Introduction to Pre-Pointing

Ted BlankIOTA Annual Conference

Las Vegas, NVOctober, 2015

Acknowledgements

• Thanks to Ernie Iverson and Steve Preston for reviewing this presentation

• Thanks to Scotty Degenhardt for documenting the pre-point process

• Thanks to Bill Gray from ProjectPluto for creating 9 versions of Guide, each better than the last

One Good Reason To Pre-Point

• Pre-pointing is a good way (and perhaps the only way) to be sure you are on the target star in an otherwise featureless field of view

Pre-pointing is “pointing your telescope at some point in the sky such that a particular star will pass through its FOV at a particular (later) time.”

As we all know, stars follow (apparent) paths like this one across the sky.The path is simply the star’s line of declination.

This means that if you point your telescope at some point on that line, all the stars east of that point on that line of declination* will eventually pass through your FOV.

x

Graphic: Stellarium

* at least until the Sun comes up

Hmmm… A Particular Star at a Particular Time?That sounds quite useful!

• Which star?– The one being occulted!

• What time?– The predicted time of the occultation for

your location!

But what’s really happening?Of course the stars are not really moving to the west – the Earth is turning,

carrying your telescope to the east.

To say this another way, if you point your telescope at some point on that line, your telescope will eventually see all* of the stars to the east of that point as the Earth turns.

*that is, all stars within your FOV and bright enough to be visible with your telescope’s aperture

x

Graphic: Stellarium

So, to capture an occultation, just follow these simple* steps:

Point your (non-tracking) telescope at the right point in the sky

at the right timeso that sometime later

at exactly the predicted time of the occultationthe earth will have rotated just the right amount

for your telescope to be pointingdirectly at the star being occulted.

Pre-point charts help you find thatright point at the right time!

* slight exaggeration

Why not just star-hop to the target star?To quote Scotty Degenhardt (circa 2010):

• 9 times out of 10 the target star is faint and/or located in an awkward star hopping point in the sky. Prepointing allows you to pick the most convenient time to aim the scope and to pick the brightest star near the most recognizable asterism (saves an INCREDIBLE amount of time, blood, sweat, tears, hair follicles, adds years to you life, takes years off your wife…. Yada yada you get the picture).

• Starting at the target star instead of prepointing means that you have to stay on the target star, i.e. your polar aligning better be good (lotsa deployment time there), your motor drive better not have errors (lotsa $ there), and your batteries better last, even in the cold… (lotsa prayers there)!

• Prepointing uses THE most accurate drive available to man… the Earth’s rotation.

• LiMovie reduction with a prepointed target star drifting through at a steady rate is a breeze to accomplish using the “Drift” Tracking Method.

Scotty Degenhardt’s Original How-To Page

• http://scottysmightymini.com/tools/How_to_make_Guide8_prepoints.htm

• Use it to get started (works for Guide9 as well)• I will discuss my experiences following his

instructions and using my own charts in the field

• Guide9 is now public domain and may be freely copied and distributed (per the author, Bill Gray, www.projectpluto.com )

When might you NOT want (or need)to pre-point?

• If you:– have an accurately polar-aligned home

observatory setup– which can reliably and accurately go-to

any point in the sky by entering RA and Dec

– with aperture and FOV large enough to see enough neighboring stars to confirm you are on the target star

– and you don’t plan to travel or go mobile

– Or if your scope FOV is too narrow to allow sufficient drift time across the camera chip

– Steve P. recommends having enough drift time to include 3-sigma of the uncertainty in the event time.

Use a focal reducer to expand your FOV and make your star images smaller / brighter

Courtesy Rich &Sue Schueller

The Time The Event Is Predicted To Occur Will Depend On Your Location

To get your exact time, use the “View Details On The Web” link in OccultWatcherand select the “Detailed Info” link there, using the longitude closest to your site.

Remember, West Longitude is Negative by Convention

LasVegas

U.T.

Setting up Guide for the event• Open Guide 9• Hit alt-J and enter “1” in “Number of test flag” to enable trails*• Set Display to Chart Mode (white background)• Display CCD frame for your planned camera and scope focal length

• Divide FL by 2 if you are using a 0.5X focal reducer• Set location to your planned observing location• Set time to predicted time of the event

– To nearest second– Always in UTC

• Add comment to Legend– E.g., 2015-10-17 (215) Oenone

• Go to asteroid or target star• Confirm asteroid “X” is on top of target star• Right click and view info for target star (this selects it as “last object viewed”)

* This is a toggle – you only need to do this once

Setting up Guide for the event…• Select Animation -> Add a trail• Clear old trails if present• Add trail (my preferences):

– Step size: -1 min– Round to nearest step– Index marks– Index freq: 1– Time labels: on– Time label freq: 4 (or your choice)– Number of steps: 450 (or however many minutes between sunset and the

event)– Increase Guide’s “Screen Font” and “Printer Font” sizes to make the

times on the trail readable– Too many labels may make it difficult to know which label goes with

which tick on the line

If the last object selected was the asteroid instead of the star, you will seesomething like this. It is the trail of the asteroid, not useful for our purposes.

Oops!

Success!

Level 3, down to m6.5

This is the line your telescope willfollow through the sky as the Earthrotates.

A Closer Look

Asteroidover starat 08:30:16 UTC

CCDFrame

Pre-point locations and times

Level 3, down to m6.5

Level 4, down to m8.5

Looking for good pre-point candidate stars

A good pre-point candidate will bea bright star with a declinationclose to that of the target star

(meaning close to the blue line).

Level 5, down to m10.5

Zooming In Some More…

Right-click on the star to identify it as Lambda Psc

CCDFrame

(Your LCD TV Screen)

For this scope and camera: ~24 arcminutes

Right-click on trail above the star to find exact pre-point time for this star.

“If Lambda Psc is here in your field of view at 7:49:45 UTC then Oenone andthe star it will occult will be centered in your FOV at 8:30:16 UTC, about 40minutes later.”

Double-check the pre-point time with Occult 4’s list of “pre-point stars” for the event.

It matches (within 1 second)!

How long will it take stars to drift completely across your FOV?

• Depends on • width of your field of view• declination of star (drift rate reduced by cos(dec))• direction the stars drift across the chip (longest on diag)

• Apparent motion of stars across the sky is:360 degrees per day= 15 degrees per hour= 0.25 degrees per minute (or 4 minutes per degree)= 15 arcminutes per minute

• So a FOV of half a degree (= 30 arcminutes) will give you 2 minutes of drift time at the ecliptic (more above or below based on cos(dec))

End of Part 1