Question about darks

[Adamar98]

If I am imaging two different objects on the same night using the same settings for each object. Can I do one set of darks and use them on both images?

Cheers in advance.

[MikeD]

Yes you can, the dark only has too be the same exposure length and around the same temp as the light.

Mike.

[Adamar98]

Cheers Mike! Just saved me a whole lot of time.

[Bizibilder]

Take your darks on a cloudy night and save valuable imaging time! If you collect a set (25-40 darks) for a range of temperatures (about every 5° or so) and at each of your favourite exposure times (I use 2min and 5mim with my DSLR) you will soon amass a set of "library" darks that can be used over and over again. I normally take mine in the garage using a timer (about £15) so I can set up and leave the camera clicking away. Just record the temperature for each set.

[VigdisVZ]

Yep you can actually build up a collection of darks from different temperatures and save for later.

[Adamar98]

Cheers all. I had no idea you could do your darks before hand. So now I set about making a darks library!

[Bizibilder]

Flats, on the other hand, (when you get around to them) have to be taken at the same time as your "lights" (or actual images of starstuff) as the optical train must be precisely the same including camera orientation and focus. As the exposures are very short it is no problem to run them off after a session of lights has finished.

[ollypenrice]

And just to back up Roger, above, be aware that flats are very, very important and helpful. They too need dark correction but the darks in question can simply be bias frames (ie the shortest exposure of which your camera is capable.)

Olly

[Adamar98]

I've had a good read up on lights, darks, flats and bias. Lights and darks are no problem. Flats are scaring the pants off me! I've read up on a few techniques how to take them. White t-shirt over the lens. Light boxes. Shooting the sky in the opposite direction of the sun. But for every technique there seems to be some criticism. I suppose it's a matter of what works for you. And am I right in thinking I don't need to bother with bias frames? If I include bias along with the lights, darks and flats wouldn't the bias be taken out twice? So much to learn. Every time I get my head around one thing something else kicks me in the ass and says learn me. Does the ass kicking ever stop?

[Bizibilder]

Flats can easily be done with a home made lightbox - mine has 4 LED's and a simple dimmer switch all inside a box made of foamboard, cost around £10 all in http://stargazerslou...er#entry1492824 If you are using a DSLR and Deep Sky Stacker to stack your images all you need are Lights, Darks and Flats. Just bung them into DSS and let it do its thing! Out pops a TIFF image that you can then get to grips with in something like Photoshop (Another expense!) or GIMP (free and quite simply not really good enough as it is only 8bit). As Olly says flats are very important and fortunately quite easy to do!

An alternative, if you have a laptop, is to open a "white screen" like notebook or wordpad and put the white screen over the end of the scope and click away - never done this myself but it apparently works.

[ollypenrice]

Flats can easily be done with a home made lightbox - mine has 4 LED's and a simple dimmer switch all inside a box made of foamboard, cost around £10 all in http://stargazerslou...er#entry1492824 If you are using a DSLR and Deep Sky Stacker to stack your images all you need are Lights, Darks and Flats. Just bung them into DSS and let it do its thing! Out pops a TIFF image that you can then get to grips with in something like Photoshop (Another expense!) or GIMP (free and quite simply not really good enough as it is only 8bit). As Olly says flats are very important and fortunately quite easy to do!

Is this quite right? DSS asks for darks for flats as well as darks for lights. My earlier point is that you don't need to faff around taking dedicated darks for your flats (ie exposure time and temperature matched ones) because there is no statistically significant difference between a dedicated 'dark for flats' and a one-size-fits-all bias. They'll do the same job.

DSS won't double-subtract your bias signal (which is indeed contained within your darks.) It will use the bias to scale the darks, as I understand it, in order to make them less temperature sensitive. (I may be wrong about this last point and don't use DSS myself.) I would suggest that you just give DSS your bias frames again in the 'dark flats' section.

I find that an EL panel or light box is the easiest way and I do find that doing them in the dark is best. Some get them to work fine in the light but I never do. I always end up with a gradient even in sealed systems such as one shot colour or with electric filterwheels.

Olly

[Bizibilder]

There was a very long thread a while ago that was ultimately inconclusive as to whether you needed bias frames as well as darks and flats when using DSS (I think DSS is designed to use whatever it "needs" from the selection of images presented to it and to ignore things it doesn't - but I could be wrong). All I can say is that I use darks, flats and lights in DSS and get results (not necessarily good results!) that allow me to produce images that appear, to me at least, to work. There may well be a better or simply different way of reaching the same result.

[ollypenrice]

Hmmm, I think calibrating your flats is very important. They do need dark subtraction but a bias subtraction is near identical and easier. I'm confident that this is so. I've tried using uncalibrated flats in Astro Art and they don't work well. For the sake of a master bias made once and for all I think it's best to chuck int in the pot!

Olly

[Bizibilder]

Fair enough - you know far more about image processing than I do! I may try making a master bias myself and seeing if there is a difference - I assume i just take a set of "shortest possible" "dark" frames and bung them into the melting pot that is DSS?

[Adamar98]

I'm going to give it ago tonight. I'm going to try the 'white screen on the laptop' method. Any advice on exposure time? I don't want to over expose but I've also read that if taken to quick the shutter can leave a shadow? I'm starting to think I should have bought a fishing rod.

[Bizibilder]

What sort of camera are you using?

PS Apologies if I have slightly hijacked your thread with the last few posts!

[Adamar98]

Canon 450d unmodded. No need to apologies. You've all help brilliantly and it saved me asking the next question about flats. If it wasn't for people like you I think I'd have spat the dummy a sold everything weeks ago. But........it's all starting to make sense now! Each day I'm getting a little more confident about the subject. Hopefully I'll be hijacking threads soon!

[Bizibilder]

To take flats with the Canon - just set up the "lightsource" over the end of the scope - without altering anything at all, especially orientation and focus - and set the camera to Av mode. Click away and the camera will automatically set the correct exposure.

Oops! nearly forgot - DO NOT switch the camera off between lights and flats - I'm not sure about the 450 but my imaging 1000D has a vibrating sensor cleaner and we want the dustbunnies to remain on the sensor so the flats will remove them in processing!!

[Adamar98]

Thank you ALL for your time and advice. It's greatly appreciated.

[ollypenrice]

Fair enough - you know far more about image processing than I do! I may try making a master bias myself and seeing if there is a difference - I assume i just take a set of "shortest possible" "dark" frames and bung them into the melting pot that is DSS?

Yup, that's all there is to it. I'll be curious to see if it helps for you. It certainly does for me.

Olly