Trius + CCDStack: Is This Correct?

[RickS (Rick)]

Originally Posted by gregbradley

Yes that is bias drift and not a good thing. Rick was using a bias drift bias drift super bias that uses the overscan region of the sensor. I am not sure how you access the overscan region to do that. I am sure there is a tutorial somewhere on the PI site.

The method for enabling overscan depends on the camera driver. In the case of my Apogee there is a "Digitize Overscan" check box in the Apogee GUI under Maxim.

The only software packages that I know support overscan calibration are PixInsight and Mira.

It's certainly worth considering on any camera that shows significant bias drift. Naskies had an SBIG camera with a KAF-8300 that also produced better results using it, so it's not just me

Cheers,
Rick.

[PRejto (Peter)]

Originally Posted by gregbradley

How do you find out you have negative values?
Greg.

I just open the dark in CCDStack., select all of the image, and the info box pops up. It's right there as a min value.

After the comment of Stan that a bias (as a dark) or a dark is required to properly subtract a flat I'm wondering how you are successful. Perhaps yours looks OK because you have a lot of signal. The image I've been unsuccessful with is an OIII with low signal. As I've said it flats fine with a dark....

Have you tried to make a pix map yet? It's pretty cool to see it work. I used a dark frame and did a hot pix rejection. It grabbed all the pix and turned them red. They really stand out against the dark. That gives you the map. Just for fun I used the dark to try interpolation. Like magic they were history and my dark looked a lot like a bias frame..

Peter

[gregbradley]

[QUOTE=PRejto;1192585]I just open the dark in CCDStack., select all of the image, and the info box pops up. It's right there as a min value.

After the comment of Stan that a bias (as a dark) or a dark is required to properly subtract a flat I'm wondering how you are successful. Perhaps yours looks OK because you have a lot of signal. The image I've been unsuccessful with is an OIII with low signal. As I've said it flats fine with a dark....



HI Peter,

I do use a bias when flat fielding. Its in the main calibration box as a checkbox to subtract a bias. The difference was to not subtract the bias when making the master flat but rather subtracting a master bias from a master flat at the time the flat is applied. It seemed a fine distinction but it gave better results on the CDK which as I say is fussy compared to refractors which are not fussy.


By the way flats are divided not subtracted.
That's why a flat with a bias does not do the bias on the image twice when you also do a bias subtract.

Thanks for the info on the hot pixel map I will try that. Does it also do a dark column?

I think also we should make it clear this discussion is primarily about calibrating a Sony sensor not a Kodak one. Kodaks need darks, biases, flats etc. A clean filtered Sony at around -20C may only need a bias and perhaps a pixel map.

Originally Posted by RickS

The method for enabling overscan depends on the camera driver. In the case of my Apogee there is a "Digitize Overscan" check box in the Apogee GUI under Maxim.

The only software packages that I know support overscan calibration are PixInsight and Mira.

It's certainly worth considering on any camera that shows significant bias drift. Naskies had an SBIG camera with a KAF-8300 that also produced better results using it, so it's not just me

Cheers,
Rick.

Thanks Rick.


A question.

If you rotate your camera and filter wheel, reducer etc together but not the telescope are the flats still valid? Adam Block and CCDstack imply they are not but I got the idea from a Roland Christen post they are. I guess the camera and filter wheel for dust donuts are still in the same orientation. My experience here would be what Roland says that the uneven illumination is more from the camera, filters dust and anything in the optical path like an OAG prism. If they al rotate together then the scope is the same as any scope I have used seems very evenly illuminated except some have a bit of bright centre spot but that rotates evenly.

Greg.

[RickS (Rick)]

Originally Posted by gregbradley

If you rotate your camera and filter wheel, reducer etc together but not the telescope are the flats still valid? Adam Block and CCDstack imply they are not but I got the idea from a Roland Christen post they are. I guess the camera and filter wheel for dust donuts are still in the same orientation. My experience here would be what Roland says that the uneven illumination is more from the camera, filters dust and anything in the optical path like an OAG prism. If they al rotate together then the scope is the same as any scope I have used seems very evenly illuminated except some have a bit of bright centre spot but that rotates evenly.

Greg,

My experience is that rotation of the whole camera/FW/OAG assembly doesn't affect flats on the systems I use. However, this would not be the case on a scope where the rest of the optical system introduced some vignetting, especially if the camera isn't perfectly centred. I guess it's something you need to test. The old calibrate a flat with a flat trick would come in handy...

Cheers,
Rick.

[PRejto (Peter)]

Hi Greg,

This is what Stan wrote:

"Sony CCDs have remarkably uniform dark current (pixel-to-pixel) and often do not need dark subtraction. In fact, dark subtraction may inject more noise than it cures (that's why it is advisable to use a very high quality dark). But when avoiding dark subtraction, it is necessary to remove the bias or bias level (pedestal) in order for flat fielding to work properly. And it might be useful to mask out a few consistently bad pixels. This can be implemented in CCDStack."

So, I don't think he is talking about the choice of bias subtraction whilst making a dark vs. doing the bias subtraction when running the calibration tool. I think he is speaking about actually using a dark (or a bias frame as a dark).

Yes, you can make a pixel map and map the columns. You just use different rejection tools. There are examples of doing it free hand in Adam Block's tutorial.

Peter

[gregbradley]

Originally Posted by PRejto

Hi Greg,

This is what Stan wrote:

"Sony CCDs have remarkably uniform dark current (pixel-to-pixel) and often do not need dark subtraction. In fact, dark subtraction may inject more noise than it cures (that's why it is advisable to use a very high quality dark). But when avoiding dark subtraction, it is necessary to remove the bias or bias level (pedestal) in order for flat fielding to work properly. And it might be useful to mask out a few consistently bad pixels. This can be implemented in CCDStack."

So, I don't think he is talking about the choice of bias subtraction whilst making a dark vs. doing the bias subtraction when running the calibration tool. I think he is speaking about actually using a dark (or a bias frame as a dark).

Yes, you can make a pixel map and map the columns. You just use different rejection tools. There are examples of doing it free hand in Adam Block's tutorial.

Peter

Thanks Peter. That's good data. Is this on the support forum for CCDWare?

Originally Posted by RickS

Greg,

My experience is that rotation of the whole camera/FW/OAG assembly doesn't affect flats on the systems I use. However, this would not be the case on a scope where the rest of the optical system introduced some vignetting, especially if the camera isn't perfectly centred. I guess it's something you need to test. The old calibrate a flat with a flat trick would come in handy...

Cheers,
Rick.

Testing is good to know for sure. Not sure what the calibrate a flat with a flat test is. They null each other out?

Greg.

[PRejto (Peter)]

Hi Greg,

Yes it is on the CDDWare forum.

I'm also curious what a flat with a flat test is.

Peter

[RickS (Rick)]

Originally Posted by gregbradley

Not sure what the calibrate a flat with a flat test is. They null each other out?

The purpose of flat calibration is to undo the effects of imperfect optics (vignetting, dust motes, etc.) and also variations in the sensitivity of the sensor. So, if you calibrate a flat with a flat or flat master you should get a result which represents the original even illumination which created the flat. If there are significant variations in illumination or visible structure then there is a mismatch.

So, take some flats at one orientation, rotate camera/FW/OAG and take some more. If a master flat from the first orientation produces a clean result when calibrating a flat from the second orientation then you know that your flats aren't affected by rotation (maybe try a few different orientations just to be sure...)

Cheers,
Rick.

[PRejto (Peter)]

Thanks Rick. That makes a lot of sense and worth trying!

Peter