The Questar 7 is a superfine instrument and will show you more on all objects that fit it's field of view then a 4" apo, or a 5" apo for that matter. That is why it has replaced my 5" Astro-Physics EDFS. Please keep in mind that I am a visual astronomer, all photons to the retina! The Questar 7 is best used for medium to high power observations, ranging from 80x - 400x, altough I feel it's general nights sweetspot is between 120x and 230x. That means it needs a widefield companion. Enter the 4" APO.

Personally, I think your Tak 106 ED is a wonderful telescope that is very hard to beat. It will bring you a lifetime of enjoyment under the stars, both visually and photographically under most any condition. I read you have had it for a few months now, even after decades it will still serve you well! So my advice here: keep the Tak 106 as your allround instrument!

I have a 4"f/8.8 fluorite doublet that I use and enjoy quite a bit and it is a great companion to the Questar 7. Perhaps my 70mm f/8 fluorite is an even better companion, being lighter, easier to transport (airplane!) and mount then the 4" fluorite. So it gets even more time for quick looks. It is great for widefield and gives beautiful images between 20x and 140x , with the 4" going a little higher to about 200x.

Enter the Questar 7. It excels at the higher powers the 4" cannot reach. The Questar 7 is wonderful scope to use but also a telescope that rewards patience and understanding of it's strengths and weaknesses. That took me perhaps 25 years to develop and appreciate. It has rewarded me immeasurably for that. For me it is the observatory-class telescope that I can easily use without the need for a permanent observatory. And it is a labor of vision and love with unique qualities that is a great pleasure to use under the stars at night.

If you can permanently mount your telescope in an observatory, please do consider a 7"-8" refractor in the f/8 (apo)-f/15(achro) range on a stable equatorial mount. They are simply wonderful telescopes to observe with on all objects. But they will make a Questar 7 look like the bargain of the week once you include the cost of the observatory!

I personally feel both the Q7 and large refractors are instruments you grow into after many many years or even decades of observing the universe with other instruments. A decision to buy such an instrument then comes gradually and naturally.

Dude!

After many decades observing Saturn with both (large) high-end refractors and more recently the Questar 7, I have come to fully appreciate what a fine combination Saturn and the Questar 7 are.

Now with the rings tilted to almost edge-on, Saturns moons fully come into view. I find the dance of the 5 brightest moons enchanting. The differences in brightness and orbit add a beautiful depth to the image. Saturn treats us with delicate shading on it's globe and a small shadowline of the rings on the globe. And the passages through the ring-plane are still to come! I am looking forward to greeting that event with the Questar 7, for even in a small APO that was awe inspiring the last time that happened in the mid 90's.

The magnifications I find useful on Saturn range from around 110x to 400x. Below 110x, there is just not enough imagescale to see details. Above 400x is seldomly supported by my seeing. This range coincidently is the optimum working range of the Q7, providing extraordinary bright, sharp and contrasty views!
To really see details well, I prefer to observe for hours. Thus viewing comfort is very important, as is tracking of the planet. Here the Q7 excels too with it's superb forkmount and viewing ergonomics with built-in barlow.
The longer fl Brandon eyepieces in the 24-12mm range provide the magnifications required with generous eyerelief, preventing fogging of the eyelens, while giving extremely clean, sharp, high contrast images.

The Questar 7 has become my most used and loved instrument for observing Saturn. No other telescope gives me the same enjoyment at the eyepiece. Thank you Lawrence, for conceiving such a visionary, timeless instrument, executed to perfection. Thank you Questar for the continous crafting of these wonderful instruments!