Affordable DSO Astronomy

Transcription

Affordable DSO
Astrophotography
Craig Stark, Stark Labs
Monday, November 23, 2009
Typical advice: Think big or go home
“To really start doing astrophotography, you’re going to
need a solid mount. Don’t even think about trying this
without something like a G-11. ... lesser mounts just
aren’t up to the task... GOTO is also a real help”
Losmandy G-11 with
Gemini GOTO: $3200
Can we get the mount, scope,
and camera for ~$2000?
$49,000 Question
$49,000 Question
Celestron C8-N ASGT
Baader MPCC
Canon Rebel XT
~$1700 Jim Solomon
Takahashi FRC-300
FLI ProLine + CFW
Paramount ME
~$50,700
Michael Fulbright
GSO 8” f/4, Baader MPCC, Canon Rebel XT, Atlas
Tony Hitchcock
Meade LXD55 SN10, Baader MPCC, Canon 300D
Jim Solomon
Celestron C8-N ASGT, Baader MPCC, Canon Rebel XT
Garrett Grainger
Orion ED80, Canon Rebel XT, Orion Sirius EQ-G
Mel Wurth
8” f/5, Canon Rebel XTi, Atlas EQMOD
Scott Anttila
HPN 8” f/5, MPCC, Canon Rebel XT, ASGT
Chuck Kimball
Explore Sci 152, Canon Rebel XT, on LX200 classic
Chuck Kimball
Explore Sci ED80, Canon Rebel XT, on LX200 classic
Chuck Kimball
Orion 6” Newt, Canon Rebel XT, on LX200 classic
Stephen Hamilton
Meade LXD-75 SN8, Meade DSI Pro II
Moral: Throwing money at it isn’t
necessary & won’t always help
Better equipment does not ensure better images.
Excellent images can be had from relatively affordable
equipment.
Learning how to image and process / learning how to best
use your gear and software as big if not bigger factors than
equipment cost.
That said, not all affordable equipment is good. How do
we choose affordable equipment that won’t limit us?
Roadmap
Telescope: Keep the focal length ~1000 mm or less
80 mm ED refractor
Newtonian or Schmidt-Newtonian (Mak-Newt)
Mount
Mass-market GOTO
Autoguided
Camera
Basic DSLR
Smaller-chip dedicated cams
Software
Freeware / inexpensive
Garrett Grainger
Telescope: Think short and/or small
Focal length
Aim for ~2 (1.5 - 3) arcsec / pixel (Your skies won’t give you
much more than this in any case).
Weight
As light as possible. Really.
No, really.
Telescopes: Short and sweet
8” f/5 Newtonian (Synta, Orion, Antares, Celestron)
OTA: $290-$400
8” f/4 Newtonian (e.g. AstroTech 8” f/4, used GSO)
OTA: $450
80 mm ED Refractor (Orion, Skywatcher, WO, AstroTech, SV)
OTA: $550-$700
66 mm ED Refractor (WO, AstroTech)
OTA: $330-360
Newt Pros: Lots of photons, no
false color, and inexpensive
Light Gathering vs. Size
No chromatic aberration
No purple halo
No IR-induced star bloat
By far, biggest aperture per $
8000
6000
4000
2000
0
3”
4”
6”
SNR in DSO images driven heavily by aperture.
Available in focal lengths well-suited to current cameras
8”
Newt Cons: Size and coma
Cons:
Weigh more than SCT OTAs or small refractors
Bigger target for wind
Tubes prone to flex with heavy cameras
Difficult to use focal reducers with
Collimation a must
Coma
Newt Cons: Size and coma
Cons:
Weigh more than SCT OTAs or small refractors
Bigger target for wind
Tubes prone to flex with heavy cameras
Difficult to use focal reducers with
Collimation a must
Coma
Coma is driven by position & f-ratio
~1 Pixel at corner of 1/3” chip
f/4
3 mm
11μ
6 mm
24μ
9 mm
36μ
12 mm
49μ
15 mm
62μ
f/5
9 mm
23μ
3 mm
7μ
6 mm
15μ
12 mm
31μ
15 mm
40μ
To use a Newtonian with a reasonable focal length, we
need to tame the coma on many chips (esp. DSLRs)
Images from James Mulherin
http://www.opticalmechanics.com/about_coma.htm
Two good coma corrector choices
Baader MPCC
$144
No shift in focal length
Big chips: f/4 limit, cleaner at f/5
TeleVue ParaCorr
$220 & $310 (Photo vs. Visual)
Extends focal length by 15% (f/4→f/4.6)
Big chips: Good to f/3.5
Baader MPCC
Corner of Trifid2
(~12 mm) at f/4.6
Corner of HX-916 (~5.5 mm) at f/4
Images from Tom Licha
http://www.licha.de/astro_review_mpcc.php
Images from Christope Demeautis
http://demeautis.christophe.free.fr/test/baader/baader.htm
Short refractor pros: Wide field, light
weight, & super-easy guiding
Short focal lengths (400-500 mm)
Let you go WIDE even with smaller chips
Guiding is easy ... really easy.
Light weight is easy on the mount
Tube flex much less an issue
Short refractors cons: Sampling
rate, color, and non-flat field
Resolution is a bit limited
400 mm, 6 u pixels = 3” / pixel
Small targets won’t show as much detail
Chromatic aberration in doublets
Field is not typically flat
Astrotech & Hotech flatteners, TV and WO reducers, ...
Think small scopes can’t go deep?
Image Sampling: 400 vs. 1000 mm
Image Sampling: 400 vs. 1000 mm
Going very wide will limit resolution (but for many
cams, going much above 1 m won’t gain much)
Other options
8” SCT
Focal reducer required (yet still only gets you to ~1300 mm)
Not big-chip friendly
AstroTech 6” or 8” RC
Explore Scientific “Comet Hunter” 150 Mak-Newt
Meade SN-8 and Orion 190 mm (if on an EQ6 / Atlas)
Whatever you have if it’s a reasonable focal length
What spending more gets you:
Stability and size of usable FOV
Mechanics:
Most focusers on these scopes have real issues with flex as
the camera weight goes up.
Newt OTAs likewise have flex issues w/heavier cameras.
Overall precision / repeatability, etc.
Size of corrected or correctable field can be limited
None pull APS-size w/o correction and APS is as big as
you’ll get corrected.
Color purity on ED doublets
More aperture and more photons cost more money.
Affordable GOTO mounts exist
Celestron ASGT: $575 (new)
Bundled with scopes like the C8-N 8” f/5
Meade LXD-75: $600 (new)
Bundled with scopes like the SN-8 8” f/4
Celestron CGEM / Orion Atlas / EQ-6
$1400 - $1500
Heavier payload
Rules of affordable mounts
Do not overload the mount
OTA < 20 lbs for ASGT / LXD-75
OTA < 35 lbs for Atlas
Guide
Balance set to just slightly east (resisting RA drive)
Adjust / tune up as needed
Eliminate all flex / loose joints
Polar align reasonably well
Polar scope
Iterative GOTO
Drift
Peter Kennett
Meade LXD-75 SN8, 30 m, Film, Manual guiding
Tuning / adjusting is straightforward
and strongly recommended
ASGT / LXD-75 / CG-5
http://www.astronomyboy.com/cg5/
http://www.lxd55.com
DIY Kit and Service
Atlas / EQ-6
http://www.beevo.com/rework.htm
http://www.jamesrobertcook.co.uk/EQ6_dismantle.htm
EQMOD: http://eq-mod.sourceforge.net/
Autoguiding
All mounts have tracking errors you must account for to
image beyond 10-60s.
Use a separate guide scope or an off-axis guider to track a
star
Manual
Automatic
Separate guide camera / chip (same pixels cannot do
both)
Computer / mount link
Software
Autoguiding Hardware
Guide Scope
Typically an inexpensive refractor
Shorter f/l can work well (e.g. 400 mm)
Often doubles as wide-field scope (e.g. William Optics
ZS-66 SD doublet)
Rings
Communication
Standard USB / serial link + ASCOM (http://www.ascomstandars.org)
ST-4 “autoguide” input + ShoeString adapter (http://
www.shoestringastronomy.com/)
Autoguiding Hardware
Camera
Webcams
Long-exposure webcams & LPI / NexImage
Less-expensive astro-cams (e.g. Meade DSI used or w/
scope purchase)
Orion Starshoot autoguider
1/2”, 1.3 megapixel
Mono
Onboard ST-4 output
$280
Autoguding Software
PHD Guiding (Free) http://www.stark-labs.com
K3CCDTools ($50): http://www.pk3.org/Astro
GuideMaster (Free): http://www.guidemaster.de
GuideDog (Free): http://www.barkosoftware.com
Mike Broussard
Atlas EQG, 6 minute, PHD
Garrett Grainger
Atlas EQG, 6 minute, PHD
“100 minute” exposure
“100 minute” exposure
Michael Garvin
Celestron ASGT, HPN 8” f/5, Zhumell 60 refractor
CCD Labs QGuide & PHD Guiding
RA: 0.16 pixel error (RMS)
350mm guide scope, 3"/pixel
1
0.8
0.6
Error (pixels)
0.4
0.2
0
-0.2
1
31
61
-0.4
-0.6
-0.8
-1
Frame (2s)
91
Michael Garvin
Celestron ASGT, HPN 8” f/5, Zhumell 60 refractor
CCD Labs QGuide & PHD Guiding
RA: 0.48 arcsec error (RMS)
350mm guide scope,
2.5
2
1.5
Error (arcsec)
1
0.5
0
-0.5
1
31
61
-1
-1.5
-2
-2.5
Frame (2s)
91
Flex-induced error is a very
common source of poor guiding
Post-guiding Error
30
Setup 1
Setup 2
25
Linear (Setup 1)
y = 0.4683x - 1.0786
Linear (Setup 2)
1 pixel in 2.1 min
Pixels
20
15
1 pixel in 17 min
10
y = 0.0589x + 0.2176
5
0
0
10
20
30
40
-5
Time (m)
50
60
70
80
What paying more on your mount
gets you: Payload & accuracy
Need more than ~30-40 lbs?
Want to image at longer focal lengths and higher resolutions?
Up to a point, magnitude of PE is not a key concern.
Key is how smooth the PE is and how much of it is at a
high frequency (rapid changes in position).
Getting round stars at 1”/pixel (or better) for 5+ minutes is
not trivial and not for the faint of heart (or light of wallet)
Cameras
One-shot vs. Mono?
Big vs. Small chip?
Cooled vs. Uncooled?
Big and Color:
Canon DSLRs: Canon Rebel XS ($500 w/lens)
Orion Starshoot Pro ($1200) and QHY8 ($1500)
Smaller
Atik 16IC ($590) and 16IC-S ($920) - cooled
Meade DSI II ($400) and III ($800)
Orion Starshoot II ($300)
Many more options used (ICX 285 and KAI-2020 cams)
Bigger chips
Chip costs rise with increasing chip size
DSLRs are by far the least expensive way to get big chips
(mass-production)
Short f/l will still get you a wide FOV.
Don’t underestimate what will fit on a “small chip”
FOV of 8 mm chips
FOV of 11 mm chips
Lower noise
Cooling
Drops dark current and dark noise to near zero (no darks
needed or very effective dark subtraction)
Makes line filter imaging in the summer possible for some
Lower and cleaner read noise
Many less expensive cameras have structure in the read
noise that cannot be removed with calibration (streaks,
bands, etc)
Atik 16IC: Adjacent bias frames
Atik 16IC: Adjacent bias frames
Canon XSi: Adjacent bias frames
Canon XSi: Adjacent bias frames
Urban M51: 350XT vs. 3 Mono
CCDs
Very large gap between the mono
astro-CCDs and the Canon.
Stack of 5 best frames, 1 m apiece, no darks.
Urban M51 Rematch: XSi, Color
& Mono CCDs
Color filters are
the source of
some of the
loss, but a
large gap
between DSLR
and astroCCD.
Single 3 m frame, Ambient ~55F, L-data only
Software: Processing
IRIS (Free): http://www.astrosurf.com/buil/us/iris/iris.htm
Windows
Very powerful. Can do most anything to process images
Mostly command-line driven - many find this difficult
Deep Sky Stacker (Free): http://deepskystacker.free.fr/
Windows
Aligning and Stacking
Pre-processing (darks, flats, etc).
Capture & Processing
K3CCDTools ($50): http://www.pk3.org/Astro
Meade, LE-webcams, normal webcams, other video
devices, SAC, Artemis/Atik
Pre-processing & color reconstruction
Stacking (incl. alt/az)
Stretching
Guiding
Aimed more towards capturing than processing.
Capture & Processing
Nebulosity ($45-$60): http://www.stark-labs.com
Cross-platform
Canon, Meade, Orion, SBIG, Starlight Xpress, Fishcamp,
CCD Labs, SAC, LE-webcams, OpticStar, Artemis/Atik, FLI,
Apogee, Atik, ...
Pre-processing (incl. Bad Pixel mapping) & color
reconstruction
Stacking (incl. alt/az, Drizzle, & standard-deviation)
Stretching (incl. DDP), Color control, sharpening, etc.
Aimed at both capture and processing
Photoshop: Even older versions are
insanely useful
Beastly expensive new ($650)
Insanely useful and widely used as a result.
Older versions: CS, CS2, etc. available on eBay and are far
more affordable.
Astrophotography-Specific Actions
Noel Carboni ($20): http://actions.home.att.net
Peter’s Actions ($15): http://petersastro.50webs.com
Excellent astrophotography-specific tutorials
Alternatives are less expensive but
a lot less powerful
GIMP (Free): http://www.gimp.org
Cross-platform
8-bit/color (32-bit total) only
http://gimp-savvy.com/
PIXEL ($39): http://www.kanzelsberger.com/
Cross-platform
32-bit/color support
Paint.NET (Free): http://www.getpaint.net/
8-bit / color
$49,000 Question
Celestron C8-N ASGT
Baader MPCC
Canon Rebel XT
~$1700 Jim Solomon
Takahashi FRC-300
FLI ProLine + CFW
Paramount ME
~$50,700
Moral
Better equipment does not ensure better images.
Excellent images can be had from relatively affordable
equipment.
Think small and / or short when it comes to the scope and
you’ll have a lot more success (no C14’s at prime on DSLRs).
Learning how to image and process / learning how to best
use your gear and software as big if not bigger factors than
equipment cost.

Affordable DSO Astronomy

Transcription

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