fine but are actually useless. The test is to set the gain you want to use and let *LOTS* of light onto the sensor - for instance 5s exposure with a torch shining into the telescope. If everything is fine then the histogram should just be a single spike at the far right. If you don't see a single spike at the far right and instead see a broad peak from about 60% to 100% then do not use the gain you have just tested - turn up the gain and try testing again until you find one that does give a histogram of everything at the 100% level.
So, unfortunately I can't point you directly at the cause, but maybe the above will give you a few directions to investigate to see if anything else of interest is revealed.
cheers,
Robin
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Moonstruck
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Re: Help needed with fireworks galaxy (NGC6946) - image capture issues?
Thanks for the suggestions. I did stack the raw frames so I guess the flats and darks in the system for livestacking didn’t cause the issue. One thing that I forgot to mention was that I was using guiding (PHD2) and I got a total error of 1.6, which is worse than it has ever been before. Could that have attributed to the noise? I think the error was so high because I was using my C8 scope, but PHD2 was calibrated based on an Esprit 100 refractor which I used last time. I think I may have to calibrate PHD2 every time I change scopes.
Anyway thanks for the advice, and I will try this again soon.
Anyway thanks for the advice, and I will try this again soon.
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Re: Help needed with fireworks galaxy (NGC6946) - image capture issues?
Hi,
I don't think that poor PHD2 guiding will cause that noise issue - problems with odd shaped stars are more likely from guiding issues. The PHD2 calibration should only need re-doing if you change the focal length (or orientation) of the guide scope - it should be independent of the main scope unless you are using an off-axis guider.
cheers,
Robin
I don't think that poor PHD2 guiding will cause that noise issue - problems with odd shaped stars are more likely from guiding issues. The PHD2 calibration should only need re-doing if you change the focal length (or orientation) of the guide scope - it should be independent of the main scope unless you are using an off-axis guider.
cheers,
Robin