Introduction
This article is for amateur astronomers that are interested in capturing images of the Moon, planets and the Sun (with a suitable Solar filter) using a video camera and telescope.
The article focuses on the components of such a system and their relationships. This is not a tutorial on how to process images captured with a telescope and video camera but rather on what is needed to successfully capture images of interplanetary objects to be later processed on a computer with the appropriate software.
Photograph by Richard Keele.
Components
While single shot images of the Moon, planets and Sun are possible using a photographic tripod and tele-photo lens this will not yield the best results or anywhere near that.
Typically, you will require a GOTO (computerised) mount capable of accurately tracking its target over a period of 20-30 seconds, most motorised GOTO mounts are capable of such accuracy. However, at higher magnifications an equatorially mounted GOTO telescope may be required. In general, you will need a telescope with a resulting focal length of at least 2,000mm for imaging the planets and a video camera with which to capture the target in a video of several seconds. Finally, you will need software to process the video you have captured and turn it into a single high quality frame superior to the best frame in the captured video.
The following diagram shows the components of an interplanetary imaging system. The necessary components include a GOTO mount, telescope, camera, accessories and PC (personal computer). The filter wheel and various filters are optional and, in some cases, desirable when imaging the planets and the Moon. A full aperture Solar energy rejection filter is mandatory when imaging the Sun. Without an appropriate Solar energy rejection filter you will permanently damage your eyes and equipment.
Imaging system components.
Mount considerations
Equatorial and alt-azimuth GOTO type mounts will work very well, and you do not necessarily need an expensive mount. Equally entry level alt-azimuth GOTO mounts would not be capable of delivering high end images, especially when overloaded. Having said that images of good quality are still possible even with moderate equipment.
Take care to properly set up and align your mount. It is important to place the mount on a solid surface and level it. If you use an equatorial mount, it is important to accurately polar align it.
Take care and be as accurate as you can when inputting the longitude, latitude and time. Use a GPS (global positioning system) device or refer to Google maps to obtain the co-ordinates. Please note that most handsets require the longitude and latitude in minutes and seconds and not in decimal notation as provided by many on-line sources.
For example, to convert 10.72 degrees already in decimal notation you will need to multiply the number on the right of the decimal point by 0.6. As such 10.70 degrees in decimal notation would be converted to 10:42:00 (70x0.6=42.00).
If your mount offers one-, two- or three-star alignment always select three-star alignment, if not select two-star alignment. Aligning on a single star is not accurate and should be avoided whether you are imaging or not. Once you have aligned your mount but decided to point the scope to a target in another meridian (from say West to East) you will need to add a reference star, this will increase the mount’s accuracy and is more relevant to equatorial mounts. The reference star must be added after your mount has been pointed to the new meridian.
During alignment and when ‘placing’ a star in the field of view use an eyepiece with a crosshair, this can dramatically increase accuracy. If you do not have a crosshair eyepiece, use a high-power eyepiece instead, a 5mm eyepiece will enable you to centre a star with four times the accuracy of a 20mm eyepiece. Eyepieces with a narrow field of view like orthoscopic and Plossl eyepieces will work better than wide field exotic 5mm eyepieces in this case.
Use a fully charged lead acid battery or a good quality regulated mains power supply to ensure a steady current flow to the mount. This will keep the motors running smoothly.
It is important to ensure that your telescope is well balanced on the mount with all equipment mounted. This will guarantee that the motors will receive the same load irrespective where the telescope is pointing.
Finally let the mount run for a few minutes, again this will ensure that all parts will reach a stable temperature.
Optical instruments
In theory an apochromatic refractor of a long focal length and large aperture would be ideal for imaging the planets, Moon and Sun at high powers. However, such instruments are prohibitively expensive at apertures above 127mm. Refactors do not suffer from tube currents, cool down quickly, are not affected as much from atmospheric turbulence and as such deliver high contrast images unlike instruments with central obstructions like catadioptric telescopes and Newtonian reflectors.
When considering centrally obstructed instruments an obstruction less than 18% of the diameter and closer to 15% will produce near ‘refractor like’ images, however mass produced catadioptric telescopes typically have large obstructions of 33%-36% that markedly reduce contrast. Still large catadioptric telescopes and Newtonian reflectors can make up by aperture to a moderate degree.
Schmidt Cassegrain telescopes will still perform better than Newtonian reflectors due to reduced internal currents and the lack of secondary mirror supports that introduce diffraction.
Maksutov designs will typically outperform both Schmidt Cassegrain and Newtonian reflector telescopes by optical design alone.
Finally, Advanced Coma Free (ACF) telescopes are excellent all-round instruments combining large apertures, high contrast and affordability.
Still, amateur astronomers have produced high quality images with all the types of telescopes mentioned above so there is nothing much to hold you back.
In general, it is preferable to start with an instrument with a long focal length when imaging at high magnifications. If you use a catadioptric telescope or Newtonian reflector longer focal ratio instruments (f15+ and f6+ respectively) will capture images of higher contrast, in general longer focal ratio instruments benefit from smaller central obstructions. However, in terms of a Newtonian reflector an 200mm F6 instrument is a long instrument and a 250mm F6 is difficult to manage due to its weight, bulk and susceptibly to the wind; a 250mm F8 does not bear thinking.
Also keep in mind that instruments with longer focal ratios (i.e., F10) are markedly easier to collimate than short focal ratio instruments (i.e., F4). A very well collimated instrument will readily outperform the same well collimated instrument when it comes to imaging.
It is most likely that you will need to use optical amplification to obtain the desired magnifications unless you own a rather large catadioptric telescope. Optical accessories and filters will be discussed later in this article.
Optical accessories
Optical accessories including barlows, eyepiece projection kits and filters play an important role when imaging at high magnifications. It is desirable to be able to image the planets at focal lengths of 4,000mm or higher to obtain large enough planetary discs with a reasonable amount of detail.
Barlows will typically increase the focal length of a telescope by a factor of x2, x3, x4 or x5 which in most cases is enough. Barlows can be stacked to deliver higher magnifications, this will however degrade the image.
Another method to obtain high magnification is eyepiece projection. Basically, an eyepiece can be used to project a magnified image onto the camera sensor. The magnification factor depends on the focal length of the eyepiece and the distance between the eyepiece and the camera sensor. Eyepiece projection kits are widely available and easy to use.
Adjustable eyepiece projection kits
An eyepiece is used to magnify the projected image, the eyepiece is held in place with the aid of an eyepiece projection kit. This method is not suited when imaging deep sky objects as the resultant focal ratios are too long with a significant loss of light.
Eyepiece projection kit.
The resultant focal length of a telescope can be calculated using the following formula.
Resultant Focal Length = (telescope focal length) x A
where amplification
A = ( S – F2 ) / F2
S = distance form the last eyepiece lens to the sensor
F2 = eyepiece focal length
The following also applies: 1 / eyepiece focal length = (1 / S) + (1 / T)
An adjustable eyepiece projection kit will allow you a range of magnifications with the same eyepiece. Eyepiece projection kits will generally accommodate physically smaller 1.25” eyepieces. Equally there is no benefit in using wide angle eyepieces when imaging the Moon, planets and the Sun as the field of view will be restricted by the physical size of the sensor.
The quality of the eyepiece is very important. For planetary imaging orthoscopic (Abbe design) eyepieces will outperform more expensive general-purpose eyepieces, as such they are highly recommended. A more affordable alternative is a good quality Plossl eyepiece.
While this method is flexible the eyepiece optics that will consist of a minimum of 4 pieces of glass will introduce aberrations, higher quality apochromatic eyepieces can be used to reduce such aberrations.
Barlows
Barlows are very well suited for imaging at high magnifications as they are optically simple and easy to use without the need for adapters. In other words, the money spent on the projection kit could be better spent on a higher quality barlow. Barlows will typically provide x2, x3 or x4 magnifications. Barlows can be stacked but this is to be avoided, a x4 barlow will deliver better images than two stacked x2 barlows. The additional glass elements will introduce aberrations of their own, amplify existing ones and ultimately degrade the final image. Aberrations result to images of low contrast with moderate colour fringing at best.
Barlow lens.
Good quality apochromatic barlows are very important in keeping aberrations to a minimum while not introducing new ones of their own. The final focal length of a telescope can be calculated using the following formula.
Resultant Focal Length = (telescope focal length) x (barlow power)
For example, a Newtonian telescope with a focal length of 1000mm and a x2 barlow will have a resultant focal length of 2,000mm (1,000 x 2).
Video imaging kits
A version of a video imaging stem based on amplification comes in terms of the Opticstar Planetary Imaging Kit. This has been designed so us to deliver high quality images at high magnifications while offering a very good degree of flexibility. The kit consists of a number of components including a high-quality apochromatic ED optical amplifier and a range of adapters and extensions.
The kit has been designed to work with C and CS mount cameras, but the ED amplifier can be attached to any nosepiece threaded to accept standard 1.25” astronomical filters. The two examples below show some of the possibilities.
Video imaging kits.
The image quality that can be achieved with this system is very high when compared to the alternatives discussed earlier. Optically the system will deliver variable magnifications using a minimal number of optical elements. The apochromatic ED optical image amplifier ensures that aberrations are kept down to an absolute minimum. All components are screw-threaded to ensure a solid arrangement.
Filters for planetary and Lunar imaging
Filters are useful as they can suppress unnecessary wavelengths. As such Light Pollution filters, UHC photo-visual filters and UV/IR block filters can be used to increase image contrast. The type of filter you use will depend on the object you intend to image. For example, while UV wavelengths are undesirable Venus emits at these wavelengths and in this case a UV block filter would be undesirable. Please note that you would need a high-quality apochromatic instrument to bring UV and RGB to focus at the same time. Triplet apochromatic refractors will markedly outperform doublet apochromatic refractors in this case. Please note that ‘Venus’ type (~350nm) filters should only be used for imaging, UV is harmful to the eyes.
To capture the maximum amount of detail when imaging the planets, a monochrome camera will capture the maximum level of detail possible. Red, Green and Blue filters can be used to image the planet in monochrome, the monochromatic R, G and B images can then be used to create a colour composite colour image in software like Photoshop or Paintshop PRO. This however requires a lot of experience, a good quality set of LRGB filters and preferably a motorised filter-wheel.
Filters for Solar imaging
Additional filters are not needed when imaging the Sun with a dedicated Solar telescope like the Coronado PST or SolarMax telescopes from Meade. However, if a non-dedicated Solar telescope is used a full aperture Solar energy rejection filter is mandatory as it will protect the eyes, telescope and camera from permanent damage. If in doubt seek professional advice.
Solar filters.
If you are not using a dedicated Solar telescope the quality of the image and amount of detail can be improved with the use of certain filters. While it will not be possible to capture prominences you will be able to markedly increase the amount of surface detail. Solar Continuum filters can be used both visually and photographically and show a green image of the Sun. You will be able to image sunspots, but surface detail will be minimal. Calcium (UV is harmful to the eyes) and narrow-band H-alpha filters on the other hand can be used photographically to show an improved level of surface detail with the H-alpha producing the most pleasing images in terms of colour where the Calcium will show the highest levels of detail.
It is worth noting at this point that a 1.25” Calcium filter and full aperture Solar rejection filter will cost over half the price of a Coronado PST dedicated Solar telescope, keeping in mind a Coronado PST will show both surface detail as well as prominences it is a difficult call to make.
To capture the maximum amount of detail when imaging the Sun with a dedicated Solar telescope or otherwise you would ideally need to use a monochrome camera. As all the filters mentioned in this section and including those used in dedicated Solar scopes have narrow bandwidths a monochrome camera will capture markedly more detail when compared to the colour version of the same camera.
Field of view and other issues
If you are new to astronomical video imaging, you will be surprised of how small the field of view is through the camera when compared even to the most powerful eyepieces. Make certain that the planet is in the centre of the field of the eyepiece you use before replacing the eyepiece for the camera, a crosshair eyepiece will help in these situations.
Also, it is unlikely that the planet will be in focus when replacing the eyepiece for the camera, as such you will need to re-focus. Please note that the planet can be invisible if out of focus and still inside the field of view.
It is advisable that you set camera exposure to manual, automatic exposure settings will not work as you would like when imaging the planets and the Moon. It is advisable that you get used to the camera’s operation during daytime by simply pointing the camera/telescope out of the window. If the outside is too bright for the camera you will need to reduce the telescope’s aperture. Use a large piece of card with a round 10mm hole in the centre to cover the aperture.
Finally, if you use a powerful barlow it is possible that you will be unable to reach focus. To reach focus you would need to push the barlow away from the focuser with the aid of an extension tube placed between the barlow and telescope focuser.
Atmospheric considerations
Good atmospheric conditions are important and will have a rather noticeable effect on image quality. Low humidity, low levels of airborne dust, a luck of wind and good sky transparency are all very important factors.
A clear, free of colour horizon just after sunset suggests that humidity is low and that there are small quantities of airborne dust. Similarly, once dark a ‘red’ horizon suggests that humidity is high where a ‘white’ horizon would suggest that humidity is low. This would only be apparent if you could look at a light-polluted horizon that is typical of urban areas.
Looking up twinkly stars suggest a turbulent conditions where steady stars suggest good steady conditions. Finally, sky transparency is more difficult to ascertain but from a dark site you should be able to observe 6th magnitude stars with the naked eye.
Imaging and processing
Imaging and processing are outside the scope of this article. The text below should simply serve as an outline of what is involved and how to go about it.
Almost all cameras can be operated in manual or automatic exposure modes. Automatic exposure modes are not applicable when imaging the Moon, planets and Sun (with a suitable Solar filter). In auto-exposure the camera’s electronics will set exposure and any other parameters like gain and colour balance in relation to the whole image frame and not in relation to the planet which is the only point of interest. Typically, when the camera operates in automatic exposure mode the planets will be overexposed and any surface detail will disappear.
The correct exposure time that needs to be set up manually and will vary depending on the camera, target, telescope aperture and focal length. You can start experimenting with exposures between 1/20 to 1/40 of a second.
For best results a video sequence of frames needs to be captured as opposed to single frames. A captured video feed of around 20 seconds would be imported in specialist software like RegiStax for stacking and processing. RegiStax is an excellent piece of software that is freely available on the Web. The stacked and processed image could then be imported into image manipulation software like Photoshop and Paintshop PRO where the final processing would take place.
It is advisable that you refer to RegiStax tutorials available on the Web and on YouTube in particular, before attempting to process your first images with RegiStax. Referring to such tutorials will save both time and frustration.
Summary
Ideally you would image at Prime Focus and simply employ barlows and image amplifiers when you require a different field of view and higher magnifications. These simple arrangements keep the focal train simple, and the number of glass elements employed to a minimum. Always use good quality barlows and power amplifiers (ED) to minimise false colour and loss in contrast.
Finally take advantage of average skies to practice and to prepare for that excellent night!
Related topics:
how-to guide, astrophotography, article
Author:
Opticstar
