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QSI 540 and Borg 101 ED f/4 Craig Stark, Stark Labs Improved Autoguiding Thursday, June 2, 2011

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Page 1: Craig Stark, Stark Labs

QSI 540 andBorg 101 ED f/4

Craig Stark, Stark Labs

Improved Autoguiding

Thursday, June 2, 2011

Page 2: Craig Stark, Stark Labs

QSI 516wsg andVixen R200SS

“100 minute exposure”Craig Stark, Stark Labs

Improved Autoguiding

Thursday, June 2, 2011

Page 3: Craig Stark, Stark Labs

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

Page 4: Craig Stark, Stark Labs

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

Page 5: Craig Stark, Stark Labs

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

Page 6: Craig Stark, Stark Labs

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

Page 7: Craig Stark, Stark Labs

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

Page 8: Craig Stark, Stark Labs

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

Page 9: Craig Stark, Stark Labs

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

Page 10: Craig Stark, Stark Labs

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

Page 11: Craig Stark, Stark Labs

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

Page 12: Craig Stark, Stark Labs

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

Page 13: Craig Stark, Stark Labs

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

Page 14: Craig Stark, Stark Labs

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

Page 15: Craig Stark, Stark Labs

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

Page 16: Craig Stark, Stark Labs

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

Page 17: Craig Stark, Stark Labs

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

Page 18: Craig Stark, Stark Labs

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

Page 19: Craig Stark, Stark Labs

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

Page 20: Craig Stark, Stark Labs

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

Page 21: Craig Stark, Stark Labs

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

Page 22: Craig Stark, Stark Labs

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

Page 23: Craig Stark, Stark Labs

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

Page 24: Craig Stark, Stark Labs

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

Page 25: Craig Stark, Stark Labs

PHD Guiding How-To

Thursday, June 2, 2011

Page 26: Craig Stark, Stark Labs

PHD Guiding How-To

Thursday, June 2, 2011

Page 27: Craig Stark, Stark Labs

PHD Guiding How-To

Thursday, June 2, 2011

Page 28: Craig Stark, Stark Labs

PHD Guiding How-To

Thursday, June 2, 2011

Page 29: Craig Stark, Stark Labs

PHD Guiding How-To

Thursday, June 2, 2011

Page 30: Craig Stark, Stark Labs

PHD Guiding How-To

Thursday, June 2, 2011

Page 31: Craig Stark, Stark Labs

PHD Guiding How-To

Thursday, June 2, 2011

Page 32: Craig Stark, Stark Labs

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

Page 33: Craig Stark, Stark Labs

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

Page 34: Craig Stark, Stark Labs

Garrett GraingerAtlas EQG, 6 minute subframe

Thursday, June 2, 2011

Page 35: Craig Stark, Stark Labs

Craig StarkTak EM-10, 100 minutes (10@10m unaligned)

QSI 516 wsg, Vixen R200SSThursday, June 2, 2011

Page 36: Craig Stark, Stark Labs

Craig StarkTak EM-10, 100 minutes (10@10m unaligned)

QSI 516 wsg, Vixen R200SSThursday, June 2, 2011

Page 37: Craig Stark, Stark Labs

Better SNR leads to more accurate locating of the star’s “centroid”

Thursday, June 2, 2011

Page 38: Craig Stark, Stark Labs

Better SNR leads to more accurate locating of the star’s “centroid”

Thursday, June 2, 2011

Page 39: Craig Stark, Stark Labs

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

Page 40: Craig Stark, Stark Labs

Good focus therefore is not always a good thing

Courtesy of Chris Peterson, Cloudbait ObservatoryThursday, June 2, 2011

Page 41: Craig Stark, Stark Labs

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

Page 42: Craig Stark, Stark Labs

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

Page 43: Craig Stark, Stark Labs

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

Page 44: Craig Stark, Stark Labs

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

Page 45: Craig Stark, Stark Labs

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

Page 46: Craig Stark, Stark Labs

Off-axis guiding is an effective way to remove differential flex

Thursday, June 2, 2011

Page 47: Craig Stark, Stark Labs

ETX-70 vs. Borg Guide Scope

Thursday, June 2, 2011

Page 48: Craig Stark, Stark Labs

ETX-70 vs. Borg Guide Scope

Thursday, June 2, 2011

Page 49: Craig Stark, Stark Labs

ETX-70 vs. Borg Guide Scope

Thursday, June 2, 2011

Page 50: Craig Stark, Stark Labs

ETX-70 vs. Borg Guide Scope

Thursday, June 2, 2011

Page 51: Craig Stark, Stark Labs

ETX-70 vs. Borg Guide Scope

Thursday, June 2, 2011

Page 52: Craig Stark, Stark Labs

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

Page 53: Craig Stark, Stark Labs

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

Page 54: Craig Stark, Stark Labs

Thursday, June 2, 2011

Page 55: Craig Stark, Stark Labs

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

Page 56: Craig Stark, Stark Labs

Before and after: Less post-guiding error after fixing this source of flex

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+$"

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$" 0" +$" +0" '$" '0" *$" *0" &$"

1234567"

84567"

9:26;<=1234567>"

9:26;<=84567>"

7 min / pixel

2.4 min / pixel

Thursday, June 2, 2011

Page 57: Craig Stark, Stark Labs

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

Page 58: Craig Stark, Stark Labs

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

Page 59: Craig Stark, Stark Labs

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

Page 60: Craig Stark, Stark Labs

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

Page 61: Craig Stark, Stark Labs

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

Page 62: Craig Stark, Stark Labs

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

Page 63: Craig Stark, Stark Labs

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

Page 64: Craig Stark, Stark Labs

Guiding on multiple stars can ameliorate the problem

Courtesy of Chris Peterson, Cloudbait ObservatoryThursday, June 2, 2011

Page 65: Craig Stark, Stark Labs

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

Page 66: Craig Stark, Stark Labs

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

Page 67: Craig Stark, Stark Labs

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

Page 68: Craig Stark, Stark Labs

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

Page 69: Craig Stark, Stark Labs

Questions?

Thursday, June 2, 2011

Page 70: Craig Stark, Stark Labs

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

Page 71: Craig Stark, Stark Labs

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