Multiple night image processing?

[Mattjohnson1978]

I have been enjoying the recent purchase of my Canon DSLR and been photographing everything I can find. I now want to pick targets and take as many images as possible to produce the highest quality possible. With the short summer (and I use that term very loosely!) nights, I was wondering what the normal protocol was when shooting the same target over different nights with regards to flats and darks. For a single nights imaging, I would collect the darks and flats with my lights and then process them through IRIS. How would I process images from 2 different sittings? Would you combine all the lights, darks and flats into the same folder and let the software sort it out or would you calibrate all respective images with their own offsets and then merge the 2 images?

Any help would be appreciated

Matt

[Bizibilder]

Lights and flats must be taken at the same time with the same optical set-up. Darks can be taken anytime and kept for future use. To process I use Deep Sky Stacker to stack as everything is automatic - just load up all your images and hit the button! With DSS you load each nights images and flats by clicking on the "group" tabs at the bottom of the image selection screen - you just load your darks into the first group and DSS does the rest. Not sure about Iris - sorry.

[Mattjohnson1978]

Thanks Roger. Someone else has recommended DSS so maybe I should give it a look. IRIS may have that function too but it isn't the easiest thing to navigate around. I will definitely give it a look. Thanks again for your help.

Matt

[ollypenrice]

Lights and flats must be taken at the same time with the same optical set-up. Darks can be taken anytime and kept for future use. To process I use Deep Sky Stacker to stack as everything is automatic - just load up all your images and hit the button! With DSS you load each nights images and flats by clicking on the "group" tabs at the bottom of the image selection screen - you just load your darks into the first group and DSS does the rest. Not sure about Iris - sorry.

No, there is no need to take flats at the same time, assuming you leave the camera on the scope or mark its position and put it back there. (Tippex is a favourite.) You might just collect an all-new dust bunny between shoots but being careful to keep things clean should avoid this. It has never, ever happened to me. I try to keep my cameras orientated along RA and Dec unless a target really needs a special angle of its own. This means that a set of flats last for many, many nights of shooting on different targets. I really do like a set of flats to last for quite a few images since they have to be done in the dark and at night where I work. (Too hot and too bright for daytime flats shooting.) This means they make a nasty dint in my already impoverished beauty sleep!

Note that dust bunnies that will be visible on an image are all close to the camera chip (ie on filters etc) and vignetting will not alter from night to night assuming reasonably similar camera orientation.

I would use one set of flats for all and try to build a darks library based on measured ambient temp, using the most appropriate ones. If the temp match is not good they will do more harm than good.

Olly

[Mattjohnson1978]

Thanks Olly.

So are you saying as long as my camera is orientated the same way on the scope each time, I could take a shed load of flats and create a master flat which I could use for multiple processing? Could you do the same with the dark files or does the ambient temperature have to be the same as the lights? I was wondering if i took a selection of darks at various ISO and set exposure times on cloudy sky-less nights and create master files for them too, it would free up imaging time on the evening plus reduce processing time......or am I talking rubbish?

Matt

[Jessun]

No, there is no need to take flats at the same time

First time I see this pointed out! I have been experimenting a bit recently and have used the same master flat for a month now, not really finding any support for this until now. I struggle to get the 'perfect flat' in terms of illumination across the image so when I finally nail a master I keep it.

For the shorter exposures I do, only chip dust is a problem. The IR/UV cut filter (protecting the CCD) sits only two inches away but does not cause bunnies for 'normal' dust.

The use of same masters over several nights also makes adding data a breeze since I only have to chuck the extra lights in the mix when preprocessing and stacking and do a run from scratch with all I've collected so far.

Anything that makes life a little easier is always welcome :-)

[Jessun]

Could you do the same with the dark files or does the ambient temperature have to be the same as the lights?

I'll let the experienced guys answer this in greater detail, but darks basically need to be taken at the same temperature as the lights for best effects. Many take sets at different temperatures, using fridges, freezers or just spare time in the open to collect the darks and then keep the different masters in a library for future use. The darks need only the covered up camera so they are simple to produce anywhere. Maybe do sets with 5 degree difference or someting.

I don't know how long these master darks last though as an effect of CCD degradation/changes over time. Again, others will be able to advice more.

One thing to make certain of is that the darks are just that - stray light will render them useless - so do all you can to ensure that the CCD is covered. A simple cap/cover may not be enough.

[ollypenrice]

I can't really say anything from experience regarding a darks library from a non-regulated temperature camera. Here we have sets of darks for all the cameras at setpoint temperatures so the library idea is perfect. Most of the darks are at -20 but sometimes the cameras struggle to get there (eg in August when the night is sometimes over 20 degrees.)

I'm surprized that more people are not keeping flats for longer. Typically I do a set and then for the rest of that dark time, maybe a dozen nights of imaging, I'll re-use them. Only a change of scope or orientation makes them redundant. The fact that most of our chips are square means that you are less tempted to change orientation! (A good reason for having a square chip!!)

Olly

[martin_h]

I have a perminant set up and reuse my flats over and over again, the mosaic im working on at the moment has over 60 hours of data taken over 3 months and I'm using the flats I took on the first night! BUT ...If I had to move the image train for any reason I would re-shoot a set, As for darks etc I tend to re do a set every 6-7 months