Backfocus using Skywatcher Esprit 120ed Reducer/Flattener - Forums

Topic starter

Aug 11, 2023

hi,

I recently purchased the reducer for my Esprit 120.
After some researches i found out that a backfocus of 60mm instead of 55mm gives a flatter image. Trying out both with the Asiair, i figured out that the focuser stays in the region of 1000 with 55mm backfocus whereas using 60mm it travels towards 7000 which is somewhere about half the travel of the focuser.
Can anyone share his experience with that combo?
thx

Brian Valente

Aug 11, 2023

Is this a matched SW reducer?

I only have the matched 120ED flattener which works at 55mm on the nose

José María Moreno Santiago

Aug 11, 2023

The backfocus of the total 0.77x reducer is 95mm. (55mm from the top of the start of the M-48 male thread). If you put a filter, which can be placed inside the reducer itself, you already know that 1/3 of the thickness must be added, in the optolong 0.6. Therefore the total with filter would be 55.6 mm.
I have never used 60mm

Patrick Graham

Aug 11, 2023 · Edited: Aug 12, 2023

If you're using the flattener/reducer that comes with the Esprit 120ED, you'll need a back focus of 95mm. For the flattener only, 75mm. This is measured from the bottom of the threads of the flattener/reducer to the sensor of the camera. See pages 8 thru 12 of the user's manual for your scope.

Patrick

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Patrick Wood

Aug 11, 2023 · Edited: Aug 11, 2023

I have both the flattener and the esprit .77 reducer for my Esprit 120. The additional back focus needed from the dedicated spacer that is suppled with the reducer and flattener is 55mm on both systems. Yes the focus position changes. In my case from 16000 steps to around 6000. This of course is dependent on the step size of your focus motor.

note: because I shoot mono and my chroma filters are 3mm thick I add 1mm to my back focus for a total of 56.

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JoeHenry

Aug 11, 2023

I have both the supplied flattener and the additional matched 0.77x reducer/flattener. The images produced by the reducer are not that flat, especially around the edges. I used 55mm backfocus for both, but I will try it at 60 to see if there is any improved field flatness. Thanks!

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Matthew Proulx

Aug 12, 2023 · Edited: Aug 12, 2023

hi,

I recently purchased the reducer for my Esprit 120.
After some researches i found out that a backfocus of 60mm instead of 55mm gives a flatter image. Trying out both with the Asiair, i figured out that the focuser stays in the region of 1000 with 55mm backfocus whereas using 60mm it travels towards 7000 which is somewhere about half the travel of the focuser.
Can anyone share his experience with that combo?
thx

I couldn't get a reasonably flat image with 55mm-62.5mm. And before anyone chimes in

the 40mm M78-M48 adapter is installed.

Even at a total of 102.5 (40+62.5) the stars are horrendous. Must have gotten one from the its 5 o'clock somewhere batch.

Anderl

Aug 12, 2023

Matthew Proulx:
hi,

I recently purchased the reducer for my Esprit 120.
After some researches i found out that a backfocus of 60mm instead of 55mm gives a flatter image. Trying out both with the Asiair, i figured out that the focuser stays in the region of 1000 with 55mm backfocus whereas using 60mm it travels towards 7000 which is somewhere about half the travel of the focuser.
Can anyone share his experience with that combo?
thx

I couldn't get a reasonably flat image with 55mm-62.5mm. And before anyone chimes in the 40mm M78-M48 adapter is installed.

Even at a total of 102.5 (40+62.5) the stars are horrendous. Must have gotten one from the its 5 o'clock somewhere batch.

Looks good to me.
i bought one myself but don’t have any finished photos yet.
From the few frames i have taken with it it looks like i have a bit of comet like stars in one corner but even taks don’t produce perfect stars when reduced and pictures taken with newtons have spikes while also having (even the expensive ones) problems with star shapes if you don’t collimate them regularly.

the esprit 120 with its flattener and the flattener reducer is pretty hard to beat if you consider the price, mobility and versatility of the scope.

btw i am using mine at 55.5 mm, same as the 1.0 flattener.

Matthew Proulx

Aug 12, 2023 · Edited: Aug 12, 2023

Matthew Proulx:
hi,

I recently purchased the reducer for my Esprit 120.
After some researches i found out that a backfocus of 60mm instead of 55mm gives a flatter image. Trying out both with the Asiair, i figured out that the focuser stays in the region of 1000 with 55mm backfocus whereas using 60mm it travels towards 7000 which is somewhere about half the travel of the focuser.
Can anyone share his experience with that combo?
thx

I couldn't get a reasonably flat image with 55mm-62.5mm. And before anyone chimes in the 40mm M78-M48 adapter is installed.

Even at a total of 102.5 (40+62.5) the stars are horrendous. Must have gotten one from the its 5 o'clock somewhere batch.

Looks good to me.
i bought one myself but don’t have any finished photos yet.
From the few frames i have taken with it it looks like i have a bit of comet like stars in one corner but even taks don’t produce perfect stars when reduced and pictures taken with newtons have spikes while also having (even the expensive ones) problems with star shapes if you don’t collimate them regularly.

the esprit 120 with its flattener and the flattener reducer is pretty hard to beat if you consider the price, mobility and versatility of the scope.

btw i am using mine at 55.5 mm, same as the 1.0 flattener.

Unacceptable to me. The only reason to use a reducer/"flattener" is to increase the FOV. At .77x you don't get enough FOV for the abberation it introduces here, when you crop it you're basically at 1.0x again or worse. I had a TAK120 with the TOA35 reducer 2 years ago and there was absolutely no curvature. You should be able to cover a full frame at least with no curvature whatsoever.

Tahir

Topic starter

Aug 21, 2023

Hi guys,
seems like, im not the only one having funky stars, using the reducer.
tomorrow i will get some spacers (1mm and 0,5mm) and i will do some trial and error from 55mm all the way up to 60mm.
i watched locamaticos YT video about the reducer and in the comments, he wrote that 59,5mm gave him good shaped stars.
Lets see, will give an update eventually

Greg Pomanti

Aug 22, 2023 · Edited: Aug 22, 2023

I think you have the same problem I had which is a dud reducer. I got all the way out to 62mm of back focus and couldn't get round stars. The focuser would stop around 1000 steps. I finally decided to just purchase a new reducer and I now get perfect (to my eyes) stars across the field with 55.6mm of back focus with filters. Now it's in focus around 7200.

If you have the ability to return or RMA your reducer and get a replacement, I'd say give that a try!

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Tahir

Topic starter

Aug 22, 2023

Thank you for your advice.
Since i bought it recently, i will definitely change it, if the spacers won’t help.
what bothers me is, that the stars are egg shaped not only in the corners but everywhere in the frame. So, its not that typical backfocus issue (too far/too close) rather „vertical-egg-stars“ even in the middle, not changing the orientation or shape.
will give an update with the final solition.

Luke Newbould

Aug 22, 2023

hi,

I recently purchased the reducer for my Esprit 120.
After some researches i found out that a backfocus of 60mm instead of 55mm gives a flatter image. Trying out both with the Asiair, i figured out that the focuser stays in the region of 1000 with 55mm backfocus whereas using 60mm it travels towards 7000 which is somewhere about half the travel of the focuser.
Can anyone share his experience with that combo?
thx

I experimented with this a lot, I bought one as soon as they were available and went through three of them while figuring it out. - 60mm is honestly tons better than the 55mm suggested by the manufacturer!
Stick to your guns mate, 55 is a waste of time with the 0.77x reducer/flattener imo, keep that 60 and enjoy!
CS

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Tahir

Topic starter

Aug 23, 2023

Hi Luke!

thank you so much, your video really helps alot! huge fan of your channel!
Tried 60mm yesterday, still no round stars

at native focal lenght though i get perfectly round stars.
Will talk with my dealer, maybe he can it or even give a replacement.

Miguel Noppe

Sep 21, 2023

Not the first time I hear complaints of the SW reducer.
I have a 0,75 reducer from TS Optics that predates the existence of the SW one.
It's this one although it since was updated and is now 0,74x.
About the back focus, I also would suggest not to pin down on the 55mm, look at the image and let it dictate what to do.
Good luck.
CS
Miguel

Dan Kohn

Jan 18, 2024 · Edited: Jan 19, 2024

hi,

I recently purchased the reducer for my Esprit 120.
After some researches i found out that a backfocus of 60mm instead of 55mm gives a flatter image. Trying out both with the Asiair, i figured out that the focuser stays in the region of 1000 with 55mm backfocus whereas using 60mm it travels towards 7000 which is somewhere about half the travel of the focuser.
Can anyone share his experience with that combo?
thx

I had to increase my back focus for my Esprit 120 when using the .77x reducer/flattener, at 55.6mm (.6 added for filters) I had elongated stars in the corners going outwards (APS-C format, ASI2600MC) . I changed out a ZWO spacer, adding 4.5mm back focus for a total of 60.1mm and that resolved it - so yes, I had a similar experience. When just using the included flatter, corners were fine at 55.6mm back focus.

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Monty Chandler

Jan 20, 2024

At 860mm fl I've never installed the reducer. Can't seem to give up the FL I bought. Pretty wide as it is. Not sure what the objective is.

Riggs Brown

Oct 11, 2024 · Edited: Oct 11, 2024

Apologies in advance as this doesn’t directly relate to the OP, but was wondering if the flattener that comes with the Esprit 120ED reduce the FOV at all? I am unfortunately finding that I don’t have a ton of options as far as imaging DSO’s go at this focal length, paired with the ASI2400. Too tight for lots of DSO’s and not tight enough for others. Trying to find some in its sweet spot seems challenging. A little buyer’s remorse at this point unless I can come up with some good imaging options.

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Miguel Noppe

Oct 11, 2024

Riggs Brown:
Apologies in advance as this doesn’t directly relate to the OP, but was wondering if the flattener that comes with the Esprit 120ED reduce the FOV at all? I am unfortunately finding that I don’t have a ton of options as far as imaging DSO’s go at this focal length, paired with the ASI2400. Too tight for lots of DSO’s and not tight enough for others. Trying to find some in its sweet spot seems challenging. A little buyer’s remorse at this point unless I can come up with some good imaging options.

I use my Esprit, except for that one time a few years ago, always reduced at 620mm.
but to each his own, I kind of knew what I wanted and I'm very happy with the 120.
For galaxy's 840mm is a bit wide in my opinion anyway, except for like a few big galaxy's (which you can count with a few fingers).

Dan Kohn

Oct 11, 2024

Hi, I have the Esprit 120 and the SW reducer/flattener, I’ve definitely had a flatter image adding 4.5mm to the backspacing. I switched the Zwo 16.5mm spacer for the 21mm spacer, so added 4.5 and it is nearly as good as the provided flattener (no reduction).

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Dan Kohn

Oct 11, 2024

Dan Kohn:
hi,

I recently purchased the reducer for my Esprit 120.
After some researches i found out that a backfocus of 60mm instead of 55mm gives a flatter image. Trying out both with the Asiair, i figured out that the focuser stays in the region of 1000 with 55mm backfocus whereas using 60mm it travels towards 7000 which is somewhere about half the travel of the focuser.
Can anyone share his experience with that combo?
thx

I had to increase my back focus for my Esprit 120 when using the .77x reducer/flattener, at 55.6mm (.6 added for filters) I had elongated stars in the corners going outwards (APS-C format, ASI2600MC) . I changed out a ZWO spacer, adding 4.5mm back focus for a total of 60.1mm and that resolved it - so yes, I had a similar experience. When just using the included flatter, corners were fine at 55.6mm back focus.

Same experience for me.