The Demands on Equipment

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As imagers advance their techniques, they become more critical of star shapes in their images. With sub-arcsecond/pixel image scales they find that keeping them tight and round into the extreme corners places a significant demand on their entire equipment setup. With film-format-sized CCD arrays, many scopes need to be modified or even replaced to obtain the quality of images one strives for. Most telescopes deliver fields adequate for the midsized series of CCD cameras, such as the SBIG ST-8 and -10 series. However, before taking the plunge for bigger CCDs, consider if the performance is good enough or can be made good enough for wider field work.

As we start the discussion on equipment requirements we should first put the tolerances of high-resolution imaging into context. Let us use the KAF3200 CCD array as an example. The individual pixels in this popular CCD are 6.8 microns square. The human hair has a diameter of 75 microns, so that means it takes 12 pixels to span the tiny width of a hair. It is immediately obvious how little mechanical movement or optical aberration would smear a faint star image spanning just a few pixels. These small errors are invisible, which makes it so difficult to track down the root problem.

Here is how I prioritize the importance of individual components in the imaging system:

Priority 1: Mount Quality. More deep-sky high-resolution images are ruined by poor mount performance than optical quality. Even on a tight budget, do not cut corners on the mount. Good mounts are harder to find than good optics, are usually more expensive to buy and are even harder to make (see Figures 8.7 and 8.8). The necessary tools and machinery to make them are not commonly available. For ATMs, making good optics requires surprisingly unsophisticated equipment. However, a mount requires precision machine tools (and machining skills)

Figure 8.7. Brian Lula and portable imaging setup. Reinforcing Brian's opinion about mount rigidity for CCD imaging is his design of a portable German equatorial mount for scopes up to 16-inch diameter with long focal lengths. It is shown here at the Stellafane Amateur Telescope Making Conference.

mechanical engineer by profession, it shows in Brian's home-built heavy-duty equatorial fork mount. Home-building a precision-tracking mount is an onerous task. Note the silver wrap insulation on the telescope pier.

mechanical engineer by profession, it shows in Brian's home-built heavy-duty equatorial fork mount. Home-building a precision-tracking mount is an onerous task. Note the silver wrap insulation on the telescope pier.

Rcos 20inch Astrophotography

to obtain the necessary tolerances for it to track smoothly and accurately at the subpixel guiding accuracy needed for high-resolution CCD imaging. I have built a number of mounts for imaging with scopes up to 20 inches in aperture and 160-inch focal lengths. I learned the hard way how difficult it is to make a strong and accurate mount and endured many reworks to get it right. A "good" mount will give you a high yield of usable high-resolution images but it will also be a treat for visual observing, especially at high power.

Priority 2: Optical Tube Assembly (OTA). Many commercial OTAs have a high enough optical quality to produce excellent images but are surprisingly poor mechanically for CCD imaging. Many advances have been made, however, over the last couple of years, by progressive telescope and component manufacturers. They have added features such as:

• stronger focusers with zero backlash motion and image shift,

• primary mirror lock-down mechanics for Schmidt-Cassegrains,

• stiffer tube assemblies with carbon composite tube or truss designs,

• active cooling systems for more rapid cool-down and better thermal control,

• zero expansion optical materials to minimize mirror distortion during cool down,

• zero expansion structural materials, such as carbon composites or invar, to minimize focus shift and

• active focusers to compensate for image shift due to temperature changes.

A growing problem, as larger CCDs become more widely used, will be the quality of the image at the focal plane for a specific optical design and the aberrations that arise from poor alignment. High-resolution imaging requires longer focal lengths, which tend to point to reflecting telescopes. Collimation and aberrations are issues to manage with these designs. Even the highly regarded f/8 Ritchey-Chretien telescope cannot provide diffraction-limited performance over the large CCD arrays now used by a number of advanced amateur imagers. Field correctors and even new OTA designs will be needed to address this problem.

Figure 8.9. Cave Nebula, Sharpless 2-155. LRGB image of 120:30:30:30 minutes total exposures, respectively (3-minute individual exposures due to the presence of bright stars) using a non-IR-blocked luminance image and Custom Scientific RGB filters. Image calibration and color combination using MAXIM DL (RC Console for Sigma Combine and pixel cleanup within Maxim DL), image registration using Registar, Ron Wodaski's gradient removal, AIP for Lucy Richardson deconvolution on the luminance, luminance layering in Photoshop for final color and star shaping processing. Equipment: RCOS 20-inch f/8 RC and Finger Lakes IMG6303E CCD camera with all images acquired in 2 x 2 bin mode for an image scale of .92 arcsecond/pixel in 2.8 arcsecond seeing and magnitude 4.9 suburban/rural skies.

A final note on the subject of OTAs. Really understand and practice collimation if you are using a reflecting or catadioptric design (i.e., Schmidt-Cassegrain). Just as you can increase the effective size of your telescope by improving your local seeing, so too does achieving good collimation. Many excellent Web resources are around to help you get the best collimation.

Priority 3: Focuser. Obviously you cannot image without a camera but without a responsive and rock-steady focuser you won't want to image anyway. Focusing at high resolution requires patience, care and practice. In the micron realm of focusing, the whole OTA structure is constantly on the move due to temperature changes and structural flexure, so accurate focus control is paramount. Many robust focusers have been built specifically for CCD imaging with the Crayford design generally the most popular. Even this design has had to go through recent improvements to be strong enough to support heavy filter wheels and CCD cameras, without introducing tilt errors. Excellent motorization is available with products such as Robofocus, which allows focusers to be remotely controlled. These can even cater for nonparfocal filters and focus position changes of the optical tube assembly caused by temperature variations.

Figure 8.10. Galaxy M101. LRGB image of 90:30:30:45 minutes total exposures, respectively (3-minute individual exposures) using a non-IR-blocked luminance image and Optec RGB filters. Image calibration and color combination using MAXIM DL, image registration using MAXIM DL, AIP for Lucy Richardson deconvolution on the luminance, luminance layered in Photoshop for final color and star shaping processing. Equipment: Homemade 20-inch f/5 Newtonian astrograph and Finger Lakes IMG6303E CCD camera with all images acquired in 2 x 2 bin mode for an image scale of 1.48 arcsecond/pixel in 3.4 arcsecond seeing and magnitude 4.9 suburban/rural skies.

Figure 8.10. Galaxy M101. LRGB image of 90:30:30:45 minutes total exposures, respectively (3-minute individual exposures) using a non-IR-blocked luminance image and Optec RGB filters. Image calibration and color combination using MAXIM DL, image registration using MAXIM DL, AIP for Lucy Richardson deconvolution on the luminance, luminance layered in Photoshop for final color and star shaping processing. Equipment: Homemade 20-inch f/5 Newtonian astrograph and Finger Lakes IMG6303E CCD camera with all images acquired in 2 x 2 bin mode for an image scale of 1.48 arcsecond/pixel in 3.4 arcsecond seeing and magnitude 4.9 suburban/rural skies.

Recently a freeware product called FocusMax was introduced to control a number of industry-standard focusers and CCD cameras, which automates the entire focusing routine. It works extremely well and is amazing to watch as it optimizes your focus automatically!

Priority 4: The CCD Camera. Last but not least is the imaging camera itself. If you take care of the preceding priorities, almost any cooled imaging camera will provide first-rate deep-sky images. No matter the choice of manufacturer, important considerations are QE (light sensitivity), low noise electronics, adequate cooling power and stable cooling characteristics, stiff mechanical assembly, high reliability, software compatibility and service support. The price of cameras varies dramatically with CCD sensor size, ranging from less than $2,000 for a TE cooled Kodak KAF400 sensor based camera to more than $20,000 for a high-end

Figure 8.11. The imaging train. This photograph details the imaging train of Brian's 20-inch RC telescope system. Starting from the telescope back plate is a threaded instrument adapter to space the focal plane at the right distance from the back plate followed by an instrument rotator (the red component) to assist in framing a large astronomical object or finding a suitable guide star. Behind the rotator is a personally built 12-position motorized filter wheel (2-6 position wheels) with an off axis guide port and autoguider to greatly help in maintaining guide star lock during filter changes and simplify finding suitable guide stars. The autoguider has its own remote focus to maintain sharp guide star focus for nonparfocal filters or straight through imaging. The camera is a Finger Lakes IMG6303E (2K x 3K x 9| pixels).

Figure 8.11. The imaging train. This photograph details the imaging train of Brian's 20-inch RC telescope system. Starting from the telescope back plate is a threaded instrument adapter to space the focal plane at the right distance from the back plate followed by an instrument rotator (the red component) to assist in framing a large astronomical object or finding a suitable guide star. Behind the rotator is a personally built 12-position motorized filter wheel (2-6 position wheels) with an off axis guide port and autoguider to greatly help in maintaining guide star lock during filter changes and simplify finding suitable guide stars. The autoguider has its own remote focus to maintain sharp guide star focus for nonparfocal filters or straight through imaging. The camera is a Finger Lakes IMG6303E (2K x 3K x 9| pixels).

camera with a Kodak KAF 6303E CCD array (see Figure 8.11). The reality is that most astronomical objects are quite small, so smaller CCD arrays are more than adequate for most high-resolution imaging.

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