[jase (Jason)]

Originally Posted by g__day

How do I decide on aperture? Is the range 100mm to 130mm a sensible choice for what I’ve just described?

As an imaging scope, aperture is only one component. Equally important is the focal ratio. Seriously, I don't think you'd notice a huge difference between the output between a 100mm and 130mm when you're attaching a high QE chip to the imaging train. There is a marked difference between 100mm and 150mm. You'd obtain more faint fuzzies and background galaxies in the 150. Potentially improved resolution, however this will vary depending on arcsec/pixel combination etc.

Originally Posted by g__day

Do I want to opt for an apo where you can add an electronic focuser as a must have, nice to have, or not really needed?

Indeed. You'll want an electronic focuser for any imaging set up. I don't know how others manage without it. You can mount a stepper motor such as a robofocus to just about any scope. I wouldn't purchase a scope that I could not fit a motorised focus on to it - seriously. When you've used a motorised that is computerised, you will never turn back and you can be assured you're always imaging in the CFZ of the telescope, even due to temperature contractions/expansion.

Originally Posted by g__day

Should I lean towards a general purpose quality apo or go for one that is more specific to imaging (e.g. the televue IS series)?

You'll want a APO that has the flattest field possible so head towards a dedicated imaging scope. Most are good for visual work as well.

Originally Posted by g__day

Should I eschew aperture and go for quality (would a 4” Televue be better for my desires than a 5” Williams Optics)?

Depending on what you want to image, but aperture isn't everything in the imaging world. Sure if you're trying to hunt down and image 5'x8' 20th mag. galaxy, you'll want aperture. However, you'll never be doing this with an APO. APOs are wide field instruments as I mentioned in my previous post. If you want deep sky (+2000mm) you need a different scope. I'd go for a quality 4" over a mediocre 5" any day. Also keep an eye on the required accessories too. If the scope has an inherent flat field into the design (Petzval), then the imaging train is simplified.

Originally Posted by g__day

Am I going to have horrible problems matching final imaging cameras to this very different range of SCT and apo? (meaning sorry its two not one S-BIG or FLI or whatever needed)?

Can't really make a call on this. Get a camera between the 7.4u and 9u pixel size. On the SCT you can bin the pixels to get a suitable arcsec/pixel combination. On the APO, simply go 1x1 (no binning). Once you've selected the telescope, come back to us regarding camera selection.

Originally Posted by g__day

I say I like planetary nebulae on the SCT but an apo is more wide field – your thinking is wrong and you haven’t taken into account… whatever

Yep, you need to work out what you want to image. The wide field APO isn't going to be the best to hunt down the faint Sharpless objects. If you're passionate about your planetary nebula goals and still want to use the SCT, perhaps look into an AO unit to counteract seeing. Alternatively sell up and get something else suited to your goals.