M33 is a close-by spiral galaxy oriented face-on to us. It's the second-largest galaxy in the sky, in terms of angular size (Andromeda Galaxy is the largest). Because it spans 1 degree in maximum extent large telescopes can image only small portions at a time, and must use mosaics to show the entire galaxy. Therefore this object is suitable for small telescopes, with short focal lengths, that can fit the entire galaxy in a single image's FOV.

Figure 1. M33, FOV = 64 x 43 'arc. LBVR exposure times 13, 33, 9 & 7 min. [Meade LX200GPS, prime focus, Starizona HyperStar, SBIG ST-8XE CCD, 2007.01.09, Hereford Arizona Observatory]

Since a photometric B-filter passes such a small amount of light compared to the other filters it is necessary to expose B-filter images longer than the others. In the above image a total exposure time of 33 minutes was used (140 15-second exposures). It's possible to create what appears like a color image using VRI filters instead of BVR filters, thus reducing total observing time. This is illustrated in the next image.

Figure 2. M33, FOV = 64 x 43 'arc. False color LVRI; exposure times 13, 9, 7 & 7 min. [Meade LX200GPS, prime focus, Starizona HyperStar, SBIG ST-8XE CCD, 2007.01.09, Hereford Arizona Observatory]

Comparing the two images shows that some regions that appear green in the LVRI image are actually red (as in the LBVR image).

All exposures were short, being 10 seconds for L, V, R aNnd I; for B they were 15 seconds. Longer exposures would have produced greater blooming streaks for bright stars. The MaxIm DL anti-blooming feature was used for L and R images (and two stars required additional editing).

The previous two images were made with at prime focus using a Starizona HyperStar field flattening lens. The plate scale was 2.58 "arc/pixel. The sharpest images had FWHM = 4.5 "arc (1.74 pixels). This sharpness is close to the theoretical limit. The next image was made with a Cassegrain configuration having a plate scale of 1.09 "arc/pixel.

Figure 3. M33 central region, FOV = 27.5 x 18.3 'arc. LRGB made without a B-filter. A B-band image was obtained by subtracting a blue-blocking filter image from a clear-filter image. Since the blue-blocking filter passes 86% as much light as the clear filter it was possible to use the image stabilizer (ABIG AO-7 tip/tilt) for long exposures, whereas with a B-filter (passing only ~7% as much light as a clear filter) there frequently is not a bright enough star on the autoguider chip to permit image stabilization or even autoguiding. Exposure times for C, R, V and the blue-blocking filter were 45 min, 9 min, 10 min and 10 min. [Meade LX200GPS, Cassegrain, x0.6 focal reducer, SBIG ST-8XE CCD, 2007.01.04, Hereford Arizona Observatory]

The greater detail in the Cassegrain image is apparent, but the sky area shown is <20% of that for the prime focus images. A mosaic could be produced from small FOV images to equal the large FOV, but it would require about 6 times as many images and more work.

This site opened:  January 04, 2007.  Last Update:  January 09, 2007