It's been a particularly dreary and overcast Winter here in the Pacific Northwest. A little while ago, we finally got a couple nights' stretch of some clear skies. With that opportunity, I took the new Starizona APEX reducer-flattener out for a spin.
First and foremost, I was very pleased with the near plug-and-play nature of the connection train. The instructions that are included are very thorough and have all of the vital information that any astrophotographer needs when using corrective optics, including metal back distance for different telescope specifications and corrected image circle.
The APEX was largely developed specifically for the SkyWatcher Esprit line of refractors, so there is an option to include an adapter that allows the reducer-flattener to be installed inside the focuser draw tube, allowing much shorter focuser travel and therefore can provide a more stable connection (available with either M42 or M48 threads on camera side).
In addition to this, the connection train for the correct back focus for very common ZWO and QHY cameras is included, so you know exactly what pieces are needed and in what order to reach that perfect back distance quickly and easily.
That back distance is variable based on the specifications of the scope that the APEX is being used on. Starizona has done a thorough job of documenting and sharing the optimal starting back distance positions for use of the APEX. Just find your telescope specifications on the table for either the small or the large version, hit that spacing, and you should be good to go!
A small caveat here, especially of you are using an APS-C sized sensor which is the maximum corrected image circle for the APEX: it can always be the case that the exact optics of a telescope behave just slightly differently. If you see barrel or pincushion distortion when you have your gear at the specified back distance, don't fret! It just may take a little bit of tweaking to get your stars round all the way to the corners.
At least for the Esprit 120 itself, I connected everything per Starizona's instructions to reach correct back focus using my ZWO 183mm, reached focus in SGP using the Posidrive from Starlight Instruments, and voila! Even correction and round stars all the way to the corners with 5 minute exposures:
When using the APEX-L out of the box with the Esprit 120, there was one thing that kind of stuck out: my focuser! I had to rack nearly all the way out in order to achieve focus. I was still able to get focus, but this means that my focuser moment arm was large and therefore stability may be affected, and if there were significant temperature variations, I may not have been able to achieve focus in different situations.
There have been reports that this is the case as well with the Esprit 150, with some people unable to achieve focus at all. Starizona was notified, and they quickly created a 2" spacer to use with the Esprit 120 and 150 telescopes. This matches the threads of the APEX adapter, and the M63 threads on the telescope side. This wonderful design means that the reducer can still be installed internally, and everything retains very sturdy threaded connections. The result is that the focuser doesn't need to be racked out nearly as far to achieve focus, allowing focuser travel in or out as needed.
Note here that the actual total distance from the back of the telescope to the camera sensor remains the same, so that moment arm is the exact same. However, keeping threaded connections significantly reduces the chance for tilt issues in the imaging train. The other benefit is that it ensures you can achieve focus without using all of the focuser travel. It can be the case that there is play and therefore deflection in a focuser that is racked all the way out. Providing a solution that prevents needing to rack all the way out also prevents that possible deflection.
Another benefit to note by retaining the ability to install the APEX internal to the focuser, is that the APEX is designed to fit into 2" connection trains. This means that the front of the APEX provides M48 female threads, so you can install your favorite imaging filter without having any weight or other part of your imaging train putting stress on the filter.
In addition to behaving as a flattener for use with refractive optics, the APEX acts as a reducer, and quite an aggressive one at that. It has a reduction factor of 0.65x, meaning it reduces your focal length by .65 times. This affects many important aspects of your imaging train: by reducing the focal length, you will get a wider field of view given the same camera, have a larger image scale, and perhaps most importantly, your focal ratio decreases as well.
As an example on my Esprit 120, the APEX converts the native 840mm focal length to 546mm. This also means that it converts the decently quick native focal ratio from f/7 to a supple and spry f/4.55. The quicker the focal ratio the better for deep sky astrophotography. That directly relates to how quickly light is put onto your sensor. A lower (quicker) f/ratio means that for a same exposure duration, you will gather more light on your camera on that frame. This results in more signal, therefore a better signal to noise ratio, meaning a nicer final image!
Put another way, a lower (quicker) f/ratio means that you will gather the same amount of light with a shorter exposure. Check out the f/ratio calculator to explore the differences.
On the Esprit 120, the difference between f/7 and f/4.55 is pretty significant. The calculator shows that for the same exposure, I am gathering 2.36 times the amount of light per pixel at the faster f/ratio thanks to the APEX. Very nice indeed!
To actually see the differences, below are two images that I took. Everything is the exact same (ZWO 183mm, gain of 120, offset of 11, temperature at -15°C, 300s exposures, Chroma 5nm HA filter) including the target: IC 410 known as The Tadpoles. These frames are calibrated and have the same stretch applied. The rotation is slightly different, but the difference in both the field of view and how much light and signal is captured is immediately noticeable. The moral of the story is: faster is better.
First up, the "original" 300s exposure at f/7:
Compare this with the "new" APEX reduced single frame at f/4.55:
The ability of the APEX to fulfill your need for speed is even more apparent on telescopes that start with a faster f/ratio. Installing the APEX on the Esprit 80 for example, pushes the native ratio from f/5 to a blinding, nearly plaid-speed, f/3.25.
Overall I've been very happy with the ease of use and performance of the APEX. It's the reducer for the Esprit that we've all been eagerly awaiting for a long time. Now that it is here, it's shown how well it performs, and its benefits will be usable on a very broad range of telescope and camera combinations. We're looking forward to gathering many more photons with this great piece of gear.
