Bahtinov – A Darker View

For some reason I have gotten along without a focus mask for the TV-76. Need to fix that… Done.

A 3D printed Bahtinov focusing mask for a Televue 76mm telescope

Just a quick project, about half an hour to design and four hours printing. First print a pleasing success, a completely functional part.

A Bahtinov Mask is used to get a precise focus, something I wrote about a few years back.

For this mask I used a public bit of OpenSCAD code from Jens Scheidtmann to generate the mask pattern, just tuning it for the correct sizing and performance with the TV-76. I added my own version of the support collar, making it a bit neater with fillets and properly sized to slip over the TV-76 glare shield.

You can download the SLT file for the mask from Cults3D if you need to print your own.

The part is one of a half dozen parts I have designed and printed over the last few weeks to reassemble the photo rig. A new guide camera mount, a mount for the ASI Air computer, a new glare shield for the guider, etc., etc… The utility of 3D printing a game changer for me.

Focus is one of the most frustrating problems when shooting in the dark. The number of astrophotos ruined by poor focus is legion. Worse, the problem is often not found until afterwards, when hours of effort have been wasted. Very slight errors in focus can spoil an otherwise great photo.

Taking photos in the dark creates a situation where the normal solutions do not work… Autofocus simply will not operate without enough light. Taking the lens off and attaching the camera to a telescope results in a completely manual focus. Simply focusing through the viewfinder is not accurate enough.

Bahtinov Mask — A Bahtinov focusing mask in use on the front of the AT6RC telescope

The simplest method of focusing in the dark is to use the live view feature of the camera. By placing a bright star or distant streetlight in the view and magnifying you can achieve a reasonably good focus much of the time. This is not perfect, even the magnified image is a little mushy near ideal focus, making perfect focus difficult to judge. If you find yourself in an after-dark situation, without any special equipment along, this is the method to use.

Do remember to switch off the autofocus feature of your lens. Once you get focus set correctly you do not want to lose it the moment you touch the shutter button. Not that I have ever done that… Never? Well, perhaps maybe.

Continue reading “Getting Focus Right”

Tag: Bahtinov

A Bahtinov Mask for the TeleVue 76

Getting Focus Right