SkyWatcher Esprit 80ED

In August 2021 a SkyWatcher Esprit 80ED was added to the set-up in order to be able to do more wide-field imaging. I started out deep-sky imaging using the SkyWatcher Esprit 150ED, but with a focal length of 1050mm (f/7) and an approximate 57' x 43' field of view that scope is great for galaxies, planetary nebulae and other small objects, but no so much for large targets like the Pleiades and most narrowband targets. The Esprit 80ED has a focal length of only 400 mm (f/5) and with that it's field of view is more than 2.5 times as large (approximately 152' x 114'). Being the smaller sibling of the Esprit 150ED the 80ED easily rides piggy back on the former.

Getting the Esprit 80ED performing well was far from easy. The one at InFINNity Deck was the third that I tested and still needed some work to produce proper images. The first one suffered from pinched optics and uneven halos around bright stars, the second one suffered from what I think is astigmatism (opposite corners show elongated stars, see this M13 image), while the third (the one now present) had again uneven halos.

As the first Esprit 80ED also suffered from these uneven halos, I wondered if it had something to do with the lens-cell. The inside of this cell is smooth and tubular and I wondered if one side would be reflecting more light into the scope than the opposite.

So I created a simple paper aperture disc with a 79mm opening and attached that to the lens-cell. As can be seen in adjacent image this immediately removed the uneven halo, but added a lot of diffraction spikes (column of images in the centre).

As there were more spikes with than without the aperture disc, they must have been the result of my sloppy circle cutting. So I created a second aperture disc, now from a piece of left-over 3mm thick aluminium sheet, bevelled and polished the aperture to a highly smooth finish and attached that to the scope (right hand column of images).

This aluminium aperture disc or DARD as I named it (Diffraction Artefact Removal Disc) fully cured the scope from uneven halos and diffraction spikes.

The full resolution image can be found here.

A quick test on M31 revealed that the scope is performing exceptionally well.

Above tests were all done with the camera that normally sits behind the SkyWatcher Esprit 150ED and as that scope is also used for daily solar observations, I needed a second camera and filterwheel for this little sibling.

So finally in January 2023 I got myself a second hand ASI1600MM Pro Cool, which still was like brand new. To that I added a new ZWO EFW and a full ZWO LRGB and NB filterset.

Last step was to modify the focuser as I had done with the Esprit 150ED. So I got myself a new bunch of robot motors with 1:16 teethed planetary gear to replace the 1:10 friction type planetary gear. The gearbox was simply separated from the motor by loosening the four screws that holds it, then the motor was taken apart to source it's central shaft with input gear. From a section of 26mm diameter solid brass rod a new gearbox housing was made to fit the focuser housing on one end and the Starizona MicroTouch focusmotor at the other end.

The whole modification was done in February 2023 and now all I need is some proper clear skies to start imaging.

As the Esprit 80ED comes with a dedicated flattener, it is of importance to get the back-focus of the camera right. Using a set of Baader filler rings, I got the right back focus as shown here. In the calculation allowance was made for the filter thickness and further allowance to get really nice round stars all over the FOV.