M57 - NGC 6720 - First Try


Subimages: 10 x 10 sec, Final FWHM=4.6"
Scope: Synta 120mm achromatic refractor, EFL=1000 mm (f/8.3)
Mount: Synta EQ4 with dual axis drive on aluminum tripod
Camera: ST7EI (1.86"/pixel) set to -10C
Guiding: None
Acquisition and processing: MaxIm DL/CCD
Imaged on: Sept 17, 2003 from my driveway in downtown Ottawa, ON.
Processing steps: No image calibration, aligned subexposures, average combine, mild ddp with no filter selected.

The power of a CCD camera is revealed in this image from a low cost achromatic refractor. The view is comparable to the "au naturel" view through my 16" dobsonian reflector. The faint halos around the stars is caused by chromatic aberration in the near IR, a spectral region to which the eye is blind, but a CCD is still very sensitive. Notice the small sine wave drawn out by a hot pixel in the lower left hand corner. The hot pixel is present in all 30 frames but after image alignment it moves about due to mount tracking error. The polar misalignment has introduced a vertical slant in the oscillation of ~7" over the 14 min of acquisition (camera read out time is 15sec + 10sec integration x 30 frames), pretty good since the mount was polar aligned with only a simple polar axis scope, no drift align was performed. The amplitude of the sine wave reveals the periodic error from the worm drive, ~30" peak to peak after declination correction, this number is fairly respectable considering the cost and sophistication of the EQ4 mount.

Raw 10sec subexposure, image contrast and brightness set the same as above.



Second Try

Imaged in 2004 with a friend's bigger telescope

Subimages L: 22 x 15 sec, Final FWHM=3.5"
Scope: Celestron NexStar 11" Schmidt-Cassegrain EFL=3010 mm (f/10.8 effective)
Mount: NexStar alt-azimuth computer controlled fork
Camera: ST7EI (0.62"/pixel) set to -17C
Guiding: None
Acquisition and processing: MaxIm DL/CCD
Imaged on: June 12, 2004 from the Fred Lossing Observatory near Almonte, ON.
Processing steps: No image calibration, aligned subexposures, average combine, linear stretch.

While at our local astronomy club observatory, about a 45 min drive from downtown Ottawa, I tried my camera on a friend's (Howard Headland) 11" SCT. This type of telescope tracked the stars on an alt-azimuth fork mount via a computer. The computer calculates the different corrections required for the right ascension and declination axis keeping a star centered in the eyepiece. However in such an arrangement the field of view will slowly rotate over time as the object tracks from the high in the sky to the horizon. Again a hot pixel in the top left corner provides us with a visual track of the image drift and rotation over the session (11 minutes). The crooked black border along the left and top is also due to this effect, the dark color is due to the minimal overlap of the frames in that region. The stars in this image are tighter than those in the refractor image (I was getting better at focusing) and lack the large halos since an SCT does suffer from chromatic aberration. The improvement in detail is substantial being due to the much larger aperture providing higher intrinsic resolution and much higher signal to noise ratio.

Raw 10sec subexposure, image contrast and brightness set the same as above. The smearing is from the unguided tracking, and is obvious due to the large focal length.



Third Try

Imaged in 2006 with my new gear.

Subimages L: 9 x 120 sec, Final FWHM=2"
Scope: C11XLT EFL=2940 mm (f/10.5 effective)
Mount: Losmandy G11 with Gemini L3
Camera: ST7EI (0.63"/pixel) set to -19C
Guiding: 1 sec exposures with an SX716 on a Taurus Tracker III OAG
Acquisition and processing: MaxIm DL/CCD v4.5
Imaged on: April 28, 2006 from my backyard in Carp, ON, just outside the suburbs of Ottawa, ON.
Processing steps: Bias and dark image calibration, aligned subexposures, average combine, linear stretch.

The gain in resolution and depth is huge when a good mount is put under the telescope. This allows the operator to consistently find good focus (instead of chasing the star after every adjustment) and integrate for very long periods without image smear. A minimal amount of processing was applied to this image to make it comparable to the two previous attempts so some hot pixels remain.

Raw 120sec subexposure, image contrast and brightness set the same as above.