Sirius Optics MV1 comparison on M42

All achromatic refractors have problems focusing light of different wavelengths to the same point.  This chromatic aberration causes images of bright objects to have colored halos (usually blue or violet) around them.  Apochromatic refractors add additional glass elements to correct this, but they are much more expensive and beyond the budget of many amateur astronomers.

The Sirius Optics MV1 filter is designed to minimize blue halos around stars in achromatic refractors.  MV stands for minus violet and it works by filtering out the blue/purple wavelengths of light.  Technical details about the characteristics of this (and other) filters can be found on their site.

To review this filter, two images were taken of the M42.  Both were 5 minute exposures on 35mm Elite Chrome 200.  All images were taken at the prime focus of an Orion 120ST refractor - a 600mm f/5 instrument designed for widefield use.  A JMI NGF-SE focuser was used in place of the stock focuser.  Images were scanned on a Nikon Coolscan IV ED negative scanner with the exposure locked to give an accurate representation of the effects of the filter.   Scanning was at 2900 dpi resulting in images of 3968 X 2648 pixels.  Various views and sizes of these two images are presented here.  With the exception of the final two images on this page, no adjustments other than sizing and cropping were done.

A similar set of exposures/comparison was done for the open cluster M35.

Central portion of the images after sizing to 25 percent.   Since the MV1 filters out blue light, it imparts a yellow cast to the photo.   The filtered image is slightly darker as well.  Blue halos are significantly reduced. Note that there appears to be an internal reflection of some sort next to the star located just left of center in the filtered image.
Unfiltered image Click the image above to see a 640 x 436 pixel version of the full-frame image	Filtered image Click the image above to see a 640 x 436 pixel version of the full-frame image

Central portion of the full size, full resolution images.
Unfiltered image	Filtered image

A different portion of the full size, full resolution images.
Unfiltered image	Filtered image

Central portion of the images after sizing to 25 percent, color corrected and level adjusted. Note that there appears to be an internal reflection of some sort next to the star located just left of center in the filtered image.
Unfiltered image	Filtered image

Conclusion
This filter significantly reduces blue halos around bright objects both visually and photographically.

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