Lets Focus

Achieving sharp focus is an extremely important goal to every astrophotographer. All those "faint fuzzies" you have seen in your telescopes eyepiece can come to life and have depth of field when "seen" through a camera if you have proper framing and sharp focus. Even the best and most expensive telescope and camera combos wont perform to their full potential without the best possible focus. Polar aligning your mount and having good guiding also figure into the equation, but today lets talk about focus.

My first images were done with a DSLR and I used the FWHM (Full Width Half Maximum) feature in BackyardEOS for focusing. This worked well for the most part but it was still difficult on some nights getting the focus just right. FWHM is a numeric interpretation of the size of a star. This number comprises of two measurements; Full Width simply means that the full diameter of the star is used, and Half Maximum is a measure of the distance from the center of the star at which the brightness level decreases to half the level value at the center. FWHM seeks to provide an empirical method of achieving focus but because of changes in the "seeing", it is not always easy to use.

HFD (Half Flux Diameter) is a similar method of focusing. This is a feature included in the program Sequence Generator Pro (SGP). I've been using SGP since I made the switch from the DSLR to the STF-8300M CCD camera. HFD is the pixel diameter of a star in which half the energy of the star is contained. This value is similar to that of FWHM but is considered to be more reliable especially in conditions of poor seeing. Still, it has its flaws and I wanted something more precise.

I had been hearing of people using focusing mask, or Bahtinov mask. These havent been around that long but seem to have made quite a splash in the astrophotography community. In 2008, Pavel Bahtinov, a Russian amateur telescope maker and astronomer, published an ingenious new way to easily focus your DSLR camera, webcam or CCD camera for astrophotography. The method is simple, very intuitive, and the device has universally been named after the inventor: The Bahtinov Mask. They work by producing a distinctive 'diffraction pattern' in a similar manner to the way a four vane secondary mirror spider vane on a Newtonian reflector produces a four pronged star or "spikes". However, the unique design of the Bahtinov Mask produces a bright image with a diffraction pattern forming a long cross comprised of two intersecting lines and a third line that moves across the center of the cross as the telescope is adjusted in and out of focus. Correct focus is achieved when the center line fits exactly in the center of the cross. At a cost of ~$20-$30 I sure didnt have much to lose so I decided to try them.

I decided on the adjustable version from Farpoint Astro. I found what I was looking for at Agena Astro and ordered the FP400 for my 80mm Levenhuk triplet. This mask is adjustable from 2.5" to 4.5". I also placed an order for the FP404 for my 8" Levenhuk RC which is adjustable from 8.5" to 10.5" (outside diameter).

Bahtinov focus mask on 8" RC

   

Adjustable focus mask that I use for my 80mm apo

                                                                             

Example of perfect focus using the Bahtinov focus mask.

I have to say that these things simply just work. They are simple to use, cost very little, and I can now quickly achieve the best focus possible. So if you are having problems getting a sharp focus, grab one (or two) of these. I think they are something every astrophotographer should have in his or her toolbox. Makes me wonder now how I ever got by without one! I feel much more confident now that my images are focused as well as they can possibly be in any seeing conditions. Happy focusing!