The main scope
The story before the Esprit
Actually I was quite satisfied with my TS 90/600 triplet apochromat. The triplet came along with a fibre reinforced carbon tube and a Baader Steeltrack focuser. At the time I purchased the scope my only intension was to use it visual. Years later I still think that the scope was excellent for this application. It comes with a smooth working focuser and delivers needle-sharp stars without noticeable chromatic aberration. All this weighs 3.5 kg only. Here I will give a description of the Skywatcher Esprit 100 ED, my way of setting work and the history how I came to it.
It took much effort to optimize the flattener to chip distance. Finally I figured out that 119 mm is the best back focus. Since the original flattener casing was a bit too loose I started my lathe and built a new one. The latter fitted inside the Steeltrack tube tight but without any tense. With the TS 2″ flattener in its new casing the image train gave excellent star-images.
Constraints when used for astrophotography
More or less some disadvantages of this scope for photographic use led to the desire for something which fits better to my requirements. What are the photographic disadvantages?
The focal ratio is f/6.7 this is fast enough for many famous objects, but too slow for fainter objects or even narrowband imaging.
The Steeltrack is 2″ in diameter and it has no thread. It is designed for clamping equipment at the eyepiece side, only. In my opinion threaded connection of photographic equipment is to prefer. The diameter of 2 inches is large enough for APS-C size chips but too small for full format. Although the lens shall be full format capable the focuser vignettes too much in that case. Hence the system is limited to APS-C, smaller or similar sizes.
Because of the focuser the scope is only suited for some certain reducers. I tried the TS 0.79x reducer and the Tele Vue TRF-2008 which both are clamped 2 inch ones. The TS reducer gave good results and increased speed up to f/5.3. The price to pay for that was slightly larger star-images, I would say.
Definition of the successor
The successor should be a high quality refractor again or even better. The most important requirement is a fast focal ratio. Searching for suited scopes it reveals very fast that only a few types are in the closer choice. Most of them are really high priced. Someday an amateur known from astronomie.de offered his Skywatcher Esprit 100 ED in a package with the original flattener and the small Riccardi reducer. I got the final impulse by the interesting thread at Cloudy Nights, which encouraged me in my opinion that the green Japanese scopes are not the only alternative.
Skywatcher Esprit 100 ED
Now the above mentioned telescope is mine and I don’t regret this step. It offers f/5.5 with the original flattener (threaded) and f/4.1 with the Riccardi (also threaded). After the first 9 months I have the feeling that the picture quality with the Riccardi is even better than with the original reducer. So I run this scope almost always with f/4.1 thus it gives a wide view angle with my KAF-8300.
Coming from f/6.7 down to f/4.1 reduces the required exposure time down to 37% for non-stellar objects. In practice this difference gives a dramatic effect, because I keep the times more or less in the old range (10 min for RGB and 20-30 min for narrowband).
Together with dew shield, tube rings and mounting plate the scope has about 8 kg mass, which is not really lightweight – don’t forget the CCD camera, flattener or reducer, the filter wheel and the guiding scope with camera, also.
It can be used visual as well as photographic. In the latter case I have the original M48 Field Flattener or the M63 Riccardi available, see below.
Reviewers of the first exemplars complained about the focuser. I can’t confirm that. The scope comes with a rotatable 3” CNC-machined 11:1 dual-speed Linear Power Focuser. I didn’t noticed focus-shifting during focusing. The load capacity seems to be by far enough for my Atik 383 with filter wheel. The smooth focus motion and 11:1 fine focus wheel allows precise adjustment.
One of the first actions was to adapt a motor focus. My first approach was to drive the fine focus shaft with a stepper motor. With that approach my system showed slip in the 1:11 planetary gear. This usually happened if the focuser had to lift the camera weight vertical. This prevented a reproduction of focus positions. The slip was only e few steps, but after moving 10 times in and out one sometimes I had a summed error of about 1/10 mm. A no go for automatic focusing! Consequently I run a second approach without driving the planetary gear. Needless to say, this behaviour does not play any role during manual focusing or visual use.
Now the final set-up utilizes a geared stepper motor and drives the focuser’s main shaft directly by a NEMA 14 motor with a gear ratio of 1:19. Transmission to the focuser shaft is done via a toothed belt drive with a gear ratio of 1:4. In total this gives a ratio of 1:72 which is by far fine enough for a precise focusing. For green light at a focal length of 415 mm the critical focus zone for my Atik with 5.4 µm pixels is about 40 µm. On the other hand the NEMA 14 motor has 200 steps per revolution which results in a step width of about 1.75 µm per step (measured 571 steps per mm). I purchased the motor earlier at Stepper Online for my GP motorization, but replaced it later by another type. The datasheet of this geared stepper can be loaded down here.
The Image Train
Matching the proper distance of the focal plane to flattener or reducer is of great importance for best photographic results. Usually for a given flattener or reducer it depends on the telescope. So the task is to find the optimum back focus distance for the Skywatcher Esprit with the original flattener as well as with the Riccardi.
The back focus distance for the original flattener given by Skywatcher is 63 mm from the M66x1 thread. During my tests I could verify this value. In the range of accuracy I reached the tested filters did not show any reproducible influence on the distance, consequently I’ve chosen 63 mm as the nominal value for following set-ups.
Related to the Riccardi I’ve had different information. All distances are taken from the M63x1 thread. Teleskop-Austria the dealer of my telescope and APM-Telescopes said 80 mm. The previous owner of the scope said 78-80mm. In order to get an overview on the used distances I collected available data, put them into a diagram and fitted a curve.
The result is shown in the figure. It reveals that the range 78-80 mm is a good value to start with the 550 mm focus length of the Esprit, but the exact value has to be figured out by experimentations. To make a long story short: During 17 nights from June to August 2016 I finally came to the best distance of 78.5 mm, if simultaneously a filter with 2.1 mm thickness is in the train. Assuming the filter accounts with 1/3 of its thickness to the optical way, the effective back focus in my case is 77.8 mm.
Adaption of a DSLR and original Flattener to the Skywatcher Esprit 100
The scope comes with a M48 adapter for the flattener which is optimal for DSLRs and their T-rings. I intend to use the DSLR with a CLS-Filter during most of my photo sessions. Clamping a XL clip-filter directly into the DSLR body is possible, but produces unwanted vignetting. Hence I realized a way to use unmounted 50 mm filters. For that I machined an adapter for the TS-Filterschublade. This filter drawer is a compromise in order to realize a short optical path length. From the mechanical point of view it is improvable. As long one works very carefully and tries to avoid touching it, it’s passable solid.
The picture shows the adapter M66x1 to M48x0.75 and the M48 filter drawer with EOS bayonet at its backside. To meet the requirement of the flattener the optical length of DSLR + Filter drawer + adapter is 63 mm. The adapter has been manufactured out of two parts: an outer part made of high-strength aluminium and an inner part made out of brass. This process was necessary in order to get a M48 thread length of only 2 mm. Due to the small optical length of the drawer the M48 thread is only 2 mm deep. If a bolt is screwed in deeper than 2 mm it collides with the filter insert.
The focuser offers a M74x1 thread this is adapted for the orignal flattener with its M66x1 by an adapter which is in the scope of supply. Everything fits well, but you now already have two adapters between focuser and flattener! This means you have many positions, where you can come into trouble because of loose connections. So one has to check many connections before taking the first frame. My system run reliable nevertheless.
The DSLR with all adapters is shown in next picture. This set-up has a focal plane distance of 63 mm to the end of the aluminium ring ready for a direct connection to the original Skywatcher flattener. The last picture gives an impression of the whole chain mounted at the telescope before a nightly session. Here is a drawing of the adapter.
Adaption of a CCD Camera and orignal flattener to the Skywatcher Esprit 100
My main imaging system is an Atik 383 with filter wheel. It needs 39.5 mm back focus. Since the optimal reducer distance was clear it was easy to design an adapter from the M66x1 of the flattener to the M54x0.75 thread of the filter wheel. The difficulty with the needed adapter is the material-saving design of the filter wheel, which has very small wall thickness and therefore only a low thread depth.
The parts for testing the optimal distance I made on my small lathe in my workshop. For the final part I wanted to assure best possible precision and quality, hence I ordered the parts from Gerd Neumann. Find a drawing of the adapter ordered here. The quality of the final adapter is out of any question and in a similar situation I will go to Gerd Neumann again.
ADaption of a CCD Camera and Riccardi reducer to the Skywatcher Esprit 100
Again my imaging system needs 39.5 mm back focus. After figuring out the optimum reducer distance it was easy to design an adapter from the M63x1 Riccardi holder to the M54x0.75 thread of the filter wheel. The Riccardi holder fits into the M74x1 thread of the focuser and fixes the Riccardi inside the focuser tube and consumes 13 mm back focus. Again the difficulty is the material-saving design of the filter wheel and its very small wall thickness. Well, ought to look for some better quality in future…
Find a drawing of the ordered adapter here.
I summarized an overview of the whole chain in a simple sketch. Maybe this is helpful for somebody. This design is at the „shorter end“ of possible the image train length. This gives the opportunity to extend it by inserting fine tuning rings, but my tests revealed that this is not the case, hence I’m using it exactly in that configuration..
Dovetail Clamp for Guider and Handle
Since one of my personal system requirements is portability, also the handling of the system is a factor. The telescope ready for shooting brings about 10 kg. I can’t hold this by one wrist while clamping it with the other hand. Hence I had to think about a handgrip.
Another point is the guider set-up. In order to be portable it should be removable as well as attachable reproducible. The aim is to reuse the PHD calibration even though the system was fully disassembled and stowed. Moreover the wiring of the system components should be separable, i.e. without firmly attached cables.
The above given requirements can be fulfilled by a dove tail clamp. As no model with matching size was available commercially, I decided to build my own. The main sets of requirements have been clear soon: The clamping shall be done by one clamping screw; the clamp parts shall be pushed apart by a spring in order to keep the empty clamp always open. Therefore a guiding of the moving part by fitted bolts seemed reasonable.
After a few hours in my workshop the clamp shown in the picture arose. A „wannabe“ drawing can be found here.
The other picture shows matching hand grip. Basically it is a kitchen hand grip out of the DIY store and a milled dovetail. It is secured against slip out by a brass plate at both sides. I had the situation that after a cold night the clamp was ice coated and even tightly clamped the dovetail slipped out! This is avoided with the brass safeguards.
The guiding scope is a standard 60 mm f/4.0 finder combined with a Starlight Xpress Lodestar – in my opinion a very powerful combination. The guider axis is always adjusted parallel to the main scopes axis. Anyway, I had never missed a guide star. Even in the dark vicinity of the Coma Cluster a guide star is available always. All parts of the optical train are threaded in order to avoid shifting effects. I modified the helical in a way that the play is reduced significantly and the position can be locked. The price to pay for is a rotation of the camera, if focused.
The only drawback of this system is the short focal length in combination with the comparably large pixels of the Lodestar. In my opinion in case of off-axis guiding or long guiding telescopes the Lodestar is out any question. However, with this set-up one has to be aware that the guide system really has to realize a precision in the order of 0.1 pixels due to the system scale of about 7.0” x 7.2”. My experience is positive, anyway.
The heating band is self-made. Is draws about 0.9 W from a 12 V supply. In combination with a neoprene overcoat upon the dew cap the lens never had even a trace of dew.
The substructure of the guider comprises a compartment for electronics. Mainly this is a USB 2.0 Hub, a switched DC converter supplying the hub with 5 V and a 12 V power distribution. The lower cap of the compartment is realized as a dovetail. In order to reduce mass the dovetail it is significantly thinner than commercially available ones. Nevertheless the 9 mm thick dovetail gives the needed amount of stability. The electronics give the opportunity to keep the cabling of two cameras, one filter wheel and two heater bands as simple as possible. Find a drawing of the substructure here.