Banding in image S/N issue using DSLR

[David010167]

Like a lot of people I am new to astro photography. My first attempt was not too bad (to me) with a stack of three images.

My next attempt has a lot more noise and banding when I stretched the image. This is a stack of 3x900 and 7x600 seconds at ISO800 on an unmodified Canon 7D. I also did 20x 900 dark frames and 20x600 darkframes.

Would this be what you would expect to see, or is this a case of poor quality subs?

I am not sure if this stack is worse due to poor subs or something else.

regards

David

[Glen]

Hi David, have you tried bias frames? Its just a matter of shooting 20 odd at the fastest exposure 1/4000 with the lens/apeture covered then subtracting those the same as darks, dss has the option.

Regards

Glen

[Glen]

Just to add that darks need to be taken at the same temp as lights/subs so no good

setting up then bringing the camera out from a warm house, running off loads of subs

then taking darks at the end of the session. Can add upto a large gap between temps.

as i found out letting the camera acclimatize gets better results.

Glen

[Psychobilly]

I would be inclined to stack it with just lights and bias and see what you get... any hot or cold pixels can be taken care of in DSS (in the cosmetic tab)

Billy...

[David010167]

I have not tried bias or flats yet. I may give this ago tomorrow and see what happens.

The darks I took were done the same night. The subs, I left the camera outside in a plastic box.

Thank you for the responses.

David

[steppenwolf]

You could take the BIAS frames now and apply them to your current data to see what effect they have. Don't underestimate the value of 'calibration frames' as these are key to getting the best results from your hard won data. 'Flat frames' for example would have an enormous effect on the vignetting on your image.

I have applied a couple or corrections to your image to simulate a BIAS frame and FLAT frame:-

[Psychobilly]

Have a look at your darks .. if the 7D is anything like the 500D you can often get away with just letting DSS take care of the hot and cold pixels... I prefer to spend more time taking lights... a sure fire way of getting more "useful" signal...

I will always use bias and nearly always try and get flats...

Billy...

[jgs001]

Dave, did you stick all the frames into the same group in DSS? I'm not sure, if you did that, quite what happens to the darks.. I'd set the 300s lights and darks in group 1 and the 600s lights and darks in group 2 and restack.

To get group 1 to appear, add something to the master group, you can remove it afterwards, but you'll see group 1 appear at the bottom. To get group 2, add something to group 1.

This forces DSS to only use the correct frames for the correct images. I normally use this for multiple nights, but it'll work just as well for the same night and different exposures.

[opticalpath]

David ...... what the others said about calibration frames.

Also, you will see a big difference if you can increase the number of frames/ total exposure time. As you stretch the summed image, you quickly see the effect of noise building up. More frames gives you a higher s/n ratio image that can be stretched further before noise overwhelms it.

If you can be bothered moving the image in the frame slightly between exposures - just a few pixels - the fixed vertical banding pattern is smeared out and less visible in the aligned summed image.

Adrian

[AlexB]

Is there a sticky post or similar which explains the use of Dark's, Flat's and Bias and how to make them? It's been a while since I read 'Make Every Photon Count' but I don't remember these different shot types being mentioned. But that's probably just poor memory

[Glen]

AlexB, Check RobH site in primers and tuts

Middlehill Observatory, Dorset UK. The Astrophotography of Rob Hodgkinson.

Hope this helps

Glen

[AlexB]

Thanks Glen, I'll give that a look.

[Glen]

Welcome , any questions just shout

[David010167]

I tried adding bias frames which helped a little, but not a great deal. I am starting to think that this set of images may not be as good as they could be, which would not surprise me with the limited breaks in the clouds I have had over in Surrey. The last few times also suffered from high humidity/dew. I have now got two dew bands and a controller for the next time I can see any sky and will try to get more images of 600 seconds and try again.

Like most things it is an exercise in learning and trying to improve each time.

Now for a glass of wine as all I can see are pink clouds out there.

Regards

David

[Glen]

Hi David, thinking about it more subs would realy help like opticalpath says.. also the ratio of darks to lights may be actually adding noise, i might be wrong.

Just to add while its overcast might be worth having a play with taking darks at diferent iso/times ect and seeing the results.

Enjoy

Glen