H-alpha all-sky survey. C. Stubbs May 25 2015
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At 85mm focal length one arcsecond spans 0.5 microns, and FOV is 28.6 16 x 24 degrees. A 10 micron pixel subtends 20 arcseconds, so we need arcminute-size galaxies with decent surface brightness. Assume the goal is to get out to redshift of 0.1, at which 1 kpc is 0.5 arcsec. Actually at 6500 A we don't need the fancy IR version, and 3-4 filters get us from z=0 to z=0.1, at 20 nm each. But the key is getting really small pixels and low dark current. So what's the smallest-pixel back-illuminated 35mm sensor? Say we want 5 micron pixels and 35mm diagonal, so 25mm on a side, which means 5K x 5K, TE cooled.
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What about using a Canon SLR camera? Have asked about conversion of 5K Mark III to B&W version, but the challenge will be dark current. Imager is 5760 x 3840, and 36 x 24 mm so each Bayer pixel is 6.25 microns. We aren't going to get images this sharp however and staying in focus is going to be tricky as well, with a fast optic in front. Assume we have a 10 micron FWHM, which is about twenty times the diffraction limit.
| lens | filter threads | plate scale on Canon 5D, color pixelscale for 10 micron FWHM
| plate scale at z=0.1 | FOV (sq deg) | speed rel. to f/2.8 | FOM | cost |
|---|---|---|---|---|---|---|---|
| Zeiss 135mm f/2 | 72 mm | 1 15 arcec | 2 7 kpc | 15 x 10 = 150 | 2 | 2.7 | $1822 B&H |
| 85mm f/1.8 | 58 mm | 1.6 24 arcsec | 3.2 12 kpc | 24 x 16 = 384 | 2.4 | 8.5 | $350 B&H |
85mm f/1.2 Canon EF 85mm f/1.2L II USM | 72 mm | 1.6 24 arcsec | 3.2 12 kpc | 24 x 16 = 384 | 5.4 | 19 | $1,999 B&H |
50 mm f/1.2 Canon EF 50mm f/1.2 L | 72 mm | 2.7 41 arcsec | 5.4 20 kpc | 40 x 27 = 1080 | 5.4 | 54 | $1,449 B&H |
| 50 mm f/1.4 | 58 mm | 2.7 41 arcsec | 5.4 20 kpc | 1080 | 4 | 40 | $320 |
| 50 mm f/1.8 | 52 mm | 2.7 41 arcsec | 5.4 20 kpc | 1080 | 2.4 | 24 | $125 |
| Zeiss 35mm f/1.4 | 72mm | 3.8 58 arcsec | 7.7 28 kpc | 2204 | 4 | 81 | $1543 B&H |
| 500 mm f/2.8 Takahashi | NA | 0.27 4 arcsec | 0.54 2 kpc | 108 | 1 | 1 | $5000 |
Well that's not a hard choice! And note that chromatic effects will be highly suppressed in a narrow band, plus the filter is probably easier to get.
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Mount Options
Bought a camera mount that can do a nice variety of scans, at sidereal and other rates, the Sky-Watcher. Also bought equatorial wedge and counterweight and camera control cable, all from B&H
| item | B&H part number |
|---|---|
| Skywatcher Star Adventurer - A | SKS20510 |
| counterweight | SKS20540 |
| equatorial wedge | SKS20530 |
| camera control cable | SKCRC5D |