craig stark, stark labs
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QSI 540 andBorg 101 ED f/4
Craig Stark, Stark Labs
Improved Autoguiding
Thursday, June 2, 2011
QSI 516wsg andVixen R200SS
“100 minute exposure”Craig Stark, Stark Labs
Improved Autoguiding
Thursday, June 2, 2011
Overview
Why guide anyway? What does it do for us and how does it work?The basics: what do I need?
What makes a good guide scope?What makes a good guide camera?How do I talk to the mount to guide it?Guiding walk-through
Fixing problems and improving guidingFinding the star: SNR and focus effectsDifferential flex: Measuring and combating itGears: Backlash and loadingSeeing: Factoring it out
Thursday, June 2, 2011
Autoguiding is a good thing. Extending your subframe exposure duration improves your signal to noise ratio (SNR).
All mounts have some error and many mounts have a lot of error.
To image on these mounts and not have star trails, you must guide.
We guide to boost our image SNR
Thursday, June 2, 2011
Autoguiding is a good thing. Extending your subframe exposure duration improves your signal to noise ratio (SNR).
All mounts have some error and many mounts have a lot of error.
To image on these mounts and not have star trails, you must guide.
We guide to boost our image SNR
http://www.starrywonders.com/snr.htmlCopyright Steve Cannistra
2 4 6 8 10
Thursday, June 2, 2011
Autoguiding is a good thing. Extending your subframe exposure duration improves your signal to noise ratio (SNR).
All mounts have some error and many mounts have a lot of error.
To image on these mounts and not have star trails, you must guide.
We guide to boost our image SNR
http://demeautis.christophe.free.fr/ep/ap1200gto.htm
AP 1200 Periodic Error
Thursday, June 2, 2011
Autoguiding is a good thing. Extending your subframe exposure duration improves your signal to noise ratio (SNR).
All mounts have some error and many mounts have a lot of error.
To image on these mounts and not have star trails, you must guide.
We guide to boost our image SNR
Thursday, June 2, 2011
Autoguiding typically uses a simple feedback loop
Take a short-exposure image of a star or star field to serve as a reference.
Loop
Take a short-exposure image of a star or star field.
Compare to the reference image to determine how far you’ve moved and in what direction.
Send corrections to the mount’s motors to compensate for this motion get it aimed well again.
Guide corrections are typically after the fact. The mount has already deviated before the correction is made
Thursday, June 2, 2011
What do I need? Autoguiding Hardware
Guide Scope
Typically an inexpensive refractor
Shorter f/l can work well (e.g. 400 mm)
Often doubles as wide-field scope
Rings
Off-axis guider
Communication Link
Standard USB / serial link + ASCOM http://www.ascom-standards.org
ST-4 “autoguide” port
Thursday, June 2, 2011
What do I need? Autoguiding Hardware
Camera
Webcams
Long-exposure webcams & LPI / NexImage
Less-expensive astro-cams (e.g. Meade DSI Pro)
Camera designed for autoguiding
Decent-sized chip (e.g., 1/2”, 1.3 megapixel)
Mono
ST-4 output
Thursday, June 2, 2011
What makes a good guide scope: Light and sturdy
Must be able to be solidly mounted to your main OTA
Focuser must be sturdy
Consider lightweight guide cams
Consider ways of stiffening the focuser
Keep it light weight
Focal length can be short (~200 mm)
Optics, schmoptics....
Thursday, June 2, 2011
An old finderscope can be a guider
Old 8x50 finderscope
T-1.25” nosepiece
Hacksaw & file / Dremel
Metal epoxy
Solid (no springs) bracket
(opt) Baader T-1.25”
50 mm f/4 (200 mm f/l)
Hard-mount, very little flex
Thursday, June 2, 2011
Cheap and cheerful: Zhumell 60
Zhumell Ion 350x60 OTA ($90)
PVC (rings), screws
Old barlow w/lens removed (extension tube)
Glue
Michael Garvin
Thursday, June 2, 2011
What makes a good guide cam? Good SNR and light weight
At least 0.5 s real exposure durations. 1-5 s ideal
Monochrome
Good sensitivity
Lightweight
(optional) ST-4 output
(optional) Image quality
(who cares?) Bit depth
Q: Why not color? Sensitivity by ~70% Resolution cut as well.
Thursday, June 2, 2011
Good, inexpensive guide cams
Orion Autoguider (CCD Labs QGuider, QHY5, MagZero MZ-5)
1/2” Mono CMOS (QE=53%, extended IR, 1280 x 1024)
ST-4 output
4.4 oz, T-thread or 1.25”
$280
Meade DSI Pro
1/4” mono CCD
10 oz, ~$150-$200 used
The Imaging Souce / DMK
1/4” or larger CCDs - great planetary/lunar/solar
9 oz, Starting at $350
Thursday, June 2, 2011
Other, inexpensive guide cams
Other options:
Meade DSI: ~$100 used, long-exposure, but color
Long-exposure webcam: Long-exposure, but color and noise may be more than DSI
Anything else you may have...
Thursday, June 2, 2011
The High End
Fishcamp Starfish (www.fishcamp.com) $700 or $1000
1/2” Mono CMOS (same sensor)
ST-4 output, Regulated cooling*, 10 bit, 11 oz
Onboard CPU with noise-reduction & guide timing
SBIG ST-402ME (www.sbig.com) $1500
~1/2” Mono CCD (QE peak 85%)
Regulated cooling
ST-4 output, 16-bit, 20 oz
Starlight Xpress Lodestar (www.starlight-xpress.co.uk) $650
1/3” Mono CCD
Body is 1.25”, ST4 output
Thursday, June 2, 2011
Star images show varying SNR across cameras
Guider Roundup, AstroPhoto Insight volume 4, issue 5
SX LodestarAtik 16IC DSI II Pro
Thursday, June 2, 2011
DSI 2 Pro DSI 3 Atik 16IC
Lodestar Orion Starfish
Star profiles show varying SNR across cameras
Guider Roundup, AstroPhoto Insight volume 4, issue 5
Thursday, June 2, 2011
Two basic ways to move the mount
ASCOM: Free www.ascom-standards.org
ST-4
Built into the camera
Parallel port
ShoeString GPINT www.shoestringastronomy.com
USB Port
ShoeString GPUSB www.shoestringastronomy.com
Pierro Astro www.pierro-astro.com
Astrogene GC USB ST4 www.astrogene1000.com
Thursday, June 2, 2011
ASCOM: A generic driver language for Windows gear
Uses the normal serial or USB link to your computerized mount.
Programs (e.g., PHD) can “speak ASCOM” and drivers convert this into mount-specific commands.
ASCOM is a standard for Windows programmers to write drivers according to.
Many mounts supported.
Guiding support can be excellent, mediocre, or unavailable. Check your mount’s driver.
Thursday, June 2, 2011
Pulse-Guide commands are more accurate than start/stop commands
Simple Guide Command Sequence
“Start North Motor at Guide speed”
Wait 260 ms
“Stop Guide”
What if the onboard computer only reads a command every 0.25 second (some do)? Guide
pulse will be ~2-3x the asked-for duration.
Pulse-Guide Command Sequence
“Activate North motor at guide speed for 260 ms”Thursday, June 2, 2011
Overview
Why guide anyway? What does it do for us and how does it work?The basics: what do I need?
What makes a good guide scope?What makes a good guide camera?How do I talk to the mount to guide it?Guiding walk-through
Fixing problems and improving guidingFinding the star: SNR and focus effectsDifferential flex: Measuring and combating itGears: Backlash and loadingSeeing: Factoring it out
Thursday, June 2, 2011
Free Autoguding Software
PHD Guiding: Most cams supported, Windows & Mac
http://www.stark-labs.com
GuideMaster: Great LE-webcam support
http://www.guidemaster.de
GuideDog: “The Original”
http://www.barkosoftware.com
MetaGuide: Best support for short-exposure webcams
http://www.astrogeeks.com/Bliss
Thursday, June 2, 2011
PHD Guiding How-To
Thursday, June 2, 2011
PHD Guiding How-To
Thursday, June 2, 2011
PHD Guiding How-To
Thursday, June 2, 2011
PHD Guiding How-To
Thursday, June 2, 2011
PHD Guiding How-To
Thursday, June 2, 2011
PHD Guiding How-To
Thursday, June 2, 2011
PHD Guiding How-To
Thursday, June 2, 2011
Overview
Why guide anyway? What does it do for us and how does it work?The basics: what do I need?
What makes a good guide scope?What makes a good guide camera?How do I talk to the mount to guide it?Guiding walk-through
Fixing problems and improving guidingFinding the star: SNR and focus effectsDifferential flex: Measuring and combating itGears: Backlash and loadingSeeing: Factoring it out
Thursday, June 2, 2011
PHD: Accuracy?
Just how accurate is software like PHD?
How well can it find a star?
What fraction of a pixel can it guide to?
How long can you expose with it?
Answer: It dependsThursday, June 2, 2011
Garrett GraingerAtlas EQG, 6 minute subframe
Thursday, June 2, 2011
Craig StarkTak EM-10, 100 minutes (10@10m unaligned)
QSI 516 wsg, Vixen R200SSThursday, June 2, 2011
Craig StarkTak EM-10, 100 minutes (10@10m unaligned)
QSI 516 wsg, Vixen R200SSThursday, June 2, 2011
Better SNR leads to more accurate locating of the star’s “centroid”
Thursday, June 2, 2011
Better SNR leads to more accurate locating of the star’s “centroid”
Thursday, June 2, 2011
Number of pixels star is sampled on can affect location accuracy
Sampling at higher rates gives more pixels to average noise over.
Very prone to noise distorting
the estimate
Thursday, June 2, 2011
Good focus therefore is not always a good thing
Courtesy of Chris Peterson, Cloudbait ObservatoryThursday, June 2, 2011
Good focus therefore is not always a good thing
Courtesy of Chris Peterson, Cloudbait Observatory
RMS
0.054 pix
0.024 pix
0.032 pix
0.153 pix
Distribution of Centroid Error
Lowest error
Very tight stars performed poorly
Thursday, June 2, 2011
Overview
Why guide anyway? What does it do for us and how does it work?The basics: what do I need?
What makes a good guide scope?What makes a good guide camera?How do I talk to the mount to guide it?Guiding walk-through
Fixing problems and improving guidingFinding the star: SNR and focus effectsDifferential flex: Measuring and combating itGears: Backlash and loadingSeeing: Factoring it out
Thursday, June 2, 2011
Differential flex is a common source of guiding errors
Eliminate all flex
Flex in your guide scope’s mounting
Flex in your main OTA’s mounting
Flex in your two focusers
Flex in your secondary
Flex in mounting the cameras
Go back and try some more to eliminate flex
Did I mention you should eliminate flex?
See why the guidescope and camera should be light?
Thursday, June 2, 2011
Measuring Flex’s Effect
Get guiding up and going. Guiding program must be keeping the star on target on average (± a few pixels).
Take a series of 15 second to 1 minute exposures for 30-60 minutes
Either:
Stack the frames without alignment (as if darks)
Stack the frames with alignment and plot the amount of shift needed for each frame.
Distance = sqrt(dx*dx + dy*dy)
Thursday, June 2, 2011
Flex-induced error limits length of exposures
Post-guiding Error
y = 0.4683x - 1.0786
y = 0.0589x + 0.2176
-5
0
5
10
15
20
25
30
0 10 20 30 40 50 60 70 80
Time (m)
Pix
els
Setup 1Setup 2Linear (Setup 1)Linear (Setup 2)
1 pixel in 2.1 min
1 pixel in 17 min
Thursday, June 2, 2011
Off-axis guiding is an effective way to remove differential flex
Thursday, June 2, 2011
ETX-70 vs. Borg Guide Scope
Thursday, June 2, 2011
ETX-70 vs. Borg Guide Scope
Thursday, June 2, 2011
ETX-70 vs. Borg Guide Scope
Thursday, June 2, 2011
ETX-70 vs. Borg Guide Scope
Thursday, June 2, 2011
ETX-70 vs. Borg Guide Scope
Thursday, June 2, 2011
Where am I flexing?
Perform the Starkian Wiggle Test
Setup on a bright star for 1-3 second exposures
Wiggle a part on your rig with a consistent wiggle
Guide tube
Guide focuser
Guide camera
Main camera
...
Examine the size of the error in your images
Thursday, June 2, 2011
Fixing the ETX-70’s Flex
Problems
Focuser moved the lens and this had a lot of flex
Focuser / flip mirror housing wasn’t securely mated to main tube.
Camera could wiggle in 1.25” holder
Fixes
Found focus position and liberally applied glue to fix lens in place and solidify flip mirror housing. (Recall, $100 spent on scope...)
Shifted to T-thread mounting of camera.
Thursday, June 2, 2011
Thursday, June 2, 2011
Still not quite perfect, but removing just this one source of flex had a clear effect on overall accuracy.
Before and after: 15 min exposures show less flex-induced error
Thursday, June 2, 2011
Before and after: Less post-guiding error after fixing this source of flex
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7 min / pixel
2.4 min / pixel
Thursday, June 2, 2011
Overview
Why guide anyway? What does it do for us and how does it work?The basics: what do I need?
What makes a good guide scope?What makes a good guide camera?How do I talk to the mount to guide it?Guiding walk-through
Fixing problems and improving guidingFinding the star: SNR and focus effectsDifferential flex: Measuring and combating itGears: Backlash and loadingSeeing: Factoring it out
Thursday, June 2, 2011
RA, Dec, & Backlash
Backlash: When gears reverse direction, some extra rotation is usually needed to get things to mesh again.
Right ascension will never reverse during guiding.
Declination may reverse
Typically, the star will drift one way.
If you’re still inside the backlash, the guide program’s correction doesn’t show up. A bigger error is registered the next time, leading to a bigger movement.
Once backlash is cleared a large jump can still made since the last error was very large.
⇒ Try to stay on one side of the worm
Thursday, June 2, 2011
Fix your gears or you’ll never guide well
Clean up / tune up your mount
Remove debris from worms
Remove backlash physically
Never use RA backlash compensation
If you use Dec backlash compensation, use less than needed
Try a slight eastward weight-bias (gravity resisting your mount’s turning)
Motors will run smoother
Less prone to wind / backlash issuesThursday, June 2, 2011
Overview
Why guide anyway? What does it do for us and how does it work?The basics: what do I need?
What makes a good guide scope?What makes a good guide camera?How do I talk to the mount to guide it?Guiding walk-through
Fixing problems and improving guidingFinding the star: SNR and focus effectsDifferential flex: Measuring and combating itGears: Backlash and loadingSeeing: Factoring it out
Thursday, June 2, 2011
Use longer exposures to average out atmospheric turbulence
Unguided Error
100 200 300 400
-4
-2
0
2
4
6
8
3s0.1s
TimeDec
Err
or (a
rcse
c)
Thursday, June 2, 2011
Use longer exposures to average out atmospheric turbulence
Residuals after LOWESS fit
100 200 300 400
-3
-2
-1
0
1
2
3 3s0.1s
Time
Dec
Erro
r (ar
csec
)
Thursday, June 2, 2011
Low frequency effects can still distort relative star positions
Star Separation vs. Time
147.0
147.5
148.0
148.5
149.0
149.5
150.0
150.5
151.0
0 1 2 3 4 5 6 7 8 9 10
Time (s)
Dis
tanc
e (a
rcse
c)
Courtesy of Chris Peterson, Cloudbait ObservatoryThursday, June 2, 2011
Guiding on multiple stars can ameliorate the problem
Courtesy of Chris Peterson, Cloudbait ObservatoryThursday, June 2, 2011
Once all this is done, tweak your software
Learn to use your guide software’s tools.
Plot your accuracy in real-time or post-hoc
Tweak settings in your software
Aggressiveness
Hysteresis
Minimum motion threshold
Handling of declination errors
Thursday, June 2, 2011
Once all this is done, tweak your software
Learn to use your guide software’s tools.
Plot your accuracy in real-time or post-hoc
Tweak settings in your software
Aggressiveness
Hysteresis
Minimum motion threshold
Handling of declination errors
Thursday, June 2, 2011
Once all this is done, tweak your software
Learn to use your guide software’s tools.
Plot your accuracy in real-time or post-hoc
Tweak settings in your software
Aggressiveness
Hysteresis
Minimum motion threshold
Handling of declination errors
Thursday, June 2, 2011
ConclusionsGuiding will make your images better.
You don’t need a fancy camera to guide with. You want a lightweight, reasonably sensitive one.
You don’t need nice, pristine optics to guide with. You want something lightweight and rigid.
Enemies
Guide star quality: Decent SNR and sampling, defocus?
Differential flex: Tighten everything and consider OAGs
Drive train: Clean gears and get to mesh well. Use a slight eastward imbalance and don’t overload
Seeing: Use long enough exposures / multiple stars to guide out the mount’s actual errors.
Thursday, June 2, 2011
Questions?
Thursday, June 2, 2011
RA: 0.16 pixel error (RMS)350mm guide scope, 3"/pixel
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1 31 61 91
Frame (2s)
Err
or (p
ixel
s)
Michael GarvinCelestron ASGT, HPN 8” f/5, Zhumell 60 refractor
CCD Labs QGuide & PHD Guiding
Thursday, June 2, 2011
Michael GarvinCelestron ASGT, HPN 8” f/5, Zhumell 60 refractor
CCD Labs QGuide & PHD GuidingRA: 0.48 arcsec error (RMS)
350mm guide scope,
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
1 31 61 91
Frame (2s)
Err
or (a
rcse
c)
Thursday, June 2, 2011
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