For a DSLR you need a T-ring and T-adapter. The T-adapter can be a flattener (for example, the Hotech SCA 2" flattener (recommended) screws into the T-ring and then the whole thing goes into the scope focuser (needs a 2" focuser, but that's pretty much the standard). No spacers needed--the flattener has 55mm back focus which is how much a T-ring + DSLR has (11 + 44 mm respectively). All you need is a T-ring for your camera (Canon or Nikon) type.
You may need an extender to your focuser to reach focus with the camera. Something like this. Depends on the scope.
There are both flatteners and reducer/flatteners. The first kind are more forgiving, the second need to be well-matched to your particular scope or your corner stars will rarely if ever be perfect. People tend to prefer the reducer/flatteners because they shave off needed imaging time (at the cost of image scale). As the Evostar is a relatively fast F/5.8, you don't really need a reducer.
The T3i was a good camera, but nowadays the Nikon D5300 is a better choice because of lower dark noise and ability to shoot at 200-400 ISO which helps dynamic range. The Canons are too noisy there, and need 800-1600 ISO. You only need the camera *body*.
Starting off unguided may be OK with your short F/L when you're just learning and *if* you limit your exposures to 60-90", but guiding will improve your images even here, is easy, and will be an absolute necessity if you get a longer F/L. I guide even at 336mm. I recommend you start sooner rather than later.
A recommendation: Read up on the calibration frames (the regular images you take are called "lights", there are also *flats*, *darks* and *bias*). All need to be taken, and it's better to get used to doing so from the start. They help with vignetting and dust motes (flats), thermal noise and hot/cold pixels (darks) and read noise (bias).
Alright, things are clearing up a bit. I think maybe I'm getting confused by different names being used for the same thing, regarding adapters. Like if I look at the RedCat 51 on FLO. Among the frequently purchased items they have the "William Optics High Precision Copper M48 T Mount" and the "William Optics DSLR-M48 Ring Adapter" but looking at them I believe they're both adapters to connect the camera to the RedCat. But one is called M48 T Mount and the other DSLR-M48 Ring Adapter.
As for the T3i, the reason I'm looking at that for the moment is because there's a guy that sells Canon cameras astromodified, so I can get one relatively cheap. It'll be good enough while I'm getting started. I'll probably move to a cooled colour camera once I feel ready.
I am planning on moving to guided imaging but, as you're aware, this hobby isn't cheap and I'm trying to get the bare minimums for starters.
I'm familiar with calibration frames. I used darks and bias previously. I wasn't sure how I would take flats with a camera lens as it was easy to bump the manual focus. But I'll definitely get more into that when I have my rig ready.
1) The telescope you pick, in my opnion, should be one that won't be seen as a waste of money in the future. Buy something cheap that is effective, light, and easy to use as a first scope. IMO good examples of this are the AT65EDQ and the RedCat51 or other quadruplets and the reason is that although these are small scopes, they are about as good as small scopes get. They can stay in your rotation as quick setups or as travel scopes even as you upgrade your medium and large telescopes in future if you do. A lot of people say to start with a slightly bigger but cheaper refractor like the SW explorer you mentioned and in my opinion that is not the best way to go because if you go that route you're going to end up replacing or just getting rid of those scopes. Additionally, a really fast and wide field refractor like the RedCat is foolproof and incredibly easy even if you're still learning to get better polar alignment or to guide. Of the two you selected, I'd get the smaller one as it would be a decent scope and might last you a while. This is a matter of opinion and of my own circumstances as someone on a budget, and if absolute efficiency of your dollars in the long run is not super important you might want a slightly bigger one simply so you can get better reach and resolution with your T3i.
2) Flatteners are required unless it's a quadruplet. Triplets or quadruplets are best for AP because of the color correction, but 4 lens elements are required for a flat field, which is why you either need a quadruplet scope or a triplet with a flattener.
3) With a DSLR, backfocus is already tight out of the box for most scopes. DSLR sensors are much deeper into the camera than dedicated astro cams and as a result adapters usually need to be minimal and no spacers are typically needed.
4) There are T-rings designed with M42 and T-rings with M48 threads. One end of the T-ring connects to your camera's respective lens mount (i.e. EF for canon, E mount, etc.). The other end has threads for either M42 or M48. Make sure you have the right one for your scope's threads because With some scopes and a DSLR, backfocus might not be enough to adapt from M42 to M48 and vice-versa as the extra space might push it too far. Make sure you get whatever T ring is necessary for your camera/telescope threads. M42 T-rings are the ones you screw into a 1.25" T adapter to slot in where an eyepiece would go. M48 usually just screws right onto something like a field flattener or filter wheel.
5) If you're going to use a DSLR, I'd recommend getting one that is astro modified. This increases sensitivity to a couple of emission lines that are everywhere and you're not likely to spend any more for this than you would for a normal unmodified one these days. I got one for $200 I believe on eBay and it came with a decent kit lens too.
Great feedback and tips. I'm definitely leaning more towards a RedCat now as it seems really handy, and as you say it'll never lose its purpose and be hard to outperform in its class. It does feel slightly pricey for a 51mm aperture scope but as it's a quadruplet I do get a pretty flat image right off the bat, not needing any extra glass. The built-in Bahtinov mask is quite neat as well. I really like what they've done. I don't think I've seen any larger quadruplets like this from WO?
I feel a bit stupid after a realized the M42/M48.. are just regular thread dimensions. I thought I was looking at some specialized astro nomenclature. It's all falling into place now. So some scopes use the nosepiece adapter and, like on the RedCat, you connect with the thread and either way you need your T-ring for your camera that fits whatever adapter the scope requires. Does that sound about right? That also explains why I needed the M48 adapter if I had a coma corrector on the Explorer 130PDS, which otherwise would take a nosepiece adapter. It's all making sense!
I would agree that there are probably better old used cameras than the T3i such as the D5300, and not really any more expensive. These days dedicated CMOS cameras are not too expensive either so I'd not rule those out. I will say that darks with a DSLR can have limited benefit but they might be worth trying. I've tried many times and never found any appreciable difference with or without them, but it usually doesn't hurt to try.
I would like the D5300 as I already have some Nikon stuff. Unfortunately I haven't found anyone selling them astromodified here in Sweden. As I said above, I'm looking at a DSLR as a temporary fix until I can get a dedicated camera. Buying a proper DSLR would probably be a good idea to have a good portable setup but right now I'm going with what I can find. I put my trust in the calibration frames and stacking to save me
For medium size targets, the normal suggestion is high quality, APO refractors of 70-80mm aperture of f/6 or faster with a field flattener and possibly a focal reducer. This can be expanded to larger, but faster scopes, such as Skywatcher Esprit 100 (550mm FL, f/5.5), and AT92 (506mm FL, f/5.5). I have the Esprit 100, which I shoot at both 550mm (native, included FF) and 413mm (f/4.13, with TSAPORED075 FF/FR), and I love it. I highly recommend the Esprit 100 ($2500, with everything included, even FF), with the idea of later getting a FR for a wider and faster option. Other good lower priced options include the Esprit 80 (400mm FL, f/5; $1650, with everything included, even FF), WO Star71 II (350mm FL, f/4.9; $1200, petzval design, no FF necessary), WO GT71 (419mm FL, f/5.9; $828 + $198 for FF/FR), and SV SVX080T-3SV (480mm FL, f/6; $2000, with everything included, even FF). TS Optics out of Germany also has many good value scopes, and the Sharpstar 61EDPH and 76EDPH look like very intriguing budget picks, especially when paired with their FF.
Get a quality APO, preferable a triplet or quad with high quality glass, such as FPL-53 or FCD-100, with a good focuser. And get a field flatner, but in severe budgetary situations, get a better scope and save the FF for later. Save money by going smaller in aperture, not by going for a larger, but inferior quality, such as Achros or those with bad glass/focusers. This way when you progress, you will still have a decent scope for larger targets.
I don't really have the budget to go for some of those more expensive scopes. Definitely nothing over $1000 and right now I feel the RedCat 51 at around $750 is pretty much my limit. I thought the ZenithStar 61 was interesting too but I would need the FF then. Good tips!
Gosh, I have never had such a difficult time deciding what to buy. I really feel like I have a lot of good options and it's tough to decide on one. I always find another good option once I learn more and maybe this new choice is a little more future proof or whatever and I feel like I need to do more research.
I really appreciate all your help and fantastic tips. Really good and sensible stuff. This is a fantastic forum and it's really fun to see such helpful members when you come here as a rookie.