How do you subtract darks & flats using Maxim?

[Martin-Devon]

Can somebody please just run through for me the key essential steps, or point me to a video (if one exists) of how you use the Maxim software to substract flats & darks from your images. I'm a beginner to astroimaging & processing and so far I have ignored flats & darks - but for my next step at making improvements I really need to get my head around these now.

I think I know how to generate the flats & darks, but I have no idea how you actually use Maxim software to subtract them. So if somebody could kindly just outline the steps you follow to do this, it would be very helpful.

Thanks,

Martin

[roundycat]

first off you might like to look here Astrophotography.

What version of MaxIm are you using?

Dennis

[steppenwolf]

You should also take some BIAS frames. Copy all your BIAS, DARK and FLAT frames into a single directory, let's call it 'calibration'

In Maxim select Process - Set Calibration

Tick Calibrate Bias, Calibrate Dark, Calibrate Flat and Bias subtract Flats

Under 'Source Folder', browse to the folder that holds your calibration files

Click on the Autogenerate Button.

Select the BIAS group in the top window and set attribute of Combine Type 'average'

Repeat for the DARKS

Select the FLATS group in the top window and set attribute of Combine Type 'median' and tick the 'apply boxcar filter' box

Click on the Replace with Masters button. Click on OK when the software finishes producing the masters.

The above prepares your various calibration files in readiness for calibrating your images.

Load in your images

Select Process - Calibrate all then click on OK

Job done!

[Martin-Devon]

Thanks for the quick replies - these are really very helpful.

Roundycat - I have V4 of Maxim; thanks for the weblink

Steve - this is a very nice & simple (appreciated) description of the steps

[roundycat]

I wrote this based on v5 but the older version is all but identical.

Calibration with MaxIm.

First of all, attend to your filing system. MaxIm has ways of collecting cal frames and assembling them into masters and assigning them a group name. Rather than wade through all that simply attach a new folder to your current image sub-frame folder. Call this new folder Cal.

Place in that folder all your calibration frames, ie 30 darks, 30 bias and (for me) twenty flats for each filter. Open all the darks and combine them using a Median output. Do the same with the Bias. Do the same with all the flats for each filter but first of all subtract Bias from each flat before combining them. (Don’t subtract bias from the darks).

At this point you have a master dark, master bias and master flats for each filter plus all the single cal frames. Delete (or move) all the single cal frames so the Cal folder has only the masters in it. You have no further need for these single cal frames.

To calibrate the light frames in MaxIm is now fairly easy. Open the Set Calibration dialogue. Navigate to the folder marked Cal which should be a sub-folder of all your lights. This approach keeps everything very simple. If nothing changes in the set-up and you move to a new target you can simply move or copy the cal folder to the new target folder ready to calibrate the new frames.

After finding the Cal source folder in the Set Calibration dialogue click on Auto Generate. This will ‘fill’ the dialogue with the correct cal frames.

On the Advanced tab you should select what you want MaxIm to do. There are five choices and the first thing you will do (maybe) is to Bias subtract your Flats.

For image calibration you first subtract the master dark and then apply (double division) the master flat so tick those boxes. MaxIm reads the FITS header to sort out the correct filter for the flats and the correct exposure time for the darks so if you have more than one master dark in the box it does not matter.

You can also use a bad pixel map during dark subtraction. Just highlight the correct map if you have more than one and it will be done automatically.

You will get told off if you try to calibrate using the wrong master. Such as wrong temperature, binning, filters and so on. If your image has been cropped MaxIm will sort out the correct part of the cal master.

If, when you first open the Set Calibration dialogue you are told there are no suitable cal frames it is just MaxIm being clever. OK the warning and continue to navigate to the correct cal folder.

Once the Set Cal dialogue is set up, OK it and go to Process – Calibrate or Calibrate All. If the required light frames are open they will all be calibrated. The Cal masters used will be listed in the FITS header.

Dennis