GB2497095A - A spectrographic device for astronomical spectroscopy including a focus aid - Google Patents

Abstract

The invention provides a spectrographic device 10 for astronomical spectroscopy, comprising an entrance slit 12 arranged to transmit a portion of a light beam received on the slit 12; a collimator 14 operable to collimate the portion of the beam transmitted through the slit 12; a dispersive element 16 configured to receive the collimated beam from the collimator 14 and to disperse the light; and a focuser 18 arranged to produce a focused image of the dispersed light. The focuser 18 is associated with a focus aid adapted to be insertable into the path of the dispersed light, preferably between the dispersive element 16 and the focuser 18, to thereby facilitate focusing of the device. The present device is particularly well suited for amateur-sized astronomical telescopes. The focus aid may comprise an image sharpener such as a slit mask. Alternatively the focus aid may comprise a Hartmann mask. The entrance slit 12 may be angled to the optical axis 20 of the device 10 to permit viewing of the slit 12 by way of light reflected by the slit 12.

Description

A SPECTROGRAPHIC DEVICE

The present invention relates to spectroscopy and particularly relates to a spectrographic device for use in amateur astronomy.

Amateur astronomy is a popular pursuit and in recent times there has been a growing number of amateurs who wish to make themselves useful in the field of astronomical research. Many amateur astronomers have computer controlled telescopes and imaging cameras, such as charge coupled devices (CCDs) etc., which allow them to do imaging and photometry.

However, it is generally the case that astronomical spectroscopy is not as common among amateur astronomers, as existing instruments for the amateur market are either too expensive or otherwise too complex to operate without expert guidance. Therefore, many amateurs may be reluctant to enter into the field of astronomical spectroscopy, as they generally believe that it is a difficult area in which to obtain satisfactory results.

For those amateurs who are willing to experiment in this area, many find that they encounter problems with accurately focussing the instrument and/or in keeping the astronomical object, e.g. star or planet etc. centered on the entrance slit of the spectrograph. Poor or inaccurate focussing of any optical instrument may degrade the resulting image, which reduces the overall satisfaction experienced by the user, and may indeed render their results useless.

Many amateur telescope mounts have limited tracking accuracy due to inherent gear errors and/or alignment errors in their mounting axes. Gear errors typically arise from manufacturing tolerances, defects andtor poor workmanship etc. Moreover, since most amateur telescopes are set up on temporary mounts (due to the absence of a permanent observing site or observatory), the problem of accurately aligning the telescope may be further compounded To obtain the spectrum of a faint' astronomical object, such as a S magnitude star, it is necessary to position the star on the entrance slit of the spectrograph and to keep the star on the slit for an exposure time of say 10 minutes. A typical slit size may be 50 microns in width by 5 mm in length.

A common aperture size for an astronomical (reflecting) telescope is generally around 200 mm with a focal length of about 2000 mm. Therefore, for a 50 microns slit this corresponds to an angular extent of 5.1 areseconds.

The typical gear errors in commercial astronomical telescopes may be around 30 areseconds peak to peak with a period of around 4 minutes.

IS Hence, during an unguided 10 minute exposure the target star will invariably move off the entrance slit of the spectrograph. A misalignment of only I degree in the polar axial alignment of the telescope produces a tracking error of around 0.261 arcseconds per second of time, so for example, in a 10 minute exposure, the target star will have drifted off the slit by about 157 arcseconds, which is considerably larger than the angular extent of the slit.

Therefore, as can be readily appreciated, the problem of keeping the star on the slit a considerable challenge for a typical amateur with modest commercial equipment.

An object of this invention is to address some, if not all, of the above problems by providing a spectrographic device that is easy to operate and use and which requires minimal set up and configuration.

A further object of this invention is to provide a spectrographic device that has an improved focussing capability andlor which mitigates against the problem of target object migration across the slit.

According to an aspect of the present invention there is a provided a spectrographic device for astronomical spectroscopy, comprising: an entrance slit arranged to transmit therethrough a portion of a light beam received on the slit; a collimator operable to collimate the portion of the beam transmitted through the slit; a dispersive element configured to receive the collimated beam from the collimator and to disperse the light; and a focuser arranged to produce a focussed image of the dispersed light, iS wherein the focuser is associated with a focus aid adapted to he insertable into the path of the dispersed light to thereby facilitate focussing of the device.

The provision of a spectrographic device comprising a focus aid associated with the focuser, and which is adapted to be insertabie into the path of the dispersed light to thereby facilitate focussing of the device, is found to be particularly advantageous. Therefore, the device is ideally suited for use in amateur astronomy, as the device is capable of achieving an accurate and reliable focus, which has hitherto been difficult to attain in conventional amateur instruments The ability to rcpcatedly achieve an accurate focus of the device is highly desirable as this enables good quality spectra of astronomical objects to be obtained with relatively little ease.

Moreover, the fact that the focus aid is selectively insertable into the path of the dispersed light means that once the device is accurately focussed, the aid may be removed from the light path which avoids any loss of light through the device, which is particularly advantageous when attempting to obtain the spectra of faint' astronomical objects.

References herein to amateur astronomy' are intended to encompass the activities of amateur hobbyists (i.e. non-professional scientists) as applied to the study of the night sky. Therefore, the term amateur astronomer' should be understood as meaning non-professional astronomers, who typically own and use modest astronomical equipment including amateur sized telescopes. An amateur sized telescope' may refer to any astronomical telescope (e.g. reflecting, refractive, catadioptic etc.) with an objective aperture usually in the range of 100 inn to 300 mm etc. However, it is to be appreciated that the device of the present invention may be used with any suitable sized aperture telescope, and even terrestrial telescopes, by both non-professional and professional astronomers (the latter case typically being for academic teaching purposes in colleges and universities etc.).

In preferred embodiments, the focus aid is adapted to be insertable between the dispersive element and the focuser. The user of the device, typically an amateur astronomer, can choose to selective]y insert the focus aid into the path of the dispersed light to accurately focus the device.

The focus aid is preferably removable from the device, and as such may be a separate standalone component. However, in other arrangements the focus aid may be disposed within the device and is adapted to be selectively moved into, and out of, the path of the dispersed light (e.g. by a pivoting mechanism etc.).

The focus aid may comprise an image sharpener. The image sharpener is preferably arranged to be operable to reduce the focal ratio of the focuser in the direction of the dispersion. In exemplary embodiments, the image sharpener is preferably in the form of a slit mask' comprising at least one slit or elongate aperture.

During focussing, the slit mask is preferably oriented such that the at least one slit is substantially parallel to the entrance slit. In this way, it is found that the focal ratio of the focuser is reduced in the direction of the dispersion, which consequently improves the spectral resolution of the device. Of course, the slit mask does reduce the amount of light which passes through the device, but by carefully selecting an appropriate width of the slit in the mask a good balance between loss of light throughput and improved resolution can be achieved.

In other embodiments, the focus aid may comprise a focus mask. The focus mask is preferably a I-lartmann mask, which is operable to cut off one half of the light passing through the device at any given time. The resulting image shift may then be used to determine an accurate or exact focus of the device.

Of course, it is to be appreciated that in any of the above embodiments, both an image sharpener and focus mask may be used intcrcltangeably within the device. Therefore, a user can select between which focus aid is to be inserted into the path of the dispersed light to facilitate focussing of the device.

It is to be appreciated, however, that any suitable focus aid may be used in conjunction with the present invention, provided it is operable to facilitate accurate focussing of the device.

During use, light from an astronomical telescope or other light gathering instrument, enters the device and is preferably focussed onto the entrance slit. The slit is arranged to transmit therethrough a portion of the light beam received on the slit, and to reflect any light that is not transmitted, In preferred embodiments, the entrance slit is angled to the optical axis of the device to thereby permit viewing of the slit by way of the light reflected from the slit.

The entrance slit is preferably positioned so as to be in the focal plane of the telescope, with the angle of the entrance slit being preferably fixed. In exemplary embodiments, the slit is preferably tilted to the optical axis of the device (and hence incident light beam) by around 22.5 degrees. Any light that is not transmitted through the slit is thereby reflected at around 45 degrees to the incident light beam.

In exemplary embodiments, the entrance slit has a slit width in the range of approximately 45 to approximately 55 microns, and is most preferably about 50 microns. The length of the slit may be around 5 mm. However, it is to be appreciated that any suitable slit width and/or slit length may be used in conjunction with the present invention.

The device may further comprise a fixed mirror, preferably angled to the optical axis of the device and to the entrance slit. [he mirror being operable to receive the light reflected from the slit and to permit viewing of the slit during use of the device.

The mirror is preferably a flat mirror, which acts to fold the path of the reflected light. In exemplary embodiments, the fixed mirror is at an angle of approximately 67.5 degrees to the optical axis, which means that the reflected light from the entrance slit at 22.5 degrees is hOW at around 90 degrees to the incident light beam.

Preferably, a second focuser is arranged to receive the reflected light from the fixed mirror to thereby form an image of the entrance slit. In this way, it is then possible for a user of the device to view the slit, without interfering with the path of the incident light beam. As a result, the user can then advantageously monitor the slit to determine the location of the astronomical target (e.g. star or planet etc.) on the slit, whereupon remedial action can be taken in the event that the target begins to drift off the slit during an exposure.

S

The remedial action' may take the form of a manual adjustment of the telescope position/tracking by the user or else may be an automatic process whereby an autoguiding device' automatically corrects the telescope's tracking and/or alignment to ensure that the target remains on the slit.

commercial autoguiders are readily available for the amateur astronomy market and it is intended for the present device to be compatible with most, if not all, of the available autoguiders.

The benefit of being able to prevent the target from drifting off the slit iS cannot be understated, as this permits the user to obtain long duration exposures (e.g. of 10 or more minutes), which enables relatively fainter spectra to the recorded.

The collimator is preferably a lens, and is most preferably a plano-convex lens, which is positioned so that it acts as a field lens that preferably re-images the entrance pupil of the telescope onto the plane of the first focuser. In this way, all of the light transmitted through the slit, including any off axis light rays (relative to the optical axis) are also made to enter the first focuser.

The dispersive element may be any type of conventional spectroscopic disperser. including, but not limited to, a prism, a grating, a grism and an echelle. Most preferably, the dispersive element is a grism, which preferably comprises a BK7 prism and a blazed transmission grating that achieves a reciprocal dispersion of about 61 nm/mm and a resolution of <2nm in the wavelength range of around 400-700 nm.

S

In preferred embodiments, the first focuser may comprise a lens and a focussing mechanism including a lockable focus mount in which to receive the lens. The lens may he a reverse telephoto type lens, so chosen because is has a long back focal length as compared with its actual focal length.

What this means in practice is that any conventional imaging detector, most particularly a CCD camera, which has a back focal length in the range of about 0 mm to about 20 mm can be used with the present spectrographic device. Therefore, the device may be universally used with most available commercial CCD cameras, which avoids the need for specialised equipment or specific types of CCD camera etc. The focus mount is preferably in the form of a focussing drum that is operable to move the lens along the direction of the optical axis of the device to thereby focus the dispersed light. Preferably, the focussing mechanism may further include a vcrnier focus ring having a scale for accurate readout of the focus position.

To attain an accurate focus, the user can employ the focus aid (as described above), which can be inserted between the grism and the reverse telephoto lens, whereupon the focus position can be reliably recorded.

The device preferably comprises a rigid housing, which in exemplary embodiments is fabricated from a solid rectangular block of aluminium -machined to house the various components. However, it is to be appreciated that any suitable housing or enclosure may be used in conjunction with the present device, without sacrificing any of the advantages or benefits of the invention.

The housing preferably comprises an entrance aperture and an exit aperture, each substantially aligned with the optical axis of the device. The entrance aperture is preferably adapted to be connectable to an astronomical telescope, and is most preferably in the. form of a conventional T-mount M42 x 0.75 telescope adaptor.

The exit aperture is preferably configured to receive a camera adaptor, and in particular a CCD camera adaptor Again the adaptor may be a conventional T-mount M42 x 0.75 adaptor.

However, it is to be appreciated that any suitable standard adaptors may be alternatively be used with the device. Indeed, it is envisaged that the present device will be provided with a se'ection of standard adaptors to enable the device to be connected to different types of telescope and/or imaging equipment etc. Preferably, the housing further comprises a slit viewing port having an axis iS aligned substantially along the path of light reflected from the fixed mirrof.

The slit viewing port is most preferably in the form of an eyepiece mount, which is arranged to receive an astronomical eyepiece or camera (e.g. TV camera. CCD video camera or autoguider etc.) depending on whether the slit is to be monitored either manually (i.e. by eye) or via electronic means.

The eyepiece mount is preferably of standard size (e.g. about 32 mm or 51 mm 1.25" or 2").

The second focuser is preferably located within the eyepiece mount and is operable to provide a focussed image of the slit to the eye of the user or camera etc. The housing preferably also comprises at least one access hatch or opening door, and most preferably two hatches or doors, to permit access to the interior of the device. Therefore, in preferred embodiments, one hatch is arranged to enable the focus aid to be inserted into the device so that the aid can be placed into the path of the dispersed light; white another hatch permits access to the focus drum and vernier scale, allowing focussing of the device to be accurately achieved.

The present invention also provides for a kit for constructing a spectrographic device according to any of the preceding embodiments.

The kit may also include instructions on how to assemble and use the device for spectroscopic purposes, particularly for amateur astronomical spectroseopy. I0

The present invention also provides for a speetroscopic telescope assembly, comprising an astronomical telescope, a mounting and a spectrographic device according to any of the preceding embodiments.

The astronomical telescope is preferably an amateur sized instrument and the mounting preferably comprises tracking motors for moving/tracking the telescope.

It is to be understood that none of the preceding embodiments are intended to be mutually exclusive, and therefore features described in relation to any particular embodiment may be used additionally and/or interchangeably with features described in relation to any other embodiment without limitation.

The present invention provides significant advantages over commercial amateur instruments, in that it preferably has a fixed slit and fixed dispersion with adjustable focus, which means that little or no configuration or set up is required for use. Hence, in essence the device has been designed to be effectively a "plug and play" device, which is beneficial to amateurs as they can use the device without expert knowledge or guidance, while still obtaining worthy astronomical results.

Embodiments of the present invention will now be described in detail by way of example and with reference to the accompanying drawings in which: Figure 1 -shows a side cross-sectional view of a spectrographic device according to a particularly preferred embodiment of the present invention; Figure 2 -shows a ray-tracing diagram of the optical layout of the device of Figure 1; Figures 3A & 3B -show respective schematic views of example slit arrangements at a preferred slit angle; Figures 4A-4C -show respective schematic views of example slit -arrangements at a different slit angle.

Referring to Figure 1, there is a shown a particularly preferred embodiment of a spectrographic device 10 according to the present invention. The device 10 is ideally suited for astronomical use, and in particular amateur astronomical usc, and comprises an entrance slit 12, a collimator 14, a dispersive clement 16 and a focuser 18. The focuser 18 is associated with a focus aid (not shown) adapted to be insertable into the path of the dispersed light to thereby facilitate focussing of the device 10.

In the example of Figurc 1, the focus aid is removable from the device and as such is a separate standalone component to that of the device 10.

During use, light from an astronomical telescope or other light gathering instrument (not shown), enters the dcvice 10 and is focussed onto the entrance slit 12 (as shown in Figure 2). The slit 12 is arranged to transmit 1.2 therethrough a portion of the light beam received on the slit, and to reflect any light that is not transmitted.

The slit 12 is angled to the optical axis 20 of the device to thereby permit viewing of the slit by way of the light reflected from the slit. The slit 12 is positioned so as to be in the focal plane of the telescope, with the angie of the sEit being fixed at around 22.5 degrees to the optical axis 20 of the device 10.

Any light that is not transmitted through the slit 12 is thereby reflected at around 45 degrees to the incident light beam, as shown in Figure 2.

The slit 12 is fabricated by electroforming a disk which is 10 mm in diameter and 100 microns thick (ef. Figure 1). The front surface of the disk is of optical quality and is highly reflective to incident light. The slit 12 is located at the centre of the disk and has a slit width of about 50 microns and a length of about 5 mm.

As shown in Figures 1 & 2, the light reflected from the slit 12 encounters a fixed (flat) mirror 22, which is at around 67.5 degrees to the optical axis 20. The mirror 22 acts to fold the reflected light such that the light is now reflected along a direction which is at an angle of 90 degrees to the optical axis 20 (and hence incident light beam).

A second focuser in the form of a short-focaL length lens 24 is arranged to receive the reflected light from the mirror 22. The lens 24 is a compound lens that serves to re-image the slit 12 in a slit viewing port 38 (as discussed below). The resulting image from the lens 24 is de-magnified, and hence brighter, than the original image at the slit 12, and the focai length of the lens 24 is chosen so that the whole of the field of view (about mm) can be imaged on an imaging device 26, e.g. CCD camera of chip size about 4.8 mm. I-)

However, the slit 12 can alternatively be viewed by eye using a suitable eyepiece in the slit viewing port 38, or a video imaging camera may be used, such as a miniature TV camera etc. S Light which passes through the slit 12 encounters the collimator 14 (as shown in Figure 2), which produces a collimated beam that continues to the dispersive element 16. The collimator is a piano-convex lens of aperture f/4.3, which is positioned such that it also acts as a field lens re-imaging the entrance pupil of the telescope onto the plane of the focuser 18. In this way, all of the light transmitted through the slit 12, including any off axis light rays (relative to the optical axis 20) are also made to enter the focuser 18.

The projected slit width' as seen by the collimator 14 is reduced by cosine 22.5 degrees, as a consequence of the tilt of the slit 12 to the optical axis 20. As a result, this acts to improve the resolution of the device 10.

The collimated beam from the collimator 14 is received by the dispersive element 16, which in the example of Figure 1, is a grism consisting of a BK7 prism and a blazed transmission grating. This dispersive element is capable of achieving a reciprocal dispersion of 61 jIm/mm and a resolution of <2nm in the wavelength range of about 400 nm to about 700 nm.

Of course, any suitable spectrographic disperser may alternatively be used depending on the desired performance and/or application of the device.

The dispersed light is refocused by the focuser 18, which comprises a lens and a focussing mechanism including a focus mount 28 in which to receive the lens. The lens is a f/2.8 reverse telephoto type lens having a long back focal length as compared to its actual focal length This means that any conventional imaging detector, most particularly a CCD camera, which has a back focal length in the range of about 0 to about 20 turn (i.e. the distance between the external mounting plate and the CCD detector) can be used with the spectrographic device-Therefore, the device may be universally used with most available commercial CCD cameras, which avoids the need for specialised equipment or specific types of CCD camera etc. The focus mount 28 is in the form of a lockable focussing drum that is operable to move the lens along the direction of the optical axis 20 of the device 10 to thereby focus the dispersed light. The focussing mechanism also includes a vernier focus ring (not shown) having a scale for accurate readout of the focus position.

To attain an accurate focus of the device 10, the user can employ the focus aid, which is inserted between the grism and the reverse telephoto lens of the focuser 18.

In an all refractive spectrograph therc will always be some residual chromatic aberration (i.e. false colour), as well as other aberrations. When a typical amateur telescope, which has a focal ratio about filO is coupled to the spectrograph, these aberrations will be small, because of the small angle of the beam through the instrument.

If the present device is used on faster focal ratios (i.e. lower f numbers), which in practice can be up to the focal ratio of the collimator (which is 114.3), the aberrations can become much more significant, especially outside of the 400-700 urn nominal wavelength range of the device.

Therefore, a focus aid in the form of an image sharpener can be used to reduce the aberrations and facilitate an improved focussing of the device.

The image sharpener can be a slit mask comprising at least one slit or elongate aperture-During focussing, the slit mask is oriented such that the at least one slit is substantially parallel to the slit 12. In this way, it is found that the focal ratio of the reverse telephoto lens is reduced in the direction of the dispersion, which consequently improves the wavelength resolution of the device 10. Of course, the slit mask does reduce the amount of light which passes through the device, but by carefully selecting an appropriate width of the slit in the mask a good balance between loss of light throughput and improved resolution can be achieved.

For example, a slit of 0.33 beam size (in the slit mask) reduces the throughput of the light in the device to 41%, which is only 0.96 magnitudes, but improves the spectral resolution of the device by a factor of 3.3.

In addition, or as an alternative to, an image sharpener, the focus aid may be in the form of a focus mask, such as a Hartmann mask. Such a mask is operable to cut off one half of the light passing through the device at any givcn time. The resulting image shift may then be used to determine an exact focus of the device, which allows the focuser 18 to accurately focus an image of the dispersed light onto a suitable detector 30, as shown in Figure 2. The detector 30 is ideally a CCD camera, but other devices can alternatively be used.

Referring again to Figure 1, the device 10 comprises a rigid housing 32, which is fabricated from a solid rectangular block of aluminium -machined to house the various components. The housing 32 comprises an entrance aperture 34 and an exit aperture 36, each substantially aligned with the optical axis 20 of the device 10. The entrance aperture 34 is adapted to be connectable to an astronomical telescope, and is ideally in the form of a conventional T-mount M42 x 0.75 telescope adaptor.

The exit aperture 36 is configured to receive a camera adaptor 36a, and in particular a CCD camera adaptor. Again the adaptor 36a is ideally a conventional T-mount M42 x 0.75 adaptor. In the example of Figure 1, the CCD camera adaptor is a plane mounted ring with a V-shaped groove for alignment and fixing. Once the adaptor is screwed into the CCD camera by a standard T-mount M42 x 0.75 thread, the alignment wilt generally be at some random angle relative to the CCD detector 30. However, the dispersion produced by the device 10 needs to he parallel to the X-axis of the detector 30. Therefore, the V-shaped grooves allow the CCD to be aligned exactly with the detector 30. Provision has also been made for approximate 0.5 mm X & Y adjustments, in order to allow some adjustment of the chosen central wavelength on the CCD detector 30.

However, it is to be appreciated that any suitable standard adaptors may be alternatively be used with the device. lndeed, it is envisaged that the present device will be provided with a selection of standard adaptors to enable the device to be connected to different types of telescope and/or imaging equipment etc. The housing 32 further comprises a slit viewing port 38 having an axis 40 aligned substantially along the path of light reflected from the fixed mirror 22. The slit viewing port 38 is in the form of an eyepiece mount, which is arranged to receive an astronomical eyepiece or camera (e.g. TV camera, CCD video camera or autoguider etc.) depending on whether the slit 12 is to be monitored either manually (i.e. by eye) or via electronic means, such as an autoguider etc. The eyepiece mount is of standard size (e.g. 32 mm or 51 mm -1.25" or 2") as conventionally used in amateur equipment. The eyepiece mount is fabricated as a separate tubular portion which is attached to the main body of the housing 32 by a threaded engagement, bayonet type connector or else welded or bond to the housing etc. As shown in Figure 1, the lens 24 is located within the eyepiece mount and is operable to provide a focussed image of the slit 12 to the eye of the user or autoguiding camera etc. (cf. Figure 2).

The housing 32 also comprises two access hatches or doors 42A, 42B to permit access to the interior of the device 10 (see Figure 1). Therefore, one hatch 42A is arranged to enable the focus aid to be inserted into the device 10, so that the aid can be placed into the path of the dispersed light; while the other hatch 42B permits access to the focus drum 28 and the vernier scale. In this way, the position of the focus can be read off the scale and recorded as necessary.

As discussed above, the slit viewing capability offered by the present spectrographic device is particularly advantageous for mitigating against the drifting of a target object off the slit of the device. The angle of the entrance slit 12 is essentially determined by two limitations, namely (I) the transmission of the slit when tilted to the incident light beam, and (ii) the position of the fixed (flat) mirror to intercept the reflected light from the slit without interfering with (i.e. blocking) the incident light beam.

In respect of (i), the slit is electroformed as a 100 micron thick disc with a slit width of 50 microns + 5 microns. Referring now to Figure 3, there is a shown the path of the incident light beam through two example slits I 2A, 12B, each angled at 22.5 degrees to the optical axis. In Figure 3A, the edges of the slit 12A are exactly square, which gives rise to an effective slit width' of only about 10 microns. In fact, due to the method of manufacture, the edges of the slit are actually more like those shown in Figure 3B, which gives a calculated effective slit width of only 33 microns.

For the present device as shown in Figure 1, the effective slit width at 22.5 degrees has actually been measured to be 38 microns. is

Figure 4 shows example slits each angled at 45 degrees to the optical axis, which represents the easiest option for viewing the slit as no additional fixed mirror would be required. However, as is apparent from Figure 4A, the slit 12A would actuafly have no transmission at this angle and therefore no light would be able to enter the device 10. But, as noted above, in practice the edges of the slits are generally curved, as shown in Figure 4B, and so some light (albeit significantly reduced) would be able to pass through the sEt 12B.

A solution to this problem is shown in Figure 4C. In this example, the slit 12C is machined from a solid block of metal and the inner surface of the edges of the slit 12C are bevelled. Therefore, there is no blocking of the incident light at a slit angle of 45 degrees. However, slits of this form would be very expensive to manufacture, as the metal, typically brass, would have to have the front surface of the slit polished to opticai quality and then the surface would need to be aluminised.

Consequently, such a component is not suitable for use with the present device, as this is principally intended for the amateur astronomy market and as such needs to be relatively cheap to manufacture.

In respect of (ii), another possible solution would be angle the entrance slit at a relatively small angle, say 10 degrees. However, it would then be necessary to position the fixed mirror further from the slit to avoid interfering with the incident light beam. As a result, the overall dimensions of the device would increase, which is undesirable as this would likely also increase the weight of the device, while also rendering the device cumbersome to use and mount to the telescope.

Although the spectrographic device of the present invention is ideally suited for astronomical spectroscopy, and in particular amateur astronomical spectroscopy, it will be recognised that one or more of the principles of the invention may extend to other applications and/or spectroscopic devices. Hence, for example the device may have application in 2D imaging spectroscopy, such as that used in aerial surveillance of agricultural crops etc. The above embodiments are described by way of example only. Many variations are possible without departing from the invention.

Claims (1)

1. <claim-text>CLAIMS1. A spectrographic device for astronomical spectroscopy, comprising: an entrance slit arranged to transmit therethrough a portion of a light beam received on the slit; a collimator operable to collimate the portion of the beam transmitted through the slit; a dispersive element configured to receive the collimated beam from the collimator and to disperse the light; and a focuser arranged to produce a focussed image of the dispersed light, wherein the focuser is associated with a focus aid adapted to be insertable into the path of the dispersed light to thereby facilitate focussing of the device.</claim-text> <claim-text>2. The device of Claim I, wherein the focus aid is adapted to be insertable between the dispersive element and the focuser.</claim-text> <claim-text>3. The device of Claim 1 or Claim 2, wherein the focus aid comprises an image sharpener.</claim-text> <claim-text>4. The device of Claim 3, wherein the image sharpener is operable to reduce the focal ratio of the focuser in the direction of the dispersion.</claim-text> <claim-text>5. The device of Claim 3 or Claim 4, wherein the imagc sharpener comprises a slit mask.</claim-text> <claim-text>6. The device of Claim 5, wherein the slit mask is oriented such that at least one slit in the mask is substantially parallel to the entrance slit.</claim-text> <claim-text>7. The device of Claim 1 or Claim 2, wherein the focus aid comprises a Hartmann mask.</claim-text> <claim-text>8. The device of any preceding claim, wherein the entrance slit is angled to the optical axis of the device to thereby permit viewing of the slit by way of the light reflected from the slit.</claim-text> <claim-text>9. The device of Claim 8, wherein the angle of the entrance slit is fixed.</claim-text> <claim-text>10. The device of any preceding claim, further comprising a fixed mirror angled to the optical axis of the device and to the entrance slit, the mirror being operable to receive the light reflected from the slit and to permit viewing of the slit during use of the device.</claim-text> <claim-text>11. The device of Claim 10, wherein the fixed mirror is a flat minor operable to fold the path of the reflected light.</claim-text> <claim-text>12. The device of any preceding claim, further comprising a second focuser arranged to receive the reflected light to thereby form an image of the slit.</claim-text> <claim-text>13. The device of Claim 12, wherein the second focuser is a compound lens.</claim-text> <claim-text>14. The device of any preceding claim, wherein the dispersive element is any one of: a prism, a grating, a grism and an echelle.</claim-text> <claim-text>15. The device of any preceding claim, wherein the focuser comprises: a lens; and a focussing mechanism including a focus mount in which to receive the lens.</claim-text> <claim-text>16. The device of Claim 15, wherein the focus mount is in the form of a focussing drum operable to move the lens along the direction of the optical axis of the device to thereby focus the dispersed light.</claim-text> <claim-text>17. The device of Claim 15 or Claim 16, wherein the focussing mechanism further includes a vernier scale for readout of the focus position.</claim-text> <claim-text>18. The device of any preceding claim, wherein the angle of the entrance slit is approximately 22.5 degrees.</claim-text> <claim-text>19. The device of any preceding claim, wherein the slit width is in the range of approximately 45 to approximately 55 microns.</claim-text> <claim-text>20. The device of Claim 10 or Claim 11, wherein the fixed mirror is at an angle of approximately 67.5 degrees to the optical axis.</claim-text> <claim-text>21. The device of any preceding claim, wherein the collimator is a lens.</claim-text> <claim-text>22. The device of any preceding claim, further comprising a housing, the housing including an entrance aperture and an exit aperture each substantially aligned with the optical axis of the device.</claim-text> <claim-text>23. The device of Claim 22, wherein the entrance aperture is adapted to be connectable to a telescope.</claim-text> <claim-text>24. The device of Claim 22 or Claim 23, wherein the exit aperture is configured to receive a camera adaptor.</claim-text> <claim-text>25. The device of any of Claims 22 to 24, wherein the housing further comprises a slit viewing port having an axis aligned substantially along the path of the reflected light.</claim-text> <claim-text>26. The device of Claim 25, wherein the slit viewing port is in the form of an eyepiece mount.</claim-text> <claim-text>27. The device of Claim 26 when dependent on Claim 12, wherein the second focuser is disposed within the eyepiece mount.</claim-text> <claim-text>28. A kit of parts for constructing a spectrographic device according to any of Claims ito 27.</claim-text> <claim-text>29. A spectroscopie telescope assembly, comprising: an astronomical telescope; a mounting; and a spectrographic device according to any of Claims 1 to 27.</claim-text> <claim-text>30. A spectrographic device as hereinbelore described with reference to Figures 1 to 4 of the accompanying drawings.</claim-text>