Michael‘s Astrophoto Blog

My old NexStar Evolution mount is a really great "grab and go" mount for visual observation. It is pretty light as it doesn’t require a counterweight, it comes with its own rechargeable battery and its own Wlan - so basically everything you need to start observing super quickly. I was surprised how far it took me on my astrophotography journey and it is probably viable with a small telescope or tele lens. However, undoubtedly it has quite substantial limitations when used for photography with my Celestron C9.25: The mount is not really balanced on any axis. So it was very hard to make guiding work in many cases. It usually took a while of adjusting parameters or guiding in only one direction depending on the weight distribution.   For higher declinations guiding did not work at all. Even when it worked tracking and guiding accuracy was not really good enough for imaging at higher focal lengths. Typical guiding errors were in the 3“-4“ range. The mount geometry requires the us...

Since a couple years I own a focal reducer for my Schmidt Cassegrain. It didn’t come with a whole lot of documentation or manuals, but I knew that it was supposed to reduce the focal ratio from 1/10 down to 1/6.3 while at the same time correcting the image. I have used it mostly to visually observe larger objects that wouldn’t quite fit in the field of view otherwise.  The reducer/corrector for changing the focal ratio to 1/6.3 When starting with photography I wanted to give it a try as I was hoping to benefit from the image correction as well as from the fact that a shorter focal length would be more forgiving in the presence of small errors in tracking and focusing. In general that worked as expected, however I was really surprised when the plate solving of my first images showed a focal ratio of 1/4.8 rather than the expected 1/6.3. That is how I started to realize the importance of the backfocus distance of a device. The corrector, like many optical devices, is calculated with ...