Image processing questions

[sadia]

Hello,
Hope this is the right forum section to ask this. I am quite new to astrophotography and wanted to clarify few things that i am not too sure about. Any input will be very helpful

Suppose i have a set of 60sec frames and set of 120sec frames.

1. Normalizing diffrent length exposure:
when i do normalize them should normalize whole lot or should i only normalize 60sec frames with 60sec frames and 120sec frames with 120sec frames

2. Stacking diffrent length exposure:
Should I be stacking all data together or should I stack them in diffrent groups (eg 60secMaster, 120secMaster) and then process those groups seperately?

Regards
Sad

[Insane Climber (Jason)]

Hello Sadia.
Im no expert but just thought i would mention some software that has helped me heaps. It's called Deep Sky Stacker and it takes care of most of the pre processing work for you, aligning and stacking different exposures and it will stack your darks and flats as well if your into that. Its freeware too.

Jas

[[1ponders] (Paul)]

Hi Sadia. It is recommended that you only operate on groups of images that have the same exposure length. If you then want to combine them you can use various masking or HDR techniques for the final image.

[Bassnut (Fred)]

Yes, process the exposure lengths seperately and then create layers for them in PS for selective blending.

[rat156]

Hi Sadia,

This actually depends on the type of data you've got.

If they're monochrome images, then calibrate them separately (Darksubtract, flats etc.). After this I combine them all together using CCDStack.

In fact in CCDStack if you've got a full set of darks flats and bias frames you can calibrate them together as well, as the software will adjust your darkframe.

If you've got one shot colour images this may be different, and the HDR or layering techniques seem to be the go. Also CCDStack doesn't like the large images from DSLRs much, it runs out of memory, even when you've got 6Gb installed...

Cheers
Stuart

[Bassnut (Fred)]

So, that tells you, dont bother with woefull QE gegapixel ABG cams, get real, go with Hi QE NABG proper astrocams, (IP is the go for DSLRs anyway, it processes off disk) , and actually enjoy processing.

[sadia]

Thank you for all the advices guys. Currently I use following set up steps

1. Image capture through Nebulosity with QHY8
2. Calibrate (dark, bias, flat) in nebulosity
3. Debayer in in nebulosity
4. Stack all captured frames in one file
5. Post processing done on Photoshop CS2

After reading all the advice I think i will modify step 4 so that i stack them in groups of same exposure length. Need to read more on layer and masking for photoshop

[jase (Jason)]

May chime in here to either provide further clarity or confuse the hell out of everyone.
Through normalisation in CCDStack, differing exposure durations are weighted differently allowing one to combine them all if they so desire with little to no ill effect on the data rejection algorithm. I regularly do this when creating a synthetic luminance from LRGB subs.

Clearly, whether you want to go down this path is going to be based on what you're trying to achieve in the image i.e. control the targets dynamic range in an effective manner is best suited to layer as many have previously advised.

I'm not sure how dark frames got brought into this equation, but I'll add that you can use one dark frame for all light frames with differing exposures...providing you have a bias frame. The bias frame is subtracted from the dark frame which turns it into a thermal frame that can be scaled to match the noise in any exposure duration light frame below the dark frame duration. However there is a general rule to this. The dark frame should be at least 2-3x the exposure length of the longest light frame you expect to calibrate to improve the scaling accuracy. So if all you intend to capture are 300s, 600s and 900s light frame subs, the master dark frame should be at least 1800s (30mins (2x900s)). The cool thing here is that if you've got a master dark of 1800s duration, there's no need to scale a thermal frame if you decide to take some 1800s light frame subs - so in many ways you improve the versatility of the calibration library.