The use of a Bahtinov Mask is my preferred method for achieving critcal focus while set up for astrophotography. A Bahtinov Mask can be bought for only a few dollars through any astronomy retailer or you can easily make your own mask via an online Bahtinov Mask generator. The task of focusing with a mask is easy to learn and quite fast once one becomes accustomed to using a mask.
- 80 mm APO Triplet and Celstron 800 HD with Bahtinov Masks Attached
The basic steps to achieving focus with a Bahtinov Mask are highlighted in examples below:
With camera or imaging device attached slew telescope to a bright star. Setup imaging software to take 2 to 5 second exposures. I use Nebulosity 2.0 in the frame and focus mode. Hang Bahtinov Mask over the end of the OTA, take a 2 to 5 second exposure. You may see out of focus view such as that in image below.
Out of Focus Bright star through Bahtinov Mask
The middle beam of light is almost centered very close to focus
The next step is to adjust the focuser and take another two to five second exposure. The goal is to be close enough to focus to achieve the spiked star pattern seen on image to the right. Once you have this pattern showing, make very small adjustments to focus and keep taking test exposures until you center the beam of light in between the diffraction spikes.
- Bright star near perfect focus using Bahtinov Mask
Perfect focus is achieved when your final adjustment looks like the picture to the left. You are now focused and ready to slew to the object you wish to image. Don’t forget to remove the Bahtinov Focus Mask before commencing image run. It is always good practice to take test exposure and preview your focus once on the object you wish to image. You are now ready to frame your object and begin the nights imaging run. This focusing method is very fast with practice and makes focusing a very simple task.