This is a discovery I made while flipping through test images of my new Explore Scientific HR coma corrector.

I noticed a small blinking red dot in the new data. Such blips are usually hot pixels, but it turns out that I stumbled upon a known variable star called RX Lyr.

RX Lyr is a Mira-type variable star with a period of about 248 days. In addition to periodic changes in brightness, star color also seems to vary between different phases of the stellar cycle, as seen in the animation below. More details about this star can be found on the American Association of Variable Star Observers (AAVSO) website.




Technical
The ES HR corrector was developed by the famous optical designer Harrie G.J. Rutten from the Netherlands, and should hopefully address a couple issues with my setup. 

1. Tilted image plane
First, purchasing a new coma corrector would allow me to verify if the persistent tilt issues with my telescope are due to a faulty GPU coma corrector or not. After using the ES HR corrector multiple nights, I concluded that my stars are bad with both the ES HR and GPU correctors! So, both of my coma correctors were likely fine.

2. Front element reflections
Secondly, the GPU front lens element will generate reflection artefacts when stray light or light pollution hits the front glass. The result is a ring-shaped reflex that cannot be corrected with flats. This topic on cloudynights shows the reflections very well. ONTC telescopes have 3 different mounting positions for the primary mirror cell. Mine was designed with the native focus either 115, 145 or 170 mm above the tube wall. I previously used the GPU corrector on the top position (170 mm) to reduce the reflections from the front element as much as possible. One downside of moving the primary all the way up, is that the camera hangs very far away from the tube and this increases the risk for tilt issues.

The ES HR corrector has a small barlow effect (it will increase focal length by 5.7%) and the focus point is thrown outside by 35 mm compared to the native focal point of the primary mirror. In addition, the corrector length is only 85mm compared to the 100mm of the GPU. Net result: the front lens element (mirror side) is completely hidden inside the focuser draw tube and better protected from lens flares when the Primary mirror is set to the lowest position (See Rev. B).

3. Flat frames and vignetting
It seems I still have a problem with the corners. There is a sharp light cut-off at the very edge of my sensor and flats do not remove the dark transition seamlessly. This is possibly due to 1.25″ filter housing of the AstroDon set, something I need to look into (See Rev. C and D).


2020 data is from my deep M57 image: 


M57 with Halo