# 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?
# 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.
# 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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