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Esprit 80 - what is the correct field flattener to sensor distance?

27 Aug, 2018 01:49
Hi all,

This probably sounds like a stupid question but what is the correct separation between the SW Esprit 80 field flattener and the sensor? I've seen it given as either 61 mm or 66 mm.

27 Aug, 2018 10:29
55mm and I have both the Esprit 80 & 100 and it is well documented that the is the correct distance, I was 5mm out and got odd stars, and until I fixed the distance then the beauty of this scope could not be revealed.

I have had to switch to a Moravian camera, OAG and FW just so that I can achieve that distance as ATIK can cannot accommodate that short a back focus.

No problem on my RC as I don't use a flattener but with the Esprits you really need to achieve this distance.

27 Aug, 2018 11:41
I think for Orion field flattener the backfocus is also 55 mm- fits exactly to DSLR with EOS adaptor, for CCD you would need to play with spacers.
Edited 27 Aug, 2018 11:42
27 Aug, 2018 13:21
According to the manual it is 61mm.
27 Aug, 2018 13:58
According to the Esprit 80 manual (SL140613 ), the optimal focal distance from the flattener to the sensor is 66mm (p.8, field flattener back focus).  This is the reason that Skywatcher provides an 11mm spacer with the Esprit 80 to extend the spacing from 55mm (standard with DSLR T-rings) to 66mm. If you are using a CCD you'll need to know the distance from the outer CCD housing to the sensor (and add the thicken of the filter wheel if you are using one) and provide the additional spacing needed to achieve 66mm to the flattener.  If this does not work, you might want to call Skywatcher USA an ask.
27 Aug, 2018 14:15
well, the SL131204 manual I got with the scope states it is 61mm !!!

27 Aug, 2018 14:19
And if you use a CCD camera you need a custom ring (part 3):

This being said, SW's manuals were kind of unreliable. smile

Go figure.,…
Edited 27 Aug, 2018 14:21
27 Aug, 2018 14:36
Now that's interesting. I wonder if since the OTA was originally released the engineering has changed and so the optical focal length from the flattener has also changed? At this point it probably makes sense to call Skywatcher and ask.
27 Aug, 2018 15:08
Being a nomad in a lousy skies region, I use a DSLR.

When I first got the Esprit back in 2014, I just plugged the camera as explained in the manual and it was perfect. I did not measure anything.

But then, I discovered that the polar alignment procedure detailed in the HEQ5 manual was wrong. ùso the credibility of SW's manuals took a bad hit. So maybe the real distance is 66 and the manual is wrong :-)

I dont know if they improved or not. And in a sense it does not matter much: the Esprit line is the best part of my equipment and I really love it.

Note that I have finally switched to a full frame sensor. The manual does not mention this possibility and I was warned about heavy vignetting by most everybody. But I took the plunge and I dont regret it at all. A little bit of vignetting that the flats and PI have no problems dealing with.
27 Aug, 2018 17:28
Right Gentlemen, The SW Field Flattener has a screw on adapter that is 11mm in width, if you use this then from the front of the screw in adapter to the CCD is 55mm for the Esprit, if you want to try and gain additional back focus then as the manual says and I quote Page 9 of 13: -

"Note: Inside the box the 11mm ring spacer is assembled with the field flattener body. When assembled, the two parts may look as if there were only one piece. When taking pictures with a CCD camera it can be useful to remove the 11mm spacer ring to gain 11mm additional back focus distance"

So unless you have made up a special adapter or configuration you will have to use this 11mm spacer and therefore the back focus distance is 55mm


Hope that helps. smile
27 Aug, 2018 23:20
we can agree to peacefully disagree :-)
28 Aug, 2018 00:00
Hi NeedsMoreCoffee. I just spoke to the guys at Sky-Watcher. I purchased an Esprit 100 today and wanted to verify the spacing for my DSLR and CCD cameras. They told me that the manuals at their website are correct for the OTA's they are currently shipping. Here is the link for the Esprit 80 ( The relevant information is on p.8. I'm guessing that since you are asking about spacing that you are not the first owner of the scope and so did not receive a manual. So, there are at least two possible spacings. For new scopes it is 66mm. For older scopes, it is 61mm, as noted by Boutros (here is a link to the older manual, In both cases Sky-Watcher shipped a spacer (step down from 66mm threads of the flattener to 48mm threads of the DSLR T-ring) already assembled with the flattener. For the 66mm version the spacer is 11mm. For the 61mm version the spacer is 6mm. So, to find out what scope you have check the spacer on the flattener. If it is 11mm the optimal spacing from the flattener to the sensor is 66mm. If it is 6mm the optimal spacing is 61mm.
Good luck, NeedsMoreCoffee. Looking forward to your images.
Edited 28 Aug, 2018 00:04
28 Aug, 2018 00:45
Thanks Gary!

this clarifies well the situation

and, Needsmorecoffee, keep up the good work! Your images are fantastic!
Edited 28 Aug, 2018 00:47
03 Sep, 2018 03:15
Sorry for the late reply and thank you all for your thoughts and advice! smile

Gary - I think you are exactly right - I had the 11 mm spacer. Thanks to your explanation I think I've solved the problem.

Cheers all!
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