Calibration frames - just checking

[Carole]

Just out of interest, How many darks, flats and Bias frames do you need, as I'm starting to build up a library of darks and just wondering how many I need to save.  

Darks
Ideally do you need 1 dark per light frame, or do you get to a point after say an hour or two when adding more darks is not really necessary.  
Also what is the minimum you can get away with if say you forget to do your darks, or you set them up and they fail half done for some reason?

Flats
I am normally doing about 15 flats per image, is this sufficient?
Bias
I normally do around 5 - 10 Bias frames and re-use them for a few months and then re-do them, is this sufficient?

How many people do dark flats?  I sometimes do them when I remember.

Carole

[MarkS]

My answers are relevant to my Canon DSLR:

Darks:  I have a single master dark (created from 16 or so 5 minute frames at a warmish tmperature) that I've used for every image I've done for the last year.  To account for temperature differences, I use IRIS to optimise the dark against each sub so I know what multiplier to apply to the dark before it is used.

Flats:  15 frames should be fine to create a master flat if you do them at a low ISO e.g. ISO 100.  I've used the same flat for the last 6 months because the dirt on my CCD has remained static in that time.

Bias: I never use bias frames - I use a synthetic bias frame with all pixels set to 256.  The main issue with my DSLR is background "curtaining".  But this "curtaining" shifts around from frame to frame so bias frames don't help.

Flat darks:  My flats are taken with a very short exposure so there's no need to have darks for them.

You can tell that I'm not obsessional about calibration because I've never found that all the extra work (creating flats, darks and biases on every single shoot) makes any difference.  But I am obsessional about using both dithering during acquisition and sigma stacking during processing - both these techniques make a lot of difference.

Mark

[Carole]

Thanks Mark that was very interesting.  I won't be able to deal with darks the same way as you, so I'll see what the others say.  

Flat's I do every time they don't take long and yes I use 100 ISO.  I was wondering whether it was worth doing flat darks as you say the image is so short, so glad to hear you think the same. 

Bias I do because I was told to, again if I am re-using them they are no trouble.

Dithering I don't do a lot of, something I need to progress to, though I do sometimes stop the guiding, move a few pixels during a run, and if I'm stacking images done on different evenings these help too, just wondering how often one needs to dither within the space of an hour to make it worthwhile.

Sigma stacking is a new one for me which I will look into as well.

Carole

[RobertM]

I do agree with what Mark has pointed out except that I do use real bias frames for all my cameras.  Bias and dark frames are used for about 6 months or more and as he says; most stacking software will scale darks very well.  Even flats can be synthesised as long as there are no dust bunnies but it's essential to take those again after the optical setup has changed (camera moved, different filter, recollimation etc...).

I've never used Flat Darks only Darks flats and bias.

Dithering is something that happens between each sub automatically with guiding.  It's not something you do occasionally, however you could do it manually if you have the patience and concentration.

Robert

[Carole]

Dithering is something that happens between each sub automatically with guiding

You mean PHD will do it automatically?
If so, didn't realise I was already dithering 

Carole

[The Thing]

Phd doesnt do dithering itself but other software eg astro photography Tool and nebulosity can control phd when you have the phd server enabled so you can define dithering in them and phd will do it.

[mickw]

I take flats and darks at same iso as the lights and usually the same quantity plus bias cos I keep forgetting I have a bunch 

[MarkS]

Dithering must happen between exposures.  The guiding oftware (e.g. PHD) must do the dither.  But PHD doesn't know when the exposures are taking place.  Therefore the application taking the exposures must signal to PHD when an exposure completes and then wait for PHD to do the dither before it triggers another exposure.  All clever stuff! 

But as Duncan says, you need a application that is PHD compatible.

Mark

[Carole]

Phd doesnt do dithering itself but other software eg astro photography Tool and nebulosity can control phd when you have the phd server enabled so you can define dithering in them and phd will do it.

Yes that was what I had thought, but Robert's post made me think I had got it wrong.

Dithering is something that happens between each sub automatically with guiding

But PHD doesn't know when the exposures are taking place.  Therefore the application taking the exposures must signal to PHD when an exposure completes

Yes that makes sense Mark.

I take flats and darks at same iso as the lights

Mick I think you have to take the flats at 100 ISO.  

Carole

[Carole]

OK, well looks like I am doing it OK, dithering would help and probably don't need as many darks as light frames if doing a long run. 

Still looking at APT, but as I am well used to Canon Utility I am being a bit lazy at sussing it all out.

Carole

[RobertM]

But as Duncan says, you need a application that is PHD compatible.

Or other astro program such as Maxim that has it integrated.

It is something you need to set but is automated once setup - sorry if my original text was a bit vague there.  With Maxim it is just a setting in the guiding tab but with phd you need to be able to synchronise with exposures taken with an external program, hence the 'server' mode.  That mode allows the external imaging program to communicate with phd and move the position of the guide centroid.

Use APT properly and it will save you a whole bunch of time.

Robert

[Carole]

Thanks Robert,

Use APT properly and it will save you a whole bunch of time

Yes, must get down to it.

Carole

[Fay]

Mark, I must remember to do my Canon flats @ ISO 100, I do them at ISO 800, which is really difficult to get the correct exposure time. What is the exposure time you do at ISO100?

Fay

[mickw]

Why 100 iso and not the same as lights ?

[RobertM]

Because flats are all about characterising the optical path.  What you want is the lowest noise images so you don't need to combine too many.  The exposure time will be dependent on your optics and the brightness of the light source used for flats.  Ideally you want the exposures for all colours to be about 2/3rd well depth (read 2/3rd the way across the histogram on the DSLR screen) and taken so that the level is the same for all of them.

Robert