Backfocus for the Esprit 100 & ASI2600MC

Jerry,
I think you're going to be very pleased with your Esprit 100ED.  I thought I would point out a couple of things I've done/learned with mine.

The Skywatcher stock focuser is OK, but you'll need something like the Pegasus Astro focus cube universal adapter to mount an external electronic focuser on it.  The do sell the apater separately, so you don't have to get their FocusCube2 EF to get the adapter bracket (although I did).  For visual viewing the stock focuser is fine.

The issue I found (and it becomes evident with the larger sensor/small pixel CMOS cameras) is that droop/flexure of the focuser draw tube become evident as the imaging train weight increases.  A ZWO 2600MM is not a light camera and its QHY equivilant is even heavier.  As a result, it has significant leverage arm in its location behind the filter wheel and other accessories (OAG-L in my case).  With the draw tube droop/flexure, it has a noticeable effect on apparent image tilt as the OTA orientation changes with star tracking/relative target position in the sky.  I don't understand why SkyWatcher made the OTA tube so short that the focuser has be extended to around 75% of its full travel to get in focus at least for imaging (never used mine for visual so maybe in a visual rig setup there is a reason)

I replaced the Skywatcher focuser/focus tube assembly with a Moonlite Express Nightcrawler (Amy Astro uses them exclusively on her many OTAs on her YouTube channel) and it is a purchase I've never regretted.  I also like the Nightcrawler since it has a built-in field rotator that makes getting the target orientation you want in your image "frame" easy.

I've had my Esprit 100ED for three years now and it is close to perfect (IMO) as a combination of speed and field of view and work perfectly with around 90% of the targets I can access.  Something with more reach is needed for Galaxy season (except for larger galactic formations like Markarian's chain).  The Takahashi is the only builder that makes a refractor with close to the same specs.  Most manufacturers chose the easier to build f7.0 spec for their refractors.  The only thing I'll knock on the Esprit 100ED is the image circle - at 40mm it is the smallest image circle of the refractors in the same class.  Not an issue for APS-C sensor cameras, but there will be some vignetting from what has been reported by users with full frame sensor cameras like the ZWO6200.  VisibleDark used to run the Esprit 100ED as well; he found vignetting with his 2600 camera when he installed the Apex-L field reducer (made specifically for the Esprit scopes by Starizona).  

For imaging, specifically for creating calibration frames, there is a potential light leak where the dew shield slides up to meet the end of the OTA when fully extended.  I didn't notice it when new, but I think it develops over time.  By necessity there is a bit of play/gap between the dew shield and the OTA to allow the dew shield to move.  In practice, the dew shield will always be extended due to how short the OTA tube is - it can only retract if the scope rings are removed.  Just be sure when making your calibration frames, to wrap the rear of the dew shield where it meets the OTA with either a strap dew heater or with black electrical tape to ensure no light is getting around that gap.