Newbie - flat dark query.

Posted by: happylimpet

Hi! New to APP - have successfully used to mosaic stacks processed and flattened in DSS/FITSWORK. However now trying to start from raw images, darks,flats.

See the attached - the flat is a fully calibrated flat (ie already stacked with flat darks) Master from DSS that I have dropped in. I dont use biases - recommended against with CMOS.

Im concerned that the flat has a 'D' against it, which suggests that the dark frame (also a Master stack from DSS) will be used with it, which would be disastrous. Can someone explain if this is the case, and if so how to prevent it?

I also find it odd and troubling that I cant upload flat darks into APP; does APP expect me to use biases or the same darks with flats?

Cheers

Hi @happylimpet,

Thank you for your question and welcome to the APP forum 😉

See the attached - the flat is a fully calibrated flat (ie already stacked with flat darks) Master from DSS that I have dropped in. I dont use biases - recommended against with CMOS.

That's fine 😉

First of all, let me point out that using masterframes from other applications can be a recipe for problems (especially for DSLR data). If you  have the original calibration frames, I would recommend to use those as a starting point in APP for data calibration. If you only have the masters then we need to work with that.

Yes, the D next to the flat, means it will subtract a matching masterdark from the flat. I can't see everything of the frame list, so I don't know what else is there.. but I think the dark was converted to a masterdark and then that would be matched to the flat because it has the same iso/gain and exposure as reported by metadata. And that's because the metadata of the DSS masters is insufficient.

What you will need to do to fix this:

1) clear the frame list.

2) only load your DSS masterdark as dark. Let APP create a MasterDark for APP from it by clicking calibrate.

3) clear the list again.

4) only load your DSS masterflat as flat. let APP create a MasterFlat for APP from it by clicking calibrate. Since this master was created properly by having subtracted dark flats like you indicate, this should be fine.

5) Then clear the list

6) load your lights, and the APP Masterdark and Masterflat. That should be it 😉

I also find it odd and troubling that I cant upload flat darks into APP; does APP expect me to use biases or the same darks with flats?

APP 1.059 and lower can perfectly address flat darks. Just realize that a flat dark is just a dark for a flat, you should load them as darks. The masterdarkflat created will be matched with the flats if they have the same exposure and ISO/gain as the flats.

(I do this all the time with my ASI1600mm-cool, for the reason that you give)

But, APP 1.060 (which I am working on right now), will have a separate dark flat load button and will also create a MasterDarkFlat, so that will be easier. And the matching of masters to lights and flats will be much easier and transparant.

Currently, the calibration rules demand that a masterdark is only applied if it has the same exposure and ISO/gain value. That will no longer be the case in APP 1.060 and it will also include dark scaling 😉

Let me know if this answers your concerns and questions.

Kind regards,

Mabula

Hi!

Thanks for the detailed answer - that makes a lot of sense. As you say, best to just go back and use the original files properly.

Cheers Nick

You're most welcome @happylimpet, let me know how it works out 😉