Noise, Pollution or Other?

[Chris996]

Hi Folks.

Just wanted to check what people think might be causing the noise in the image below (NGC2403). 

I keep getting similar problems with noisy background and would like to improve what I'm doing. I get this a similar pattern of noise on all filters, I'm imaging from just outside Bournemouth, but the back garden is really dark - this image is from last night.

I've tried different background subtraction options in Pixinsight, but it feels like it needs correcting before that?

Camera: Atik 314+

Scope: ED80 w/ Focal Length reducer & manual filter wheel

Image: 30 x 2min subs. Lum Filter. Stacked in PIxinsight

Processing: No Dark, Bias or Flats applied - although I've tried these before and they didn't seem to help much, other than with the vignetting.  Easy could have done something wrong tho!

Any help much appreciated - great forum btw!

 

 

[vlaiv]

Proper calibration is very important.

You need to sort that out first.

What is wrong with your calibration, can you describe the process and explain why you think it's not working?

[Chris996]

Hey, I was just wondering whether the uncalibrated image is looking right.  Calibration steps I've done before are:

Dark calibration I havn't done so far with the Atik because it seemed the least of my worries having moved from a DSLR - the odd hot pixel felt livable for now, until I get the overall noise sorted.

Master bias I've built up (I use the same master for each image & session) and have used that in calibration, but it makes little difference to the final image.

Flats - I havn't taken these so far, mainly because I havn't got a decent lightbox I can use for each imaging session at night and I was concerned my lights weren't up to scratch / worth it.  Am I right in thinking I need a separate flat for each filter, for each session? (before changing focus)

 

[vlaiv]

I don't see nothing obviously wrong with the image apart from background. Stars look good, and there is "normal" amount of noise. Image is certainly stretched beyond what the data allows - but it's a good way to show any problematic areas.

Proper calibration should yield nice uniform background (noisy, but uniformly noisy). So no gradients like present in this image.

You have right side that is darker - like a band on right hand side. There is also one on the left, but further away from the edge. Bottom edge is brighter than it should be.

I suspect that all of that is due to lack of calibration performed.

If you have permanent setup and filter wheel that is accurate in positioning - you can take on set of flats for each filter, just make sure you do it when your scope is properly focused. Otherwise you will need to do flats for each filter at the end of lights (this will make sure your focus is right).

I tear down my setup each time and I do a set of flats on each filter change. Luckily I work with CMOS sensor (fast download time) and have very strong flat box - it takes something like 10-15 mins to do full set of 256 flats.

[Chris996]

Ok thanks for the info - really useful to know.  I’ll get to work on proper calibration.