There are many approaches to the dark sky wide field refractor thing, Whenever I am out under dark skies, I set up at least one, sometimes two, short focal length fast refractors to go with the Dob I have chosen.
Here's some thoughts and experiences..
- Achro versus ed/apo. If the scope is only going to be used for deep sky, an achromat offers similar views to an ed/apo at a much more affordable cost. If the scope is going to be a dual purpose scope that will see backyard use for the planets and doubles as well as deep sky from a dark site, then an ED/apo is worthwhile.
- 80mm versus 102mm. For me, these are two apertures I find to be the best companions for a Dob. The 4 inch typically goes some deeper, the 80mm goes some wider. Typically the 80 mm will be lighter and more manageable than the 4 inch. In my mind, for this sort of observing, there is a noticeable difference between an 80mm and a 100mm but like an 8 inch Dob and a 10 inch Dob, they are much more the same than they are different.
- Over the past 20 years I have owned a number of wide field refractors, currently I have 5, two achromats and 3 ED/apos, an ST-80 fitted with a 2 inch focuser, a Celestron 100mm F/5, an 80mm F/6 doublet, an 80mm F/7 doublet and the TeleVue NP-101. Each one has it's place, has it's virtues. Other scopes I have owned that are worthy of consideration are the ES AR-102 achromat, the AT-102ED, the Orion 100mm F/6 achromat.
- The 102mm F/5 achromat and the NP-101 are at opposite ends of the spectrum. The 102mm F/5 achromat is just simple and crude, significant field curvature, chromatic aberration at higher powers, the NP-101 is as perfect as it gets for a wide field scope. It's corrected for field curvature, the color correction is essentially perfect, whatever you want to look at, it's just about as perfect as perfect can be.
And yet, when observing deep sky, particularly at lower magnifications, one sees pretty much the same things in both scopes, 4 inches is 4 inches.
The NP-101, even used, is not within the budget. A used Genesis or SDF might be but I'd probably skip those.
I see two approaches. Something very capable and very affordable, the 102mm F/5 achromat. Get your feet wet, with the 30mm ES 82 degree you're looking at a 4.8 degree TFoV at about 17x, that is going to be simply awesome. Pop on an UHC or O-III filter and go to town.
The other approach is the Astro-Tech 102ED. It doesn't offer the widest field of views, about 3.4 degrees with the 30 mm ES 82 Degree, but they are plenty wide for most purposes and it's better at high magnifications on DSOs and it's a dual use scope, it is a grap and go doubles and planets scope, it's more versatile.