SkyInspector.co.uk

Astronomy and Sky Website of Martin Lewis

This is a fuller version of an article which appeared in the Nov. 2012 edition of the BBC Sky at Night magazine.

Note that I have a second page on ADCs on my website. This is of a more specialised nature and explores the subject of aberrations introduced by the ADC particularly when used on low altitude objects

This page was last updated 12-2-2017

Planetary Imaging with an ASH Atmospheric Dispersion Corrector (ADC) sited between my old DMK camera and my 3x TV barlow

Introduction

In late 2011 I started using a device called an Atmospheric Dispersion Corrector (ADC) during planetary video imaging sessions. This device dramatically reduces the prismatic smearing caused by the passage of the light through our atmosphere and allows planetary images to be taken which show noticeably finer detail.

Commercial ADCs are relatively new and currently in the UK are only used by a handful of planetary imagers. Their significant benefits, however, justify more astronomers knowing about them. As well as being an advantage to imagers, they can also help visual planetary observers, increasing the amount of surface detail seen through the eyepiece

Why you would use an ADC?

Light from any star or planet that enters our atmosphere at an angle will suffer refraction effects which bend to light to a slightly steeper angle. This refraction effect makes the object appear higher in the sky than if the atmosphere wasn’t there. The degree of refraction depends to a small degree on various things like temperature, humidity, observers and height above sea level, but does increases strongly with decreasing altitude of the object – for example, the image of the sun at sunset can actually be elevated by over 0.5°, which is slightly more than one solar diameter.

Like most optical media the refractive (bending) power of the atmosphere is actually dependent on the wavelength of light. This is called optical dispersion. Atmospheric dispersion means that actually the degree of ‘lift’ that the refraction causes depends not only on the angle of the light, but also the colour of that light. Atmospheric dispersion spreads the light from any point source into a vertical spectrum of colours whose length increases the lower the object is in the sky.

The schematic below illustrates this dispersion effect as light enters the atmosphere and also the lifted elevation of the image. The lift is wavelength dependent and hence the image is separated into different colours. A planet exhibiting bad atmospheric dispersion effects will have a blue fringe on the top edge and a red fringe on the bottom edge.

You can see an example of this dispersion effect on the image of Polaris below for an altitude of 52° and taken with a colour digital video camera with no atmospheric dispersion corrector in place. The airy disc is spread into a small vertical spectrum whose height is considerably longer than the diameter of the airy disc.

Polaris spectrum
Here we see Polaris at an altitude of 52° showing the effects of dispersion which has spread the Airy disc of the star into a vertical spectrum (222mm scope).
For a great comparison between real and modelled dispersion of stars at different altitudes and with different filters please have a look at the webpage of Andre Paquette

A correctly adjusted ADC placed between the camera or eyepiece and a barlow lens will reduce this spectral spread, improving image resolution as a result. It does this by applying the opposite amount of dispersion to that caused by the atmosphere, reconverging the light of the different wavelengths at the focal plane.

Principle of Operation of an ADC

Atmospheric-Prism-no-atmosphere
Here we see the simplistic situation of light from a distant star being brought to focus by the telescope
Atmospheric-Prism
In the real world things are not so simple- the light from a star at low altitude in the sky is actually bent by the atmosphere which acts like a large prism. Because the bending power of this prism is wavelength dependent, the light gets spread into a spectrum with the light from the star seemingly elevated in altitude- Blue is elevated the most and Red the least. As a consequence the telescope brings the light from the star not to a focus at a single point but instead focuses it into a thin spectrum spread in the vertical direction
With Barlow
Adding a Barlow to increase the image scale for imaging makes things even worse by increasing the separation between the red and the blue in the same proportion as the increase in magnification

To recombine the separated colours caused by atmospheric dispersion an ADC generally uses a double-prism arrangement to generate a user-settable dispersion of the opposite direction to that induced by the atmosphere. The clever double-prism arrangement allows the correcting effect to be smoothly varied over a wide range by moving one prism clockwise and the other anti-clockwise so they are always symmetrical about the vertical and the corrective dispersion is in the vertical plane too.The user adjust the prism pair so that at the right setting the separated images of different colours are brought back together again at the focal plane.

As an example of an ADC, that made by Astro Systems, Holland, houses a matched pair of thin, 2° deviation, multi-coated BK7 prisms, which are circular in shape and arranged face-on to the optical axis. These prisms are arranged so that the rotational position of each prism is independently controllable using a lever. By using these levers to alter the angle between the two prisms, one can vary the degree of overall dispersion of the ADC between zero and double that of a single prism.

The double prism arrangement can be seen in the schematic below. This first diagram shows the prisms at a position of maximum dispersion correction, where the two thick ends of the prisms are together and the levers are 180° apart, and then at no dispersion correction, where the two thick ends of the prism are opposite each other and the levers are together

With-Barlow-and-ADC-at-Max
In the ADC, a pair of adjustable prisms are able to rotate about the optical axis of the scope. This allows you to generate a prism pair which when correctly aligned gives a variable bending power in the vertical plane. The ADC prism material is chosen to give a similar dispersion characteristic with wavelength to that induced by the atmosphere but the prism set is upside-down compared to the one that effectively is in the sky. The prisms bends the blue light more than the red light so that at the correct setting the different wavelengths all reconverge at the focal plane….. Note that the recombined (corrected) image is displaced vertically upwards away from the optical axis by the action of the prism – the red image was originally sitting on the optical axis of the scope……In use the setting for good correction would usually be at less than maximum but this schematic where the images recombine at maximum is clearer.
With-Barlow-and-ADC-at-Zero
Here the prisms are rotated 90° each- the LH one clockwise and the RH one anticlockwise, so that the levers are now together. Like this the effects of the two prisms cancel each other, leaving the light of the star still spread into a vertical spectrum at the focus. Like this the ADC has no correcting effect and this is the null or zero position

This photo shows an ASH ADC with the two prism handles equally placed either side of the null (handles together) position.

As well as allowing a variable magnitude of overall dispersion, having pair of identical prisms that are both adjustable allows one to maintain the direction of the dispersion correction applied as one homes in on the optimum setting of the magnitude. This is done by moving the levers equally either side of the central position. This is vital as both the magnitude and the direction need to be correct to properly cancel the atmospheric dispersion.

It is important to realise that the ADC does not try and set to zero the angular divergence of the different colours that has been induced by the atmosphere (schematics often show the emerging colours as parallel). Actually it needs to introduce a reconvergence of the angularly separated colour images so they all meet up again at the focal plane.

As well as introducing the correcting dispersion that brings the colours back into coincidence, a pair of separated narrow-wedge prisms, arranged as above, will introduce an overall deviation of the light beam away from the optical axis. The actual lateral shift of the image is dependent on the prism settings used but can actually move the object right out of the field of the camera during adjustment. This movement is an inconvenience, but after a bit of use one can usually make a good guess at a provisional setting for the prism and small offsets then can be readily corrected for by moving the mount slightly. In use this issue has turned out to be less of a problem than I had anticipated.

Atmospheric Dispersion and Altitude

Traditionally monochrome imagers take videos through separate red, green and blue filters and then combine the three final processed images into one full-colour image. The method does actually partially suppress the effects of atmospheric dispersion without resorting to using an ADC. This is because the dispersion spread within each separate colour band is significantly less than across the whole of the visible spectrum and so by realigning the three separate red, green and blue images, prior to creating the final colour image, the overall amount of atmospheric dispersion is reduced. This method works reasonably well for the red and green bands, especially when the planet is quite high in the sky, but is less successful for the blue wavelength band. Because the magnitude of the dispersion increases at shorter wavelengths, for the blue band atmospheric dispersion is great enough to cause significant vertical smearing within the blue image, even at quite high altitudes of the planet.

The plot below is kindly reproduced with permission of Jean Pierre Prost (see www.astrosurf.com/prostjp/Dispersion_en.html) and shows the amount of vertical smearing of the image caused by atmospheric dispersion in arcsecs calculated as a function of an object’s altitude. The plot is for Red, Green and Blue Astronomik brand filters and also for an Astronomik L filter (which allows the full visible spectrum through except IR and UV).

You can see here that the smearing of the image caused by atmospheric dispersion is unacceptably large at almost all altitudes for the L filter (which accepts the full visual spectrum) and also is significantly larger for the blue filter than the green or red filters. Blue has the worst smearing because the change in dispersion with wavelength is greater in the blue this means that even though the blue filter has the narrowest bandpass of the three colours (88nm compared to 93nm for red and 90.5nm for green) there is the biggest difference in dispersion from one side of the wavelength band to the other side. 

For my 222mm reflector, where my finest resolution detail is about 0.4″, we see a benefit in the blue for planets when below an altitude of 66°, a benefit in the green when below 42°, and a benefit in the red when at altitudes below 33°.  For larger aperture telescopes the resolution is better and then the use of an ADC becomes beneficial at the higher object altitudes.

From the UK the ecliptic is never more than about 60° high so this is the maximum altitude that a planet can attain. Below is a list of the maximum altitudes of Mars, Jupiter and Saturn at opposition from latitude 52° N over the coming few years;

  • Mars
    • 2020 Oct 5th = 43.5°
    • 2022 Dec 8th = 63°
    • 2025 Jan 16th = 63°

  • Jupiter
    • 2020 July 14th = 16°
    • 2021 Aug 20th = 24.5°
    • 2022 Sept 26th = 38°
    • 2023 Nov 3rd = 51.5°
    • 2024 Dec 7th = 60°
    • 2025 Jan 10th = 60°
  • Saturn
    • 2020 July 20th = 16.5°
    • 2021 Aug 2nd = 19.5°
    • 2022 Aug 14th = 22.5°
    • 2023 Aug 27th = 26°
    • 2024 Sept 8th = 30.5°
    • 2025 Sept 21st = 35°

Methods of Setting the ADC prisms

The two prism arrangement in an ADC needs to be correctly set to give the right amount and also the direction of dispersion to most effectively cancel out the dispersion caused by the atmosphere. First one needs to set the orientation of the mid point of the prism pair and then one sets the overall magnitude of the correction by adjusting their angular separation.

F-Ratio and Location in Optical Path

ADCs are designed to work best at high f-ratios. At short f-ratios or with large amounts of prism correction, aberrations can be introduced which can start to offset the overall benefits. To reduce aberrations on short focal ratio scopes it is best that they are placed directly after the Barlow lens and a reasonable distance from the camera. For longer f-ratio scopes, such as SCTs, putting the ADC before the Barlow is less problematic and has some advantages in increasing the available amount of correction for low altitude objects.

I have a dedicated page on aberrations introduced by ADC if you want to read more.

Orientation

Atmospheric dispersion always spreads the light out in a vertical direction so the ADC needs to be orientated so its overall dispersion axis is vertical too. For my ADC this is simply done by ensuring that the mid-point of the two prism adjustment levers is horizontal as seen through the focuser. 

For any scope where there is no right-angle mirror, such as an SCT or refractor (without a star diagonal) the horizontal axis as seen through the focuser is the same orientation as the horizon relative to the observer, and so this makes the task quite straight-forward as the mid-point of the levers is also just horizontal. The mid-point of the levers can be horizontally pointing left or horizontally pointing right. In one of these cases adjusting the ADC will make the atmospheric dispersion worse but in the other orientation it will make it better. You need to find which of the two possibilities is correct by experimentation (the correct one is where the fat end of the prisms is at the top as seen in the earlier ray diagrams).

If you have a right-angled mirror in the telescope, such as for a Newtonian reflector or a refractor or SCT with a star diagonal in place, the horizontal orientation through the focuser may well not be parallel to the real-world horizon. How you ensure that the ADC is correctly oriented so that the mid-point of the levers is parallel with the horizontal direction in the focuser depends on the type of mount you have;

For an equatorially mounted Newtonian scope, the orientation of the horizontal direction in the focuser will change as you move around the sky as the telescope tube will rotate about it central (long) axis. As you track an object across the sky the sky orientation will remain fixed with respect to the focuser but because the angle of the horizon is changing with respect to that bit of sky this means the horizontal direction will move around the outside of the focuser. Lining up the ADC correctly becomes difficult for this sort of telescope. Methods such as using a cardboard mask to block the bottom half of the scope aperture are possibilities. In this method the top edge of the card is made horizontal and out of focus stars in the field will be half moon-shaped with the straight edge indicating the horizontal direction. The mid-point of the levers need to be oriented to match this direction.

For an alt-az mounted Newtonian scope the task of orienting the ADC becomes much easier because the horizontal direction through the focuser is fixed with respect to the outside of the focuser and the tube does not rotate about its axis as you move around the sky. This horizontal direction will likely be some odd angle to the bottom edge of the focuser, as it depends on how far around the tube the focuser is sited, but once found it won’t move around like with the equatorially mounted Newtonian.

To place the prism mid-point marker correctly for future use an alt-az Newtonian you’ll first need find the horizontal axis of the sky as viewed through the eyepiece. The image below hopefully explains this a bit more clearly and shows a simplified version of the view through the focuser for my Dobsonian mounted Newtonian reflector with the eyepiece removed. The horizontal axis is marked with the yellow dotted line. At the ~4 o’clock position on the focuser face I have placed a white triangle marker to denote the horizontal position and it is this which I align with the mid-point of my pair of ADC levers.

Marking the location of the horizontal direction on the focuser for correct ADC orientation when using an alt-azimuth mounted Newtonian reflector

Invariably I set the levers equally either side of their zero position and align the zero position with the yellow triangle. To make this zero position easier to find and to enable me to tell if the levers are approximately equally-spaced either side of the mid-point in the dark by feel alone, I have added a fixed rod on a 1.25″ ring at the null location.

You can see this fixed zero marker on my previous ASH ADC at the bottom in the picture above. You can also see the prism levers set equally either side of this zero marker

Below shows a sequence of images to help explain the lever positions and shows my PA ADC with prism settings ranging from zero up to maximum adjustment. This image set is based on the reference image just above, where the horizontal line as seen through the eyepiece was marked in as a dashed yellow line. The positions of the prism levers are marked with yellow spots to make them easier to see.

Degree of Correction- Magnitude of Adjustment

The dispersion of the atmosphere depends primarily on the object’s altitude, with some small adjustment for temperature, humidity and barometric pressure. One might think therefore that the device could come with a scale marked for different object altitudes. Unfortunately, it is not that simple. The optimum ADC prism setting for a given altitude of object depends on several things outside of the ADC manufacturers’ control such as;

•    the effective focal length of the telescope

•    the distance between the ADC and the focal plane

The best way to determine the most suitable prism settings for an object at a given altitude is by experimentation. This can be a bit of a chore to start with, but once the optimum setting is found the same setting can be used for the same optical set-up provided the planet is at a similar altitude – which it usually is if you observe with the planet close to the meridian each time during an apparition.

I have used two methods for finding the best prism settings for the ADC;

Determining the Amount of ADC Adjustment Needed- Wratten 47 filter method

A Wratten 47 filter allows violet light through but also has a strong leak in the infra-red. Without the ADC in place, atmospheric dispersion for wavelengths so far apart can be quite significant and cause a bright star away from the zenith to appear as two well-separated images; a bright IR one below a fainter violet one. With the ADC in place one can adjust the two prism levers (ensuring they are equally displaced about the mid-point lever) until the violet and IR image are brought together. The method works okay but relies on a reasonably bright star being at the same altitude as the planet you want to image. There can also be issues with the fact that the IR image is generally so much brighter than the violet one.

The comparison below shows a star at about 28° with various ADC settings and a W47 filter using a colour camera to show the violet and IR images. 

A 1.25″ W47 filter can be purchased in the UK from; www.astronomica.co.uk/wratten-47-violet-colour-filter

Different ADC settings for a star at 28° when using a W47 filter showing the varying separation of the violet and IR images transmitted by this filter. For clarity this is shown for a colour camera. Of course such a method can be equally used with a mono camera in which case both the UV and the brighter IR images will be monochrome which makes it doable but harder to determine the position of optimum correction.

Determining the Amount of ADC Adjustment Needed – Edge-Tinge method

In this method you use a colour camera in image preview mode to view the planet with the ADC and barlow in place and the ADC set to zero. Once the planet is in best focus, increase the gain or exposure time to make the disc of the planet overly bright (even just burnt-out/white). The dimmer limb of the planet should not be burnt-out but instead it should have an obvious coloured tinge, red on one side and blue on the other, caused by the atmospheric dispersion effect. To find the optimum setting of the prisms you just move the ADC levers equally about the zero point until the colour is even all round the limb.

The comparison images below, taken with a colour Toucam, show the variation in the edge tinge distribution with lever position. The optimum position of the levers is ±3.5 divs. At ±4.5 divs the planet has a blue tinge at the bottom due to too much correction, whilst at ±2.5divs the blue tinge is at the top because there is too little correction applied (in my set up I have the planet orientation uninverted in the preview screen).

This method is more accurate than the W47 method and because it is done on the actual object you want to observe the altitude is exactly right. You may get better results by increasing the colour saturation for the camera but be careful to keep a good colour balance so that blue and red are equally seen.

If you are temporarily using a colour camera just to set the ADC and then want to replace it with your main mono camera to image with, it is important that you determine the correct prism lever settings at the same focal plane to ADC distance that you will be using for your imaging camera. This is because the correct prism settings are dependent on the separation between ADC and the camera. Generally I put my mono camera in place and get the right focus, then swap to the colour camera to find the ADC settings, then swap back to my mono camera. I focus the colour camera just by moving it in an out of the ADC’s 1.25″ eyepiece holder, without disturbing the focus knob, until the image is sharp. I then lock the camera in this position, adjust the prism levers until I get the right settings before removing the colour camera and switching back to the mono camera, which should still be close to optimum focus.

A refinement of the edge-tinge method uses a nice feature of the camera control software Firecapture, which is the strong colour-saturation setting under the pre-processing tab (see ringed feature on right hand side of screenshot below)- this makes the task of setting the ADC easier still.

By setting saturation to maximum and increasing the exposure to massively overexpose the planet you get high sensitivity to small changes in correction. This can be seen below for an image of Jupiter at an altitude of 60° using my Pierro Astro ADC with a 2x Meade barlow and my ASI120MC colour camera. Here the effects of a change of just 0.5 divs. in the lever position (=7.5° of lever rotation) can be clearly seen.

For very low altitude objects the amount of dispersion obtained with the levers at maximum setting (levers at 180° apart) may not be enough to obtain full correction. The answer in this case is to increase the distance between the ADC and the focal plane effectively increasing the power of the ADC.

NB. The edge-tinge method needs to be used with caution on Mars. Here bluish cloud at the limb of the planet can be confused with the colour tinging from dispersion. You may need to play around with the levels of correction, over-correcting and under-correcting to see the dispersion tinge appear and disappear to be able to separate it from cloud edge-tinge.

Determining the Amount of ADC Adjustment Needed- Visual Method

If you are not imaging planets, but just visually observing them, you can use the simplest method of all to set the ADC. Set the orientation of the ADC as described previously with the mid-point horizontal (as seen through the eyepiece) and then simply put your eyepiece in the ADC and adjust the levers until you find the setting where the image does not have a red tinge on one side and a blue tinge on the other side.

For more critical visual work one could use the edge-tinge method with a colour camera as described above, prior to eyepiece use. The colour camera used in the set-up needs to be at the focus of the eyepiece so the procedure is;

  • Focus the eyepiece and then do not disturb the focus knob
  • Focus the colour camera by just moving the camera adaptor in and out of the ADC 1.25″ eyepiece adaptor. Lock the camera in the adaptor.
  • Adjust the prism levers to remove the edge tinge.
  • Remove camera and reinsert the eyepiece which should still be at the right focus.

The converse to all this is to use the visual method to set the ADC for imaging ensuring that the eyepiece’s focal plane is at the same position as the camera’s focal plane using a method similar to that described above. Such a method is very quick and is useful for say imaging Venus in bright twilight where the blue tinge would be lost in the background sky. This rough and ready method can also be used for a quick setting when you are imaging in the red where the actual setting does not need to be so accurate due to the low dispersion in this wavelength band.

A useful accessory to help here is an eyepiece set to be parfocal with your camera so that the focus doesn’t need to be changed between visual setting and camera use. This is a very useful accessory anyway to speed up setting up for imaging and well worth the trouble of making. This is shown here and is referred to on my page on imaging cameras;

Eyepiece with parfocalising ring so that it is the same focus position as camera
Focusing eyepiece inserted where camera would go. From L to R set-up is eyepiece/1.25″ adaptor/filter block/ADC/barlow

Running Out of Adjustment

You may find that for low altitude objects your ADC does not have enough power to fully correct for the high levels of dispersion that the atmosphere is inducing. Some ADC come in different strength versions where the deviation angle of the prisms can be 2° or 2.5°. Before concerning yourself about possibly buying the stronger version there are other solutions you can try which are illustrated in the schematics below;

Moving the ADC closer to the Barlow will increase the ADC correction and bring the red and blue star images together. There are two reasons for this….At the nearer ADC position there will be less physical separation between blue and red images to correct for and also the difference in deviation between red and blue beams introduced by the ADC is able to act over a greater distance to achieve correct convergence.
Moving the Barlow further inside prime focus by racking the focuser inwards pushes the focal plane further out and increase Barlow to imaging plane distance (as well as usually increasing magnification). For the same Barlow to ADC distance as in the top schematic there is now much greater correction as the difference in deviation between red and blue beams introduced by the ADC acts over a much longer distance. Here even a prism setting for the ADC of less than maximum still achieves good dispersion correction. Note that the vertical displacement of the reconverged image is greater for solution 2 than solution 1 which is not surprising as the magnification is greater too.

Adding a Third Prism

Another solution you can try if you want to boost the ADCs max correcting power is to add a third fixed prism before the pair of adjustable prisms. The prism should have its orientation fixed so that its deviation direction (an axis of symmetry) is in the vertical direction as seen through the eyepiece.

I have used an extra fixed prism for low altitude objects when using a coma-correcting barlow. In this case the barlow to ADC distance was minimised and I could not increase the ADC to camera distance without affecting the coma correcting properties of the barlow. Stuck for a solution to increase the ADC power without having to fork out for the latest PA ADC which has 2.5° prisms compared to my 2° ones, I added a third fixed 2° prism as shown below, which worked very well.

The normal range of the PA ADC I had was 0° to 4° deviation (for two 2° prisms). With the extra prism added in (which is easily removable) the range can be temporarily increased to 2° to 6° deviation if required.

Barlow on LHS with fixed third prism in place. Prism orientation is fixed with pins and slots. PA ADC is on RHS
Breakdown of components showing from L to R.; third prism/double 2.7x APM barlow arrangement/ADC with filter block and 1.25″ adaptor

Different Modes of Use

The benefits of using an ADC depend on the mode in which it is used. Currently ADCs are mainly used by those doing planetary imaging with monochrome cameras, but they can be used in other ways too.

Mono Camera Imaging

Currently ADC devices are mainly used by those doing planetary imaging with monochrome cameras. They image through separate red, green and blue filters and then combine the separate images into a final full colour image. Dispersion effects cause separation of the red, green and blue images at lower altitudes and this can be corrected in the process of aligning the separate colours, however, this alignment cannot remove any dispersion smearing that occurs within each of the three colour bands. This smearing with in each colour was seen earlier in JP Prost’s graph of dispersion with altitude (click below to enlarge) and shows the smearing to be worse the lower the altitude and the shorter the wavelength. 

As the worst dispersion effects are seen in the blue band for low altitude planets then in mono imaging the greatest benefits from using an ADC in the blue when the object is low in the sky.

As an example of the benefits of monochrome imaging, see the image below. This compares Jupiter at an altitude of 48° taken on a night of good seeing (19-11-2011) when images are taken through red, green and blue filters, with and without the ADC in place. You can clearly see the improvements in the image quality in the blue image whilst red and green are virtually identical. This is all as predicted by dispersion versus altitude plot above.

Colour Camera Imaging

The colour filters used in RGB mono imaging are of high quality where the primary consideration in the manufacture of the filter is the wavelength transmission window. Colour cameras also have red, green and blue colour filters but these are permanently patterned onto the front of the CDD chip in what is called a Bayer matrix.

One would think that a colour camera would benefit from use of an ADC to the same degree as a mono camera with RGB filters, but actually it benefits more. This is because the filters in a colour camera are not nearly as good as the filters used in RGB mono imaging. The filter bandwidth is wider for the Bayer matrix filters as there are far more considerations to be taken into account when choosing the filter material apart from the wavelength window (ability to photographically pattern then for example). This wider wavelength window of the filters for the colour camera make them more prone to the smearing caused by atmospheric dispersion, hence the benefit from using an ADC is that much greater.

The plots below show the transmission spectra of Astronomiks R, G, B filters versus the filters in a Toucam colour camera. The improved control of the spectral band for the former is strikingly obvious.

Astronomik Type II filters transmission Spectrum (above) for Red, Green and Blue filter as well as Luminosity (=yellow curve)
Toucam (Sony ICX-098BQ chip) Transmission Spectrum (above) in Red, Green, and Blue
As an example of the improvement in image quality when using an ADC with a Philips Toucam colour camera see the comparison image below for Mars at an altitude of 48°. The increase in detail with the ADC is striking even though the red, green and blue images have been aligned in Registax in both cases.
Another colour camera ADC comparison is shown below for Saturn. These were taken on the same night with an ASI120MC colour camera when Saturn was at an altitude of only 27°. The improvement achieved with the ADC is again striking.
Here we see another comparison of the use of an ADC on an OSC camera for Saturn- this time at only 17° altitude in July 2016. The RGB align feature in Registax helps, but cannot correct for dispersion occurring within each colour band. Comparison done using a ZWO ADC at f13.8 and an ASI224MC camera with an L filter. ADC was at max. setting (levers 180° apart) & 3x TV barlow in front of the ADC. Aligned horizontally with N at top.

L(RGB) Imaging

In Deep Sky imaging it is fairly standard practice to take a long exposure luminance (L) image, having minimal filtering, and combine this with a lower resolution RGB colour image to end up with a high resolution colour image. This is called L(RGB) imaging. The much reduced loss of light in the filter during the luminance image allows longer exposures with less noise and so is generally higher resolution than any of the colour images.

This technique has not generally been used in planetary imaging as the image taken though the L filter (which just filters out the IR and the UV) is so badly affected by atmospheric dispersion that it has much less detail than the colour images.

The use of an ADC changes all that and allows a detailed L image to be taken, despite the broad wavelength range, and then this can be combined with a colour image assembled from separate R, G & B images to make a high resolution RGB image. This L(RGB) approach is especially useful for low surface brightness planets like Saturn, where light is at premium.

Extra light allows several benefits which can lead to improved image quality when used singly or in combination;

  • Higher magnification without excessive image dimming to enlarge surface details
  • Lower gain and hence lower image noise without excessive image dimming
  • Shorter exposures leading to higher frame rates and more frames to stack leading to lower noise or higher quality stacks

The image compilation below illustrates the benefits of this type of imaging on Saturn and uses images taken in May 2012 with the planet at an altitude of 30°. In the top row we see the huge benefit of using the ADC with the L filter, transforming the level of detail seen. In the lower row we see a standard colour RGB image created in the same imaging session (also with use of the ADC) which is improved significantly by making an L(RGB) version. Here the L image is used as the luminance layer and the RGB as the separate colour layer in Paint Shop Pro.

ADC comparison using L filter to create L(RGB) image . The capture details of the individual images were;
L= Gain 850, exp. 32.5msec, 1200 of 3000 frames captured at 30fps; R=Gain 950, exp. 61.7msec, 1200 of 2000 frames captured at 15fps
G=Gain 950, exp. 63.9msec, 1200 of 2000 frames captured at 15fps; B=Gain 950, exp. 128msec, 1200 of 2000 frames captured at 7fps
So you see the L image was captured with a lower gain, a shorter exposure, a faster frame rate and the stack rejected a higher percentage of poorer frames. All these contribute to the higher quality of the L(RGB) image compared to the straight RGB image which was particularly let down by the poor blue image which bled light into the Cassini division in particular.

Visual Observation of Planets using an ADC

The two L images of Saturn taken on the 13th May with and without the ADC (top row in previous image) are interesting from another angle. The luminance view is the view that most closely matches that of the visual observer looking at Saturn with no filters in place and shows the significant benefits of using an ADC in such a situation.

Such a benefit from use of an ADC during visual observation of Mars, Jupiter and Saturn is backed up by personal experience where I have seen a significant improvement in the presence of fine detail by moving the levers between a position of no correction to a position of optimum dispersion correction.

One of the downsides of visual observing with an ADC is that for some scopes you are restricted to using the device after a barlow to keep the f-ratio high and reduce astigmatism effects. This many only mean a small change in procedure for example for my 222mm f5.8 scope where I would normally use a 5mm Nagler eyepiece for planetary viewing I now would use a 3x barlow, the ADC and a 16mm Nagler to give a similar viewing scale.

Commercial ADCs

There are currently four companies making ADCs that I am aware of;

Astro Systems Holland (ASH)

This was the first ADC I owned and was bought from Herman ten Haaf at http://www.astrosystems.nl/. The device uses multicoated BK7 prisms and has a 21.8mm clear aperture. It is a simple design and easy to use with 1.25″ male and female ends. There are regular markings on the barrel to be able to set the prism levers equally either side of zero and to be able to set the device to the same settings as used previously, which is handy.

The latest version of this device separates the 1.25″ barrel, ADC body and 1.25″ adaptor and individual parts are T-threaded much like the Pierro Astro version below. The price for the main barrel is 295 Euros with the other components available at additional cost.

An uncoated version is available for UV imaging as well as a version with quartz prisms.

My first ADC by AstroSytems Holland (ASH)
Latest version of ASH ADC showing piece parts

Pierre-Astro (PA)

This is the ADC I currently use and which I have used exclusively for almost all my planetary images since mid-2013. This exceptionally well engineered device is produced by Pierro Franquet (Pierro Astro) of France and sold in the UK by Astrograph. It uses better than 1/10 wave, 25mm diameter multicoated fused silica prisms, and has a 24mm clear aperture. The prisms used in my version have a 3.52° mechanical angle (typically 2.25mm at the thin end and 4.0mm thick at the thick end) giving a 2° light deviation angle, although the version now sold has stronger prisms, with a 2.5° deviation angle.  Although only the UV transmitting version is sold now, I did pick this more expensive option when I bought mine and this has been useful for imaging cloud detail in Venus using a Baader Venus filter. The transmission of the prisms is quoted as >92.5% in the range 300-700nm (see plot below) and its full bandwidth is 170nm-2500nm.

The levers controlling the prisms are able to be locked in position by a simple twist. In addition, the lever slots overlap allowing you to move the prism levers together over a large range to compensate for rotation of the vertical direction with time (better than rotating the ADC body which could throw out the focus). This ADC has a separate scale ring which screws onto the top to provide markings at 15° intervals (10° on most recent version) to be able to record the lever positions and ensure they are equally displaced about the null position. This scale ring can be seen fitted to my PA ADC under the section called ‘Setting the Prisms’.

The MkII PA ADC with 2.5° prisms and UV transmitting fused silica prisms is available from Astrograph/Pierro UK for £329 or is available in a kit with 1.25″ or 2″ ends (see below) for £368. A MkIII version is now available where both prisms are moved by using just one rotating knob.

Assembled Pierre Astro ADC kit with 1.25″ ends and T2 threaded top
ADC main body- end-on view with levers at maximum correction (180° apart)

Parts of 1.25″ Pierre Astro ADC kit. Left is 1.25″ T2 threaded nosepiece. Middle is main ADC body which is 30mm long and T2 threaded male at top and female at bottom. Right is T2 threaded 1.25″ eyepiece adaptor.

ZWO ADC

This is the newest commercial ADC, and is available from ZWO in China, for $128 or £118 in the UK from 365 Astronomy.

Like the PA and newest ASH version, this ADC is in 3 parts; the ADC body, a 1.25″ barrel adaptor for the bottom and 1.25″ eyepiece adaptor for the top (with brass compression ring). The two ends of the ADC body are female T-threaded. Like the PA version the two slots overlap to allow both levers to move together over a wider range to accommodate a changing orientation of the null location (without having to rotate the body). Because the ADC body is female threaded on both sides you will have to use the male T-threads on the top adaptor if you want to connect your T-threaded camera to the ADC which is not an issue because if you screwed it directly to the ADC the ADC to camera separation would be unworkably small giving minimal corrective power.

A nice feature of this ADC is the white locking screw on the scale marking the horizontal position and the rotatable scale. This allows the null point to be readily seen to set the levers symmetrically either side and allows you to finely position the null point without having to rotate the body which may upset the focus and camera alignment. It is only the angle between the levers that matters for the correction not their absolute position relative to the ADC body so the movable null point and the overlapped slots allows the null point and the levers to move together as a set to account for the horizontal direction drifting with time. The black plastic prism adjusting levers also have a nice adjustable frictional resistance using O-rings at their base.

The two H-K9L (BK-7 equivalent) prisms in the device each have 2° deviation angles and are said to be of 1/10wave accuracy. Experiments I carried out with Venus showed about a 20% light loss in the UV compared to my PA ADC using a Baader Venus filter which indicates that the multicoatings are pretty good at these wavelengths (see plot below).

Complete ZWO ADC showing white scale locking screw and T-threaded top
Top, body and bottom parts of ZWO ADC- all T-threaded

ZWO and PA ADC transmission plots. The PA has a superior transmission in the UV, making it more suitable for imaging Venus cloud detail

Gutekunst Optics

This is the Rolls Royce of ADCs with each prism made of a crown and flint pair to eliminate image shift and reduce aberrations but retain the dispersion correction effect. The device is available from Gutekunst Optics in Germany. Two versions are available, both with 1/30 wave optics, a professional one with a clear aperture of 38mm at 7500Euros, and a compact one with 28mm clear aperture at 3500Euros.

Gutekunst Optics Professional ADC
Gutekunst Compact ADC

Further Reading