I would advise against a DSLR lens for DSO Imaging, even the venerable L series. Using a camera lens for telephoto astronomy is a classic newbie mistake, and I can say from experience I’ve been there (10 years ago). Reasons to avoid a lens for DSO Imaging :
I agree with a lot of your reasons, but man, honestly, it really doesn't matter for someone just starting out. (and for experience ... 47 years - I win. )
They should go out and learn to shoot with what they have. See if they have fun doing it. If they do, and they want to get better, then then can buy a telescope and real mount.
And, in the real world, and even for the most experienced and even the pros, you're going to be shooting with camera lenses anyway for shorter focal lengths for wide angles, so get used to it.
And the Dragonfly Telephoto Array team doesn't have any problems using 48 Canon 400mm f/2.8 telephoto lenses in their cluster that they do real science with.
As far as your specific points, there are some things to think about though...
1. I can't find a full res of M42 by tjones 84(and I looked but might have missed it), so I can't make a judgement on this. I've shot with the Canon 300/4 and I didn't find it that bad at all when stopped down a small bit.
2. It's really about air-to-glass surfaces in terms of light loss. The simple 70mm ED refractor will have 4 air-glass surfaces initially, but once you add a field flattener you have more than that, and a lot more elements too. And if you really want to compare a 70mm refractor with 420mm of focal length to a 300mm camera lens, you also need a focal reducer, and those are more complex and expensive, which adds to the cost which I will get to in number 3. BTW, I recommend a small refractor like this as a first scope for astrophotography too. It's just that you don't have to go straight to a scope when you already have camera lenses that you can test at lest to see how good they are before you drop the cash on a scope and decent mount.
3. Astro-Tech AT72EDII - $469 +$120 for focal reducer = $589 probably can find cheaper maybe on used forum here.
Canon 300mm f/4 non-IS EF - $439 used on EBay.
The cost is really a wash. A used 300/4 is not that expensive.
4. Some of them flatten the field pretty well. You are right about not being designed to shoot stars. But some camera manufacturers are now considering that when producing lenses such as the Sigma Art lenses (which cost a fortune) and Rokinon.
5. Much harder to use the refractor to shoot wildlife or sports. And they don't autofocus.
If someone said I have $450 to spend on an optical system and all I want to use it for is visual and photographic astronomy should I buy a used 300mm Canon lens or a used 72mm doublet ED refractor with focal reducer - I would unquestionably recommed the refractor.
If someone said I already have this 300mm f/4 lens, I would say use it, but don't expect it to be quite as good as a scope on star fields.
And don't forget to figure in $1,500 for the mount if you get the scope, because the line on small portable equatorial mountings like the SkyTracker and Sky Adventurere is going to be at about 300mm in terms of what they can carry, and I'm remembering (foggily) that the 300/4 weighs a lot less than a 72mm/0.8x FR refractor.
Edited by Jerry Lodriguss, 23 December 2018 - 10:12 PM.