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Hi Ezio,
I wouldnt recommend a cheap barlow if you are trying to get some reasonable results with the Mak and the ASI camera. You will get horrible colour fringing on the planet as cheap barlows will refract the light wavelengths differently for each colour.
Your best bet is to start with a 2 x or 3x Televue barlow. I use a Televue 5x barlow with my F5 (1000mm Focal length) skywatcher newtonian reflector to effectively give me F25 and my Imaging source DBK21AU618.AS webcam but with the longer 1500mm focal length of the Maksutov, you probably would be better off with a 2 or 3x magnification to give you F22 or F33 (F33 might be pushing it too far for the seeing (atmospheric turbulence))
If your camera comes to focus with only the barlow in (and NOT the diagonal, dont use this!) then you shouldnt need the extension tube at all.
Otherwise, you would put the extension tube in the telescope focuser first, then the barlow and then the ASI camera and see if that comes to focus. The extension tube is effectively replacing the diagonal in the image train, so you might need that distance in order to simulate the distance the light would normally travel through the diagonal.
There have been some lovely images taken with the ASI cameras lately and I am sure the MAK should do a nice job on planetary, even if it is a modest 5 inch aperture, so give it a go!
Cheers
Chris
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