Color vs. Mono Camera Match
SBIG ST-2000XCM color camera vs. the ST-2000XM mono w/filters

In Summary

Image Resolution
  Viewing the images at 200% & 300% scales, the differences between the two cameras were relatively small for the emission and reflection examples as well as the star cluster example and probably would not be noticeable when displayed at the native scale or even noticeable for web display after completing the post processing. The M1 Crab Nebula color example taken with the XCM showed the most visible loss in fine resolution compared to the other examples.
  Comparing the H-alpha examples, differences between the gaseous nebulas NGC 6992 and the Bubble were minimal with the XM having the lower noise levels and slightly higher details. The more finer detailed structure objects such as the M1 Crab Nebula, the differences become very evident and reveal the XCM's limits with this form of narrow band imaging.

Color Accuracy
  Color differences between the Bubble Nebula and M45 reflection nebula examples were minimal. Similar differences can be seen with mono cameras using different astro filters from different manufactures. The red H-alpha emission colors were close due to the very similar red spectral curves and QE of the Bayer and mono w/red filter. The blue reflection nebulas were also close in color and would require some additional processing of the XCM data to correct the shift, though minimal. But, the big differences are objects emitting strong O-III emission lines, and the XCM displaying the greatest shift away from the correct color. This is due to the cross over points of the Bayer matrix green/blue dyes and has to be compensated for with much additional processing. This is one of the biggest differences you will see between a mono w/ astronomical filter and a Bayer masked sensor and require more processing time to correct.

Luminance Blending
  Using a mono camera with a luminance filter (or clear filter with or without a secondary UV/IR blocking filter depending on OTA), you should spend more time getting a good high S/N luminance image and later combine it with a lower resolution color image either from the XCM, or in the case of using the XM in 2X2 bin mode, for a composite image. This technique can be applied to either type of camera producing outstanding results by improving the S/N, fine detail resolution and dynamic range as well as increasing the star field population in some cases. This technique is where the mono camera shows one of its full potentials when using the LRGB method compared to the color cameras lack of acquiring a separate luminance channel ability.

Time Savings

  Exposures- There isn't any time savings with the use of the color camera over the mono when it comes to getting similar quality S/N data so expect to spend at least the same amount of overall time as the XM for a given exposure requirement. The XM, with its higher QE displays less noise and brighter image structure when equal DDP stretches were applied to both examples. This can be improved with the XCM in most cases however as the noise reduction techniques and packages available provide very good results. But, this does add to the overall processing time. Also, depending on the object being imaged, you may need longer sub-exposures or more shorter exposures to get adequate S/N and this is the prefer method instead of over processing. If you have to set up and tear down after each session, the camera selection is something you should seriously consider if the amount of time you can dedicate is of concern. This is where the mono camera would be preferred.

Ease of Processing-   This is a 2 part comparison breakdown:
  1- Here is where the scales tilt towards the mono XM most of the time. Because the mono camera produces better S/N and slightly better details for the same exposure time as the XCM and with more accurate colors (assuming you are using good astronomical filters), it will require less post processing time. I have spent much more time with certain XCM images, especially ones with O-III content, to get the correct colors and also to coax out more details where needed.

  2- Some say that "with a one-shot color camera you spend less time gathering flat frames and you don't have to deal with 3 or 4 separate color channels to calibrate, align and stack". Well, that's true! But when you use a mono with separate filters, collecting flat frames that are on the order of 1-10 seconds each for each color channel is the very fast part of an imaging session and doesn't add much time. Calibration, align and stack is automated these days with processing software. Images Plus has an excellent automated bulk file calibration feature that saves me a lot of time.
The additional time spent processing the XCM data to correct color imbalance, highlight details and clean up star artifacts can quickly surpass the time saved by not collecting and calibrating separate RGB (and maybe L) data with the mono. But processing time is a moot point and shouldn't be used as a purchasing decision because it's the one thing you can do on a rainy day. I know many mono camera users who spend days on processing an image.

Setup-   The one time saver that favors the XCM camera is the infamous 'finding a suitable guide star' when using the built in auto-guider CCD. Using the XM w/filters, I have to find a guide star bright enough through the blue filter before settling on the final camera position and at times this can be a major pain especially if using an AO unit. Then as the filters are rotated throughout the session there usually aren't guide star issues. Of course the mount you have and it's PE profile will have the biggest effect on this when using the mono camera w/ filters so the longer guide exposures you can use, the less of finding a good guide star is a problem. Also, tools like The Sky with it's imaging/guider CCD overlays and/or using a rotator can assist in finding a good star during the pre-session planning stage. With the color XCM, the guide star you see is the guide star you will use through the session and the only thing you need to be concern with is occasional focusing.

Final Judging

  So, which camera won? When someone asks me which camera is best my response depends. These tests show that it depends on the objects imaged, its emission line contents, amount of details and more importantly how well either one will fit into your budget. If you plan on doing narrow band imaging or photometry work then the obvious answer is you definitely need to go with the mono camera. There are other factors that this test did not evaluate such as the amount of detail loss difference using a longer focal length OTA where the photons are spread out over more pixels so the sampling rate would be much finer. This would also reduce or eliminate the star edge artifacts in the XCM images.

 The results displayed here were based on the equipment I have, my local seeing conditions and processing techniques so your results will vary.

  For a grab-n-go camera, the XCM is certainly the one to use for simplicity hands down. Also, for objects like comets and very bright DSO's such as the Orion Nebula or M45 Pleiades, the XCM performs very well and can provide very satisfying images with good processing techniques. I mention processing techniques because many people who buy the XCM are new and still developing still their processing skills and may expect too much at first.
  My experiences with both cameras has me getting rid of neither. When I want to show someone what astro photography is like, I use the XCM for ease of use, quick setup and fast color results to display. I use the XM mostly because of the finer details I want to see in an image and I find it much easier and less time to process. Take you time and gather the necessary data sets to build a high quality image.

The important thing to remember is you really need to understand and accept your cameras capabilities and its limitations when selecting an object to image and work within that circle to get the best results and more importantly, have as much fun and satisfaction as possible. After all....this is only a hobby!

 I hope you find these series of tests interesting and informational. I learned a lot doing this and also proved that even though the XM mono camera is better in many areas, it isn't the "blow the doors off" type of results some would suggest. If you would like to make comments, suggestions or point out errors, please send emails to:
mike"at"helixgate"dot"net   replace "at" with @ and "dot" with a . (period)

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© 2008 Michael A. Siniscalchi