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How good is a SW Esprit 100 ED without its Flattener?

17 Sep, 2018 10:34
Hi all, I am going to buy a Sky-watcher Esprit 100 ED , and I know that using SW Flattener there is a really problem with large optical train, because it has a very short backfocus. So I wonder how good is it without its Flattener?.

Backfocus with SW flattener is only about 55 mm, but witout Flattener backfocus is larger than 185 mm, my current optical train is about 150 mm.


17 Sep, 2018 20:47
The Esprit 100 is a fast f/5.5 refractor. Such a telescope design will always have very strong field curvature without a flattener, no matter how expensive it is. I have personally used an 80mm f/5 refractor without a flattener, and most of the field is pretty much useless for imaging. You can only get good sharpness near the center. So I would never use the esprit 100 without a flattener, unless you're using a camera with a very small sensor like a guide camera.

150 mm is a lot, can you describe your imaging train?
Edited 18 Sep, 2018 07:34
17 Sep, 2018 20:54
Thank you for your replies… my optical train is: Optec TCF-S + AO-8 + CFW9 + ST-2000XM.
Maybe I would not use AO-8 because my average seeing is about 2-3" and I would not get much benefits using it. But this only  will reduce my optical train 50 mm.. so I will keep out of back focus.
Edited 18 Sep, 2018 20:28
18 Sep, 2018 02:14
You can also use a Riccardi 0.75x reducer, I believe backfocus from corrector to sensor is ~ 87mm

How on earth do you have a 150mm image train? My QHY163m + filterwheel still needs an 18mm spacer for 55mm of backfocus
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