horsehead nebula – DSLR astro-mod comparison

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The horsehead nebula is another winter target easily found next to alnitak, which is a bright star in orion’s belt. I photographed this target on two recent occasions. Once before and again after making some equipment modifications. For both of these imaging sessions, I used my canon t3i, but I performed a full-spectrum astro-mod and added a Stellarvue field flattener reducer and Optolong 2″ L-pro light polution filter. Several key optical upgrades in this one, but let’s look at the images to see if they made a noticeable difference in image quality.

I performed the astro-mod to the camera myself after watching a few online tutorials and getting familiar with the parts I would need to buy in advance. I went with the so-called “full-spectrum mod” opting to remove both the low-pass filter 1 and the IR/cut filter. I replaced the IR/cut filter with an Astronomik MC clear glass cover from This modification will result in bloated stars due to excessive photons from IR and so I chose the Optolong L-Pro 2″ filter to serve both as an IR filter and a light pollution filter. This 2″ threaded filter screws right onto the end of the Stellarvue SFFR102-2 flattener/reducer that I also purchased from

The stock canon t3i is on the left and astro-modded canon t3i is on the right:

The changes to the image are as expected: the overall field has been expanded due to the 0.8x reducer. The image shows more fine Ha nebulosity and the contrast is improved due in-part at least to the L-Pro filter. The table below provides more details on the conditions behind each image.

stock canon t3iastromod canon t3i
imaging cameraCanon t3ifull-sectrum astromod Canon t3i
guide cameraZWO ASI290mm MiniZWO ASI290mm Mini
mountSkywatcher EQ6-R proSkywatcher EQ6-R pro
telescopeExplore Scientific ED102 APOExplore Scientific ED102 APO
flattener/reducernoneStellarvue SFFR102-2
combined flattener and 0.8x reducer
filtersnoneOptolong L-Pro 2″
exposure settings180s
iso 800
iso 800
stacking38 lights
25 darks
25 flats
30 bias/offset
60 lights
30 darks
50 flats
30 bias/offset
imaging session detailed comparison.
Horsehead Nebula captured with astromodified Canon T3i

I am happy with this result and feel the effort of upgrading the optics in my system were worthwhile. You will also notice that I have resolved the “pinched optics” that was causing odd diffraction spikes in the stock Canon T3i photos above. Thanks to Explore Scientific customer support, I was able to loosen three set-screws intended to hold the objective lens securely during shipping. I’m glad that simple fix corrected the diffraction spikes. See the instructions to loosen the set-screws HERE.

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