A Solar Astrograph Refractor optimized for Hydrogen Alpha: SarHa.

After building my first refractor from an Surplus Shed lens, I realized it was a fun process, and it could be made as simple (or as complex) as one wants, depending on skillset and time available. Then, building my Newtonian Astrograph MOANA I stopped being afraid of the process of designing, manufacturing and coating metal (aluminum) parts. Last, I wanted to enter solar astronomy, with a home made astrograph of large aperture, based on an Istar cell. SarHa was born.

Optical design

Here is the optical train, from the entrance to the exit pupil:

  • Optional Aperture limiter: a large circular sheet of laser cut aluminum acting as a heat shield to reduce the aperture of the scope. When not actively imaging, the shield is on, reducing the sun energy allowed in the tube, limiting over heating of the various optical parts.
  • ERF, or Energy Rejection Filter. I am using the 160 mm diameter Baader ERF, aka: DERF-160 (Baader 2459245D). This is an optical piece of glass with a coating rejecting all solar energy but a broad band centered around H-Alpha.
  • Istar H-Alpha Optimized doublet, 150mm or clear aperture, SIO2 multi coating, focal 1480mm. This lens is a one trick poney: it is optimized for the H Alpha band. Because the goal is monochromatic imaging, we are not worried about color correction, therefore do not need a triplet. The advantage of the doublet is : less expensive, less internal surfaces, hence less internal reflection.
  • Telecentric system. I use a “Research Grade TZ-3 Telecentric System (3x focal length) (Baader 2459257), described as follow by the manufacturer. “Telecentric extenders are designed so that the exit pupil seemingly is positioned at infinity, which means that the center ray from any point in the field appears to come from infinity and is therefore perpendicular to the image plane and parallel to the optic axis. This means that the off axis beam arrives at the image plane with the same angular geometry as the axial rays. All field elements look as if they where on axis, across the image plane.”
  • Solar Spectrum filter: Solar Observer (25mm clear aperture) x 1 0.50 /- 0.05 Angström SS-SO.5. This is the H-Alpha ultra narrow band tunable filter.
  • Mono CMOS camera: my ASI 1600MM for now.

Mechanical design

  • Heat shield: various laser cut aluminum sheet that stack together, to protect both the telescope, the ERF cell, and when necessary, provide a stop to the optical train.
  • ERF cell: to reduce weight, the ERF cell is a rather complex 3D printed multi element assembly, with dust cap, and which can be separated and capped on both end forming a storage box for the ERF.
  • Doublet cell: aluminum cell, 2 elements, one element coming with the doublet, one designed and machined to the tube size.
  • Tube: a high strength carbon fiber tube.
  • Baffles: laser cut aluminum baffles positionned in place by 3D printed TPU “baffle positionners”. Robust, light, easy to install, remove or move along the tube, easy to manufacture. No holes needed. Calculated with my trusted “baffle calculator” Python script.
  • Focuser dove tail: aluminum part designed and machine to the tube size.
  • Focuser: Moonlight SCT focuser, aka: MoonLite CHL 2.5 inch Large Format Crayford EDGE Focusers.
  • Finder: Sol Searcher by televue, aligned with a hole in the heat shield.
  • Rings: the mounting rings are some stock 6″ rings from Explore Scientific.

I provide a zip file with:

  • the OpenScad designs, mechanical drawings and step-files ready for machining.
  • the ERF cell and the various dust cap to be 3D printed in TPU: OpenScad design and stl file.
  • The heat shield and baffle to be laset cut off aluminum: dxf files.
  • The baffle script and result, including a spread sheet for baffle position.
  • Various document relative to the scope.

Above, the front of the telescope, with the heatshield and ERF. To the left, the scope if diaphragmed to the “tracking/finding’ aperture. In the middle to the testing aperture, and to the right the diaphragms are removed and the scope is full aperture, while the cell and the rest of the scope are protected by the heat shield.

Front of the scope. From left to right: carbon fiber tube painted white, black anodized aluminum cell receiver, Istar aluminum lens cell, TPU 3D printed ERF cell, aluminum laser cut heat shield.
Back of the scope, from top to bottom: Moonlight focuser, black anodized aluminum dovetail connecting the focuser to the tube, carbon fiber tube painted in white.
The various TPU 3D printed dust cap: for the scope and to form a closed box around the ERF cell when disconnected.
The aluminum baffles and theri TPU 3D printed positionners.
Baffles in the tube.
The baffle positioning diagram.
Baffle positioning table.
Doublet spot diagram from Istar
Corrected drawing of the Istar cell. The actual cell is longer than described on their website.

Focuser tube adapter and dove tail.

Lens cell receiver, for the Istar lens cell.

The 3D printed ERF cell. The ERF is is pink element. The ERF cell is yellow and closed with the red part. Blue and purple are dust caps. On the left, the cell is closed on both end to transform a storage box. On the center and right the ERF is ready to be positioned on the Istar cell.

At this point I am waiting for my solar Spectrum filter, ordered 1 year and a half ago!! I will update when I receive it with actual solar imaging.