Wednesday, December 30, 2009

Helping focus.. Bahtinov Mask

In my long line of posting fun stuff that helps make my astro-life easier... I decided to write a short one on gaining perfect focus quickly and easily. While performing focus with an eyepiece, you can normally get pretty decent focus by taking a little bit of time and moving the eyepiece back and forth. In all likelihood you can get a pretty good focus (at least good enough for your viewing pleasure) just by moving in and out of focus, perhaps pretending your at the optometrist and chanting 'how about now' or 'this one or this one' :-) until the image looks good.

When attempting to take photos however, this becomes a bit more problematic as the camera and associated photos are generally less tolerant of less_than_perfect focus, this becomes more apparent with longer exposure times. It is also more difficult and time consuming gaining focus when you are using a camera (either prime focus or via a computer) as you need multiple exposures to test the focus, this takes a very long time to get perfect for your next frame-able photo of M42. One evening I spent almost 40 minutes moving my focuser back and forth trying to gain perfect focus. :-( Spending this much time on focusing, takes valuable time away from capturing those photons on your image sensor... not a great use of time.

While looking for focusers (dual speed Crayford types), I ran into some sites that discussed products they made to make focusing easier ( / Astrozap). I decided to give one a try, selecting the astrozap version.

I ordered it from Astronomics and received it a couple of days later, on a cloud-less evening no less! So I took it right out and placed it on my CPC, hooked up my CCD camera and gave it a run.

In less than 3 minutes (after alignment) I had the selected star in perfect focus! My shots came out just as planned and even better than expected. The more you use the mask, the faster your focusing becomes (as with all things, the more you practice something, the more efficient you become).

You can find more detail about how these things work, but I will supply a brief overview here:
The mask creates diffraction spikes (like an astrograph, but instead of 4 spikes you will see 6) that can be used to dial in your focus.
If the spikes are not perfectly aligned, then the star is out of focus.. but when aligned you will have a perfectly round star. If you have a dual focuser, it will be easier to put the diffraction spikes in perfect alignment, if you do not, it will still get you right on the 'money' it just might take a few seconds longer.

This turned out to be a pretty good purchase overall, as it reduced my focusing time from 40+ minutes to less than 2 minutes. You can go to to see how they work, at less than 100 dollars (USD) it is probably the least expensive
accessory I own but one that saves me quite a bit of time (and time is money right?).

Happy Photon Hunting!

[note: I am not getting paid by astrozap or any other vendor for publishing my views here]