How to add all arrays from all .fits files in a directory?

Question

I have a directory on my computer with several .fits files that I am trying to work with in IDL. For some reason I am unable to add all of the arrays together correctly, as I'm getting negative numbers in the totaled array when all of the individual elements in all files are positive.

In IDL, I tried the following code:

  flatsfiles = file_search(Vflats, filter)
  addedflats = make_array(1530,1020,value=0)
  
  FOREACH element, flatsfiles DO BEGIN
    flatsarray = readfits(element)
    addedflats = [addedflats + flatsarray]
  ENDFOREACH

Pretty simple code, yet the values in addedflats are negative, while all the arrays have ONLY positive elements on the order of 10^4. Can anyone see where I'm going wrong, or have a different way of doing this?

I've also tried adding the arrays one by one to see where it goes wrong:

  array = readfits(flatsfiles[0])
  addedflats = [addedflats + array]
  array2 = readfits(flatsfiles[1])
  addedflats = [addedflats + array2]

Here, the first addedflats array shows the same thing as array, which is expected since it's just being added to an array of 0's. The second addedflats, however, gives negative numbers again. For reference, the first element of array is 25189, the first element of array2 is 24030, but the first element of addedflats is -16317 rather than the expected 49219. TIA!!

Answer 1

Be careful of integer types that might not be able to hold the sum. Your example is probably 16-bit signed integers (the default integers in IDL):

IDL> print, 25189 + 24030
  -16317

To fix, declare at least one of the arrays that you are adding to be an integer type that is big enough to hold there result. Here, 32-bit longs are sufficient (just doing scalars for simplicity, but the same works for arrays):

IDL> print, 25189L + 24030L
       49219