QHY8L OSC artifact problem

[Carole]

Thanks for all your help Mark, I think if you can't solve it then it is unlikely I am going to find the answer.  Gonna have to find a way around it somehow, or just carry on processing out those artifacts.

Carole

[MarkS]

Sorry - I meant I was using 3.3.2

I've now downloaded 3.3.3 Beta 51 and still see the same posterisation.  I think this is a combination of 2 problems:

Problem 1:
Carole, I think you have a problem with your darks.  The ADU count in the dark files is 2600 whilst the bias files are around 2300.  So the dark current is giving an ADU count of 2600-2300 = 300 in 600seconds.  I think the gain on the QHY8 is around 0.5e/ADU so that means the dark current is 150 electrons in 600 seconds.  That is about the same as my uncooled Canon 350D.  It tells me that the camera's cooling was not operating when the darks were taken.

Problem 2:
I'm beginning to think there is something strange happening in DSS's calibration calculations.  Maybe I've got a setting wrong somewhere because I now have a set of files that ought to produce sensible results but I'm still seeing clipping to black and posterisation. I might design some test files to put through DSS to reverse engineer its calculations and determine once and for all if anything is amiss.

Mark

[Carole]

Thanks for all your work Mark.  The darks are old ones from the library, I could run another set and see what I get. 

APT temperature control wasn't as good as Artemis when I did these and normally I watch the exif readings, but being darks I might have walked away.

Carole

[MarkS]

I forgot to say Carole, that in addition, I don't understand why AstroArt is giving you those bright patches.   I suspect that it is being upset by your faulty darks and is trying to cope as best it can.  Shoot a new set of darks and you might see a big improvement.

BTW,  your file names indicate that your bias frames are shot at 0.1s.  Is that the fastest shutter speed available?  If not then you should also shoot a new set of bias frames at a faster speed.

I don't think this will solve the DSS problem but it might give you much better calibration in AA.

Mark

[Carole]

Thanks Mark, I'll see how low APT will go in shooting Bias. 

Not had a chance yet to do another set of darks, I was puzzled too by the bright spot in the centre as I didn't get that with the first stack I did with DSS on the laptop.  All very weird.

I also don't understand why I am getting the posterization problems with various bits of stacking in DSS with the Atik camera, but that's a different issue.  I'd switch over permanently to AA except sometimes strangely DSS does a better job than AA, so I use whichever one works best on each object.

Carole

 

[MarkS]

I also don't understand why I am getting the posterization problems with various bits of stacking in DSS with the Atik camera, but that's a different issue. 

I'm hoping to get some time tonight to investigate the DSS "background clipping" and "star posterisation" problems by generating some synthetic test files.  It will allow me to check the calibration calculations that DSS is performing.

[Mike]

I'm sure APT automatically chooses the shortest exposure for Bias frames. Same as when doing flts it automatically chooses AV mode.

[JohnP]

Blimey Mark its a good job you don't charge by the hour Carole would owe you £1,000's...

Carole - no idea what's causing all your problems but if Mark can't solve it I doubt there's a man that can - have to agree with Fay though need to give your kit a bit of a clean...

[Carole]

I've cleaned it John, I need to take some more flats to see if it's made any difference.  Cleaning the IR glass did not get rid of the artifacts, so I cleaned the chip as per QHY instructions. 

Carole

[MarkS]

I've solved the " background clipping" and posterisation problem I was seeing with DSS.

Experiment shows that if you provide only one dark, bias or flat then DSS assumes they must be the Master dark, bias or flat.  The trouble is that it assumes the Master dark has already had the Master bias subtracted because when DSS creates Master darks, it subtracts the Master bias.

So if you provide only one of each file it does the following:
  calibrated light = (light - dark - bias)/(flat-bias)

So the bias gets subtracted twice (because the bias is already inherent in the dark) and hence the background clipping.

I was providing single calibration files for speed!

However, Carole was seeing background clipping for a different reason:  the darks had higher pixel values than the lights - possibly because the cooling was not operational for one reason or another or possibly for some other odd reason, yet to be determined.

So that's that almost sorted!

[Fay]

 

[Carole]

You are so good Mark, you never let a problem beat you.  I thought something else was going on because I kept getting a message from Dropbox saying various master files had been updated, I had a feeling you were doing some more experimentation.

Experiment shows that if you provide only one dark, bias or flat then DSS assumes they must be the Master dark, bias or flat.  The trouble is that it assumes the Master dark has already had the Master bias subtracted because when DSS creates Master darks, it subtracts the Master bias.

when I was in France I was using Ollys stock master darks (probably claibrated in AA), so I have no idea whether bias had already been subtracted, however it was only the Ha I had problems with posterisation, no problems with any of the other filters. 

I've had rather a hectic few days and still have for the next couple of days, so will re-shoot those flats and bias over the weekend. 

I'm sure APT automatically chooses the shortest exposure for Bias frames. Same as when doing flts it automatically chooses AV mode.

I have a feeling that I forgot how to create a new file from the list in APT (i.e. when no imaging plan has been set before in the Bias) and just experimented with length of subs for the Bias in the darks menu, I've now remembered how to do that so will do them again.

Carole

[MarkS]

You are so good Mark, you never let a problem beat you. 

I'm obstinate, that's all.  Sometimes it's a good thing and sometimes it's a bad thing. 

[RobertM]

So, let me get this straight - it was operator error 

[Carole]

So, let me get this straight - it was operator error

No Robert, Mark hasn't got to the bottom of the artifacts problem, only another DSS stacking problem he found along the way.

Carole