The Tom Osypowski equatorial platform
Part II: The guiding / imaging setup
|No telescope drive can eliminate the need for guiding in long exposure astrophotography.
Guiding is necessary to correct for small tracking errors caused by the periodic error of the gear, atmospheric refraction, inaccurate alignment of
the telescope mount and flex of the telescope structure. There are two choices to make the tiny corrections needed: Using a seperate guidescope
or use an off-axis guider.
I use an off-axis guider. With a dobsonian telescope or a Newtonian telescope in general, flexure of the telescope structure is likely to occur during guiding. A seperate guide scope cannot account for all the flexure, especially when working with long focal lengths (in my case: 2070 mm for the 16" f/4.5 Ninja and 2300 mm for the 16" f/5 Zambuto). Off-axis guiders divert a part of the light cone of the main imaging telescope to a separate ccd camera. The ccd camera analyzes the position of a star ("guide star") in that light cone and sends correcting signals to the motors of the mount.
I first used the well-built TSOAG9-EOS off-axis system. It performed flawlessly but in order to save weight (69 g instead of 149 g), get a bit closer to the coma corrector (by 2 mm) and to have less screws to accidentally loosen, I switched to the Lacerta off-axis guider for EOS cameras. Both off-axis systems can be purchased at Teleskop-Service in Germany.
As for the autoguider, I chose the Lacerta MGen standalone autoguider from Hungary. It guides the equatorial platform without a PC or a laptop and can control the Canon EOS 450D camera (number, time and pause time of the exposures, waiting time after camera mirror lockup) at the same time. One 4 Ah lead acid battery feeds the MGen autoguider, another one the equatorial platform.
Short focus Newtonians show off-axis aberrations known as "coma". Stars in the center of the field are not affected by coma, but the effect grows stronger toward the edge of the field. Stars affected by coma are shaped like tiny comets instead of points. To prevent this, a coma corrector has to be used.
|I started with the TeleVue Universal Paracorr PLU-1106 coma corrector. Unfortunately this correcting lens cannot be used visually so I
later replaced it by the new TeleVue Paracorr Type-2 coma corrector VIP-2010, which can be used both visually and for imaging. The VIP-2010 is longer
(80 mm instead of 50 mm) but slightly lighter (200 g instead of 204 g).
Until 2012, I used a Canon 20Da camera for astrophotography. The Canon 20Da was based on the successful Canon 20D camera. It was released in 2005 and had a modified low-pass filter with increased hydrogen-alpha sensitivity. In addition, it offered a live view mode. It had an 8 megapixel APS-C sensor (15.0 x 22.5 mm) with a pixel size of 6.4 micrometer. It was a very good camera for long-exposure deep-sky photography. Unfortunately, when stacking a large number of Canon 20Da images, the stack exhibited significant amplifier glow which was almost impossible to remove, even with a large library of dark frames.
In 2012, I acquired a second-hand, astro-modified Canon 450D. It has a Baader BCF H-alpha intensifier filter for enhanced hydrogen-alpha sensitivity. The 12 megapixel APS-C sensor is 14.8 x 22.2 mm across with a pixel size of 5.2 micrometer. The Canon 450D shows no signs of amplifier glow and produces less noise than the Canon 20Da. In addition, it is a bit lighter (524 g / 18.48 oz instead of 770 g / 17.16 oz).
Below, you can find photographs of my current guiding / imaging setup and further down the page, you will find images of the guiding / imaging setup which I don't use anymore but which did a good job as well.
|Top left: TeleVue Paracorr Type-2 coma corrector VIP-2010, Teleskop-Service M65-M48 adapter ring TSTV-M48a, Teleskop-Service Lacerta Off Axis Guider for Canon EOS cameras.||Top right: Coma corrector, adapter ring and off-axis system put together.|
|Top left: astro-modified Canon 450D, coma corrector attached to off-axis system, Lacerta MGen autoguider.||Top right: Lacerta MGen autoguider attached.|
|Top left: The complete guiding / imaging system consisting of astro-modified Canon 450D, coma corrector, off-axis system, Lacerta MGen autoguider put together. The whole unit (camera, corrector, adapter ring, Lacerta off-axis-system, MGen autoguider) weighs 900 g (31.68 ounces).||Top right: The complete guiding / imaging system consisting of astro-modified Canon 450D, coma corrector, off-axis system, Lacerta MGen autoguider disassembled.|
|Top left: Upper tube of the Ninja 16" f/4.5 dobsonian telescope with Starlight Feather Touch 2" Crayford focuser.||Top right: The complete guiding / imaging system attached to the focuser.|
Older guiding setup (not used anymore, but worked perfectly)
|Top left: TeleVue Universal Paracorr PLU-1106 coma corrector, Teleskop-Service M65-M48 adapter ring, Teleskop-Service off-axis system TSOAG9-EOS and Canon EOS ring.||Top right: Coma corrector, adapter ring, off-axis system and Canon EOS ring put together.|
|Top left: Canon 20Da, coma corrector attached to off-axis system, Baader T2 extension tube (7.5 mm, #25C), Lacerta MGen autoguider.||Top right: Lacerta MGen autoguider attached. Before mounting the autoguider with the Baader T2 extension tube, a T2i-M28i adapter (available at Teleskop-Service) and a Kasai Ortho 9 mm without its metal sleeve are attached to search manually for guiding stars.|
|Top left: The complete guiding / imaging system consisting of Canon 20Da, coma corrector, off-axis system, Lacerta MGen autoguider put together.||Top right: The complete guiding / imaging system consisting of Canon 20Da, coma corrector, off-axis system, Lacerta MGen autoguider disassembled.|
|Top left: Upper tube assembly with MoonLite Crayford focuser.||Top right: The complete guiding / imaging system attached to the focuser.|
Part III: The photos