127mm Vs 72mm

[Rod-AR127 (Rod)]

I made a comment the other day that l had bought this little Sky-Watcher ED72 to compare the optics to my 127mm Bresser. That's a $659 question right there.
Well, I'm glad l did.
I don't know the maths of how it all works, l think FOV plays a big part in it though.
With the same Orion Edge On Planetary 5mm EP as l used in the 127mm achro, l just viewed Jupiter bigger and clearer in the ED 72mm. Isn't it supposed to be the other way around?
Huge field of view with Jupiter and four moons drifting slowly across it. Then l remembered l received a 2x Barlow yesterday.
Even bigger now and still able to focus, just.
Plus my coloured planetary filters worked well, yellow being the best.

The ED 72 looks and feels like it's made to a price point from whatever is available from the parts bin on the day.
I will review it separately after a few viewing sessions but l like it better than my 127 already which has answered my question, what are the 127mm optics like?
Answer. Crap.

I have been deciding which go-to mount to buy, having a a 2kg and 10kg ota, choice is easy now.
Ioptron Sky Hunter with IPolar. Two more OT shifts and it's mine.

[Joves (Aaron)]

What are the focal lengths of each scope?

[Rod-AR127 (Rod)]

AR217 is 1200mm FL and the ED72 is 420mm FL.
I understand magnification, FOV is something l should sit down and draw and learn.

EP's are 30, 20, 15 @ 68°AFOV and 5 mm at 55°AFOV.
I'm just happy cruising the sky.

[Rod-AR127 (Rod)]

So, last night while l was viewing Jupiter with the ED72 l did a quick swap with the AR127 achro to compare.
While the 127 showed a slightly bigger Jupiter, with more detail visible, the CA was quite bad around one side of the planet. The ED was a far better view with no CA and pinpoint moons.
Lovely view.

[raymo]

Something strange; should have been MUCH larger in the 127, not slightly,
[using the same eyepiece].
The CA is bound to be obvious through the 127, which at f9.4 is very short on focal length for an achromat, but the manufacturers try to keep the scopes manageable. A traditional f/15 version would be about 1.8 metres long, and
somewhat heavier, necessitating a MUCH higher and sturdier tripod. Depending upon the quality of the lens, an f/15 would show very little if any CA. Very long focal length achromats can rival apochromats, but are hugely
inconvenient to use, anything over about 125-150mm usually being a
permanent installation. An easy way to get a more equal comparison between the two, lens quality wise, would be to make two aperture stops for the 127
out of matt black stiff paper, one 100mm aperture [f/12], and one 80mm aperture[f/15]. The CA should be noticeably less at f/12, and very slight at f/15. No point in using the scope at 80mm, but you might find the loss of aperture an acceptable exchange for less CA at 100mm.
raymo

[Saturnine (Jeff)]

Also, for the 127mm achromat, add an Minus Violet Filter, like the Baader Semi Apo or Fringe Killer. Other brands have similar MV filters as well. When fitted to the eyepiece or Diagonal they help to reduce the colour fringing.
I have an home assembled Surplus Shed 127mm Achro among my collection of scopes and I find it performs 80 / 90 % as well as my 127mm triplet, for 1/4 the price. The 127 will also be ahead in resolution and on nights of good seeing, have pushed it to 250 / 300X mag before it gets too soft an image.

[Rod-AR127 (Rod)]

Originally Posted by raymo

Something strange; should have been MUCH larger in the 127, not slightly,
[using the same eyepiece].
The CA is bound to be obvious through the 127, which at f9.4 is very short on focal length for an achromat, but the manufacturers try to keep the scopes manageable. A traditional f/15 version would be about 1.8 metres long, and
somewhat heavier, necessitating a MUCH higher and sturdier tripod. Depending upon the quality of the lens, an f/15 would show very little if any CA. Very long focal length achromats can rival apochromats, but are hugely
inconvenient to use, anything over about 125-150mm usually being a
permanent installation. An easy way to get a more equal comparison between the two, lens quality wise, would be to make two aperture stops for the 127
out of matt black stiff paper, one 100mm aperture [f/12], and one 80mm aperture[f/15]. The CA should be noticeably less at f/12, and very slight at f/15. No point in using the scope at 80mm, but you might find the loss of aperture an acceptable exchange for less CA at 100mm.
raymo

That's very interesting and something I'll look into doing.
When l get another mount I will have them side by side and then the fun will really begin.