Anyone want a go at this

[Ant]

a 36meg Tiff straight from stacking!

Ant

[SteveL]

downloading...

[MikeD]

Hi Ant.

I had a quick look but only got as far as the second pull on curves before streaking from hot pixels started to show through.There is a lot of very good data in this, but it needs darks or at least to be sigma clip combined.

How did you stack the (I am guessing 20 odd subs from the hot pixcell track) subs; Sum, Average or Median?

The cleaner the raw stack the better the final result will be and this data looks worth the effort to get it right.

Mike.

[Ant]

I didn't count, but i asked the camera for 100 and the battery died so I'm guessing at around the 60 - 70 mark.

How many darks do you need? I thought just one at the end would do? Not that I even took one Can I bodge one tonight -

I stacked using Deep Sky Stacker. Just open the files and clicked on stack image (or whatever the term as used).

Ant

[SteveL]

Some strange streaking in the image when levels/curves get pulled around... not sure why...

[MikeD]

Can I bodge one tonight -

yes you can,10 darks should do at around the same temp if you can.

Try stacking in maxim and use sigma clip that may be enough.

Mike.

[Ant]

Excuse my ignorance - but why multiple darks?

Do I need flats? If so how do I take them...

Luckily I know what the temperature was, 11c. So I should be able to get close.

Ant

[SteveL]

Multiple darks - To average out any noise, and at the same time, bring out the detail in any hot pixels

Flats - dont go there

[MikeD]

Flats are a good thing and not to be feared :nono:

You just need a white evenly illuminated area to point your scope at and take 10 shots

The flat field must be taken at the same temperature as the dark, bias & image frames. The time should be as short as possible whilst at the same time getting the correct saturation level. For the correct saturation level the flat fields average value should be approximately one third of the maximum saturation level of your camera.

If black= 0 and white=255 in a photoshop histogram, then you want a histgram where the spike is around 85

Mike.

[Ant]

Somthing like a white peice of paper or card would do then - does it have to be under a certain type of light or daylight?

Suddenly observing seems like a good idea

[Ant]

Here's my attempts so far!

(click to enlarge)

(click to enlarge)

(click to enlarge)

Going to take the Darks tonight and then see what happens...

Ant

[MikeD]

This is about as far as I would push the data as is.

[MikeD]

A full set of Darks flats and bias frames would transform this image IMHO.

[Ant]

Dark frames are designed to remove the electronic noise from the CCD image.

Flat Frames are designed to remove imperfections in the optical system - dust and the like. Don't worry about these for now is my advice - concentrate on the darks.

Bias are also taken with the lens cap on, but are as fast as possible - ideally 0 length, but I think only dedicated CCD's do 0 length.

HTH

Found this from back in June, I just want to make sure that I get this right.

1) I have the light frames already.

2) The darks I will do tonight - I should be able to match the temperature.

3) Flats, might be a problem untill the weekend and I can get some daylights - but these are exposures as fast as possible? on a plain white background - this also means setting the scope up etc

4) Bias, with lens cap on again quick as poss. So take these in Manual mode with a setting of 1/4000 sec should be ok?

Ant

[MikeD]

Yes, you got it but flats could be a problem unless you can set up so the camera/scope is in the exact same orientation as it was for the Light frames ( so all the dust in the imaging train lines up the same).

Mike.

[Ant]

OK I'll have to forget the flats then - I think that the chances of getting that right is non existant!

Cheers for all the advice so far!

If the Bias frames are very short with the lens cap on - what do I gain from them, what do they do for my image?

Ant

[MikeD]

What does a bias frame do.

A CCD chip has thousands of separate collectors called pixels on it that measure the amount of light that falls on them. When you download an image you are downloading the value of each individual pixel, then you put them all together on the monitor screen in the right order to make an image. If you zoom in on an image you can see the individual pixels. So a pixel with a low number is darker than a pixel with a large number. The problem with this is that the pixels do not all start with a value of zero. This means that if two pixels close together receive the same amount of light they will have different values. The value of the light received plus the the value they started with.

Pixel 1.

Start-up value (50) + light received value (1000) = total value (1050)

Pixel 2.

Start-up value (20) + light received value (1000) = total value (1020)

To remove these unwanted values we simply subtract the bias frames from the dark frames and the image frames. This means that every pixel starts with the same value.

Not my words but you get the idea

[MikeD]

Something like a white piece of paper or card would do then - does it have to be under a certain type of light or daylight?

Suddenly observing seems like a good idea

Sorry missed this one.

White card will work, white light is best.

A good method is to put a white t-shirt over the scope and point it at an evenly illuminated bit of sky, (Not at the sun) don't let any shadows fall over the s-shirt.

Before I made a proper light box I used to use a staggered stack of double layered grease proof paper ( use a box and cut a window in each end and cover in double latered grease proof paper)and point that at a light, with this the flats can then be taken at any time, even at night before packing up while you still have the cam on the scope

[MartinB]

Just one point Mike, I wasn't aware flats had to be taken at the same temperature as the lights.

[Ant]

Excellent, thanks for the info.

Things are slowly becomming clearer!

I think I finally understand what all these things do.

Lights, Darks, Flats and Bias should all be taken at the same time (to match temp etc.). But Darks and Bias can be done at a later date if needed. But as soon as the camera is revolved / removed from the scope you cannot do Flats...

Cheers

Ant

[MikeD]

Just one point Mike, I wasn't aware flats had to be taken at the same temperature as the lights.

You are probably right( googled the quote to save time typing ), mine always are as I take them after lights and before darks.

Mike.

[MikeD]

Excellent, thanks for the info.

Things are slowly becomming clearer!

I think I finally understand what all these things do.

Lights, Darks, Flats and Bias should all be taken at the same time (to match temp etc.). But Darks and Bias can be done at a later date if needed. But as soon as the camera is revolved / removed from the scope you cannot do Flats...

Cheers

Ant

Thats it, unless you mark the cam/ scope interface so you can line it all up again.

Another piont: the focus for the flats has to be the same lights.

I always do them at the same time, it,s just part of the packing up routine.

I put the scope away in the shed with the camera Still attached and then do the flats and bias. Then I set the darks going and go to bed (as they sometimes take an hour or two)and they are all waiting for me in the morning and that way the whole thing only adds 10 Min's to the imaging session.

Mike