[PRejto (Peter)]

I agree with previous comments. I have had very good results from the Televue 3X Powermate with my TEC140. I have seen excellent images with the TEC140 and the 5X Powermate so I invested in one only to find that I was normally defeated by the atmosphere and I consistently obtained better results with the 3X. But, it isn't only atmosphere. As the image gets dimmer you will find that you cannot capture at a high enough frame rate, and that is not a small consideration. I think on a good steady night, on Jupiter, the 5X would possibly get me good results, but with Saturn, and a 5" scope it is probably not going to make you happy, at least very often. If you get excellent focus, steady air, fast frame rate, and a long enough capture, you will be surprised at what you can get. In my view 3X on a 5" f21 or so would be the way to go. Later I will attach 2 images that I took a year or two ago with the TEC140, one at F35 of Saturn on a lucky night (for me).

Edit. I just realised your scope is f5. So, what you get with a 3X is close to what I would get at f7 and a 2X barlow. Perhaps you would be ok with a 4X barlow putting you at f20...this would be very close to what I have imaged at, though the TEC140 has near perfect optics with no obstructions and an extra .5" Still, on a good steady night it might get you a very good result.

Oh, you did not say if you were capturing with a mono or colour camera. I know most serious planet photographers go for mono; I would say your problems are increased 3 fold, but for an appreciable gain. Having said that, however, perhaps cut your teeth so to speak with a decent colour camera as you will probably get a better result sooner and not get too discouraged! (The photos I posted are from a colour camera)

Edit #2: You can also experiment with your existing 2X barlow. Just get an extension tube and increase the distance from the barlow to your CCD. You may get 3X or more depending on the distance and still have pretty good quality. There are ways to calculate the exact power. Shouldn't be hard to locate the formula.