For imaging at long focal lengths I use a 14.5 inch f/7.9 Ritchey-Chretien telescope manufactured by Optical Guidance Systems (OGS) The OTA is constructed of carbon-fiber material and the mirrors are made from zero-expansion ceramics. This results in no significant focus change during the night. Temperature changes during desert evenings can be quite pronounced so this new construction is a welcome improvement over my first RCT. John Stiles of OGS has recently retrofitted this telescope with a Robofocus unit on the motorized secondary focuser which allows for automatic computer-controlled focusing. In addition to the excellent optics the telescope also has a generous back-focus of 9.75 inches. Adjustable spider vanes and tip/tilt adjustments for both primary and secondary mirrors makes accurate collimation achievable. The instrument has a native focal length of 2909 mm at f/7.9. An Astrophysics 0.75X focal reducer is conveniently threaded into the compatible rear focuser draw tube to yield an effective focal length of 2182 mm (f/5.9). For wide-field CCD imaging I use a Takahashi FSQ-106 f/5 refractor which is also fitted with a Robofocus unit.
Click on the thumbnail images below for an enlarged view.
Figure 1: The Optical Guidance Systems 14.5 f/7.9 inch Ritchey-Chretien and Takahashi FSQ-106 telescopes mounted on the Byers Series II at Ash Meadows Observatory. Also shown is the SBIG STL-11000M connected to a nearby PC which is set on a small desk with wheels. The mounting sits on a platform which is welded to the pier using four struts. The pier is in turn welded to and supported by struts atop a heavy duty pallet. The entire structure is constructed out of aircraft quality aluminum and is extremely rigid and stable.
Click on the image below to view an enlarged picture of my recently purchased Paramount ME from Software Bisque. Go HERE to see the 'first light' image of M51 taken using the new mount. The image was taken on April 4th and 5th, 2005.
CCD Imaging Cameras:
The ST-10E is used at 1X1 binning for the high resolution Luminance images used to prepare the final LRGB images. Some of my earlier images were done with the ST-10E binned 2X2 without any noticeable difference in resolution, presumably because of the long focal length of the telescope.
The FLI-DM is used to obtain the RGB images. This back illuminated 1024X1024 imaging chip has 24 micron pixels and is an extremely fast camera, making it ideal for imaging using RGB filters. RGB images with exceptional signal-to-noise (S/N) are obtained in remarkably short times with the camera binned at 1X1.
An Optec RGB-IR-blocking filter set was used with the FLI CFW-1. Optec has since offered a new, improved set of RGB filters (Refer to Don Goldman's website at www.astrodon.com.). Exposure times for the three filters with the FLI-DM were obtained with photometric CCD measurements on a G2V star at the zenith. AIP4WIN imaging software was used to white balance the exposure times for correct color balance. The correct exposure ratios are: R:G:B: 1.00:1.53:1.56
May, 2004 - present;
In January 2004 I ordered an STL-11000M, SBIG's large format CCD camera. The imaging chip is an amazing 36 x 24.7 mm in size with pixel dimensions of 9 x 9 microns for a total of 11 million pixels. The imaging area is equivalent to a 35 mm film format. When imaging with my Takahashi 106mm f/5 refractor I can cover the identical area of sky which my Astrophysics 155mm f/6 refractor covered on a 6x7 negative. I am now using this camera for all of my LRGB image acquisition. I use the 48mm Astronomic Type 2C LRGB filter set with built in IR-blocking. The filters thread into the internal color filter wheel (CFW) of the STL-11000M. An Astronomic 48mm H-alpha filter (13nm bandwidth) is threaded into the fifth slot of the CFW. The L filter, unlike the clear filter in the ST-10E, blocks both UV and IR wavelengths and matches the UV/IR spectral trasmission of the RGB filters. This gives a more accurate Luminance component for LRGB compositing. The five filters, LRGB & H-alpha, are parfocal. The Astronomic RGB filter transmissions match the CCD chip's quantum efficiency curve very nicely which allows for the same exposure times for each color filter. The Astronomik parfocal LRGB & H-alpha filter set can be purchased from Adirondack Video Astronomy.
The STL-11000M is binned 1X1 for the Luminance and H-alpha images and binned 2X2 for the RGB images. White-balanced R:G:B weights were obtained from photometric CCD measurements on a G2V star near the zenith using AIP4WIN. The correct exposure ratios are R:G:B: 1.15:1.000:1.10. The individual raw RGB exposure times are identical (usually 10 minutes) The weights are used to adjust the color balance after RGB-combining in CCDSoft as described in Image Processing.
Camera Contol Software: CCDSoft V5 (Software Bisque) is used to control both the ST-10E and STL-11000M. Exposures are autoguided using the self-guiding feature of these dual-chip cameras. Maxim-DL is used to control image acquisition with the FLI-DM when using the FLI- CFW for RGB imaging. The telescope is guided during RGB FLI-DM exposures using a securely mounted guidescope (see above illustration) and the SBIG-STV autoguider. The imaging cameras are focused by first placing the chip very close to proper back-focus (9.75 in) using the rear focuser drawtube and spacers if necessary. Fine focus is achieved using the motorized focuser on the secondary mirror assembly, which is fitted with a Robofocus digital focuser unit, and FocusMax software. A desktop computer (Dell 4600C) is used to run the image acquisition software.
A light box using white LED lights is placed over the front of the telescope to obtain flat fields.