darks and flats?

[nightfisher]

could some kind person explain just what these are and how best to achieve them

[RikM]

From a DSLR perspective:

Darks capture all the hot pixels etc. They are just the same as lights except you put the dust cap on (so it's dark), same exposure length, same ISO, and the same environmental temperature if you can. There is a bit of leeway here but closer is better. Aim for 20 - 30 to get a good average when they are stacked. I note the sensor temperature and reuse them for lights of matching temperature and exposure time. Darks also contain bias, which is the baseline electronic noise of your camera which is in every shot you take.

Flats capture the variation in illumination over the FOV. You basically take a short exposure of an evenly illuminated something. I use a light panel, but you can use a laptop screen displaying a blank white page or string a white t-shirt over the scope and point it at the sky. They should also be the same ISO as your lights but the exposure time is different. Mine are usually about 1/250 sec at ISO1600, but you should look at the histogram and adjust the exposure until the peak is 1/3 to 1/2 way across. Another way to do it is set the camera to AV and let it sort itself out. The key thing with flats is that you must not change the optical train from when you took the lights. The camera / filter orientation must be exactly the same and even the focus position should be the same if you can. If you adjust the focus or move the camera, any specks of dust will shift and the flats won't match up with the lights. I take mine the morning after my lights, before I pack everything away. Again 30 or so is good. These are quick to take so I normally take 51. These also contain bias.

Bias frames (also called offset) are the same ISO as everything else, dust cap on and set the shortest exposure time you can 1/4000 sec in my case. Temperature is not important here (I don't think). Take loads, they are quick and easy. You can reuse these for months at a time, you don't need new ones for each image.

If you use DSS for stacking, just load everything and it will sort it out. It uses a slightly different method to other stacking applications. If you just use darks, you don't need bias since it is in the darks already. If you take flats, you also need to take bias frames because DSS can't extract the bias info from the darks to apply to the flats.

[nmoushon]

Rik beat me to the punch. But what he said is good advise.

[nightfisher]

gentlemen, very helpful, gonna throw a curve ball now, IF i go down the dslr route with my ZS 70, could i get away with just lights and darks, i have to state this will only be for globs like M13 and maybe a poor attempt at M42 and M45

[nmoushon]

Yes you don't have to take flats. You can process your lights and darks just fine. You'll more than liking have gradient issues when you start processing your images to their max. Which is where flats come in. Its actually not hard to do. I found a way thats really easy, basically because if I have to shot anything I have to travel out of the city an hour so I like to pack as little and as light as possible.

The way I take flats is I stretch a white t-shirt tight and smooth over the front of the telescope and hold it in place with a couple tight rubber bands. I have a little head lamp that has several light settings so I flip it to the brightest setting and try to look straight down the tube. I try to adjust the distance my distance so that the the light illuminates the t-shirt as evenly as possible. Then switch the camera to AV mode and snap 30-40. Its not the best but it works pretty decent and its easy. I do that the very last.

[RikM]

You could get an image without flats but they make processing so much easier that it is well worth the effort. I reuse bias and darks wherever possible but new flats are a must for me since I set up from scratch each time. They make the most obvious improvement to an image of all the calibration frames.

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[squeaky]

Sorry to butt in - but the only thing in there I didn't catch was "set your camera to AV"... what's AV, please?

[AndyH]

Oh dear Jules. Have you fallen for the lure of 'the dark side'.

Step away from the ledge, step away.......

[nightfisher]

Oh dear Jules. Have you fallen for the lure of 'the dark side'.

Step away from the ledge, step away.......

Yes and no, having the Olympus pen, and not really finding a use for it, decided on the WO ZS70 for some wide field work, grab an go (okay just an excuse to own a lovely frac:D), so i figured it might not hurt to use this combo to take some really awful shots of bright globs, but i wont be venturing into the proper dark side, not enough money, not any where near enough time

[RikM]

Sorry to butt in - but the only thing in there I didn't catch was "set your camera to AV"... what's AV, please?

Aperture priority mode on Canon cameras. You choose ISO setting and aperture if using a lens - at prime focus on a scope this is f/0 - then the camera automatically sets the correct exposure time. I sometimes take one shot like this to see what exposure time it sets, then manually set it at that so it stays constant for the whole run. With aperture priority, it is recalculated for each exposure and if you are using the t-shirt method you can end up with a load of unmatched exposures.

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[BigBlueOne]

Hi Rik,

I,ve a westcott softbox could I use that do you think for some flats.

Thankyou BigBlueOne.

[RikM]

Hi Rik,

I,ve a westcott softbox could I use that do you think for some flats.

Thankyou BigBlueOne.

I guess so. You are aiming for an evenly illuminated field so if you shoot straight into it you should be fine.

[BigBlueOne]

Thankyou Rik.

[squeaky]

Aperture priority mode on Canon cameras. You choose ISO setting and aperture if using a lens - at prime focus on a scope this is f/0 - then the camera automatically sets the correct exposure time. I sometimes take one shot like this to what exposure time it sets, then manually set it at that so it stays constant for the whole run. With aperture priority, it recalculated for each exposure and if you are using the t-shirt method you can end up with a load of unmatched exposures.

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Ah, right. I'll have to find the right terminology for my Nikon

Thank you.