Darks & Flats

[ScubaMike]

Hi,

Having just started on the imaging trail, I am on a very steep learning curve, so please be gently as I drag my knowledge up the hill.

I am starting with Lunar & Planetary (Jupiter at the moment).

Will I need darks & flats for these targets?

How do I take them?

My understanding is that I put the scope cover on and take an exposure = dark, and I illuminate with an even light field and expose = flat.

What length of exposure do I take for each?

How many of each?

Is there anything else I need?

Would be really grateful for any advice people can give.

[Ben Ritchie]

Will I need darks & flats for these targets?

No, as a first rule of thumb i'd view darks and flats as something to do for DSOs (like most rules of thumb it's not entirely true, but at the start of the learning curve it's a reasonable assumption).

My understanding is that I put the scope cover on and take an exposure = dark, and I illuminate with an even light field and expose = flat.

What length of exposure do I take for each?

How many of each?

That's pretty much right. For darks they should generally be the same integration time as your lights (they don't have to be, but if not you need bias to scale them). For the flats, exposure time should be enough that it's reasonably well exposed but not saturated, i.e. typical ADUs should be around half maximum, something like that. It doesn't need to be exact, just not saturated or deeply underexposed.

I usually take around half a dozen and median combine to form a master dark/master flat.

[ScubaMike]

Thanks for the input Ben,

There are a couple of bits I don't understand though.

For darks they should generally be the same integration time as your lights (they don't have to be, but if not you need bias to scale them).

What is meant by "Integation time"?

For the flats, exposure time should be enough that it's reasonably well exposed but not saturated, i.e. typical ADUs should be around half maximum, something like that. It doesn't need to be exact, just not saturated or deeply underexposed.

I do not know what an "ADU" is, nor how to measure it, or know when an exposure is "half-maximum". Could you give a layman explaination?

I am slowly making my way through "Making every photon count", but I'm not at that point yet...... and tonight looks like it will be clear

[daz]

Integration time = exposure time x number of exposures

i.e. if you take 20 x 120 seconds lights, you need 20 x 120 seconds darks. You will read a *lot* of discussion around this topic, but for starting out, this is fine.

ADU = the number of photons captured. Every sensor has a maximum amount of light it can capture. Easiest way to check is by looking at the histogram of the image. The peak should be central-ish.

Flats will be in the order of seconds in terms of time.

Remember that you must not move the camera between taking the lights and flats!

The darks you can do when it's cloudy!!

But as Ben says, you don't need darks and flats for planetary/lunar imaging

HTH

[Ben Ritchie]

My suggestion to anyone starting out would be do experiment a lot with calibration frames (i.e. darks and flats), get a feel for how they work with your CCD, what happens to the final image when you do/don't use some of them, then work out your preferences. There's a lot of variation in how much work you need to do, what's essential, and what's optional. As a couple of examples:

- I had a Starlight-Xpress SXV-H9 that had such low dark current that taking darks was pretty much unnecessary, in that case I would just subtract bias frames (dark frames are a combination of time-independent bias and time-dependent thermal noise). But the field of view was large enough that I did need to use flats.

- another CCD was a SBIG ST-402ME. This was very noisy and had to have darks, but setpoint cooling meant that I could build up a library of darks while it was cloudy (or even daylight) and reuse them, which made the process rather easy. The field of view was also small enough that illumination was effectively constant, so it didn't need flats (...as long as it was clean enough to not have 'dust donuts'...)

So there aren't really hard rules, just general themes you pick up with a bit of experience