Processing help

[paul mc c]

Hello,again dont know what i am doing,i have only got gimp to post process so probably thats the bigest problem.

I deceided to image te North american neb as it is big and in a good position,my subs were 300 secs,and guidin was very good,i used a UCH filter,Hence the green ting.

I was happy wit the single subs,but wen i stacked them there wasnt much difference between 5 mins and almost 2 hours,why.

First is single sub other is tif file.

 

Autosave004.tif

[tooth_dr]

I might have a look at this later Paul when I get home from work.

[paul mc c]

Nice one mate,im basicly playing just about,so any help would be great.

[vlaiv]

You need to do some calibration on these subs.

Signal is very faint and if you try to pull it out you will get into zone where problems from missing flats / darks etc start to show.

I tried basic stretch in gimp and ran into trouble straight away:

[tooth_dr]

Vlaiv is right, flats are pretty much essential.  Can you maybe do them this evening?

[paul mc c]

Never done flats,i used darks and was doing bias and my battery went,can i still do flats,and will i have to open the obsy to bo them.

[paul mc c]

20 minutes ago, vlaiv said:

You need to do some calibration on these subs.

Signal is very faint and if you try to pull it out you will get into zone where problems from missing flats / darks etc start to show.

I tried basic stretch in gimp and ran into trouble straight away:

Thank you,when you say calibration on the subs,what does this mean....sorry still very wet behind the ears.

[vlaiv]

11 minutes ago, paul mc c said:

Thank you,when you say calibration on the subs,what does this mean....sorry still very wet behind the ears.

I just mean standard darks, flats, etc ...

Point of calibration is to take away any signal or signal defect that is not related to accumulated light. Darks remove thermal buildup in pixels, Bias remove any bias signal when reading pixels. Flats correct for uneven illumination (either vignetting, or dust).

Signals that we work with are really small, so any defect will be comparatively large, and we want to remove all those defects, so we are left with only pure target signal and unfortunately all random noise that remains - stacking will reduce noise component.

[paul mc c]

Would you do all this with every new set of subs.

[vlaiv]

5 minutes ago, paul mc c said:

Would you do all this with every new set of subs.

That depends on gear that you are using, here are some guidelines:

- If your bias files are stable (some CMOS cameras recalibrate on every power cycle and give different bias - these are unstable) - take one set of bias subs and reuse them. In case you have unstable bias, either don't use bias files, or shoot bias files every session. If in doubt about bias subs, take one set and do simple stacking (Average), power off your gear, power on, take another set of bias files, again simple stack and subtract two stack - you need to get very uniformly random image with 0 average pixel value over image. This means your bias files are stable.

- If you have set point cooling camera - one set of darks at certain temperature can be reused many times (I tend to have winter set and summer set - because of ambient temperature difference and maximum cooling that I can achieve - winter -20C summer -15C). In case you don't have set point cooling then darks are problematic (they depend on sensor temperature). Best you can do is use "dark optimization" feature - algorithm tries to guess how to scale darks to proper temperature. In this case you need a set of darks close to working temperature. Maybe take one for each 5-10C shift in ambient temperature. So if you are working in 10C use one set of darks (shot at this ambient temp), when temp drops to around 5C take another, in 0C have third, etc ...

For dark optimization you need to take bias files for it to work properly.

- Flats need to be done per filter, but you can have set of flats if you have permanent setup and don't alter your optical train. This is useful if having filter wheels, or if shooting with single filter (OSC + LPS filter, or OSC unfiltered), and leave your gear unchanged between sessions. I have filter drawer and dismantle my setup (kind of back yard mobile setup) every time, so I take flats each session. Any time you change something in imaging train if you have permanent setup (clean filters, reorder them, anything that can change orientation of optical elements or allow new dust to settle), you need to retake your flats.

- Couple more things - darks need to be exact match to light subs - same gain/ISO, same offset (if your camera supports it), same temperature (if you have set point cooling), same duration. Flats are best shot at 75-85% histogram, and don't need to match lights in gain/iso and other settings, but flat darks (if you are going to use them, or you can use bias files for flat calibration) need to be exact same match to flats. Bias should be taken with same Gain/ISO settings as lights (and set of bias if you are going to calibrate flats with bias - to match flat frames).

There are couple of methods of calibration, only one is "proper" calibration (meaning mathematically it removes all signals apart form light signal), but others can work well under certain circumstances.

Many people use bias instead of darks for DSLR, because darks are not reliable. You can calibrate your flats with bias only since flat subs are very short (order of 1s or less, but that depends on strength of flat panel) and there is no enough time for thermal signal to build up to significant level.

 

[paul mc c]

Thank you,thats very helpfull.

[carastro]

Yes you need to take flats every time you either move your camera, or change your filters (if you have them).  It's a bind but we are all stuck with it.  Trying to process images with no flats applied is a nightmare.  

I will give your image a go but I fear it will be difficult. 

Also GIMP is not the best for processing.  I know some people use it, but I downloaded it on holiday once when I did not have Photoshop with me and could not get on with it at all.

Carole 

[carastro]

I managed to do a bit of processing in Photoshop.

First thing I did was convert to 16bit.

Colour balancing made a huge difference. 

I managed to overcome the Vignetting by doing a gradient exterminator (excluding the NA) 

Did one iteration of levels but could not stretch much as it was very noisy with horizontal banding.

I treated the horizontal banding with the relevant tool in actions (I don;t use actions much, but its nice to have it to fall back on when needed) and finally increased the star colour slightly. 

 

[paul mc c]

Thank you,that makes such a difference,i have just taken some bias frames and will take some flats if i can open the obsy roof if the rain ever stops.

[tooth_dr]

19 minutes ago, paul mc c said:

Thank you,that makes such a difference,i have just taken some bias frames and will take some flats if i can open the obsy roof if the rain ever stops.

What scope is it Paul?

[paul mc c]

Its a c80ed and 1100d non modded.

[tooth_dr]

2 minutes ago, paul mc c said:

Its a c80ed and 1100d non modded.

Thought so   Using the same scope I take my flats using a laptop screen sitting in front of the scope with a white t shirt stretched over it held wrinkle free with an elastic band.  This is done with the shed roof closed. 

[tooth_dr]

Same ISO as lights, and use AV mode on camera to set the correct exposure. 

[paul mc c]

Thank you will do that later,i havnt moved anything or touched the camera so how many should i take,i just took 40 bias,is that enough?

[carastro]

Quote

Same ISO as lights, and use AV mode on camera to set the correct exposure. 

A word of caution.  When I had a 450D AV worked every time for flats, but that eventually had to be replaced and I now have an 1100D and found AV did not work at all.  So I have to experiment with the length of exposure until I get the histogram right.  

When you are taking flats as a newbie understanding what you need is paramount to getting the right flats.  If you have Digitial Photo professional installed (part of the Canon suite) you can look at the histogram and it needs to be 1/3 to just under halfway from the left side.  If you are using APT you can look at the histogram on there too, but when I was helping a new imager the APT flats plan produce flats that were far too bright, so we had to change to taking a "shot" and looking at the histogram.

HTH

Carole

[paul mc c]

5 minutes ago, carastro said:

A word of caution.  When I had a 450D AV worked every time for flats, but that eventually had to be replaced and I now have an 1100D and found AV did not work at all.  So I have to experiment with the length of exposure until I get the histogram right.  

When you are taking flats as a newbie understanding what you need is paramount to getting the right flats.  If you have Digitial Photo professional installed (part of the Canon suite) you can look at the histogram and if need to be 1/3 to just under halfway from the left side.  If you are using APT you can look at the histogram on there too, but when I was helping a new imager the APT flats plan produce flats that were far too bright, so we can to change to taking a "shot" and looking at the histogram.

HTH

Carole

Its getting scarier by the minute...lol,i will bear this in mind and see what works.

[tooth_dr]

13 hours ago, paul mc c said:

Its getting scarier by the minute...lol,i will bear this in mind and see what works.

How’s you get on last night?

[ollypenrice]

Being an idle dog I often use luminance flats for everything and 49 times out of 50 it makes no difference at all...

Olly

[MartinFransson]

49 minutes ago, ollypenrice said:

Being an idle dog I often use luminance flats for everything and 49 times out of 50 it makes no difference at all...

Olly

That's something I want to know more about. Do you take flats for the L filter and use it for all filters? Even Ha?

I find flats such a hassle that I don't even use them at the moment...

[paul mc c]

2 hours ago, tooth_dr said:

How’s you get on last night?

Didnt get a chance,i meant to ask you,when you use the laptop screen do you have to have a white screen on it?