Strange Artifacts in Ha Image

[Jonesdee]

Hi All

Last night I had a relatively cloud-free sky albeit strongly moonlit, so I tried imaging the Pelican in Ha.  The sky quality was not great but I shot 15 subs at 300s duration with my ASI 1600MM camera set to unity gain and cooled to -10*C.

This morning I quickly stacked the data in Astro Pixel Processor without calibration frames, simply to see if the subs were worth keeping, given the poor conditions. Here is a simple screen grab of the integrated stack with default stretch, but no post processing:

 

When I looked at the magnified image, however, there are myriads of small trails, which don't appear to be due to tracking issues, as the stars are relatively round by comparison:

I frequently process data in this manner to quickly assess the quality of the image but have never seen this before - can anyone shed any light on this phenomenon please?  

Can it perhaps be due to drifting water droplets in the atmosphere lit by the Moon (the sky was quite hazy)?

Thanks for any advice

Dave

 

[geordie85]

Streaky hot pixels? 

[Jonesdee]

I wondered that - I dither every second frame, but with dithering the entire image should move a few pixels from frame to frame.  Looks like it might be clear tomorrow night, so will have another try.  Very odd, unless of course, something has gone seriously wrong with the camera sensor.  I might try shooting some more dark frames to see if anything odd appears in them.

[Laurin Dave]

Maybe dithered hot pixels not being rejected when you stack... I'd just calibrate with matched darks, experiment with rejection and see what happens.  If you weren't aware with the ASI1600 its important to use exposure, temperature, gain and offset matched darks.

Dave

 

[Jonesdee]

Hi Dave, yep with the 1600 I use exactly matched Darks plus Dark Flats and use Sigma Clip when integrating (no BIAS frames in accordance with the recommendations for this camera).  With my 'standard' parameters set in Astro Pixel Processor I integrated several sets of data from the last month or so, and it is only the two most recent sessions that exhibit this issue.  I struggle to believe that it is due to the atmosphere, although seeing was far from perfect and the Moon was bright, hence was using narrowband filters.

As soon as I get a chance I will make some more Dark frames and Bad Pixel Map to see if there has been a recent change in the sensor characteristics and have another look at the Ha subs.  A quick inspection showed consistent results from frame to frame (i.e. no rogue images with trailing or similar which could have been picked up in processing).  I will, however do a 'proper' integration using matched Darks and BPM to see if that rectifies it.

Cheers

Dave

[Jonesdee]

Oh dear - integrating the Ha data again with darks and BPM yields this very streaky result - I believe that this is a known issue with the ASI 1600, so I suspect that mine may have broken

Will send the files to ZWO for their assessment

 

[david_taurus83]

Have you tried using DSS to rule out a stacking issue?

[Jonesdee]

Hi David, I was just hunting through my downloads to see if I still have DSS before downloading it again to do exactly that

Long time since I used DSS so probably will be scratching my head for a while.

Images shot prior to the weekend of 17th/18th Nov all stack fine in APP, with and without calibration frames; something seems to have changed last weekend.

It is forecast to be clear at my place tomorrow evening, so will have another try.  Thinking of imaging with and without dithering, just in case

Cheers

Dave

[Jonesdee]

Same issue after stacking with DSS - a quick stretch of the Autosave file in PS:

I also tried (in APP) stacking  random sets of subs, 4 at a time, and the phenomenon is visible in all stacks.  It is not, however, visible in single subs as far as I can see.

Many of the strange trails seem to be better focused than the rest of the image - I'm wondering if there is a clue there?

Cheers

Dave

 

[Laurin Dave]

Hi...  maybe have a google of asi1600 walking noise, there are some threads on CN and the Pixinsight forum that might help..     

[Jonesdee]

Thanks Dave, I have also seen similar reports on the Astro Pixel Processor Forum - several cameras were replaced, it seems.  I sent the details of my problem to Zoltan at 365 Astronomy (very helpful guy) as I bought the kit from him and he has passed that on to ASI already.  Hopefully will hear back from them soon.

Will Google the threads you mentioned also

Cheers

Dave

[Jonesdee]

Update - I think I have found the cause of this phenomenon.  The guys at ASI responded very quickly and asked me to send some files for examination, including a Flat Frame shot at a Gain setting of 300.  I have always used unity gain (139) so opened the camera ASCOM driver to change this.  I was surprised to see that the gain had defaulted to 1 and could not be altered.  A quick check of the FITS headers of recent images showed that the change from 139 to 1 had happened between late October and early November, during which  time I had upgraded APT to the latest version.  I  uploaded my stored configuration settings without paying sufficient attention to the changes in functionality and completely overlooked the fact that CCD Camera Gain is now controlled by default from the Camera Control panel. To allow the Driver settings to take priority as was previously the case, requires a configuration change which I did not make

Given the low S/N resulting from such a low Gain setting with an Ha filter, I think this explains my issue - hopefully will be able to confirm that tonight if the clouds part for a while.  

Also explains why the seeing has seemed particularly bad lately!

Wiping egg off face . . . . .

[Laurin Dave]

Good news that the camera isn't broken, this lark is complicated enough without software changes messing things up. Hopefully be clear for you later..  it is near Reading and the clear skies are blowing your way

 

Dave

[Jonesdee]

Cheers Dave, about to get the kit outside in anticipation.  I do hope that it is not a camera issue - the gain setting would certainly explain away most ofthe issues I have seen - not sure a bout the trailing, but some images tonight should prove it one way or another

Dave

[ollypenrice]

3 hours ago, Jonesdee said:

Update - I think I have found the cause of this phenomenon.  The guys at ASI responded very quickly and asked me to send some files for examination, including a Flat Frame shot at a Gain setting of 300.  I have always used unity gain (139) so opened the camera ASCOM driver to change this.  I was surprised to see that the gain had defaulted to 1 and could not be altered.  A quick check of the FITS headers of recent images showed that the change from 139 to 1 had happened between late October and early November, during which  time I had upgraded APT to the latest version.  I  uploaded my stored configuration settings without paying sufficient attention to the changes in functionality and completely overlooked the fact that CCD Camera Gain is now controlled by default from the Camera Control panel. To allow the Driver settings to take priority as was previously the case, requires a configuration change which I did not make

Given the low S/N resulting from such a low Gain setting with an Ha filter, I think this explains my issue - hopefully will be able to confirm that tonight if the clouds part for a while.  

Also explains why the seeing has seemed particularly bad lately!

Wiping egg off face . . . . .

I don't think the egg should be seen only on your face. I take the old fashioned view that a product should be sold with its own software and that this software should be good and fit for purpose. I think the customer needs to know where the buck stops. Do ZWO offer a competent control software of their own? I don't know, it's a genuine question but, if they don't, they should get off their backsides.

Olly

[david_taurus83]

1 hour ago, ollypenrice said:

I don't think the egg should be seen only on your face. I take the old fashioned view that a product should be sold with its own software and that this software should be good and fit for purpose. I think the customer needs to know where the buck stops. Do ZWO offer a competent control software of their own? I don't know, it's a genuine question but, if they don't, they should get off their backsides.

Olly

I think they do something called ASICap though I've not used it. Only APT and Sharpcap.

[Jonesdee]

Yes, ASICAP is a capture program from ZWO/ASI which offers full control over all functionality of their cameras.  I use it if I need to image through my scopes in daytime as it is the easiest method I have found to use short exposures on the 1600 MM Pro (for setting back-focus, setting up OAGs, occasional bird watching, etc).

It does not, however, offer the full astro image acquisition functionality that APT, Sharpcap, or SGP provide within a single application.  By that, I mean integrated focus aids, plate solving, Polar Alignment tools, guiding, etc which make those programs so convenient.

Possibly, if I had experimented with different gain/offset settings I may have recognised the impact of minimal camera gain before worrying that the camera was faulty, but with so much to learn and so few opportunities to see a clear night sky in UK, I had only ever used unity gain/50 offset until this was inadvertently changed.

If my weather conditions matched the Met Office forecast, I would be testing the camera again right now, but once again cloud is covering my bit of North Wiltshire.

Dave