To build your own device
Purchase 2 sets of 2 lenses with different focal lengths f1 and f2 (4 lenses total, 2 with f1 focal length, and 2 with f2 focal length)
Separate the first 2 lenses by the sum of their focal lengths (So f1 lens is the first lens, f2 is the 2nd lens, and they are separated by t1= f1+ f2).
Do the same in Step 2 for the other two lenses.
Separate the two sets by t2=2 f2 (f1+ f2) / (f1— f2) apart, so that the two f2 lenses are t2 apart.