Flats? Darks? Bias? It's all Greek to me.....

[SCOOB_81]

Hi All,

Can I ask some advice with DSO imaging using a Sony A350 DSLR on my 200p EQ5, please?? The mount is not motorised at present, but I’m hoping that the birthday fairy will be kind.

I’m sort of at the point that I was with SPC web cam a year back, where I’m new to it and (in honesty) haven’t got a clue. I draw encouragement from just how bad my first few shots were, but I’ve improved a wee bit since. I’m trying to use our recent monsoon weather to learn how to, and just what type of shots are needed to image and process DSO’s. By that, I have seen terms such as darks, lights, flats and bias. Also, for example, multiple exposure times with a single image, such as 10x 5mins and 15x 2mins.

If someone has a moment to briefly explain I would really appreciate that. I’m just looking for that starting point, if that makes sense.

I apologise if this has been asked many times previously, I don’t seem to be able to get much out of the search function with this one.

Many thanks,

Sean.

[Viv]

Sean,

First off I would suggest buying every photon counts, brilliant book for everything you need to know (especially when starting to image)

the exposures (subs), are just the individual lights that are taken. You need to have the shutter open as long as your light pollution allows.

So the ones you have : 10x 5mins (10 subs at 5mins per sub) and 15x 2mins (15 subs at 2 mins per sub).

(appologies to who wrote this initally, I can't remember) but I found it very helpful :

A few definitions for you:

"Lights": These are the pictures that you actually take of an object. They are also called "subs". EG a single one minute exposure of a galaxy is a "light" or "sub". If you take 20 one minute exposures you have 20 "subs" making a total exposure time of 20 minutes. You should record the exposure length, ISO value and temperature for each set of "subs". Why two names? Most folk talk of "subs" but Deep Sky Stacker calls them "Lights".

"Flats": These are short exposures taken using your camera/ telescope set up. It is essential that you use the set-up EXACTLY as it was for your lights - especially focus, so don't touch anything!. Normally I take flats immediately after I have finished taking my Lights. To take them you will need a light box or EL panel (electro luminescent panel) placed over the end of the scope to give an even illumination. Set your DSLR camera to "Av" so it will automatically expose correctly and fire off around 20-30 shots. These correct vignetting and dust bunnies in the optical train.

"Darks": These are picture that you take with the lens cap ON the camera! Yes really - pictures of nothing at all!! They do actually record artifacts produced by the camera's sensor during an exposure. Darks must be of the same exposure length and the same temperature as your "Lights". Take around 20-30 of them. I take mine on cloudy nights so I don't waste valuable imaging time - often I put the camera in the garage or observatory where the temperature is near to that of outside. You can build up a library of darks - which is why you need to keep careful records of things like temperature. (I have darks taken every 5° from -5 to +25 deg C. I use the nearest one to the temperature at which I took my Lights).

[sologuitarist61]

Yes, I second 'Make Every Photon Count' - brilliant book that walks you gently through astro photography.

[SCOOB_81]

Wow, thanks so much for the detailed explanation, Scarlet!! Thats exactly what I was hoping for and I really appreciate your time. Your descriptions make perfect sense, even to myself with no experience of DSO photography.

Thanks to you both for the book recommendation, too. I'll put it on the ever expanding birthday list.

Cheers!

Sean.

[Viv]

Sean - with Astro photography you have birthday & Christmas sorted for years !

Keep us posted, it's really fantastic, when you start to see images you create.

[SCOOB_81]

Haha! You're not wrong there! At least it's nice and easy for the Mrs to pick something up, I have a wish list as long as my arm.

Will do, it'll be good to see what the seasoned vetrans make of the first efforts.

Thanks again!

[ollypenrice]

Only multiple exposure time has gone unexplained so I'll address that. I can see no point in mixing different exposure times in the same stack. A rare sub-set of objects has a huge dynamic range and does need a set of short, medium and long exposures. In my own case it's a sub set of one! (The Trapezium within M42.) I have tried short exposures for notororiously bright objects like the star Alnitak and the core of M33 and never ended up using them. What I do instead is perform a very gentle stretch of the raw data to avoid burning out the bright stuff and blend it onto the full stretch in Photoshop. This will make more sense when you start doing it.

I use CCD but I think that with DSLRs there might be a case for doing some short exposures to try to retain star colour. But that's a refinement for further on...

Best of luck.

Olly

[SCOOB_81]

Hi Olly,

Many thanks for the information! It makes sense what you've all said, and hopefully when I've picked a copy of the book up it will help further still.

I just need some clear skies up here to make a start, as you can learn far quicker with some experience. If you've seen any of the weather on the news you'll know that it's not happening any time soon!

Thanks again,

Sean.