[peter_4059 (Peter)]

Originally Posted by pvelez

Peter

I had something similar. Turned out my exposures were too short ie the light box was too bright. This resulted in shutter artifacts. This was apparent when I stepped down the light with some plain white material.

Looking at yours however, it seems only the corners so it may be different. How long are your exposures?

Pete

Pete,
The flats I've been using are 4 second exposures however the qhy8 doesn't have a mechanical shutter so it's not a shutter issue.

Originally Posted by RobF

I don't have all the answers (any answers ) Peter, but will be watching for people's thoughts with interest. From a dark sky site I find I can usually wing it better just using dynamic background correction in Pixinsight, but from a city location with greater skyglow or through my Ha filter the vignetting is too pronounced. I think I've seen Marc suggest a way of "backing down" flats to subdue the effect.

Sorry for lack of hard facts though

Rob,
Pixinsight does do a nice job of removing the gradient with the dynamic background extraction however I'm trying to get to the bottom of why the flat is not correcting properly in the first place. The problem with dynamic background extraction is where to put the sample points when the object you are imaging fills the whole field of view (often the case with my setup). Marc did suggest using pixel math to "adjust" the flat and this is the workaround I referred to - if I add 28000 adu to the flat it corrects perfectly. I'm just trying to understand why I can't get the calibration to work with the un-adjusted flat.

Originally Posted by Ross G

Hi Peter,

I had a similar problem with my new QHY8L when I started using it early this year. My flats would give me light corners.
I found doing bias frames as well solved the problem.

Hope this helps.

Ross.

Ross,
I'm using bias and have also tried full calibration - ie lights, flats, bias, dark flat, darks but get the same result.
A few questions:
Are you using a light box?
What software are you using for calibration?
What ADU level are you shooting the flats at?
How long is your flat exposure?
How long is your bias exposure?

Originally Posted by Alchemy

I've used a qhy8 for years no problems....

2 things come to mind

Firstly use a full calibration set, bias flat darks etc, ..... I always do

Secondly color is no issue, mine are always light blue as I do sky flats, it's only the luminosity thats applied, however make sure your flats are a mid range not too bright or too dark.

Clive,
I've tried full calibration but get the same result. What software are you using for calibration? I think I've read DSS calibrates each channel seperately. My flats ahve a range of about 16000 to 36000 ADU. I've also tried iincreasing and reducing the exposure time for the flat however this doesn't appear to make any difference.

Originally Posted by gregbradley

I get this sometimes. My Planewave CDK17 setup is very sensitive to correct flat fielding.

Firstly, I take flats with a white cloth cover over the end of my scope and take about 6 flats in my observatory during the morning usually. The slight filtered light that makes its way into my observatory is perfect for flats as it turns out.

I aim for about 20,000 ADU. If the flat is too bright then I find it overcorrects.

Similarly I take a dark for the same exposure time for each filter's flat.These are subtracted when the flat is done. This is all at an exact temperature - the same as the light exposures you took. Also at the same focus and with the camera in the same orientation as the light exposures.

If you do adaptive darks - that is using a longer exposure dark for a shorter light but at the same temperature you also need a bias. That stops the weird corrections as the dark is scaled properly.

I take 5 or 6 flats darks and flats and then use median combine for the flats and subtract the sigma rejected combined darks from them. I shoot for 20,000 adu and make sure the exposure is long enough so there are no shutter artifacts from too slow an exposure that captures the shadows of the shutter opening.

I sometimes can get away with older flats if I haven't changed anything but I would update my flats if anything changed - camera orientation, filters, camera, changed telescope etc.

At a guess I would say your darks are mismatched to your lights and then your flat is not flat dark subtracted. Or your flats are overexposed.

Greg.

Greg,

Is the 20000 ADU the mean or the maximum value of the flat? My 4 sec flats range from 16000 to 36000 ADU with the mean around 26000. I take 20 flats for each imaging session to ensure same camera orientation and focus position. All the lights, flats, darks, bias etc are done at the same temperature.

I've tried shorter and longer flat exposures and get the same result.
I've always used darks, bias and dark flats and have also tried without dark flats and get the same result.

Based on all the testing I've done it appears the issue is the difference in contrast over the flat image - ie the difference in ADU count between the centre of the image and the corners. My understanding of the calibration process (ignoring darks and bias to simplify) is:
Calibrated light = Light / (Flat / mean of Flat)
(some software uses median or mode instead of mean but I believe these values will be similar in my case as there is a reasonably even distribution of ADU values across the flat according to CCD Inspector flat analysis).

So this means at the corners the light is being divided by 16000/26000 = 0.615 which is equivalent to multiplying the light by 1.625 - ie the dark corners get lightened by 63%
In the centre of the image the light is being divided by 36000/26000 = 1.385 which is equivalent to the centre being darkened by 28%
Somewhere between the dark corner and the light centre where the mean value is the light gets divided by 26000/26000 = 1 - ie. no change.
　
By adding 28000 to the flat using pixel math I've changed the calibration maths to:
at the corners the light is being divided by (16000+28000)/(26000+28000) = 44000/54000 = 0.815 which is equivalent to multiplying the light by 1.227 - ie the dark corners get lightened by 23%
In the centre of the image the light is being divided by (36000+28000)/(26000+28000) = 64000/54000 = 1.185 which is equivalent to the centre being darkened by 16%
Somewhere between the dark corner and the light centre where the mean value is the light gets divided by (26000+28000)/(26000+28000) = 1 - ie. no change.

What this means is the resulting calibrated image has the dark corners lightened by a similar amount to the amount the centre is being darkened whereas with the raw flat the corners are being lightened by twice as much as the centre is being darkened.
　
In any case when I adjust the flats this way and then review the resulting calibrated image using threshold I get a much more even illumination across the image. All the trials I've done so far are on an image of M20 as this was the only recent image I had with some plain "sky" in the corners. My next step is to take some images of a reasonably dark piece of sky with no structure and see if I get even illumination right to the centre.

Originally Posted by multiweb

Your flat needs to have the bias subtracted from it or it won't scale correctly when you divide it in your subs. That's quite an important point.

With the QHY8 I aim for 9500 ADU or where about in the brightest part of the flat.

Marc,

I'm definately subtracting the bias and have tried shorter flat exposures but still get the same result - adding using pixel math is the only thing that has provided a satisfactory result so far. Changing exposure is equivalent to multiplying by a constant (in the linear part of the sensor response which doesn't change the ratio I describe above as all values get scaled by an equal amount.

Originally Posted by Terry B

The other important thing is for the flat to be only exposed within the linear part of your CCD. If you have an antiblooming CCD then it will start to have a non linear response somewhere over 1/2 the full wells. Keeping the max exposure to 1/2 the full wells is a safe way of preventing this. There is an advantage to having the brightest exposure you can as it reduces noise but you must keep in the linear part of the CCD- a fine balance needs to be made.

Terry,

I ran some tests of the response vs exposure duration and I'm pretty confident I'm in the linear range. The results were in this thread...

http://www.iceinspace.com.au/forum/s...highlight=flat