About John A. Blackwell

Director of Phillips Exeter Academy Grainger Observatory, research astronomer, astronomy & education consultant.

A Visit to SALT: The South African Large Telescope

During the last week of June 2018, I was in South Africa working with a school as they brought focus to their interests in students-centered, discussion-based learning. My host, Shaun Hudson-Bennett, a member of their Maths Department is a wonderful person and shared an interest in visiting the South African Large Telescope. We were based in the town of Somerset just east of Cape Town… about a 4 hour drive south of SALT. SALT is located in the town of Sutherland in a region that is largely rolling hills and flat open terrain reminiscent of what we would call “high chaparral” here in the American West. Sandy and silty soil is mixed in with scrubby shrubs and prickly plants that all struggle to get water to survive. The geology of the area is lovely to behold: layered metamorphic and sedimentary mountains, and one can even find a volcano remnant. Much of the area is a fossil lover’s dream-scape.

DSC_1179.JPG

Heading out of Cape Town, one must first get through some of the mountainous regions. It is very colorful, even in winter, with grape arbors and other fields planted.

DSC_1186.JPG

Headed into the high chaparral nearing SALT. The rods are very straight for a good length of time.

DSC_1187.JPG

Entering the town of Sutherland. Lots of eateries and dark sky observing sites are available here.

DSC_1204.JPG

Some of the smaller domes on the summit.

SALT itself is surrounded by what I call dark sky country: there are dark sky reserves in the area, and most businesses in the town of Sutherland are very much aware of the value of their dark sky commodity.  Small bed-and-breakfasts are spotted throughout the town, each advertising their private star-gazing pads and fields. Climbing the road out of Sutherland, one arrives at the summit region of the South African Astronomical Observatory, home to SALT and quite a few other facilities owned both by South Africa and a number of other foreign consortiums (Japan and Russia to name but two). At 1798 m (5899 ft), SALT is well placed for its work, primarily spectroscopy. The air is not too thin, and walking around was easy enough.

DSC_1237.JPG

The SALT dome and alignment tower.

SALT Facts:

  • Location: Sutherland, South Africa in the semi-desert region named the Karoo.
    • Latitude: 32°22′34″S
    • Longitude: 20°48′38″E
  • Altitude: 1798 meters (5899 feet).
  • Optical Design: Optical with a total of 91 hexagonal 1m diameter mirrors making a composite 11m diameter hexagonal primary.
  • First Light in September 2005.
  • Primary Science: Spectroscopy, polarimetry, imaging:
    • SALTICAM
    • Robert Stobie Spectrograph (RSS) (née Prime Focus Imaging Spectrograph).
    • The Berkeley Visible Image Tube camera (BVIT).
    • Fiber-fed High Resolution Spectrograph (HRS).
  • Website:  http://www.salt.ac.za/
IMG_1270.JPG

My host, Shaun (right), Frigid (the Penguin) and myself in front of the SALT primary mirror. The mirrors are very thin and lightweight. Combined they equate to an 11m diameter primary.

DSC_1229.JPG

The instrument payload package at prime focus (black painted objects). These move for tracking in both RA and Dec by sliding on rails.

The optics of the telescope are most interesting. The system has a fixed altitude for the primary, so pointing and tracking are not done by moving the primary and all attached equipment, as is done with most other systems. At SALT, tracking is accomplished by moving the equipment package at primary focus. So, to image, the camera system is slewed across the top of the observatory as the target moves through the field. The telescope can be rotated in azimuth to view other regions of the sky. Each segment of the primary is adjustable using remotely controllable actuators. This allows the system to be aligned rapidly. Alignment is accomplished using what I call a collimation tower, an interesting structure that looms high on the outside of the dome. Essentially the telescope is rotated until the tower-top is at the center of curvature for the primary. Each of the 91 mirrors of the primary are then tipped and/or tilted until they are precisely aligned to form a spherical primary system using lasers sent from the tower to each mirror on the primary.  To maintain alignment, the whole structure is air-conditioned to maintain conditions as close to ambient as possible. The interior was pretty chilly the day we arrived, as it was winter there (in June).

DSC_1231.JPG

The whole telescope can rotate in azimuth on air-bearings like this one. Once aimed, the bearings are deflated and the telescope remains static throughout integrations. Only the instrument payload set moves during data capture.

DSC_1223.JPG

Another view of the massive primary assembly.

Visiting?  They do allow visitors during daylight hours, twice daily (at this time), and reservations are very highly recommended. More information may be found on the SALT website:   https://www.salt.ac.za/

 

Testing a Heliochronometer

IMG_1108.jpg

Pilkington & Gibbs Heliochronometer

Back in the early 1900s, heliochronometers were all the rage for those seeking some extraordinary piece for their garden or other vista-filled location. This particular instrument was built by the Pilkington & Gibbs company of London. Only about 1000 of these were made by the company – sadly much of the brass was melted down to support the war effort and the making of artillery shells. This unit is not only intact, but is in 100% working condition, another rarity, as many of these were left exposed to the elements where they would slowly get coated by a green oxidation patina.

IMG_1098-1.jpg

Alignment with True North

Using these instruments was quite simple once it had been properly installed. Installation requires that their primary rotation axis be parallel to the rotation axis of the Earth, aimed at true north and with the correct angle to account for one’s latitude.  I accomplished this easily enough with the gentle use of a wrench, a screw driver and compass, being sure to dial in first the correct magnetic variation for the compass for our location.

IMG_1096.jpg

Setting the date.

In use, one first sets the calendrical date on the smaller inner dial: June 16th. This slides the gnomon back and forth to account for the equation of time(!): quite a remarkable design!

projectedsun.jpg

Before and after alignment: Casting the pinhole image of the Sun onto the screen by rotating the main dial.

One then rotates the larger dial to project the dot of sunlight cast through a hole in the gnomon (sight vane) onto the opposing vertical line on the center of the screen vane. One then reads the hour and minute on the brass gauge on the outer edge of the main dial. Simple and accurate! In this case, it was within two minutes, likely because I have yet to adjust the minute readout-scale to its proper position for our longitude within the time zone.

IMG_1104-1.jpg

Aligned and reading very close to the correct time… and yes, the watch is set to local daylight savings time, one hour ahead of the solar time read on the heliochronometer.

IMG_1102.jpg

Reading the minute across from the hour. Very accurate!

Polar Alignment with a CCD Imager

This is not for the faint of heart. You need to know how to use a CCD imager and your telescope. I write this here, because there are so many websites with INCORRECT information about this process. Some miss the point completely. All this does is mess with your head and mess with your gear. This method works. Trust me.

Polar Alignment with CCD

This is for permanent installations, as it can take up to 3 or 4 hours to complete the whole process to the point where your mount is perfectly polar aligned. The result is that you no longer have to worry about declination drift during imaging. You still have to worry about periodic error from your mount’s drive, but any good autoguider will take care of that. These instructions assume that you know how your CCD is aligned on the scope (which way is N, E, S, W on the images). In all cases, you will be monitoring North-South changes in a star’s position on your CCD images. IGNORE any east-west drift.

Polar Axis Altitude Alignment:

1. Manually polar align your mount to the best of your ability. Some mounts come with polar alignment scope in the polar axis shaft. Use it! This will actually get you close enough to take 5 to 10 minute integrations without doing the rest of this list! You will depend on your autoguider.

2. Aim your telescope at a star low on the eastern horizon and on the celestial equator (close is good). Faint stars are ok. You do not want them blooming.

3. Take 15 30-second long integrations of that star. Some people prefer to take one 5 minute long shot of the stars to see its path, but it works just as well to see the star’s position move due to drift via snapshots. DO NOT move the mount in any way during these shots. Just let the RA motor do its job.

4. Now to fix your drift! In this step you move the mount’s permanent polar axis depending on the way the star has drifted on your images.

a. If the star drifted north on your images, then move the mount’s polar axis down a tiny(!!) bit.

b. If the star drifted south on your images, then move the mount up a tiny(!!) bit.

5. Repeat steps 2 through 4 until you see NO DRIFT in 5 minutes. Want better? Go for longer. You will find that you can use the centroid tool in your image processing software and get excellent results in about 30-40 minutes.

Polar Axis Azimuth Alignment:

1. Aim your telescope at a star on the meridian and on the celestial equator.

2. Take 15 30-second long integrations of that star. Some people prefer to take one 5 minute long shot of the stars to see its path, but it works just as well to see the star’s position move due to drift via snapshots. DO NOT move the mount in any way during these shots. Just let the RA motor do its job.

3. Now it is time to fix your polar alignment’s altitude to perfection! Be sure to make VERY SMALL adjustments to the polar axis at this time.

a. If the star drifted to the north in your images, then slightly move the mount to the east.

b. If the star drifted to the south in your images, then move the mount to the west.

4. Repeat steps 2 and 3 above until there is NO DRIFT in your 5 minute series.

Congratulations! Your mount is polar aligned. You will likely not need to adjust this again until you swap out telescopes, have an earthquake (more common than you think!) or someone fiddles with a knob or two on your mount (also not all that uncommon as you think).

Taking Flat Fields

Introduction:

When taking CCD images, and particular, when trying to use those images for scientific purposes, it is important to reduce the amount of unwanted signal and unwanted noise from each image. Optical path “noise” (some of which is actually signal), is such a problem that many astronomers really never come to grips with it. Their data suffer, and the end result is poorer science. This treatise will spell out the simplicity of taking good flat fields to reduce optical path noise and CCD sensitivity issues and will also walk you through a couple of methods to get flats done.

Optical Path Noise:

Telescopes, CCD chips and filters all block light as well as transmit light. They also harbor dust, finger prints, and other unwanted shadow producing things in the light path. The result of such optical path obscuration is an unevenly illuminated CCD chip. This is a real nightmare for anyone doing photometry, in which a standard star of known brightness might measure a bit faint one night because it was being imaged on top of a dust speck on the filter glass! Optical path vignetting and other physical path obstructions will also cast large, non-discernable shadows onto your CCD, causing poor even illumination.

CCD Sensitivity:

In the spatial realm both on the multi-pixel and single-pixel level, a CCD chip will display uneven sensitivity to incoming light. This can depend on the thickness of the substrate and uneven cooling among many other issues. This creates issues very much like those mentioned already in the optical path noise section above.

The Solution:

Take flat field images and divide them out of your images. A flat field is an image taken of an evenly illuminated object like the dusk sky, or a special illuminated white card hanging on the wall of the observatory. These images are taken through the telescope:

· at the same temperature as your nightly work,

· through the same filter/s as your nightly work,

· at the same focal point and at the same rotational angle being used all night,

· and with integration times to allow the flat to reach an average of between 20 to 50% full well capacity of your CCD chip. Flat images should never bloom, but should also not be less than a second in integration time.

For precision work, 20 to 30 or more flats through each filter should be taken each night you are collecting science data. Each flat of a given filter should then be averaged together to create a master flat which is then divided out of your light frame on a pixel by pixel basis. These details are usually all handled automatically by your software. I will assume you are using MaxIm DL software revision 5+ for the following examples.

In Practice – Taking Sky Flats:

Taking flats is easy. Here is a step-by-step method to take sky flats which has worked well for me for years. You need no special equipment other than that you already own to take CCD images.

1. Wait until the sun is setting, but still just above the western horizon.

2. Turn on your observatory: EVERYTHING. The mount, the fans, the CCD, the PC, lights normally on, etc.

3. Cool down your CCD to the night time working temperature. Wait 10 minutes for it to settle to the working temperature.

4. If you are using filters, you should take flats in order of densest filter to most transmissive. I work in the order of Ha, B, V, R, then lastly I. Set your filter wheel to the first filter.

5. Set the focal point of the system. Minor adjustments through the night are ok in order to allow for temperature changes of your optical tube assembly. Do not make changes more than a mm or so. You’ll have to take new flats if you do make larger changes.

6. Set the CCD camera’s angle to the system. Leave it here all night.

7. Point your telescope at the blue sky towards the western side of the meridian. Avoid areas of sky where there are bright stars (which will not be visible yet, as the sun is still up).

8. Take a 1 second integration.

9. Once it downloads, use MaxIm DL’s Information Window in Area Mode to inspect the average pixel count of the image. If it is too bright, some pixels will be saturated, and you will have to wait until the sun sets some more. If you have an image that reads about 20-50% of the full well count, then proceed immediately to the take a series of flats.

a. Generally the Sun is at a point in the west where its light might just be still touching the top of the treetops on the eastern horizon. Stars are not visible to the eye, nor generally to the camera yet.

b. My full well count with an SBIG camera is 65535, so I aim to get flats with an area average of 20000.

c. You can use MaxIm DL’s image series command to take a set of flats with any given filter. Repeat all the steps above as needed until you have flats for what you need.

10. You can use these flats for as long as you wish, but for precision work, flats are taken every night and sometimes in the morning after your imaging is complete. If you are not after precision work, then taking flats once a week is enough. Some would say that’s sacrilege!

A helpful hints:

If you want to start taking flats earlier, just to give yourself some time, cut out sheets of frosted mylar (used in silkscreening) to cover the objective of the telescope. Use 5 to 10 sheets of this milky white plastic material to basically dim the incoming sky brightness to the optics.

You can take flats while aiming at evenly illuminated clouds. This is ok!

I have gotten away with as few as 6 averaged flats. For truly accurate work, I have gotten up to 40 averaged flats.

Here is a flat. Look how ugly it can be! The donuts are dust. The edge darkening in the corners is caused by vignetting.

flat.jpg

A typical flat field frame. This one is of the evening sunset sky taken through an H-alpha filter. Note the dust donuts and uneven illumination of the chip. This is what we use to correct our images for these issues.

A Meeting of the Moon and Venus

Mark your calendars for June 16th 2018: the Moon and Venus will slowly get to within 2.3 degrees of each other making for a lovely sight. All you have to do is head out in the early evening just after sunset and look to the lower western horizon.  You might also catch some bright later-Winter stars as well.  Here’s the view (click to make larger):

Venus and the Moon June 16th 2018

Venus and the Moon June 16th 2018 looking west after sunset.

While you are enjoying the view, take a close look at the Moon. You might just see something special, some Earth-Shine.  When the unilluminated portion of the Moon is visible, this is due to sunlight reflected off the Earth, bouncing back to light up the Moon, and making it appear a faint eerie blue color. Binoculars will make this really prominent. Enjoy!

Spring: the time for planets. They’re Back!

Now mid-March 2018 and there are planets in the sky!  Here are some of the notable moments.

If you look to the west right after sunset you will catch bright Venus and fleeting (and fainter) Mercury. On March 18th, just after sunset, you might also be able to catch the very young, sliver moon, low on the western horizon.

March 18th 2018 looking west right after sunset.

March 18th 2018 looking west right after sunset.

Are you and early riser? Then you will be able to catch the other bright planets, Mars, Jupiter and Saturn.

Jupiter, Mars, Saturn and the Moon visible at about 5:30am looking southeast on March 12th 2018.

Jupiter, Mars, Saturn and the Moon visible at about 5:30am looking southeast on March 12th 2018.

 

HDR of Eclipse Totality

With some more time back at the regular computer, I have had the ability to merge some of the eclipse images together to make a solid HDR image of totality. This is what I call quite the Photoshop workout. This is a merge of 12 images ranging from innermost corona out to the outermost corona that did not wipe out the moon’s dark disk.  Some 20 hours went into this. The star to the lower left is Regulus in Leo.

2017-08-21 Eclipse

HDR of the 21 August 2017 solar eclipse.

With just a little more editing, the image can come out even better…..

2017-08-21 Eclipse Totality HDR refined.

2017-08-21 Eclipse Totality HDR refined.

Totality from Casper, Wyoming

The eclipse was a perfect success! Clear skies prevailed throughout totality, with forest fire smoke and clouds coming in after it was all done. The temperature dropped from 88 to 68F, stunning! The corona was larger than those I had seen before. We had prominences and more. MANY images to deal with and not fast internet, so please be patient while I get these things edited and out there. It might take a while… a week or more. In the meantime, here are a couple of unedited shots.

Casper was most generous: we had an excellent space from which to observe: green grassy fields on to the hill to the east of town. The parking lot lights and grass watering sprinkler systems had been disabled for the day. The medical center even provided a lunch at the end!

Mid Corona 2017

Totality: This is the mid-corona using a longer exposure to show the fainter regions.

 

Inner corona 2017

Totality: The inner corona. Note the fain, pink solar prominences on the right limb.

 

Partial stage 2017

Partial stage as the eclipse begins. There are a few sunspot groups to see there.

 

Diamond ring 2017

The diamond ring as the eclipse reaches totality.

A Casper Saturday: People Arrive!

The town of Casper, WY is now hopping along. A lot more people are here today. We started out with a visit to the Geology Museum on the southwest side of things. They have an excellent display on geologic time along with representative minerals and fossils from each period/era. They also have their very own T.rex skeleton, a nice surprise. It is being slowly picked out of the encasing rock….

So, there was a lot of good paleontology to enjoy there for sure!

Back into town, the place is swinging. Lots of people wandering about with geeky T-shirts… yes, these are eclipse watchers for sure. The town has closed off the central area for shops to show off their goods, for people to mill about, to have a quick bight to eat/drink, and relax in the summer sun, a hot summer sun, pushing to 90F. The skies today: crystal clear. Absolutely lovely.

Arrived: Casper Wyoming

All, I am safely here at Casper, Wyoming, right on the line for the eclipse. We DID have rooms in the local motel (YAY!) and have been rambling through town a little to get oriented. The town is divided in a couple of neat ways: the Platt River runs through as does a major train route frequented by the BNSF RR and others. The town has a basic grid layout with a couple of highways to help with faster circumnavigation. It is clear, sunny and hot, and the weather is supposed to stay that way through the eclipse. Restaurants and businesses are all out in full force with eclipse gear: shirts, hats, pins, logos, stickers, drinks. One can not make way through town without seeing some reference or another. Yesterday as we drove into town, there was little in the way of traffic, so I surmise that there will be a LOT of traffic today and tomorrow. In short: ALL is GO for eclipse 2017 here.