TV-Autoguider

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autoguider

Prerequisite for a successful astrophotography is good autoguider. The principle of device is easy. Autoguider input is fed by signal from a sensitive TV camera, which captures stellar field in the auxiliary (guiding) telescope, which is attached to the main telescope. LCD panel of the TV autoguider displays an artificial cross and stellar field captured with the camera. By means of an arrow keys on the autoguider, cross can be moved in the star field and placed over some sufficiently bright star. If the star due to inaccurate mount operation starts to drift, guider begins to send correction signal to the mount over a cable and corrects inaccurate operation. This TV autoguider is designed by one friend of mine Martin Myslivec

In order to autoguiding properly fulfill its purpose, it must be ensured that the main telescope and guiding telescope assembly is sufficiently rigid and there is no mechanical cross movements or deflections. Important is also rigid fixing of autoguider camera to the guiderscope. Some hi-tech equatorial mounts are so accurate that don't need autoguiding, but this kind of equatorials are quite expensive.

Cross positioner

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pozicionér

Positioner is a handy alternative to the guiding circles which allows easier catching of a suitable guiding star. Positioner allows X-Y movement of camera in guiderscope focal plane and increases the probability of finding of a sufficiently bright guiding star. If one take pictures somewehere in the Milky Way, there is not problem to find suitable guiding star near optical axis of the guiderscope, however in the spring or autumn with stars poor sky, it is sometime challenge even with positioner. Compared to the guiding circles, positioner has the advantage of easier and faster handling, reduced risk of rolling fields and the greater robustness of the whole assembly. Plans for this handy widget gave me my friend Martin Myslivec.

Flat-field panel

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pozicionér

Most of the optical systems, especially those with higher aperture suffers from uneven field illumination at the sensor surface, which appears as light loss from the field center to the edge. The phenomenon is known as vignetting. At the normal daily images, except apparent extremes, it is usually unnoticed. However at the photographs of space objects, vignetting is manifested in a very messy way that is demonstrated on one of the pictures below. Severity of the vignetting is worse, when weak parts of an objects should be visible in a picture. Vignetting can be more or less sufficiently eliminated with suitable calibration using artificial image, which is called "flat-field."

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Picture without "flat-field" correction. Strong and irregular vignetting is apparent on image edges.

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Picture with applied flat-field correction

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Animation which shows positive effect of flat-field correction


Principle of vignetting correction is quite easy, only problem is finding of perfectly even illuminated area for its taking. Strategies for taking of flat field images are various:

  1. Use of even iluminated twillight sky without stars
  2. Use of flat field box
  3. Use of computer monitor with large white area on screen

For my purposes, I decided to modify possibility from point 3), so I took damaged Hewlett-Packard LCD screen and modified it as flat field-panel for using without need of computer. I had to replace original circuitry designated for supply from 220/120 V mains with CCFL driver which operates at 12 V DC. Original brightness of LCD panel back-illumination had to be decreased with three sheets of matt white self-adhesive plastic foil, which also cancel light polarisation from the screen.

On the images below is another cracked LCD panel which I have as a backup

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LCD screen from Compaq Armada V300

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CCFL driver with MP 1010MBF circuit

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Dismantled block of polarization foil, fresnel lenses and CCFL lamp


Nowadays, electroluminescent foils become popular for flat-field panel construction, mainly due to ease of use, regular illumination of panel area and possibility to buy it in large dimensions suitable for high aperture telescopes. Disadvantages are higher price, need for power supply with higher voltage and predisposition to easy destruction by voltage spikes.

electroluminescent panel

Electroluminescent foil


CCD filters for narrowband imaging

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EOS Clip filter inserted to special holder in CCD camera G2-8300

EOS Clip filter inserted to special holder in CCD camera G2-8300

EOS Clip filter H-α (12 nm)

EOS Clip filter H-α (12 nm)

EOS Clip filter O-III (12 nm)

EOS Clip filter O-III (12 nm)


Narrowband filters for astrophotography transmits only a very narrow segment of the spectrum, which is centered on given wavelenght. Bandwidths of a transmitted wavelenghts are different for different filters, but the narrower the transmitted range, the more contrast can be achieved in the image. Filters are designed at a wavelenghts at which different types of objects in the space glows. The most common is ionized hydrogen (656.3 nm, 486.1 nm) and oxygen (500.7 nm). Other less common filters are designed for sensing at a line of ionized sulphur (672.4 nm) and nitrogen (658.4 nm). If we use narrowband filter, we can achieve significantly higher contrast of the object, better delineation of the object structure and obtain information about chemical composition of the object in its different parts. Another benefit is possibility to take pictures of deep space from light polluted areas, where traditional broadband astrophotography is impossible. The best results are achieved, when using these filters with a sensitive monochrome CCD camera. The resulting monochromatic images are presented in shades of gray. If the object is photographed using multiple narrowband filters, we can create color composites (false color) in a PC. Stunning appearance of color composites taken through narrowband filters is known from the Hubble telescope.

Filters on the upper photographs are EOS-clip filters from Astronomik company with a 12 nm bandwith, one at line of ionized hydrogen and one at line of ionized oxygen. The filters are designed for direct insertion into the body of Canon EOS cameras. I use them in combination with a CCD camera G2-8300, to which special clip-filter holder was made. The advantage of this solution is quick and easy filter replacement during the night, without need of composition re-adjustment after filter change. Below is a sample of images taken through narrowband filters.

propustnosti RGB filtrů Astronomik

Image of NGC 281 in false colors made with bicolor technique from images taken in H- α and OIII

propustnosti RGB filtrů Astronomik

Image of the NGC281 taken through H-α filter

propustnosti RGB filtrů Astronomik

Image of the NGC281 taken through OIII filter


Cable release with timer

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Kabelová spouš s časovačem

Another useful tool in sequential shooting in astrophotography is a cable release with a timer. If we are acquiring greater sequence of images during the night, it can save us time and exposure monitoring. This release is DIY stuff, made by me according to original instructions on the website of "Marek Pecka", where complete documentation can be found, including circuit diagram, printed circuit board and AT microprocessor source code, all for free. The timer lets you set the number and length of exposures, period between frames, mirror lockup and some other functions. I use the timer for many years and never let me down even in extreme weather.

In my beginnings with astrophotography, I used this timer for deep space photography with DSLR. Now, after DSLR replacement with CCD camera, I use the timer for star-trails and timelapse video shooting, which is another highly addictive activity :-)

Power supply for Canon EOS 350D

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Power supply for Canon EOS 350D

This power supply is one of DIY stuff designed and made by me for feeding the camera from external battery. I use it for long sequential photography, where capacity of original battery (NB2-LH) is not sufficient. The camera is conected by means of this source to external accumulator (11-13V). Switching power supply was chosen because of significantly higher efficiency if compared to linear voltage stabiliser. For feeding of this source I use either 12 V lead battery or 13.2 V LiFePO accumulator. The use of lithium-phosphate batteries are especially useful due to the significantly lower weight compared to lead-acid gel battery, which I appreciate particulary in case when equipment must be brought to mountains on my own back.

Detailed description of the construction can be found here (only in Czech). I tested this power supply on the Canon EOS 300D, 350D, 400D, 450D and 40D. It is likely that it will operate satisfactorily on other camera models that use batteries NB2-LH or BP-511 (7.4-8.2V voltage). Using this supply on a camera that uses a different voltage can lead to its destruction. At this point, note, that if you decide to build it and use, you do so at your own risk and I take no responsibility for any damage to camera or injury.

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PCB with electronic parts

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Circuit of the switching power supply (click for larger image)


Telescope dew controller

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Telescope dew controller

At a time when I started with astrophotography I thought that heating management of a telescope is a total superfluous jewelry. The first few nights in autumn, when I first got the technique under the sky convinced me otherwise. Front lens of my Pentax refractor was dew covered in course of few tens of minutes, so I recognized that telescope dew management is absolute necessity. When I took into consideration the cost of commercially available solutions, I decided to made my own design. The control unit operates on the principle of PWM (Pulse Width Modulation) and is based on the NE555 timer circuit. The unit is powered from 12 V battery. Regulation of heating power is carried out by changing the width of voltage pulses which goes to the heating tape from the control unit.

As a heating tape is used piece of resistive wire (6.93 Ω/meter) with lenght about 1.4 m. Wire is fixed to a special glassy fabric covered with layer of PTFE. All is hiden in the layer of Velcro.

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Glassy fabric covered with PTFE

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Glassy fabric in detail

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Cutted tape of fabric prepared for use

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Tape with marks for wire passage

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Resistive wire RD 100/0.3

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Tape with resistive wire


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Cotton fabric used as one side of heating tape, second side is made from Velcro.

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Tape components prepared for stitching

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Final assembly