Deepsky imaging focusing

[RedLeader1]

I am not sure if there is a focusing issue I have or a combination of things such as brightness etc when I image deepsky objects as the stars seem out of focus in the attached image.

I have the Celestron focus motor and thought I had excellent focus but finding it tricky to get the stars nice and pin sharp. You will notice on the attached image that the more distant stars are blurred but is this a focus issue or something else? What is best practice to achieve excellent focus?

I have a Celestron AVX 8* SCT and use a 0.63 reducer and back focus is set to 105mm.

Here is the camera settings used during the live stack:
[ZWO ASI585MC (2A1F060402020900, via USB3)]
FrameType=Light
Debayer Preview=Force RGGB
Pan=0
Tilt=0
Output Format=FITS files (*.fits)
Binning=1
Capture Area=3840x2160
Colour Space=RAW16
Exposure/Gain Shift=0
Temperature=20.9
Hardware Binning=Off
High Speed Mode=Off
Turbo USB=100(Auto)
Flip=None
Frame Rate Limit=Maximum
Gain=252
Exposure=10.464s
Timestamp Frames=Off
White Bal (B)=50
White Bal (R)=50
Brightness=50
Auto Exp Max Gain=300
Auto Exp Max Exp MS=30000
Auto Exp Target Brightness=100
Mono Bin=Off
Mask over exposed pixels=Off
Flip (after dark/flat)=None
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=None
Hot Pixel Sensitivity=7
Subtract Dark=Hot Pixel Removal Only
DisplayStretchEnable=1
NegativeDisplay=0
Display Black Point=0
Display MidTone Point=0.5
Display White Point=1
Notes=
CPWI Focuser=28335
CPWI=RA=20:00:46,Dec=+22:50:46 (JNOW)
TimeStamp=2024-08-27T23:20:27.4462384Z
SharpCapVersion=4.1.11591.0
TotalExposure(s)=523.2
StackedFrames=50
LiveStack.SaveRawFrames=None
LiveStack.AlignFrames=True
LiveStack.Derotate=True
LiveStack.StarsForAlignment=15
LiveStack.StarDetection.Sensitivity=75
LiveStack.StarDetection.SuppressHotPixels=True
LiveStack.AlignAutoDisabled=False
LiveStack.ReduceNoiseAmount=0.5
LiveStack.BrightnessLimit=89.36543
LiveStack.FilterBrightness=False
LiveStack.AutoBrightnessLimit=True
LiveStack.FWHMLimit=4
LiveStack.FilterFWHM=False
LiveStack.WarningThreshold=5
LiveStack.StackingMode=0
LiveStack.SigmaClipInitialCount=5
LiveSack.SigmaClipThreshold=3
LiveStack.SigmaClipLowLimit=1
LiveStack.AutoSaveReset=False
LiveStack.AutoSaveResetMinutes=5
LiveStack.LogarithmicHistogramHorizontalAxis=False
LiveStack.AutoSave=True
LiveStack.BlackLevel=9.12365
LiveStack.WhiteLevel=100
LiveStack.MidLevel=20.028837
LiveStack.WBB.db=0.34215227
LiveStack.WBG.db=0
LiveStack.WBR.db=0.06948291
LiveStack.Saturation=1.4963253
AutoStretch.Strength=2
LiveStack.StarDetection.FaintStarsOptimization=False
LiveStack.StretchMode=2
LiveStack.CometStarRemovalSize=50
LiveStack.AlignOnComet=False
LiveStack.CometThreshold=0.1
LiveStack.StartOnClear=True
LiveStack.AutoDisableDisplayStretch=False
LiveStack.SaveStackLog=True
LiveStacking.MaxStarSize=50
LiveStacking.NewStackOnReopenAfter60s=True
LiveStacking.Guiding.RequireGuiding=False
LiveStacking.Guiding.AutoDither=False
LiveStacking.Guiding.ShortExposuresWhileAdjusting=True
LiveStacking.Guiding.CountSecondsOrFrames=1
LiveStacking.Guiding.AdjustmentInterval=0
LiveStacking.Guiding.Enabled=False
LiveStacking.Guiding.RecenterEnabled=False
LiveStacking.Guiding.RecenterMovementThreshold=20

[admin]

Hi,

I can see what you mean in the image that the stars are not quite sharp, and there is also a bit of non-roundness to the brighter stars that might be down to collimation being slightly off or possibly tube currents in the telescope.

Anyway, SharpCap has a number of tools to help you find best focus without having to judge by eye - in this case you would probably be best using the 'Multi-Star FWHM' focus tool, which measures the size of a number of stars across the image and shows how that measure changes as you move the focuser using the motor focuser. If you have an ASCOM driver for your focuser to allow SharpCap to control the movement then you will get a nice graph showing (hopefully) a 'U' shape with best focus at the lowest point in the U.

There is a whole section of the SharpCap user manual on focusing, which you can dip into and find the relevant information - https://docs.sharpcap.co.uk/4.1/#Focusing

Hope this helps,

Robin

[RedLeader1]

Thanks for the reply and advice on what to try and will give things a try tonight hopefully.

I did do a collimation a few days ago using an out of focus star and it looked pretty even throughout so will check again.

I suppose for tube currents I would need to have the telescope outside earlier in the day?

[admin]

Hi,

tube currents on SCTs are tricky to deal with - even if your telescope has been outside for hours. At night, the sky is cold and the ground is warm, so the top of the telescope will tend to cool down (radiating heat to the sky), but the underside of the tube will stay warmer. That can lead to continuing tube currents however long you leave the telescope out for. Wrapping a heat insulating mat over the top of the tube *may* help by preventing the top of the tube from cooling, but my SCT rarely gets used because of this exact problem

cheers,

Robin

[RedLeader1]

I actually managed to improve things a bit more with collimation as I removed the f/6.3 reducer and you could see it was still out.

I can still seem some distortion afterwards and was able to get the focusing a bit more fine-tuned but there is still a little more that could be done. Will look into the insulating but need to spend some time on working with the multi-star focus tool.