Stacking darks/bias/flats in Photoshop/GIMP?

[fragmonkey]

Hi there. Since DeepSkyStacker doesn't seem to ever want to stack my images due to a lack of stars or something, I'm manually stacking them.

I've got a couple of questions. I use GIMP, and I understand that the light frames are to be added together and the Dark frames subtracted from that.

However, how does this work when I'm aligning as well? As a result of aligning, I've got these consistent hot pixels and other noise that move all over the place. Is it just a matter of lining up the darks with each area of hot pixels/noise and subtracting? 

Additionally, what is done with Bias or Flat frames? I'm not sure on the layer mode that is supposed to be applied to them.

Finally, I read a few times that images are to be stacked with an opacity of 100% for first, 50% for second, 30-something% for third, and so on. Most guides I've read only have like 10 images to stack so they use those numbers. How should I set the opacities for dozens or maybe even hundreds of images to manually stack? 

Thanks

[msh1]

have you tried nebulosity from starklabs its not free but not expensive but very good 

[johnrt]

Crikey, that all sounds like hard work! Bias are also subtracted, and flats are divided. Don't forget to subtract the bias from your darks and flats!

If I were you I would work on why DSS won't stack your images rather than manual stacking in GIMP.

[Sam]

Have you played with the threshold settings on DSS. There's a percentage setting that you can lower to identify more stars, and test it to see if it works. I think it's an advanced button next to the stacking parameters button.

[jgs001]

That's a lot of work when there are tools that will do the hard bits for you... DSS requires at least 11 or 12 stars (I think) to be identified to be able to stack the images. Try adjusting the star detection threshold where Sam mentioned it, then you can use DSS to count the number of stars, make sure you have at least 20, adjust until you get that number. If after adjusting the threshold value to get that many, DSS still will not stack, then the implications are a problem with the focus of the images. I'd suggest also posting a sample of a single sub.

[r3i]

have you tried nebulosity from starklabs its not free but not expensive but very good 

+1 for Nebulosity for stacking your images.  The latest version (V3) makes the process of assembling bias, darks, flats and lights for stacking very straightforward.

It's great for image capture too.

[Daniel-K]

You need to adjust the star threshold under advanced settings move it to the left to detect more stars

[fragmonkey]

I have tried messing with the detection threshold and other settings but have had no luck. I'm using an f/10 newtonian reflector on an EQ2 mount with a basic drive motor and (currently, still practicing) shoddy polar alignment. As a result, my images usually contain faint and/or smudged stars that DSS cannot seem to detect ever. The images do look a lot better after manual alignment, but it seems I've got the photography issues to sort out first before I can hope to process in DSS (hence why I've been using GIMP)

[lensman57]

Hi there. Since DeepSkyStacker doesn't seem to ever want to stack my images due to a lack of stars or something, I'm manually stacking them.

I've got a couple of questions. I use GIMP, and I understand that the light frames are to be added together and the Dark frames subtracted from that.

However, how does this work when I'm aligning as well? As a result of aligning, I've got these consistent hot pixels and other noise that move all over the place. Is it just a matter of lining up the darks with each area of hot pixels/noise and subtracting? 

Additionally, what is done with Bias or Flat frames? I'm not sure on the layer mode that is supposed to be applied to them.

Finally, I read a few times that images are to be stacked with an opacity of 100% for first, 50% for second, 30-something% for third, and so on. Most guides I've read only have like 10 images to stack so they use those numbers. How should I set the opacities for dozens or maybe even hundreds of images to manually stack? 

Thanks

Hi,

As suggested give Nebulasity a try, the process seems a bit long winded in the begining but you will soon get the hang of it as it is almost automated and doesn't give you headache like DSS, it is also great for capture with some good tools in there and works with a lot of CCDs and now with Canon cameras too. It also has some basic image processing capability ( basic compared to the big boys, Maxim, PI , StarTools and others) so for $80.00 it is a bit of a bargain. PS can stack frames but you really do not want to spend a whole day messing about with layers and opacity sliders, what happenes you took 100 subs of 30s each? BTW why can't you get the DSS to stack your subs?

A.G

[dph1nm]

As a result, my images usually contain faint and/or smudged stars that DSS cannot seem to detect ever.

Try superpixel mode in DSS - this reduces the size of the images by 2x2 and more importantly makes the stars smaller. I find that DSS can then sometimes detect bloated or trailed stars.

NigelM

[dph1nm]

I use GIMP, and I understand that the light frames are to be added together and the Dark frames subtracted from that.

No, you should subtract the dark from each frame before stacking the lights (ditto biases and flats - they all need to be applied to the individual lights before you align them). I have only had limited success with GIMP and flats - the 8-bit limitation can make the image end up with rather quantised levels when you divide. Adding and subtraction seems to work OK though.

NigelM

[fragmonkey]

No, you should subtract the dark from each frame before stacking the lights (ditto biases and flats - they all need to be applied to the individual lights before you align them). I have only had limited success with GIMP and flats - the 8-bit limitation can make the image end up with rather quantised levels when you divide. Adding and subtraction seems to work OK though.

NigelM

Ah okay. Is it safe then to reuse individual dark frames? I know people often recommend taking 20-30 of them, but I may have more lights as a result

[dph1nm]

Well usually folks stack all the darks (or biases) first to make a master dark (or bias) with less random noise in it, then subtract this from each light (I think this is what DSS does). The reason for doing this is that when you subtract a dark (or any other frame for that matter) the random noise in the dark (which is always present) will increase the random noise in the light by some (hopefully small!) amount. I guess this might seem slightly paradoxical, but the purpose of the dark is really to remove hot pixels, amp glow etc. Hopefully removing these will have a bigger beneficial effect that the detrimental effect of adding that extra random noise!

Hence it is a good idea to start with a dark with as little random noise in it as possible, which is why folks go for this master dark idea.

NigelM

[Zakalwe]

I have tried messing with the detection threshold and other settings but have had no luck. I'm using an f/10 newtonian reflector on an EQ2 mount with a basic drive motor and (currently, still practicing) shoddy polar alignment. As a result, my images usually contain faint and/or smudged stars that DSS cannot seem to detect ever. The images do look a lot better after manual alignment, but it seems I've got the photography issues to sort out first before I can hope to process in DSS (hence why I've been using GIMP)

IMHO you should concentrate on getting the basics right first. Manually stacking is a load of work and the time and energy spent in doing it is probably better spent in getting the basics correct.

Even if you get them stacked, then all you will end up with is a load of poor quality images stacked (remember the old adage: Garbage in, Garbage Out GIGO). Get better quality at the input and you will end up with a better result.

[fragmonkey]

Try superpixel mode in DSS - this reduces the size of the images by 2x2 and more importantly makes the stars smaller. I find that DSS can then sometimes detect bloated or trailed stars.

NigelM

Where can I find this superpixel mode?

[dph1nm]

Where can I find this superpixel mode?

It is one of the debayering options for RAW images

http://deepskystacker.free.fr/english/technical.htm#rawdecod

You find it under Options - Settings - Raw/FITS DDP Settings...

NigelM