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Flats problem


martin_h

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I have never had problems with flat fields before but today for some strange reason its all gone belly up. I use flats around 22K ADU and have never had a problem, but after last nights imaging run my 22k flats made a right mess! and seem to be over correcting - but - 50K flats seem better, but not perfect........ ideas?

The attached image shows no flats, 22K, 50K

flats.jpg

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It's really not possible to say what the cause may be from the end result of a preprocessing run.

To evaluate a problem like this you really need to be able to see the raw data for lights, darks flats and bias.

You could try running again with only flats and lights, no darks or bias, to eliminate those from the possibilities.

In the end you will probably have to inspect and measure the flats individually to ensure they meet the requirements for a good calibration, look at the fits headers for the flats and check was the cooler running and is it at the correct set temperature.

There is a post here that details how to measure a flat using P.I. it was written for checking and adjusting DSLR flats but the process applies just the same to mono greyscale, you just measure and record the single luminance channel for each filter instead of being able to check all three colour channels simultaneously as in a RGB DSLR or OSC flat.

Sometimes you just "acquire" a bad set of flats, maybe the light source was not quite the same, maybe a different colour temperature etc etc.. next time out you follow the same procedure and everything works just fine. Careful examination of the individual flat frames should reveal the answer.

The use of ADU or histograms is often quoted as a panacea to obtain good flats and I have been guilty of that in the past myself but unless you know the distribution of those average digital units in the flat it can lead you up the garden path. Once you measure and understand the distribution of pixel values in the flat and can relate that to a particular ADU or histogram shape then you can use the ADU in future or look at the histogram shape to determine if the flat exposure was good or not, but this will only be relevant to one particular telescope, camera, set of filters, light source for flats etc, you can't say for example 22K ADU works for my 8" Newt and DLSR and expect that to be true when imaging with a QHY9 and 100mm frac.

I have been imaging recently with a TS 100mm Quad and it has the most bizarre flat field I have ever seen, not a gentle rolling landscape like the South Downs, more like Mont Blanc, nothing wrong with the scope, it is just a result of the optical configuration of the quad lens assembly, but it took a lot of experiment with different flat exposures before I was able to capture the dark vignetting edges with sufficient headroom above the noise floor and not clip the centre peak.

H.T.H.

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On 11/3/2016 at 12:04, Oddsocks said:

It's really not possible to say what the cause may be from the end result of a preprocessing run.

To evaluate a problem like this you really need to be able to see the raw data for lights, darks flats and bias.

You could try running again with only flats and lights, no darks or bias, to eliminate those from the possibilities.

In the end you will probably have to inspect and measure the flats individually to ensure they meet the requirements for a good calibration, look at the fits headers for the flats and check was the cooler running and is it at the correct set temperature.

There is a post here that details how to measure a flat using P.I. it was written for checking and adjusting DSLR flats but the process applies just the same to mono greyscale, you just measure and record the single luminance channel for each filter instead of being able to check all three colour channels simultaneously as in a RGB DSLR or OSC flat.

Sometimes you just "acquire" a bad set of flats, maybe the light source was not quite the same, maybe a different colour temperature etc etc.. next time out you follow the same procedure and everything works just fine. Careful examination of the individual flat frames should reveal the answer.

The use of ADU or histograms is often quoted as a panacea to obtain good flats and I have been guilty of that in the past myself but unless you know the distribution of those average digital units in the flat it can lead you up the garden path. Once you measure and understand the distribution of pixel values in the flat and can relate that to a particular ADU or histogram shape then you can use the ADU in future or look at the histogram shape to determine if the flat exposure was good or not, but this will only be relevant to one particular telescope, camera, set of filters, light source for flats etc, you can't say for example 22K ADU works for my 8" Newt and DLSR and expect that to be true when imaging with a QHY9 and 100mm frac.

I have been imaging recently with a TS 100mm Quad and it has the most bizarre flat field I have ever seen, not a gentle rolling landscape like the South Downs, more like Mont Blanc, nothing wrong with the scope, it is just a result of the optical configuration of the quad lens assembly, but it took a lot of experiment with different flat exposures before I was able to capture the dark vignetting edges with sufficient headroom above the noise floor and not clip the centre peak.

H.T.H.

I have just run a test using your method of assessing a flat and it has made no difference, the flat was well below saturation and above the noise baseline, I averaged out 30 new flats and calibrated them with 100 new bias frames averaged to get a master, and visually the stack looks just the same with or without the flats... hey ho! 

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Did you try a calibration run using only bias, flats and lights? do not include darks in the calibrate sequence ands see how it comes out.

If you have a Dropbox or Google Drive set up you could post a link to four folders containing around ten each bias, darks, flats and lights in fits format, If I don't pick them up I'm sure one of the other imagers on the forum will take a look for you and try and come up with an answer.

It might even be that the offset and gain for the camera needs to be re-characterised to account for ageing or drift of A to D and amplifier boards, or the camera offset and gain settings have been corrupted or altered in some way in the acquisition program due to a Windows Update or acquisition program update etc.

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1 hour ago, Oddsocks said:

Did you try a calibration run using only bias, flats and lights? do not include darks in the calibrate sequence ands see how it comes out.

If you have a Dropbox or Google Drive set up you could post a link to four folders containing around ten each bias, darks, flats and lights in fits format, If I don't pick them up I'm sure one of the other imagers on the forum will take a look for you and try and come up with an answer.

It might even be that the offset and gain for the camera needs to be re-characterised to account for ageing or drift of A to D and amplifier boards, or the camera offset and gain settings have been corrupted or altered in some way in the acquisition program due to a Windows Update or acquisition program update etc.

I only use Bias and flats because I use a bad pixel map. I will put up some data later........... I have just cleaned the sensor and scrapped the lights from the other night.

Will check the Gain and Offset tho.

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