How To Make A Hartmann Focus Mask for Seestar S50How to build and use a mini Hartmann focus mask for use on the ZWO Seestar S50 smart telescope to achieve perfect focus every time.
As with any photography, astrophotography requires pin-sharp focus. Even the slightest bit out of focus will ruin all your time and effort capturing long integrations. Because our targets are very faint, we must focus on a brighter target, such as a nearby star. These are so small that finding the sweet spot of focus can be challenging. A Hartmann mask can help us with this due to the unique diffraction pattern it creates.
What is a Hartmann Mask?
A Hartmann mask, named after the German astronomer Johannes Franz Hartmann, is used in astronomy to test and align optical systems, particularly telescopes. It is a simple and effective tool designed to aid in the precise focusing of optical components, such as mirrors or lenses, by producing a distinctive diffraction pattern.
The basic principle behind the Hartmann mask is that the diffraction pattern produced by the holes or slits provides a way to assess the optical alignment of the telescope components. The diffraction pattern appears symmetrical and centred when the optical system is well-aligned. Any misalignment or optical aberrations will cause the diffraction pattern to shift or become distorted.
Why use a Hartmann Mask on the Seestar S50?
The Seestar S50 features a good autofocus system, but it only works well for bright stars. If you want to focus on an object that only has dim stars nearby, it may not be able to achieve perfect focus. The Hartmann mask will allow you to manually focus the telescope using the advanced controls and get perfect focus every time on any target.
How to Make a Hartmann Mask to fit Seestar S50
Unlike regular telescopes, where a Hartmann mask sits over the aperture and is held in place with a ring, the Seestar Hartmann mask is required to fit inside the aperture with a ring, preventing contact with the lens. I made this Hartmann mask with materials around the house, some of which you may or may not have. You can use anything for this.
I found an old toilet roll to be the perfect size to fit inside the Seestar S50 aperture with a little fabric tape to provide a friction fit. A narrow fabric tape such as that used to wrap wire bundles (~8mm width) provides an ideal "seat" for a 55mm rubber washer that fits snugly over the toilet roll and is wide enough to cap the telescope aperture.
Trace around the washer onto a piece of thin cards, such as the back of a cereal box, then glue the washer to the cardboard tube so that it is resting on the fabric tape seat.
Divide this circle into quarters, then each quarter into thirds. Each slice should be 30°
Draw an equilateral triangle from the top point to the 4 and 8 o'clock positions. Where the lines of the triangle intersect the dividing lines, draw a line to make a smaller triangle which points outwards. Shade this in for clarity, then carefully cut out the circle and the smaller triangles with a knife.
You should now have the cardboard disc with three small triangles cut out and the tube with tape and washer glued on. Trim the tube to flush with the rubber washer, then glue the cardboard disc to the washer.
If all goes well, you should have a Hartmann mask that fits snugly inside the Seestar S50 aperture.
How to Use a Hartmann Focus Mask on Seestar S50
To use the Hartmann focus mask on your Seestar S50; you must first enable the use of the manual focus system if you haven't already. Go into the "My Seestar" menu by tapping on the picture of the telescope. Find the "Focus" setting and tap to enter the menu, then toggle the "Focus Panel" On.
When you start stargazing, you will see some extra arrows on the side of the screen. These are the manual focus controls.
Use the goto function to locate a bright star near your target object, then carefully insert the focus mask into the telescope filter aperture.
Depending on how in or out of focus the telescope is, you will see three triangle-shaped blobs (out of focus) or one single blob with a star pattern (in focus). If you see three blobs or one large blob, adjust the focus until you see a single object with lines radiating out from it.
Please remember to remove the Hartmann focus mask after focussing. Otherwise, your imaging will be bad. Ensure the mask is removed before turning off the telescope; otherwise, it will crash when parked.
The Hartmann mask is a valuable tool during the collimation (alignment) process of telescopes, especially for reflectors. Collimation is crucial for optimising the performance of telescopes by ensuring that all optical elements are precisely aligned. Regular use of a Hartmann mask allows astronomers to maintain the quality of their observations by keeping the telescope in proper optical alignment. It is a relatively simple but effective method for amateur and professional astronomers alike to enhance the performance of their optical systems.
