I was provided with a Spike-a™ Bahtinov Mask. This mask is similar to a Hartmann Mask but more accurate and is rather easy to use to get a precise focus. There are diameters available from 4" ($70) to 14" ($100), so pretty much any telescope within those sizes should work OK. This would also include the ETX models. The mask is made of metal and has three plastic screws to hold it in-place on the OTA. I did my testing with an 9-1/4" diameter mask on my 8" LX200-ACF.

Here you can see the mask attached to the 8":

Here is a star way out of focus (notice the lines on the right are not centered over the star):

When proper focus has been achieved you will see the lines of light coming out of the star and equally spaced:

And here is the star properly focused (without the mask):

The above images were cropped from the full-frame versions to better show the details. They were taken with a Nikon D70 DSLR at Prime Focus on the 8" LX200-ACF. I also did some tests with an Antares f/6.3 SCT Reducer/Corrector. These images of the star Regulus are not cropped to better show what the eye sees in the camera viewfinder.

Way out of focus:

In proper focus:

Stars nicely infocus (no mask):

Depending on the brightness of the star and the camera/imager being used, you may find it somewhat a challenge to determine precise focus. But when you see it you know it! I found it difficult to see the lines in the D70 viewfinder with the focal reducer but by moving the focus knob back and forth a few times, it was evident where the best focus was achieved. The photo of Regulus and other fainter stars shows the perfect results.

Here is one example to show how well the Spike-a Bahtinov Mask worked. I had focused on Regulus, locked the focuser in place, removed the mask, and slewed to M51, the Whirlpool Galaxy. This single 5 minute exposure (ISO 1250) is cropped from the full-frame image. M51 was not precisely centered in the full-frame image. You can see how nice the central stars appear. The stars near the edge of the image were distorted by the focal reducer.

To see this image and a couple of other images at full-frame, see my report for 28 February - 2 March 2009 on my Oracle Observatory web page.

I am pleased with my initial use of the Spike-a™ Bahtinov Mask and look forward to a lot more use. If you need to improve the focus of your astrophotographs, I can easily recommend the Spike-a™ Bahtinov Mask as a solution.

Go to the LX200-ACF Table of Contents.