Another invaluable use of the RONCHI TEST is to perform a star test at night with any telescope. It is much easier to perform than a Foucault Knife Edge Test and gives just as good a result. Perfect optics will show up as parallel shadows with no waviness. If you are about to purchase a telescope then applying this test will show up any defects. The interpretation of the shadow patterns is an essential part of assessing the quality and many examples are given in the instruction sheet.
Two versions are available: 80 lines per inch and 120 lines per inch. Both come complete with instructions. The 80 is marginally easier to use. They are £9.50 each, post included.
Three special Meteor Camera Shutters have been supplied to the BAA for use in meteor triangulation, height and velocity determination. This device chops the light going to a camera so that a meteor, if recorded, will show up as a sequence of dashes on the film. Knowing the chop speed you can work out the angular speed.
Special circuits have been designed and built by AWR Technology to make a large shutter or paddle made out of balsa wood rotate at over 10 revolutions per second. Small but powerful stepper motors were used. When the load was calculated it was found that the inertial torque presented was large and the shutter would need to be accelerated up to working speed. Also the final speed had to be frequency locked to a crystal in order to achieve the required accuracy in the meteor determinations. The shutter is directly connected to the motor shaft.
Normal stepper motor drive circuits based around the SAA1027 have a problem in that the mechanical torque available drops off with frequency. However AWR use a bi-level drive circuit meaning that the motor can be worked up to much higher frequencies and maintain the torque. The meteor camera shutter application actually requires more torque at higher speeds as there is air resistance. If you calculate the tip speed it is about 40 miles per hour!
The final specification for the units are as follows:
Lock frequency 500Hz +/- 0.01% Chopping period with 2 vanes 48.0ms Design acceleration time 15 seconds Motor shaft diameter 4 mm Motor type ID35-114, 48 steps per rev, 6.5Ncm torque Power requirement 0.6A 12.0V DC Inertial load presented Balsa wood 0.45m x 0.10m x 5mm
There is excess power to cope with operation in a stiff breeze. The maximum output frequency before stalling was 560Hz with a supply of 12.0V DC. The minimum time to reach lock speed was 4 seconds. (PRICE)
Also known by the trade names TEFLON and FLUON this is a remarkable material noted
for its chemical inertness and heat resistance. The full name is polytetrafluoroethylene
(PTFE). It has very low friction and can be used as low friction blocks in
Dobsonian telescopes etc. N.B. It is very difficult to get glue to stick to it, but it can be screwed.
We do not have an easily available source for etched PTFE.
Max working temperature . . 250 deg C Density . . . . . . . . . . 2.2g/cm3 Not attacked by UV, strong acids or alkalisWe sell PTFE cut from stock sheet by the square inch.
1.0mm thick . . . 25p per square inch 1.6mm thick . . . 25p per square inch 6mm thick . . . . 75p per square inch 10mm thick . . . £1.00 per square inch
Here are some ideas for its use:
WARNING - Above 250 deg C fumes can evolve. If this is likely to occur, such as when the material is machined then SMOKING IS PROHIBITED. DANGEROUS decomposition products can result.
TUFNOL is a proprietory laminated plastic based on a medium weave fabric (WHALE brand) or
fine weave fabric (CARP brand), light weight but extremely strong.
It can be machined easily and is widely used for bearings, gear wheels and pulleys.
It is stable and can be used for surfaces which rubbing blocks would travel over,
eg in Dobsonian telescopes, Poncet mountings etc.
Max working temperature . . 120 deg C
Density . . . . . . . . . . 1.36g/cm3
We can obtain TUFNOL in sheet and rod forms of various thicknesses. It is not
that expensive and costs a lot less than PTFE.
We have 1mm, 2mm, 4mm, 10mm and 15mm thicknesses in stock.
Contact us with your requirments for a quote.
An alternative material for rubbing against PTFE is the brand name 'FORMICA' used on kitchen work surfaces. It is available in thin sheet from wood merchants. It can be glued onto mating surfaces.