Darks, flats and bias

[zicklurky]

Hiya,

If you take shots of say 2 DSOs in 1 session, do you take a set of darks, flats and bias for each, or 1 set that you can use for both?

Sent from my Galaxy Nexus using Tapatalk 2

[jimmyjamjoejoe]

Should be able to do one batch, so long as temps/conditions dont change.

[Adamar98]

I asked this same question a few weeks back. Your fine to use one set of dark on both images as long as the settings are the same. Flats and bias need to be done separately for each image. I've started a darks library. I've taken 20-40 darks at 2 mins and 3 mins (exposure times I most use) and a range of different temperatures. Pop them in a folder stating the exposure time and temp. Now all I do I go collect my data and match up my darks depending on my temp and exposure. I hope this makes sense?

[Richie092]

I think that's a good idea Adam.

So could I take out just my DSLR with the body cap on ( no lens or scope) and shoot different sets of darks on the next cloudy night to build a library or would it have to be set up as per the imaging run?

[Adamar98]

I think your scope or lens has to be attached. But yes that's about it. You'll save so much time. You'll need to update your darks library every 6-8 weeks.

[GaryG]

Hi,

You should be able to use the same darks (electrical noise of the chip) & bias (camera read out noise) frames for both objects. Flats would need to be done per object if you've changed anything in the imaging chain eg focus, rotation, filter.

Darks & bias frames do not need the telescope to be attached - i often do my dslr darks & bias frames with a camera lens and lens cover atached, in the case of dslr's you do however have to ensure you get no light in throught the view finder.

Gary

[mart rich]

I would have thought that the darks should be OK if you just take one set per session. I do worry a little about having a library as I tend to run my DSLR with about 20s gap between exposures, and I suspect the chip heats a bit over an imaging session, so unless the darks have the same exposure time and gap time, then the conditions won't be replicated.

For the flats I have read many time on SGL that the imaging train focus and camera position (rotation) need to be the same as for the image frames, so one set may not be appropriate if you have two targets each night. So far I have only used flats when imaging one target and I have had some success taking them by leaving the setup until the morning and using a tshirt stretched over the front of the scope early morning before the sun has risen properly. How much difference it will all make is perhaps a subject for another thread.....

Bias should be easier though.

As you can tell I am far from an expert but thats my twopence worth. I find it all very confusing, so I apologise if I haven't helped much!

Good luck

Martin

[Richie092]

I think your scope or lens has to be attached. But yes that's about it. You'll save so much time. You'll need to update your darks library every 6-8 weeks.

Sounds good. It won't hurt to stick a lens on and take a couple hours worth of darks then. That will really help next time we get clear skies as I will be able to spend all available time collecting data.

[zicklurky]

Ahhhh ok, thanks, that makes a lot of sense.

Great idea Adam! I love the idea of a darks library.

SGL is awesome! Thanks so much to everyone for all the help so far!

Chris

[Adamar98]

I'm just passing on the information that was passed to me. Your right about SGL. It is awesome! If it wasn't for this place and the friendly people on here, I think I would have packed it all in and taken up fishing. I wonder if there is a Pond Gazers lounge?

[ollypenrice]

Bias is bias. The shortest your camera can take. Average combine a decent set (say 40) and you won't go wrong. You certainly do not need to do them for each image. Off the scope is fine.

Darks need to be at the same temp, bin mode, ISO and exp time. How do you know your chip temp with a DSLR? You don't, which is one reason for choosing CCD! But an idea of the ambient temp is a decent starting point. A darks library is great but I update mine twice a year at most. A hot pixel filter is a wonderful thing.

Flats image the imaging train. Ideally you should do them for each new image. Whether you really need to do so depends on a number of things, but if I have to rotate the camera I always shoot new flats. How important are flats? Very important. You do not need to shoot darks for flats. They must be calibrated (this is very important) but a master bias will do as a dark for all flats done with a reasonably bright light source.

Olly

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2277139556&k=FGgG233