Calibrating. At wits end

[LukeB]

Hello,
I hope someone can point out what I am doing wrong. 

I have been trying to calibrate a set of three Ha subs but no matter what I do I can't seem to remove the hotter pixels in my images.

They are 900 seconds subs and I also created a new master dark(10frames) and bias(20frames).

All were acquired at -15C and the darks also at 900 seconds.

Basically I followed all the normal steps.

I even went back to the turorial files and followed the steps and setting as suggested to create my master dark and bias frames.

I did not use flat frames.

Following the calibration, I open the resultant _c file to examine but it appears that apart from making the frame appear noisier it does not seem to have removed any of the hot pixels.

If I examine the master dark frame, all the hot pixels overylay the light frame perfectly.

I know I must be doing something stuipd but I just can't seem to figure out what 

I used the following settings as outlined in Vicent Peris tutorial to create the master dark and bias frames.

Any suggestions would be greatly appreciated.

Cheers,
Luke

[georg.viehoever]

Hi Luke,

In my frames, it is not calibration that removes hot pixels. That generally happens due to pixel rejection in integrating the calibrated lights. The key here is that (thanks to my less than perfect mount), the hot pixels appear at different locations in the star aligned images, and thus are rejected as outliers.

When I am not using the mount (e.g. when shooting star trails or wide fields), the hot pixels are always at the same location, and appear in the integrated pictures despite the use of bias, darks and flats. I guess that the darks alone are not sufficient to remove them. I use the Defect Map process in this case to remove them. You can use your master dark to create a defect map.

Georg

[LukeB]

Hi Georg,
Thank you for replying to my post.

I haven't tried to do that before.

I guess I always thought that the whole purpose of dark frames was to remove defects like hot pixels that occur at the same place in every frame and so I was really surprised to see them still there.

I makes me ask if it's really worth all the time and effore to take dark frames if they don't seem to do very much?

Of course when hot pixels are not removed by the dark frame and I use Star Alignment, due to a slight shift in each image (caused by less than perfect polar alignment), I end up with a hot pixel trail in the integrated image.

I'll have a look at creating a defect map and see if that helps.

I just can't understand why I'm having so much trouble with this at the moment.

Dark subtraction seemed to be working a treat before for my nebula images, but now that I'm starting to try my hand at some Galaxies, I've come to a grinding hault.

Thanks again for your suggestion.

Cheers,
Luke

[Silvercup]

Hi Luke:

Try a median combine, this should get ride of hot pixels and there is no great difference with Average.

Silvercup

[georg.viehoever]

I agree with Silvercup: Median combine usually takes care of any hot pixels.

Darks are still important: There are often relatively cold hot pixels, and even properly operating pixels show different dark currents. These are taken care of by darks.

For your initial experiments with stacking I would recommend DeepSkyStacker http://deepskystacker.free.fr/english/index.html, that has is much easier to use than PI for this purpose. PI has many more tuning knobs than DSS, but the DSS results are usually not far from the best you can get with PI. General procedure:
- load lights, darks, bias, flat to DSS
- Select all images
- register selected images
- check recommended settings
- select "Stack after register", select 80% for stacking
- press OK and wait for some minutes
- Save result as TIFF
- Do the postprocessing in PI.
That's what I do in most cases.

Georg

[LukeB]

Hi Georg, Silvercup,
I have tried Median combine and although I get varing results, it's still not very good.

The strange thing is that the normal calibration procedure works really well with low noise subs (high signal targets) like M42 and Carinae.
Here almost all the hot pixels are removed by calibration.

My problems started with Centaurus A and even at 900 seconds the Ha still has a low signal to noise ratio.

I did manage to get an almost satisfactory result with image integration with a sigma high value of around 1.000.
Here it managed to get rid of most bad pixels but not the ones in a part of the image (at the 'top' of the image) that displays some light polution.

Another interesting experiment I did was to use DBE on the raw light frames before calibarting.
I thought that by evening out the background I may be able to get integration to remove the pixels in the light polution area.

Bad idea. The results when I calibrated the DBE frames was a totally light gray image with the stars and galaxy all bloated white.

I will persist for a while longer to try to sort this out.
I suspect that it may be because even at 900 seconds, the subs signal to noise is too low for the cal to work properly.

I will also give DSS a try and compare the results.

I'll let you know how I go.

Thanks again for your help.

Cheers,
Luke

[georg.viehoever]

I always do DBE on the integrated result. I dont think it makes sense to use DBE on the subs.
Georg

[LukeB]

Hi Georg,
I know, but I was desperate 

I just tried DSS and the result was great.
It removed ALL the troublsome pixels. But, for some reason it created an RGB result 

That is, all the background looks RGB.

I've never really tried DSS before and I'm sure that it is just the settings.

I'll have a look at the DSS manual.

I would like to get PI to do as well though.

Cheers,
Luke

[georg.viehoever]

I dont know what type of camera you have. DSS works perfectly fine for my Canon EOS40D (.CF2 files). If your camera uses FITS files, you'll probably have to experiment with the RAW/FITS settings,

PS: The gradients that you still see may also be caused by the missing flats. Flats are always a good idea.  The remaining gradient then may be from light polution.

[LukeB]

Hi Georg,
I have a QSI583ws.
It's a monochrome CCD camera.

It looks to me that DSS is really for DSLR cameras rather than mono ccd cameras.

I can't find anything that allows me to tell it that I'm using a mono camera and it only seems to produce RGB files.

Looks like I'll have to keep trying PI. 

Maybe one of the PI gurus will chime in and offer some ideas?

Thanks again for your help. 

Cheers,
Luke

[pfile]

i think DSS can handle mono files... i've defintely calibrated mono files with DSS, can't remember if i stacked them.

[LukeB]

Hi pfile,
I checked the DSS calibrated files and they are still mono and so it must be in the stacking process the for some reason turns them into RGB?

Not to worry though, I have ImagesPlus and that did a good job of both calibration and combining.

I think that most of my problem is that I don't have enough data to get the snr up to a reasonable level.

Thanks to all for your replies.

I'll keep learning 

Luke

[Nocturnal]

DSS works equally well for mono and RGB images. Of course you need to know how to operate the program.

[LukeB]

Hi Sander,

Ahhh! I thought there was something I was missing

Cheers,
Luke

[Nocturnal]

Hi Luke,

join the deepskystacker yahoo group if you need assistance making it work. I co-moderate it.