QHY8L OSC artifact problem

[Carole]

I've been struggling with my QHY8L with 4 problems, which is why I haven't used it much.  I know you guys like a bit of analysis so I was hoping you might be able to tell me what's going on here.  

1. I've cured the regularly disconnecting cables problem by fitting a retainer clip (poor design fault).
2. I think I have sorted the focus/looping/live view problem, as Ivo has brought out a looping facility with the latest version of APT, though I have yet to try it with a live sky.
3. N.B. the original problem of the faint lines was (I think) due to using the wrong download speed, I have not had the problem since as far as I can see using normal download speed.
4. My final problem is that of dirt/artifacts not getting removed by flats.

I wonder whether you guys could give an opinion as to what is causing this and why aren't flats removing the problem.

Should I get some lens cleaning tissue and try cleaning the lens?

It's not the scope as I have used (on the same evening) a different camera on same scope, and no artifacts there, ditto reducer/flattener. It's not the LP filter as I get the same with or without. I have tried cleaning the glass in the front of the camera, but maybe it could be on the underside. But even if it is a dirty sensor, why aren't flats removing it? In fact it looks like flats are possibly over-compensating, really not sure what is going on.

Don't try to fit the flat exactly over the image as I've cropped them to make it easier to see, and the crops are not necessarily precisely the same.  

Any ideas?



Carole

[RobertM]

It looks a little like your calibration is going wrong somewhere.  As a first step I would take one of your flats and calibrate that as if it were your image.  That would show whether it's the calibration process since it should produce a perfect flat frame.  If it doesn't then you'll have to check the calibration process as something is going wrong (for instance the flats aren't being scaled to the image correctly).

I'll be out the rest of the weekend so can't reply further but I'm sure someone else will chip in somewhere along the line.

HTH
Robert

[MarkS]

As you said, the flat is clearly "over-compensating".
There are two possible explanations:
1) The flats are not being applied properly by the processing - this is unlikely.
2) The most likely explanation is that more scattered light was reaching the sensor when you were imaging than when you were taking the flats.

Is it possible the flats were taken with a dewshield on and the imaging was done with it off?
Was the moon or some other light getting into the scope?

There's some useful (fairly technical) background reading on flats here:
http://forum.orpington-astronomy.org.uk/index.php?topic=9342.0

[RobertM]

Just had a look at the flat in Maxim and it looks a little too flat, can you check a single flat against the master flat.  Flats generally look quite a bit brighter in the middle and not very uniform like that master.  The centre to edge brightness usually varies by around 8-20% depending on optics and sensor size.

Robert

[Carole]

In quick reply Mark and Robert (thanks). 

2) The most likely explanation is that more scattered light was reaching the sensor when you were imaging than when you were taking the flats.

I have had this problem ever since I got this camera, so I can't imagine this would happen every time in differing conditions. 

Is it possible the flats were taken with a dewshield on and the imaging was done with it off?

The flats were done with an EL panel and light box.  I don't have this problem with my Atik camera using the same method.

Just had a look at the flat in Maxim and it looks a little too flat, can you check a single flat against the master flat.  Flats generally look quite a bit brighter in the middle and not very uniform like that master.  The centre to edge brightness usually varies by around 8-20% depending on optics and sensor size.

The flat I posted up was only slightly stretched Robert, this is it stretched quite a lot, and as you can see there is vignetting and it has a lot of dust that it has removed. 



Normally I just process out these artifacts, but it's not the answer really, and I would like to be able to remove the worst bits but do you think they are on the sensor itself?  QHY talk about cleaning it:

You can open the front cover of CCD to clean the surface of chip. You don't have to open the front
cover of CCD( this can be delete because ccd front cover can not be open)
Screw the front cover of CCD counterclockwise, and clean the dust off of the surface of CCD with a manual air pump. If there is dirt that doesn't come off with the air pump, you can use lens paper or a commercially available SLR camera cleaning kit.
Here is the correct procedure to follow with the lens
paper:
1. Wash your hands with soap.
2. Take a piece of lens paper,
and fold it once or twice
(do not fold too many times as
the lens paper will get very
sharp and the edges may
scratch the CCD glass surface)
Cleaning tips
3. Blow at CCD in a breath and use lens paper to
clean the CCD surface. Make sure to maintain an
appropriate pressure for CCD with your hand.
4. Finish cleaning and reinstall the front cover of
CCD. If there is higher relative humidity in your
environment- it is necessary to dry the inside of the
CCD sealed chamber.

I have tried the puff ball method. 

Carole

[Carole]

As a first step I would take one of your flats and calibrate that as if it were your image.

I've done that Robert and the result is clean as a whistle.

I'm mystified.

I have also had a clean of the glass in front of the CCD and it was a bit dirty even though this did not appear obvious to the naked eye, however this still doesn't solve the cause of flats not subtracting these artifacts. 

Carole

[MarkS]

Do you use an IR filter when imaging with this camera?

[Carole]

Do you use an IR filter when imaging with this camera?

I don't add any filters to it Mark except a CLS filter when at home.  However the glass in front of the chip seems to be IR block.

Carole

[Mike]

Is the histogram for the flat clipped?

[Carole]

Is the histogram for the flat clipped?

The unstretched flat isn't clipped Mike.

The one above which I stretched a lot for demonstration purposes I am not sure about:

[MarkS]

Yes, it does appear the QHY8L has IR block:
http://qhyccd.com/ccdbbs/index.php?topic=2972.0

This is a bit of a mystery. 

I'll take a look if you can post a light or two and the master calibration files (flat, dark, bias) in Dropbox or similar.  Make them FITS or TIF files so that there is no loss of data.

Mark

[Carole]

I think they are already in dropbox Mark as some-one else wanted to practice on them on Astronomy shed.

I've shared the folders with you on Dropbox. 

Carole

[RobertM]

That flat does not look right !  Could you post an UNSTRETCED version of both master and a flat I'll take a look at it, , if not resolved before I get back, tomorrow night.

[MarkS]

Carole,

There's a problem with your master dark.

The individual darks have 16 bit integer pixel values of around 2600 but the master dark has 16 bit pixel values of around only 300

The master bias is fine - it has values of around 2300 which is the same as found in the individual offset frames

Using that faulty master dark causes the over-compensation you are seeing.

You need to review what you did to create the master dark.

Mark

[MarkS]

That flat does not look right !  Could you post an UNSTRETCED version of both master and a flat I'll take a look at it, , if not resolved before I get back, tomorrow night.

The flat was OK - it was simply stretched to show the dust.