New to CCD imaging - help please

[michaelmorris]

I've recently acquired an Atik 490EX mono CCD camera.  All my previous imaging experience is with DSLRs and webcam/DMK cameras.  I know this is going to be a steep learning curve. Here are my first (of many) questions.

Q - What simple guides are there to taking flats and bias frames with a mono CCD camera?

Thanks

 

[gnomus]

Bias: set exposure time to lowest possible (I think you might manage '0').  Shoot 100-200 exposures.  Stack them.

Flats: You need to shoot seperate flats for each filter.  You may find that your CCD is so sensitive that you might need to put sheets of paper over the dewshield because your illuminated panel (or your sky) is too bright.  Aim to have an exposure peak around half-way along the histogram.  I usualy aim for around 25,000 ADU.  I take 25 flats.  If you have SGP it comes with a Flats wizard that helps determine exposure length.  

Good luck.

[Robp]

Agree with everything Gnomus posted. 

The Bias you can take a set off the scope and use them for a while but make sure you get no light ingress.  I renew the Bias set every 6 months.

Flats need to be taken each session without changing focus. I usually take around 50-75 flats as they are pretty quick to take.

[Skipper Billy]

APT (AstroPhotography Tool) also has a flats wizard - just type in the desired ADU and it works everything out and produces a perfect set of Flats.

 

[ollypenrice]

If you're using Atik's very likeable Artemis Capture then all you need to do when setting up flats is look at the white point value in the Display window. As Steve says, get this to about 25000 and that's fine.

Darks and bias should not, in my view, be done on the scope but off it, using the metal screw on cap provided by Atik. When I experimented with light-excluded on-scope darks I found they were very different from darks done off scope. 30 minute darks really showed the difference but it will also be there in shorter ones. That's the price of a sensitive chip. I have no idea how the light got in, but it did.

When you've shot your set of flats and made your master bias, use the master bias as a 'dark-for-flats' or 'flat dark' when stacking your flats. There is absolutely no need to bother with dedicated darks-for-flats because, at short exposure times, they are insignificantly different from bias.

If you are observatory based and don't change your kit around I would say that re-shooting flats every time is a waste of effort. I find mine may last for six months if I'm lucky. It soon becomes obvious when a new dust particle has entered the light path. Also, if your scope has a well made rotator and focuser, rotating the camera will not invalididate your flats because everything that imposed itself on the flat is being rotated as well. Nothing on the objective appears on flats. (Just think of secondary mirrors...) If your focuser sags or your light cone is assymetrical then you will need to re-do flats after rotating but on our three imaging scopes the flats work perfectly after rotation.

You might find that you get better results without darks. I do, as do many others. I use a master bias as a dark and a bad pixel map. This gives me consistently cleaner results on our very noisy Kodak full frame chips then I ever had using darks. A combination of dither guiding and 'master bias as dark' might well work sweetly on your nice clean Kodak chip.

Have fun!

Olly

[AlistairW]

Can i ask - what is meant when its stated - "I use a master bias as dark" ? 

In DSS for example, you can select lights, darks, bias , and flats - and let the program sort the stacking out - but in the absence of darks, what does having a master bias as dark actually mean ?

Thanks

Alistair

[ollypenrice]

15 minutes ago, AlistairW said:

Can i ask - what is meant when its stated - "I use a master bias as dark" ? 

In DSS for example, you can select lights, darks, bias , and flats - and let the program sort the stacking out - but in the absence of darks, what does having a master bias as dark actually mean ?

Thanks

Alistair

It means that you give DSS a master bias in the place where you would normally put a master dark. Simple as that. For your own clarity of mind (if, like me, you are easily confused!) then you could make a copy of your master bias and call it BiasAsDark or something. That way the programme won't be confused by seeing the same file name in different places. It might not like that.

I don't use DSS myself and stack in AstroArt, which I can heartily recommend. Although you can throw all your 'from camera' files (sets of bias, flats, darks, lights, etc) into it for it to deal with en masse, I never do that. I make masters of darks, bias, and flats first. I know I'm going to want to re-use them so it's easier and saves disk space. I will just put all my bias into the box for images (not bias) and ask it to average combine them without alignment. That's a master bias. Same thing with darks, if you use them. When I make my flats I put the flats into the images box (not the flats box) and the master bias into the box for darks and again average combine them without alignment. That's a master flat.

Olly

[red dwalf]

i understand that the darks need to be the same temperature as the lights, but what about the bias and flats ? do these need to be taken at the same temperature ? 

[gnomus]

3 minutes ago, red dwalf said:

i understand that the darks need to be the same temperature as the lights, but what about the bias and flats ? do these need to be taken at the same temperature ? 

I don't see why not.

[swag72]

I have found that the bias and flats do need to be taken at the same temperature as the lights - to this end I use -15 degrees as a standard temperature as I can easily get to that all year round. When I tried bias without cooling, they gave me odd lines all over the place.

[carastro]

That's interesting Sara, I had a lot of problems with Bias frames causing streaking, much more than odd lines, in fact I have given up using them on my CCD cameras.  I have never however heard that flats and Bias need to be the same temperature as the light frames.   What has made you come to this conclusion, in particular the flats?

Carole 

 

 

[swag72]

I found that if they weren't at the same temperature they didn't work...... put the cooler on, cooled down to temp and that was that!

Did you try them at the same temperature as the lights Carole? To see if it made a difference? 

[AlistairW]

Thanks Olly for that explanation. I have been considering AstroArt for a while and I am going to give the trial a good go. 

Simple question, when the masters are combined do you only use the lights and master biasfordark. Or does the normal master bias get added in. Think i am getting confussed ?

[martin_h]

No you only use one bias master, in your case the 'biasfordarks'

[ollypenrice]

I would always run the cooler for taking flats but haven't ever found them to be particularly temperature sensitive. I think the issues will be 1) that flats can introduce noise into an image so should be taken cool to reduce this and 2) that the bias needs to be reasonably well matched when used as a dark for flats.

14 hours ago, AlistairW said:

Thanks Olly for that explanation. I have been considering AstroArt for a while and I am going to give the trial a good go. 

Simple question, when the masters are combined do you only use the lights and master biasfordark. Or does the normal master bias get added in. Think i am getting confussed ?

As Martin says, don't double subtract the bias. I hope you like AstroArt. I certainly do. If I can help on getting going just drop me a PM.

Olly

[Star101]

15 hours ago, swag72 said:

I have found that the bias and flats do need to be taken at the same temperature as the lights - to this end I use -15 degrees as a standard temperature as I can easily get to that all year round. When I tried bias without cooling, they gave me odd lines all over the place.

Thanks Sara, 

Its nice to know the professionals do same

Yesterday it was 10C. I decided to create a new darks/bias library due to noise and streaks on some images. I found that I could not get my ATIK down to -20C. Maybe I didn't give it enough time! I even placed it in a cool box with ice packs from the freezer ( making sure the camera would not get wet from condensation). I got it down to -18C...so decided -15C which is attainable most of the time, especially in summer, will have to be the norm. So started the sequence.

[carastro]

Quote

Did you try them at the same temperature as the lights Carole? To see if it made a difference? 

No, never tried it, I'll give it a go and see if it makes any difference.  

Carole 

[Uranium235]

1 hour ago, carastro said:

No, never tried it, I'll give it a go and see if it makes any difference.  

Carole 

It should make a difference.

I take all my calibration frames with the cooling on. Especially with flats since without cooling the hot pixels are counted as signal (in my experience), and therefore giving an inaccurate ADU count (maybe not such an issue with a sony sensor, but the 8300 chip definitely needs cooling).