A dedicated Startest telescope
By many if not all telescope makers the Startest is considered to be the ultimate and single most important test to verify the quality of optics. It consists in visually analyzing the intra- and extrafocal diffraction images of a star.
One of the problems the ATM usually faces in relation to the execution of the startest is that he might not yet have built a finished telescope when he needs to test the optics. Another complication is that mirrors of different sizes and focal lengths will need different telescopes in order to be "star-tested".
In consequence, optics are often star-tested after the telescope has been completed and the mirror has been coated.
Faced with the dilemma, I had been thinking for some time about a dedicated startest telescope when I came across Jerry Oltion's Startest telescope. Immediately I was hooked on the idea of having an adaptable and dedicated start-test telescope. I started drawing up some plans with Alibre CAD and adapted Jerry's design to my necessities.
My design requirements were:
- Mirror support for optics up to 24 inches
- Adjustable three-point mirror support
- Centred secondary/detector position
- Single pole design
- Lightweight and quick/easy setup
- Data acquisition with WebCam or other CCD imaging system
- Sliding focuser with digital readout
Thanks to the design and simulation capabilities of Alibre, I arrived fairly quickly at a feasible 3-D model and in consequence at a working prototype.
As it happens so often with prototypes, they tend to work so well that there is little incentive to rebuild them into a finished product, which is what happened to this telescope as well.
The following photographs shall illustrate some of the construction details. Since completing the telescope in autumn 2010 I have added some features like a laser finder and counterweights under the base board to give the telescope more stability when testing small mirrors.
- The mirror cell. The side-view includes an 8 inch mirror.
The mirror cell features:
- Two point frontside collimation. The third point is a spring-loaded screw.
- 90° separated edge supports in the form of two brass discs that have been machined to a very fine rounded over edge. The edge supports are adjustable in height so they can be located on the mirror blanks centre of gravity.
- The mirror rests upon three felt padded screws.Those screws are separated by 120° and can be adapted to different mirror diameters thus providing at least basic back support for being able to evaluate realistically the figure of the optics.
- To set the telescope to the correct altitude for testing on a star, I constructed an extendable foot with included fine motion. The leg is made of 20mm aluminium tubing and the fine motion consists of a M8 threaded rod that runs inside the tubing.
- On the top end of the single pole "telescope tube" is the support for the stabilisation strings. There are five strings in total that help to dampen out vibrations. I use normal off-the-shelf nylon strings as they are commonly used in construction work.
The hole in the strings support board provides the opening for the cables that connect the camera and the motorised focuser.
I use or 3 cm aluminium tubing for the single pole focuser support.
- I revisited and modified my original sliding focuser design for this telescope. As no eyepiece holder is required, the focuser board can be relatively small.
The focuser is driven by a small 3V geared motor which in turn drives a M6 threaded rod. The focuser board attaches to that threaded rod with a drive nut.
The longitudinal displacement of the focuser is measured by a digitally calibre.
The whole assembly attaches to the drive pole with a standard pipe flange.
- The focuser is shown here with a mounted laser collimator. The camera support arm is a 12 mm aluminium tube and can be adjusted in length. The camera support mount is fully adjustable and collimation usually takes very little time since no reflections from a secondary mirror have to be taken into consideration.
First experiments with a Meade™ PC camera showed that common WebCams are not sensitive enough to effectively startest uncoated mirrors.
In order to be able to use a WebCam for this task it has to be modified first for higher sensitivity. The Phillips Toucam and SPC900 cameras have been used traditionally with big success for astrophotography and modifications for long exposure and even CCD replacements are widely documented on the web.
I am in process of adapting a SPC900 for startesting and will document it on this webpage. The modifications include long exposure mode (LX) and swap of the standard CCD for a SONY ICX618ALA black and white sensor that will almost triple sensitivity.
- This startest video of a 6" F/7 mirror has been taken with a Meade USB Pc-camera eyepiece stacked on a 2x barlow lens. It was a test to try out the capacity of that simple camera.
Seeing was quite bad and the mirror had not yet reached thermal equilibrium. Nevertheless one can already see the potential of this way to execute the startest once the setup and parameters are optimized.
- This is a startest video of a 8" F/6 mirror. The mirror was coated and had many defects, the most prominent being lots of astigmatism and spherical aberration. The first part of the movie shows the diffraction pattern 2mm outside focus. The second part is at "focus" (whatever focus could be achieved with that mirror). The inside focus sequence was so washed out due to SA that it has not been included in this video.