Guide Scope/Camera Necessary?

Well schmaks…like most things there is a lot to this answer.  Basically, guiding attempts to correct shortcomings in the manufacture of our equipment when we ask more of it than it can deliver.  No mount is perfect. So there will be some movement of the mount, any mount, that is not in sync with the sky. Also, other things interfere with the mounts tracking, like wind for example.  If you have a very long focal length scope, taking a very long exposure, that movement will show up as streaky stars and blurry targets.  If you are using a short focal length and taking a quick exposure, that unsynchronized movement may well not be obvious or even visible.  We used to talk about the 500 rule in photography, that says divide 500 by your focal length…if you exposure time is below that value you likely will not notice star trails in an image, and that was for a still camera with no tracking.  So, even if your tracking isn't perfect, a set of fast exposures with a short focal length probably will not need guiding.  For example, I frequently use a star tracker (which is not considered a very accurate mount) with a dslr camera and short lens (say 85mm). No guiding.  I can take a 30 sec exposure like that and they are perfect.  But, with the same mount, if I put on a 250mm scope with an APSC camera and attempt to take a 120 sec exposure without guiding, I will absolutely not get a good image. My long scopes, like my 2030mm SCT need guiding (or encoders…different solution to the same problem) for any shot.

Guiding works by having a separate camera connected to software on the computer that looks at one single star. If that star moves around in the image, the guiding software sends a signal to the mount to nudge it back on track.  It watches this star through the entire imaging session, looking at that one star, and keeping it in the same place in the frame by nudging the scope forward, back, up, down, by tiny increments, all night.

To my knowledge, there is no way to do this without a computer, but that computer can be almost anything. There are products that use very small inexpensive computers that you access through your phone.  It doesn't have to be costly or high powered.

Finally, you asked if there are any DSOs that you can capture without guiding.  There are a few, but they have to be bright and you will likely have to be at a short focal length.  Things like M42, or the milky way core are frequently imaged without guiding.

Hope that helps.
Tim

Depends greatly on the scope. You might want to check out a Redcat 51 from William optics (250mm Petzval).  It is a very nice scope and not to pricy. As a guide scope you could use a ZWO Mini Guide scope and pair it with a ZWO ASI174mm Mini or a ZWO ASI120mm Mini. This is a package that I own (actually I own 2) and it is very capable. Also, 250mm is a good focal length for someone just starting out. I made the mistake of starting out with a long focal length SCT and it was way too much scope for a beginner.

Since I purchased my Redcat they have introduced a new model that looks to be much improved. Check it out. You can see mine in my profile Here . I have a focuser on mine but you can ignore that is you're not looking for a focuser at this time.

The HEQ5 is way more mount than you need for this scope. You may need to be careful about balancing because this scope is on the small side, physically.

If this proves to bee too small a package look for a main scope around 500mm and a small refractor as a guide scope. Orion makes a nice little 50mm guide scope. It works very well. Also, a Stellarvue F50G is a very nice little guide scope. I own both and both are great. I prefer the F50G. The two guide cameras that I mentioned above are great little guide cams. Of course, the traditional answer to the guide cam question is a Lodestar.

You can't make a wrong choice with any of the gear that I've suggested. Just make sure you choose a pairing that all works well together (don't put a ZWO mini guide on a 2000mm main scope, for example).

Best.
Tim

It is perfectly possible to capture unguided DSOs, the only limitation is your exposure will not be as long. If you don't mind taking, storing and processing 400×30" subs instead of 100×120" then unguided will arguably prove better: four stacked 30" exposures, each well tracked, will be sharper than a single 120 second exposure where the autoguider sent twenty correction pulses, sometimes for the wrong reason such as a slight breeze or a small fluctuation in seeing that it should have just ignored.

I do both guided and unguided. I use a Raspberry Pi 4 with Kstars as the "guiding computer" but since I have it, I also use it in unguided captures, as an intervalometer.

I've always used Nikon DSLR cameras, they have their own intervalometer but the problem is it has only 30, 25, 20, 15, 10 second exposures (and shorter of course). If you want some custom duration, e.g. 35 seconds you must use a shutter control cable and a timer. I have done it in the past and I guess it is not so bad. But if you want to fire up a sequence of 50 images obviously the Pi is a major convenience.

The error of the tracking mount which has no mechanical issues, that is to say no loose screws, no undervoltage and no mechanical damage, should not increase significantly over time. If it does, it means your polar alignment is not so good, or the equipment is too heavy for the mount, or not properly balanced. If everything is set up correctly, the error should be periodic, meaning it becomes zero every 5-10 minutes (depending on how long it takes your mount's internal mechanism to complete a rotation).

In this regard I would say there is an advantage in guided compared to unguide: when you guide, you see a graph indicating the error in RA, the error in DEC and the corrections sent by the autoguider. Normally the errors should be more or less sinusoidal, +, -, +,-,+,-, averaging out at zero in the long run. If you see that the DEC error is consistently positive (or negative), this means you have a polar alignment issue. If you see that the RA error is consistently positive or negative, it means you have a weight or balancing issue. With unguided it is quite difficult to understand what is going on simply by looking at the pictures, I mean obviously there will be star trails but go figure why

I would summarize the situation as follows:
Guided: much longer subs but slightly less sharp, also impossible without a computer and note the guiding equipment *is* extra weight and set up complexity.
Unguided: sharper subs if done right, significantly shorter exposures, possible without computer, a little more difficult to understand what the problem is when there is a problem.
Both: you need to do accurate polar alignment, good balancing and have a mount that is comfortable with the weight of your equipment.