Darks, flats ????

[spaceboy]

Hi All

Darks and flats ? Do darks have to be done in the scope and for the same exposure etc? Flats, same exposure and in scope etc ?

Also onece done at an exposure and ISO is that it? can these then be used for other stacks ? once done they are done so to speak. So could I spend a cloudy night doing various darks with different exposures and ISO ?

Sorry if this all sounds dumb

[jgs001]

Flats need to be with the identical setup... spot on orientation and focus position. Use the same ISO, but you need to adjust the exposure to suit.. I've not tied with a CCD, but with an SLR, using Av mode works well. If nothing has changed between sessions, then you can reuse the flats.

Darks, should be taken at the same temperature, ISO and exposure setting, but with the camera capped... this doesn't have to be attached to the scope however, and I've often taken darks with a normal lens fitted back on my SLR whilst packing away everything else. If the ISO and exposure times and operating temperatures are the same, you can reuse the darks.

[spaceboy]

Thanks John. Regards the flats, you take these during the day don't you ? So with out sounding too stupid how dose getting the correct orientation work ??

[SteveA]

Your best bet is to get one of these.... or similar

First Light Optics - Flat-Field Panels

It makes the whole deal with taking flats a piece of cake!

That way you can take your flats at night before breaking your kit down....

[spaceboy]

£125 for the 8" version. I'm to tight to spend that kind of money.

[jgs001]

I take mine just after the capture run, using a laptop monitor with notepad on it maximised. As I use my slr for daylight photography a well and have to take down at the end of the night, I can't really wait till the following day.

My sequence is lights, flats and darks.

[ollypenrice]

You can try T shirt flats. A white T shirt goes over the scope to diffuse the light and you point it away from the sun into the daytime sky, ideally when it isn't too bright. Expose so that the histogram peak is between 1/3 and 2/3 of maximum. I must say I find they are hard to get right but with a lightproof DSLR you have a fighting chance.

Take about fifty and also make a master bias of at least fifty. Use the master bias as a dark for flats.

Olly

[spaceboy]

Thanks for all the advice guys. How do you make Bias ??

[jgs001]

cap the camera, shoot the same ISO as the lights, and the shortest possible exposure. My 450d for instance, fastest shutter speed is 1/4000s, that's what's used to shoot bias frames.

[cyborg421]

I am using a Nikon D40. Is there an idots guide to how to take darks/flats/lights/bias that anyone knows about?

Thanks

Simon

[jgs001]

Simon take a look at the help for deep sky stacker, that's pretty good.

[cyborg421]

Hi John,

Thanks, I've had a look through that. I still wasn't 100% clear though.

I was looking for a kind of step by step guide.

Simon

[jgs001]

Lights - These are the actual image data exposures

Darks - Identical to the lights but with the lens/scope capped

So these would both be for example... ISO800, 60 seconds, lights uncapped, darks capped.

Flats - Set the camera to Av mode, make sure no exposure compensation is set, and shoot an evenly lit subject (I use a laptop running notepad maximised), use the same ISO

Bias - Use the same ISO but set the camera shutter speed to the fastest the camera can manage, and cap the lens/scope. The consumer models seem to be 1/4000s, higher spec cameras, seem to be 1/8000s

[RikM]

Overview for DSLR users

Lights = take as many as you can. 20 minimum but 50 is better. ISO between 400 - 1600 but they should all be the same for a given stack. Exposure time as long as possible with round stars or until the histogram is 1/3 - 1/2 the way across from the right. Once you choose and exposure time, keep it the same for all the frames in a given stack. Turn off long exposure noise reduction and high ISO noise reduction, set white balance to daylight (or try tungsten if you have light pollution but no filter). Shoot in RAW format rather than jpg.

Darks = take a good number, 20 minimum but again 50 is better. Same ISO and exposure time as used for the lights but with the dust cap on. Ideally, should be taken before or after the lights because the temperature of the sensor needs to be the same for darks and lights (± 5°C should be okay but the closer the better). These can be taken with the camera on the scope or with the camera removed and a normal lens & cap fitted. Also soot RAW rather than jpg. If you know the chip temperatures (exif temp), these darks can be reused for other images with matching temperature, ISO and exposure time.

Flats = take a good number, 30 minimum but again 50 is better. These are quick to do so it's not too difficult to take plenty. Same ISO as the lights and darks but the exposure time needs to be reduced. These are taken either in the morning / evening with a t-shirt over the end of the scope as a diffuser or with a light panel or laptop screen as a light source. Set the camera to Av and let it control the exposure time. To check, the peak in the histogram should be 1/3 - 1/2 the way across. The camera should not be removed from the scope nor should focus be radically changed between taking lights and taking flats. If you shoot multiple targets without demounting the camera, the flats can be reused for all (probably).

Bias = take a good number, 30 minimum but again 50 or even 80 is better. These are very quick to do so it's not too difficult to take plenty. These should be the same ISO as the other frames, with the scope or camera cap on and the shortest exposure possible. The camera does not have to be on the scope, the temperature doesn't really have to match. You can re-use these for 3 months or so.

Flats make the most obvious difference and are the biggest help making processing easier. For DSS, if you shoot flats you also need bias. If you shoot lights and darks - no flats - you don't need a separate bias because the bias signal is also included in the darks. DSS processes files differently from other packages, it can't extract the bias info from the darks to apply to the lights but if you include all four file types it doesn't subtract the bias twice. Forget about flat darks.

Probably something wrong in here somewhere if so ...sorry.

[spaceboy]

My god you guys deserve medals I take it I can only shoot one DSO a night then ? (if I'm lucky) Makes you appreciate how much effort goes into some of the images on SGL. I knew it's not a walk in the park but wow.

[jgs001]

That depends on what you're shooting... a star cluster without faint stuff (neb etc) you can get away with 6 to 12 shorter subs... so could do several in a night. If you want a galaxy or faint stuff, then you could be looking at multiple nights of capture adding into the final image (one a week ??)

[spaceboy]

How would this then work with the sensor temperature ??

[jgs001]

With DSS you can load each nights subs and calibration frames into separate groups, each group is calibrated separately and then all the calibrated images are combined into an end result.

[spaceboy]

Thanks John

[cyborg421]

Thank you John, Rik,

That is a real help.

Simon

Sorry I couldn't reply before Stargazers is blocked at work.