Focusing at night is never easy, particularly if you cannot have real-time feedback.
With modern DSLR and Mirrorless cameras, you can count on the live view to see in real-time how focus improves (or not) while manually focusing the lens or the telescope.
Things are a bit different if you are using an astro camera that is controlled with a computer. In particular, if you are controlling the camera over Wi-Fi with the ZWO Asiair or similar gear.
The problem, for me, is the lack of real-time feedback: you set the program to take an exposure every 1 or 2 seconds, wait for the image, move the focus, wait for a couple of images to be sure that what you see in the screen is not affected by you changing the focus and iterate the process over and over.
Is frustrating. In particular, if you are using your telescope in many different configurations: to observe, to photograph the Moon with a Barlow Lens, to photograph deep-sky objects with a focal reducer, etc.
And the aforementioned configurations do not share the same focus. And this is the problem: sometimes you are so out of focus that you can’t even see the typical donut from a defocus stars. You are so out of focus that you don’t see anything on the screen.
And if you, like me, are using an SCT or Maksutov telescope, you know the focus travel is huge, and hunting for the proper focus is frustrating.
Last time I lost one hour to finally see a donut turning into a star with proper focus.
So I asked the internet and a nice chap on the Facebook Group Astrophotography for Beginners, Ian Humphreys, suggested a neat trick I was not familiar with: it is brilliant and I want to share it with you.
If you are a beginners, I strongly advise to join this group, anyway…
If you have a telescope you may be familiar with the fact that if you focus, say, the Moon with a 25mm eyepiece, when you decide to go closer using a 10mm eyepiece, you have to refocus.
Except if you have a set of parfocal eyepieces, all sharing the same focus point. If you have normal eyepieces, you could achieve a similar result by fitting your current eyepieces with parfocal rings.
A Parfocal Ring.
The idea is simple: suppose your Moon is in focus with a 25mm eyepiece. Now, remove the 25mm eyepiece try focusing the image with the 10mm eyepiece fitted with the parfocal ring.
You do this by inserting slowly the eyepiece in the diagonal: when you see the Moon in focus, simply hold the eyepiece in position, push the parfocal ring against the diagonal and lock it.
My 25mm Plössl eyepiece fitted with the parfocal ring.
Should you need more back focus, i.e. the eyepiece tube is too short, note that with many eyepieces you can replace the tube with a longer one by shopping for a suitable eyepiece extender tube.
This way the ring will create a “hard stop” for inserting the eyepiece in the diagonal so that the next time, when you will swap the 25mm eyepiece for the 10mm one, you will not have to refocus.
This way you can swap different eyepieces and barely adjust the focus with your telescope, rather than hunting for it.
The same principle can help us with the astro camera: in daylight frame a distant object and bring it into focus using your telescope focuser. Then, remove the camera and replace it with an eyepiece fitted with a parfocal ring and try to find the proper focus by sliding in/out the eyepiece. Once you find the focus, lock the ring.
Next time you are under the star, pre-focus with the eyepiece in place enjoying the real-time feedback from your focuser. And when you will replace the eyepiece with your astro camera, you will be very close to the ideal focus.
“Refine it instead of hunt for it” is your new motto.
Ideally, you want to have a set of cheap eyepieces with parfocal rings, one for each type of setup: one for when using a Barlow lens, one for using the focal reducer, etc.
Video Tutorial
To clarify the process, I made this short video. I hope you'll enjoy it!