High Resolution Photography Requirements
High resolution photography requires lots of practice, and a little luck. Before you are set up to capture detail within Jupiter's Great Red Spot, you may want to study this list of requirements. It is based on both calculations and experience!
Image stabilization
compared to adding short-exposure images
Image stabilization is equivalent, in many respects, to co-adding many short exposures. The image stabilization is accomplished by using very short exposures, and the single-image noise is reduced by adding many exposures together. Each image must be shifted before adding, using either automated software, or manually. This technique is less useful for color filtered images, since each exposure through a filter will either be very noisy or unacceptably long. For black-and-white images, however, this technique is very useful. For a terrific set of images that demonstrate this technique, see Antonio Cidadio's web page, including experiments with webcams, at:
General requirements: For best results, the following suggestions are recommended. Your photographic results will vary, but try to adhere to as many of these as possible. These first suggestions are difficult to quantify, but if all of these are done, you will be on the way to diffraction-limited photography! Note that while SCTs have limited visual resolution, high resolution photography is possible with the hints and techniques described here.
- wait for seeing good enough to see the Airy disk; you
may need to wait for another night
- use a good orthoscopic or Plössl
eyepiece for eyepiece projection optics
- use projection magnification to get F/50 -F/150 (film)
or F/30-F/50 (CCD)
- use a camera with a mirror lockup feature, and a
massive camera body (metal body is preferred)
- use an air bulb cable release, draping the cable to the
telescope, exiting on the tripod
- mount the telescope on a sturdy mount (if not on a permanent
pier)
- use a direct-view focus magnifier (with optional T-ring
adapter) for precise focus (if using film)
- locate telescope away from area heat sources such as
buildings or paved parking lots
Additional requirements: The following list is more detailed. Note that uncorrected blurs typically add up to 5 to 10 arcseconds; after the easiest corrections are made, the blurs are typically reduced to 1 or 2 arcseconds. The best correction requires the AO-2 adaptive optics system to reveal 0.2 to 0.7 arcsecond features!
a) to reduce Telescope Collimation errors, center the image within 1 mm at prime focus
The telescope produces large coma unless the image is centered within about 1 mm of the optical center. See your telescope manual for collimation instructions. Cold weather and focus backlash can produce mis-collimation of several millimeters on commercial SCTs, where the primary mirror is moved. For an 8" SCT, fine adjust the collimation by turning the secondary mirror screws in increments less than about 1/50 turn. Inspect a stellar Airy disk on a night of good seeing to verify collimation.
b) to reduce Focus errors, focus the image within (F/#)² microns at camera
A focus within (F/#)² microns results in an acceptable blur diameter not much larger than the Airy disk. For an 8" SCT, focus the primary mirror within 1/100 turn of the focus knob. To get the best focus, move the camera, not the telescope mirror. For example, using the formula, the camera focus should be within ± 6 mm at F/80. The F/2 primary requires ± 4 microns = 0.004 mm!
c) to reduce Atmospheric Image Motion, use the AO-2 adaptive optics system
Adaptive optics was invented to remove this rapid image motion! The blur diameter depends on the seeing; uncorrected blurs of 1 to 2 arcseconds are typical, practically independent of the telescope diameter. If the Airy disk is visible, watch it move with a reticle eyepiece under high magnification. The eye will see the slower image motion; actual motion is up to 30 Hz.
d) to reduce Refraction errors, use color filters
Atmospheric refraction causes about 1 arcsecond of red-blue color blurring for planets at 45° above the horizon. Use color filters with CCD cameras or film to reduce image blur. Color images require separate exposures through three different filters, with computer or darkroom processing to combine the three images back into one. All SCT and refractor telescopes are designed to have large blurs in the IR spectral region due to the glass optics. All CCD cameras have large blurs in the IR spectral region because of transmission through the silicon substrate. An IR blocking filter is required for high resolution visible light photos.
e) to reduce Polar Misalignment errors to less than 1 arcsec blur per 4 sec exposure, polar align with less than a 1° error, or use the AO-2 adaptive optics system.
Polar alignment within 0.1° is usually required, unless an AO-2 is used to remove image drift.
f) to reduce Clock Drive error, synchronize your shots with the gear train, or use periodic error correction (PEC), or use the AO-2 adaptive optics system.
Commercial SCT clock drive errors produce typically 3 arcseconds of blur in a 10 second exposure. Clock drive errors can be minimized by taking photos during the brief period each gear cycle when the clock drive rate is coincidentally accurate. PEC drives may reduce errors to a few arcseconds, if the motion is smooth. The AO-2 reduces all types of drives errors.
g) to reduce Planet Rotation effects, use short exposures and quick color filter changes
Jupiter rotates 0.1 arcsecond per 23 seconds. Mars rotates 0.1" per 110 sec. Saturn rotates 0.1" per 57 seconds. Try to minimize the time required to change color filters. Don't use such narrow-band filters that require very long exposures.
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