Tele Vue Ranger has excellent optics at its age, once considered to be the reference standard at its own class together with its brother, the Tele Vue Pronto. However, we have got many small apochromatic refractors come out to the market these days, Tele Vue finally discontinued their Ranger/Pronto.
My Ranger has served me extremely well, from wide field visual observation, to medium power planet/lunar observation. When I bought my SM40/BF10, it also served me as an excellent solar scope, too. I've some special relationship with it, it's not my first telescope (which is a C90), but I used it the most, I didn't use my first telescope as much as it is. I would say, it's my first telescope which I really use and learn astronomy.
So, it's normal to think of how to upgrade it. Several vendors are providing 80mm f/7 APO lens in cell to the market, to name two, say William Optics's APOGrade lens which is selling at around $400 USD, and another vendor in ebay, selling a lens in cell at around $300. People reported that the William Optic's one is better in terms of quality.
So, one will say, Ranger is a 70mm instrument, so how to use a 80mm lens with it?
Yes, Ranger is a 70mm f=480mm telescope and the lens is going like 80mm f=560mm. Ranger has a 77mm thread lens cell, and therefore, one can make a small extension tube to make up that 80mm difference in focal length, and with a 77mm male thread to mate with the original lens cell! Of course, you will need to remove the original Ranger lens first. Simple?
One can also adjust that 80mm difference in focal length to create more or less back focus, according to your needs like your imaging device, binoviewers, etc.
In order not to vignet the 80mm aperture, one can use similar triangle to calculate the light cone size before entering the Ranger tube which is 77mm at its longest. Given a 80mm f/7 lens, the light cone will be 77mm wide when it's ~21mm from the objective. Therefore, the new lens cell should at least 21mm when joining the old Ranger lens cell with 77mm thread.
To preserve the original focus travel of the Ranger, we will need the extension to be 560-480mm = 80mm and for it's longer than 21mm, it will not vignet the 80mm objective. However, the Ranger lens is recessed from the front thread, we should taken it into account. It's around 2cm recessed, and therefore, the extension tube should be at most 60mm if we want to keep the original focus travel.
Therefore, we can go from 21mm to 60mm as we wish. If we want more back focus like for binoviewer, we should make it shorter, like 21mm or 25mm to be safe. If we simply want to preserve the original focus travel, we can simply make it 60mm.
A pretty simple project, I suppose, provided that if you have a friend who can machine the adapter for you.