Sunday, 6 November 2016

A Review Of The Solutions To Newtons Rings In Solar Imaging

Newtons Rings are a problem that have afflicted solar imagers, in a variety a wavelengths, for some time, resulting in a series of alternating light and darker bands or concentric rings that appear super imposed over the target image.  Noticeably fickle in their apparition they can appear for some people in a particular optical / camera configuration, then not be visible for other using exactly the same setup.  Imagers have come up with a number of methods to remove or reduce them:  

One method involves letting the image drift or 'drizzling' the image across the field of view when taking the capture file, and then relying on the stacking software averaging these out in the stacking process.  Whilst it can be an effective way, it is not always convenient, feasible or desirable to do this, and the results can be variable depending on the orientation of the banding.

An alternative is to use a flat field; where the imager places an opaque material e.g. cling film or a clear plastic bag over the objective of the telescope and takes an image (or series of images).  This 'flat field' is then applied and subtracted from the final image during the stacking process automatically by the stacking software.  This has the benefit of it also removes the effect of 'dust bunnies' the dark shadow spots created by dust / detritus on the camera sensor.  Whilst this would appear to be all that is needed to remove the effect of Newtons Rings, sadly it is not.  The technique cannot be applied to full disk images, like the one above, it is only suited for close ups.  It is not wholly suited for making animations either as the effects of Newtons Rings also vary with the temperature of the camera being used, and if the camera is warming up, the effect of Newtons Rings will vary with time until he camera reaches thermal equilibrium.  

 Another method is to use a tilt adaptor as shown to the left.  This consists of 2 plates with 3 sets of adjusting screws allowing the user to offset the tilt between the 2 plates relative to each other.  This works by then effectively tilting the path of the light path relative to the chip / face plate on the camera where the newtons are occurring.  This tilting alters (makes longer) the effective path length of the light in the offending chip / face plate and as a result the constructive - destructive interference of the light no longer occurs and the problem is resolved.  With this method a solar imager needs to make a light shroud to go over the gap between the 2 plates, as, in the daytime this leaks light and will reduce the contrast in the solar image.  This is easy to do with something simple such as electrical tape.  In addition this type of adaptor adds about 10mm to the optical path which, if a Barlow lens is being used will result in a slightly higher magnification (than without), which may or may not be an issue for the user.  This type of adaptor works well if you do not need to adjust the tilt, but for different wavelength / camera / optical configurations changing the tilt can be fiddly and not really an effective way of doing it.  In addition this type of adaptor is available in a range of connectivity options including T2, M48 and SCT threads for at the time of writing ~£50 from a variety of international retailers.
 A variation of this is the one offered by Rowan Astronomy , shown to the left, which offers a spherical joint between the flanges to stop light entering the device, this is at the expense of additional back focus, which, may or may not be an issue.

Another alternative is the Daystar Interference Eliminator shown to the right and currently retailing for £/$149.  Whilst initially this may seem to be an expensive alternative to the adaptor above it offers several benefits.  Firstly the 2 curved surfaces remain closed at all times and as such there is no light leak to reduce contrast of solar images.  Secondly the 2 large thumbscrews allow easy 'real time' adjustments for different camera / wavelength / optical configurations.  The third aspect which most people over look the significance of is the fact that this works by tilting the camera off normal while keeping the centre of the image sensor stationary, so that focus, framing and vignetting are not changed with tilt.  The tilt changes the angle of incidence and optical path lengths inside the optical sensor, mitigating the interference.  As with the adaptor above this solution adds about 15mm of additional back focus, and if used with a Barlow lens will result in a slightly higher magnification, which, again, may or may not be an issue for the user.  The Daystar Interference Eliminator is the technically best available solution for Newtons Rings, but is also the most expensive, but this is reflected in the quality and effectiveness of the engineering employed.  It is available in T-mount and C-mount variations.

A final alternative is the use of a 'wedge prism' or 'Risley' prism.  This is an optical piece of glass where the 2 opposite faces are set at an angle rather than being parallel.  This has the effect of deviating the beam of light away from the normal axis, so that when it passes through the optical sensor it has a slightly different path length and so the interference effects of Newtons Rings are negated.  The wedged face of the prism is the one that needs to face towards the sensor.  The prisms are available in a range of wedge angles, usually 2, 4, 6 degree beam deviation angles, with the former 2 being suitable for solar imaging.  They are available with broad band optical coating from Thorlabs for the very cost effective price here in the UK for £24.  Being 25mm in diameter they are easily adapted to fit in the C-mount nosepiece of a camera, with a variety of retaining rings etc available from Thorlabs to secure these in place.  Unlike the 2 tilt adaptors above these add no additional back focus to a setup, with no change in the resultant magnification of Barlow lenses that may be used, which, again, may or may not be of concern to the user.  On the down side for this solution the tilt of the system is fixed, unlike the variable option of the 2 options above.  Also, unlike the Daystar solution, the tilt is not from the centre of the optical sensor, so, as with the first tilting adaptor, there can be focus issues at the edge of the frame or subtle vignetting.  The image at the top of this post was taken with a double stacked PST at ~500mm focal length using a 4 degree tilt Thorlabs wedge prism with a PGR Chameleon 3 USB camera.  If you look carefully at the full size version of the image you can see the left hand side of the image is softer than the right, a result of the tilt which would not be apparent with the Daystar method.

Other alternatives that have been used with some success are the use of atmospheric dispersion correctors, however these are expensive and add a considerable amount of back focus.  If the 1.25" - C mount adaptor you own is made of plastic, cross threading this to fit into the camera nosepiece to introduce tilt is an option, but with the usual caveat of focus gradients across the image and also potential vignetting. 

To sum up; Newtons rings is a problem that will always be with solar imagers, however as can be seen above there are a number of solutions.  By far the best method is the Daystar Interference Eliminator, but this is also the costliest, but for the serious solar imager cost should not be the deciding factor.  For those not wanting to spend any money flat fielding can be effective within reason and limitations.  For those wanting to spend some money, but not prepared to pay the premium of the Daystar route, for me, the wedge prism followed by the 'standard' tilt adaptor are the options to follow.