#1  
Old 05-08-2012, 02:17 PM
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Flats and the pesky B filter

Stop me if you've heard this before. I can't get my flats right.

It presents as an excess of blue in one side of the image - the left, particularly the lower left corner.

I had thought it was LP, a dodgy filter, a dodgy lightbox, poor calibration technique or something else. So far, I can't pin it on anything.

The colour gradient is almost exclusively in blue - I don't think LP is wavelength sensitive. I'm happy to be told that I am wrong on this.

The lightbox is excellent - one of Exsfo's. I've rotated it and the flats and results are the same.

I have reduced the lightbx brightness, thereby avoiding shutter artifacts. No change.

Internal reflections? Nuh. I have the same issue with the Tak FSQ106 and the RC8.

The filters are ok I think - I plan on rotating them between sets of images to see if there is any difference. When the clouds part I'll try this out.

I'm very new to PI and have still to learn the ropes. Having said that, I have the same result in Maxim. The flats have master bias and darks subtracted. The darks are scaled.

I've looked at the raw flats - the dynamic range of the B flat is no different from the R.

So what else might it be?

To give you an idea, I've attached a screenshot of 2 subs. On the left is B, the right G. Both are 180 seconds and have been calibrated. This was last night near a full moon so there is a gradient obvious.

If you look closely, the dark section to the left in the corner is much smaller than the corresponding corner of the G sub. In other words, there is more signal in the B than the G. When combined, I end up with and indigo background here.

I have stretched both images to the same extent using the STF in PI. The images were taken consecutively.

So what am I doing wrong?

I'm open to suggestions - very open.

Pete
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  #2  
Old 05-08-2012, 02:27 PM
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In case it helps, here is a comparison of calibrated and uncalibrated images.

The image on the left is the raw image, the right is calibrated. You can see the difference in the bottom right corner.

Pete
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  #3  
Old 06-08-2012, 06:35 PM
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These were taken with the QSI camera?

This looks like a shutter problem.
How many seconds were each flat frame?

I know that my QHY camera needs a good 2 seconds of exposure to not capture the shutter. Anything less and i get a darker bar down the side of an image.

You can check this by taking a flat file, then stretching it. If there is a black "bar" there then try increasing exposure time to over 2seconds. Is the black bar gone or still there?
If it is gone, make sure your ADU for the flats are OK. If it is too high then you will need to find some way to reduce the light coming in to the scope from your lightbox/source.

If the black bar is still there @ 2seconds,try 4 seconds. If it's still there, then you may have some vignetting caused by something (only other thing i can think of).

My first thought though would be the shutter.
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Old 06-08-2012, 06:56 PM
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Thanks Nathan

I'm pretty sure its not the shutter. With the B filter, I'm out to about 5 seconds which is plenty long enough for the QSI. My old SBIG had difficulties at anything less than 1 second but the QSI can handle that.

But I appreciate the suggestion.

Pete
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Old 06-08-2012, 07:05 PM
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So I've been doing some testing with this issue over the last few nights.

The attached screenshot compares the initial image (right) with the same subject with the B filter rotated 90 degrees. The images are the background extracted by the DBE function in PI. They have the same level of screen stretch.

To me, the deep blue has been shifted clockwise.

I've just imaged with the camera rotated to see if its an issue with the camera. Next is to image after rotating the filter by an extra 90 degrees ie 180 degrees from the initial position.

Will be interesting to see what happens. My money is on LP rather than CCD defect or filter problem.

Pete
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Old 07-08-2012, 11:35 AM
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Are you able to upload a single R, G or B frame and a flat frame so we can have a look?
unedited that is, raw FITS file.
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Old 09-08-2012, 09:24 PM
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Licked - I think

OK - I think I understand my problem.

Its very simple really - and has nothing to do with flats, filters, LP etc etc.

My camera, like all CCDs, does not respond equally at all wavelengths of visible light. The camera is most sensitive in the green and tails off gently towards the red. In blue however, it has a rather sharper cut-off.

So when I take images of the same integration time, I have more signal in green, slightly less in red and much less in blue. How much depends on the colour of the target.

All image processing programs cater for this. Maxim allows manual adjustment to the channel weights when combining - the G2V star calibration provides a good guide to the appropriate weightings. PI allows for this too although the autostretch function in STF does this for you by allocating a weighting to R, G and B.

The key however is that the level of signal in B is still quite low and needs to be stretched to match the G and R ie normalised. This level of stretching is excessive, particularly in low signal areas - I guess where SNR is high. When this happens in the bright areas of the image - no problem as there is still plenty of signal to play with. But in the dark areas, there is less margin for error. The noise in the image gets stretched as well ie its brightness is boosted - so that areas that should be dark are in fact lighter than they need to be. The same thing happens in R and G but because the B is that much fainter, the stretch is that much greater. So I am boosting the noise much more in B than R and G.

This explains why the excess blue is in the fainter areas of the image - which happen to be near the edges of my images.

I am sure I'm not explaining this well. Hopefully the attached images illustrate the issue.

The images are, in order, raw uncalibrated frames in R, G and B respectively. Next to them is the stats for the frame. Note how much lower the Median is for the B frame ie overall it is much darker. I have stretched each image by the same amount. The brightness of the R and G are similar while the B is much dimmer. There is less dynamic range to work with so the dark areas get dragged up too much - resulting in a blue hue to the shadows.

Now I am sure that applying a flat to the images complicates things. Unlike the lights, the integration times for the flats are scaled. This is because the integration time is matched across all filters by the ADU in each flat. So that I have normalised the flats through changing the integration times. In contrast, the lights are not normalised in this way. So - I think- that when PI flat calibrates, it applies a high dynamic range flat to a low dynamic range light. So flat calibration compounds the problem.

Does this all make sense?

I picked this up while playing with the raw subs. At the suggestion of a wise mate I combined a single R, G and B frame of 180 seconds - uncalibrated - in a variety of orientations (rotating filters, rotating the camera etc) and found no real pattern other than that the excess blue was always in the shadows.

So how to deal with this? I have 2 ideas.

The first is to simply up the integration times for the B filter. Taking a few G2V measurements, my weightings should be roughly 1:1:1.6. So I increase my B subs to 288 seconds from 180 seconds. That should give me a better dynamic range (and reduce the SNR in the darker areas). I can then let PI autostretch and use DBE to clean up any residual gradient.

The second is to increase the integration times across all subs. This seems less attractive for me. I am near the Sydney CBD and LP is heavy duty here. I'm also near a flight path and longer subs means greater prospects of plane trails.

I'd appreciate any thoughts on this - but I'm pretty sure I'm on the right track. Among other things, it explains why the colour balance in bright areas of my images are usually OK.

Pete
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  #8  
Old 09-08-2012, 09:30 PM
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Just to add to that last (long winded) post - here are the raw G (left) and B (right) frames. Note how much noisier the B frame is.

You'll also see (if you squint) that while the G frame has many more stars, the background in the B looks more extensive (see in particular the top half of the right edge of the frame and also the bottom right corner).

Pete
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