The Infinity or Reflex Sight
by Graham Wood
This article describes the principles and construction of an Infinity Sight.
General Description
When using an astronomical telescope, even at relatively low magnifications
(x50), it is remarkably difficult to find a particular star or comet owing to
the very small field of view - typically 0.5 degrees. Some sort of 'finder' is
required so that a larger field of view can be seen. Often a smaller
telescope, of similar perfomance to binoculars, is employed as a finder and a 6
x 40 or 8 x 50 unit is often employed. (In specifications like 'M x D', M is
the magnification and D is the diameter of the objective lens.)
The 'finder' device described here is also known as a '1X' finder since there is no magnification on the direct optical path from the celestial object (star) to the observer (eye). It is exactly like looking at an object through a small window. Here, the window is a plastic membrane, made with 'Cling Film' (*)stretched over a rectangular aperture in a flat plastic sheet which sits at 45 degrees to the optical axis. This forms a 'see-through mirror' which is very much more 'see through' than it is a mirror!
However, it is sufficiently reflective to present an image of 'target circles' or 'cross-hairs' within the field of view so that the body of the finder can be aligned on the object of interest. The image of the 'circles' is created by a mask, illuminated by LEDs, in which the clear areas transmit the red light to form the image.
* I have lately discovered that the 1mm approx polystyrene sheet for double-glazing provides a good alternative to the 'Cling Film' and being rigid, it is far less affected by wind. It is also easier to make since it doesn't need to have a rectangular hole cut out of it.
Design and Construction Notes
Details of the Electrical System
The light source is a pair of large red rectangular block LEDs. The amount of light is controlled by a variable resistor which also has an on/off switch. The variable resistor is in fact a standard 10k ohm 'volume control' with a DPST or SPST switch and is mounted on the side of the plywood box and fitted with a knob. The block LEDs require current limiting resistors and 220 ohms is a satisfactory value.
These block LEDs have two actual LED chips in them, the connections of which are brought out to separate pins. The resistor is wired between the +ve of one and the -ve of the other. This leaves the other two pins, one +ve and one -ve, to be wired to the volume control and battery. Note that block LEDs frequently have no markings for the pins in which case the constructor needs to discover the polarity for him/her self. Therefore construct the sub-assembly with the 220 ohm resistors and try connecting it either way round to the battery. One way round will be correct whereupon the LEDs will light up.
Design of the Pattern Mask
The mask is placed immediately on the front of the block LED pair and should be large enough to cover the whole area and a bit more, say 1/16". It can be made from a computer artwork that is printed on 'OverHead Projector' film (OHP) or by photographic means. The white areas in the illustration are produced clear so that the red light behind the mask is allowed through.
The circles mask is probably the best one to use. It has circles that represent 4 degrees, 2 degrees and half a degree. This combination allows easy navigation around the sky by reference to known points on the celestial sphere.
To calculate the physical size of the circles on the mask, we need to know the focal length (f) of the lens used in the system. Using this value we can establish the actual diameter of the circles from the equation:
In the case of the cross hairs which should be 4 degrees long and have a half
degree separation, the formula becomes:
diameter = tan( separation or length in degrees ) x f
Note that in both cases, the answer is in the same units as the 'f' parameter.
The centre of the mask is black which means that the object of interest can
aligned into the center of the pattern without the illumination getting in the
way.
The Front Surface Mirror
The front surface mirror is obtained from a small ladies' make-up mirror which
should be about the same size across as the diameter of the lens. The reason
we need a front surface mirror rather than use it as it is, is that when used
conventionally the 45 degree mounting angle causes the main image from the rear
surface to be 'shadowed' by a weaker pattern displaced from it by 1.4 times the
thickness of the glass owing to reflection from the front surface of the glass.
This effect can be aleviated completely by using a front surface mirror.
To achieve this, the paint that protects the back needs to be removed. The
easy way is to brush on a paint remover like 'Nitromors' or other product
containing methylene dichloride, wait for the paint to shrivel up and then
rinse under the tap. Bear in mind that the revealed surface is very easily
marked and should only come into contact with cotton wool when cleaning under
the tap. The colour of the silvering may turn to copper after exposure to the
air although this has very little effect on the system when in use. Please
observe manufacturer's procedures when dealing with toxic materials like
methylene dichloride.
The Lens
The lens is required so that the image from the mask can be brought into focus
at infinity. Since the Mark I Eyeball is also focussed at infinity when
looking at the sky, the image will appear sharp and clear. Furthermore, it
will exhibit the property that it seems to be projected onto the sky and even
if the observer moves his head, the image seems to be 'locked' on to one point
of the sky. This property means that no backsite is required nor must the
observer place his eye in any particular position to obtain the correct result.
As long as the image can be seen in the field of view, the finder will allow
the telescope to be properly aligned. This is the same principle as the 'head
up display' used in aircraft and other military applications.
The lens should be 1.5" to 2.5" in diameter and have a focal length of 5" to 8" inches although 7.15" is the longest focal length that a 4 degree circle will fit on the mask for the 0.5" size of the block LED. Make sure that the lens is glass or a very good quality plastic one. There are some quite acceptable 'Hand Magnifying Glasses' that cost around 50 pence available at market stalls. But there are some awful plastic ones that cost the same and have a very distorted lens shape and these are totally unsuitable for this purpose. To see which is which, ask to try one out outside of the bubble pack it comes in. An item that feels 'heavy' will have a glass lens. If the image through the lens seems clear and sharp all across its diameter then it is probably OK to use in this project.
To establish the actual focal length is quite easy. Take the lens and a white card and a ruler outside at night and set up to focus the light from a not-too-near street light on the white card. When you have the image focussed at its best, measure the distance between the white card and the centre of the thickness of the lens. This is the focal length of the lens.
The See Through Mirror
This is the element that the observer looks through at the heavens and sees the
image of the mask superimposed on the sky. The requirement is to have a thin
sheet of material that detracts very little from the view of the sky and is
virtually flat across its height and width.
The reason that it should be thin is too avoid the separation of the front and
rear side reflections of the image which, otherwise, would result in seeing two
separate images at equal brightness. The sheet needs to be flat otherwise the
shape of the image will become distorted particularly at greater viewing
distances.
Both of these requirements can be met with a piece of 'cling film' stretched
across an aperture cut into a sheet of rigid plastic e.g. Perspex or Plexiglass
or similar. The glossy finish on most pieces of these plastics allows the user
to evaluate their flatness before any work on producing the aperture takes
place. Look at the reflection of an object to the side of you as you slowly
rock the plastic sheet to pass the reflection across the usable area. If it
seems OK across the width, rotate it 90 degrees and and perform the test again.
Unfortunately, 'cling film' comes in various optical qualities as well. To
test the film, stretch a sample piece across the aperture cut in the plastic
sheet and assess if the view of a distant scene in daylight has lost its
clarity or sharpness. If the view is degraded, discard that piece and find a
different source. Sometimes, the film that the Supermarket uses to seal its
meat portions into the plastic trays can prove to be suitable. The film needs
to be handled reasonably carefully to remove it from the tray and the easiest
way is to pierce it close to the edge at one corner with a sharp knife and cut
it down to the other corner. Repeat this on the other side to obtain a pair of
parallel cuts. Now, with the scissors, cut from one corner to the other. The
piece removed may be washed in warm soapy water and laid to dry between two
sheets of kitchen roll. When it has dried, it can be evaluated for use.
Other films used in packaging have been tried. The film that cigarette packets
are sealed with is usually quite good optically, but it is difficult to fit it
across the aperture since it needs narrow strips of sticky-tape to affix it.
Consequently, ripples across the surface appear, which play havoc with the
image. These ripples may be shrunk by the application of heat from a hair
drier, but my experience is that they come back after a couple of days.
When using cling film to cover the aperture, simply stretch it across from side
to side and from top to bottom. Since there is no outside edge at the bottom
of the aperture, simply allow a half inch or so of surface for the film to
stick to. I have never found it necessary to use glue to hold the film down.
At the sides and top allow the film to fold over the sheet onto the other side.
I have recently discovered that the thin (1mm) polystyrene sheet used in some
DIY double glazing systems is a good replacement for the assembly described
above. It needs no cutout in it, simply trim to overall width and length and
use it as it is. It has an advantage in that being rigid it isn't affected by
the wind which can cause image problems with the cling film method.
This concludes the description of the individual elements of the design and we
move on to the design of the finder.
Design of the Finder System
The general layout is shown in the sketch below where the various elements are
colour coded to help in identification. Please note that this schematic is not
drawn to scale.
The LED subassembly is mounted on a small piece of curtain rail to allow it to be moved closer to/further from the mirror to achieve the correct focus. The volume control is fitted through a hole in the side of the plywood housing. The four AA batteries can be placed in a 4-cell holder and affixed to the floor of the box. The front surface mirror is mounted on a block of wood, cut at 45 degrees, with a blob of clear silicone glue. This blob holds the mirror in place yet provides a flexible mounting so that the three adjusting screws can bear on the back of the glass and tilt it to the correct angle to bring this system into optical alignment with the main telescope. The details of this activity are covered later.
The lens is a slightly more difficult item to secure but the best way is to cut a pair of thin plywood squares that are slightly larger than the diameter of the lens and cut circular holes say 1/8" smaller than the diameter of the lens in these squares leaving a narrow edge to support the lens. The distance between the upper and lower squares can now be measured and some small pieces of scrap can be cut to fit. These may be glued in at each corner of the square to snuggly trap the lens. This should result in the lens being constrained from moving about sideways or rattling up and down. The square lens holder is now easily mounted between a pair of parallel rails made with 1/4" square beading and fixed to each side of the box.
The see through mirror is mounted similarly between parallel pieces of beading. These need to be fitted so that the plastic sheet is firmly held at 45 degrees. Note that the bottom of the sheet should not interfere with the optical path between the mask and the bottom mirror. Therefore I constrained the position of the bottom of the plastic sheet to project not lower than the top of the bottom mirror. (The schematic is accurate in this respect !) The top of the plastic sheet should align vertically over the rear edge of the lens aperture.
The size of the box that houses these components may now be determined. The internal width is that of the lens holder plus a smidgeon of clearance, say 1/32". The base plate of the box is a piece of deal 1/2" thick and is planed all round and brought to the width above.
The length is determined by the focal length of the lens ('f' and the dotted line in the first schematic). Since this length is folded by the front surface mirror a certain amount should be subtracted which is typically the diameter of the lens but be certain that your arrangement uses exact values. To this remaining length is added about 1.5" to 2" for the front surface mirror's mounting block and adjusting screws at the rear and about 1.5" to 2" at the front for the block LEDs and its pins and components. The deal is cut 1/8" shorter at the front and rear to allow cover plates made from 1/8" ply to be fitted. The extended top rail of the lens holder mounting is also fitted 1/8" below the top edge of the box again to allow a cover plate at the top to be fitted.
The mounting block for the front surface mirror is glued to the deal floor, as is the battery holder and the curtain track for the block LEDs. The two sides are fitted to the deal block at each side with three screws each. The see through mirror slides down its channel until it hits the bottom stops. This item forms the front stop for the lens holder which slides into its parallel track from the rear.
A spacer, if required, follows the lens holder into the track to occupy the remaining space to the open end of the track. The rear cover plate then conceals the tracks and holds the lens holder in place. This method presents a neat arrangement when viewed from above. The external surfaces of the wooden items within the sighting path should be painted matt black. The outside surfaces of the box may be varnished, oiled or french polished (my choice) to water proof them and provide an attractive finish. Pale coloured plywood may be stained before applying the finish.
Calibrating the Finder
Firstly, if there is any doubt as to the alignment or colimation of the main telescope, fix that first. There is no point in aligning the finder if the next thing you do is alter the telescope. Set the scope up in daylight and find a distant object that is clearly identifiable with the naked eye. Chimney pots (or TV aerials) are fine as long as it is not possible to have more than one in the field of view. (It is quite easy to set the finder on one chimney pot and the scope on another - after all, the image is inverted and most chimney pots look the same anyway !). So try and find a unique object.
Find the object in the scope and bring it into the centre of the field of view using a low power eyepiece. Change to a higher power eyepiece and re-centre it. Now look through the finder and switch on the LEDs. Adjust the brightness so that the image of the circles are clearly visible. Without moving the telescope, carefully adjust the three mirror screws in turn so that the distant object is brought into the centre circle. From time to time, look through the main eyepiece to ensure that the telescope hasn't been pushed off its aim. If it has, re-centre it and return to the finder adjustments with more care. After a practice or two, you will find that the whole exercise can be done in less than five minutes.
Things only you know how to do
Mounting the finder to the telescope. I mounted my box firstly to a base whose underside matched the curvature of the 'scope body. To this I attached a hinged securing band which has a toggle catch to pull the two ends together and clamp the whole thing to the barrel of the 'scope. I stuck down a piece of tape on the OTA barrel along the side of the base so that I could remove and replace the finder and maintain a good alignment. This also allows me to verify that the finder hasn't been biffed in transit and knocked off aim. I'll add more points here as they arise.
Points arising should be addressed to