EbolaBooze’s Astro Corner: Eye of Newt(onian) and Banks of Fog

Seeing is crap and the weather’s a dog.

Half a point to whoever gets the reference.

So! Still cloudy and windy, and seeing in the range of 4-5 arc seconds in the clear breaks. Here’s a (somewhat) recent synoptic chart for those interested.

IDY00030.201412061200

It’s summer now, for the record.

I did promise a breakdown of what Newtonian telescopes (Newtonians, Newts) were good for, and here it is!

 

Newtonian Telescopes

Named after their inventor; Sir Issac Newton, mathemator of the letter x, lord of apples, the deadliest son-of-a-bitch in space, the Newtonian is a versatile design, and one of my favourite types of telescope.

A classical Newtonian is a purely reflective system, composed of a parabolic (or spherical, or hyperbolic) concave primary mirror, and a flat elliptical secondary mirror angled at 45° to the light path.

16in Newt raytrace

Newtonians in their purely reflective form suffer from coma, which is the tendency for point sources of light (like stars) that are not in the centre of the field to be distorted, appearing to have a tail (coma) like a comet. In Newtonians, this is due to the fact that though a parabolic mirror will focus light rays to a common focus; this is true only for parallel, on-axis rays. Light rays from objects that are not on the optical axis i.e. the light rays strike the primary mirror at an angle, are not focused to the same point. This is only really a big problem on Newtonians with a focal ratio of f/6 or faster (lower), as the curve of the mirror increases with decreasing focal ratio.

The second major drawback of Newtonians is the fact that they have a central obstruction, caused by the secondary mirror required to reflect the light rays into the eyepiece/camera. The decreases the effective light-gathering area and the contrast obtainable by a scope of any given size, and is dependent on how large the central obstruction is. Again, the faster the focal ratio, the larger the secondary obstruction as the light cone has steeper angles and the mirror needs to be larger to intersect the whole of it.

Newtonians are also rotationally asymmetric, what with the focuser hanging off the side of the tube. If you’re mounting one on a German equatorial mount, more often than not the eyepiece will end up in an awkward position. For astrophotography though this doesn’t matter as much since the tube can be rotated to face the camera towards the mount axis.

That being said, the Newtonian is a design just as versatile as a refractor, and cheaper to boot. They can be had in apertures from 152mm (6”) all the way to massive monstrosities in the metre-class.

Aperture for aperture, a Newtonian is cheaper than almost any other type of telescope, and is the only option for amateur astronomers past around 50cm (20”) aperture.

The Dobsonian variant is one of the more popular types of telescope – in essence a Newtonian telescope placed on a simple alt-azimuth mount. Motorised tracking and computerised versions are available for ease of observation as well. These are primarily visual telescopes, able to offer impressive views of most objects in the sky for reasonable prices, up to apertures of half a metre.

DobCollapsible14Z

For astrophotography purposes, a Newtonian of focal ratio f/4 or faster on a good German equatorial mount is recommended. However, for decent images a coma corrector is a mandatory purchase – this will add around $300 to the price of the telescope, if a good quality one is purchased.

Focal lengths can range from 800mm for a fast 200mm aperture f/4 telescope, with the upper limit really determinant on the mirror size. A good upper limit for general astrophotography would be around 1600mm, achievable with a 406mm (16”) f/4 mirror, or a 306mm (12”) f/5 mirror. Various tube designs also exist, from solid metal tubes to open carbon-fibre truss designs.

254 174

Most Newtonians tend to be in the f/3 to f/6 range, and depending on aperture are suitable for imaging most nebulae and some deep space objects. A caveat is that faster Newtonians tend to be significantly more expensive, as the mirror geometry becomes increasingly complex to grind.

My current primary imaging rig is a 254mm (10”) f/4.7 customised Newtonian. The 1200mm focal length is suitable for most nebulae, star clusters and the closest few galaxies to us. Larger nebulae need a focal reducer for an increased field of view and further-away galaxies need a Barlow lens or a tele-extender. The medium focal length doesn’t lend itself well to planetary imaging. Ritchey-Chrétiens and other Cassegrain telescopes, with their much longer focal lengths are much more suited for planetary imaging.

More on those next week.

 

Clear skies!

EbolaBooze’s Astro Corner: Telescopes! Huh, yeah, what are they good for?

Absolutely no-

Let’s stop there.

Since I bought some telescope parts a few weeks back, it’s been nothing but storms, hail, cloud and rain. Well, at least I’ve kept my 100% hit rate for telescope purchases to inclement weather!

So instead, let’s have a pictorial guide on the different types of telescopes, and what they’re good for – strengths and weaknesses!

Let’s start with a classic – the refractor!

 

Refractor Telescopes

A refractor telescope at its most basic consists of two lenses – an objective (the light-gathering element) and an eyepiece. Refractors suffer primarily from chromatic (failure to focus all colours) and spherical aberration (failure to focus all light rays at a single point). Modern refractors can use up to six glass elements of varying optical dispersion and design to correct for these aberrations.

For astronomy purposes, two refractor designs are prominent: achromatic refractors and apochromatic refractors, with accompanying sub-designs.

Achromatic refractors are the cheaper of the two types, correcting for some spherical aberration and bringing only two wavelengths of light to a sharp focus. These are good beginner visual astronomy telescopes, as apertures of up to 152mm (6”) can be cheaply bought (~$700), and due to the lack of additional glass elements are lighter than apochromats.

starwave_152mm_ota5_m

Apochromatic refractors have two or more glass elements, with an extra-low dispersion (ED) element to bring three wavelengths of light to a sharp focus, and remove virtually all spherical aberration. Obviously, using more elements and more expensive glass raises the cost and weight of these telescopes significantly. 80mm aperture apochromats are $600-$800, with price increasing significantly once apertures are larger than 120mm.

esprit120

As a rule of thumb, most affordable refractor telescopes tend to be of short focal length (<800mm), and small aperture (<152mm). Refractors tend to be the most versatile telescope design, useable for visual, as well as planetary, wide-field and deep-space imaging. For a given aperture, their views are more contrasty and brighter than pretty much any reflecting or Cassegrain telescope type, due to the lack of central obstruction. The larger apertures are prohibitively expensive though!

For planetary views, good colour correction and a long focal length is needed – 1200mm is about the lowest you want to go. You can get this focal length by adding a Barlow lens or a tele-extender to your visual train. This will allow you to see the bands of Jupiter and the rings of Saturn with ease on a good night. For deep-space views, aperture is king – a 152mm achromat will serve well.

 

The list of refractors that are good for imaging are as long as my arm, but what is important is that the majority of mid-price apochromatic refractors project what is called a curved focal plane. This has no bearing on visual astronomy, but is important to note if you want to use one for imaging. The effects of a curved focal plane means that the stars in the centre of your image will be in sharp focus, with the stars further out being blurry and distorted – the amount dependent on the degree of curvature. Below are two examples, at the high and low-end of imaging refractors to demonstrate what some extra glass can do.

My imaging refractor is a SkyWatcher ED80, a two-element ED semi-apochromat at f7.5, with a focal length of 600mm, so a pretty slow scope. It needs a field flattener to produce decent images.

The Takahashi FSQ-106ED (on my Lotto wishlist, at a cool $5300), is a four-element apochromatic Petzval design refractor, and it’s corrected for astigmatism, coma, chromatic aberration, spherical aberration and field curvature. No correctors are needed, as the correction is part of the optical design.0038147_takahashi-toa-150nfb-double-ed-triplet-ortho-apochromat-refractor

Hopefully that was informative; I’ll be talking about Newtonian telescopes next time!

 

Clear skies!

EbolaBooze’s Astro Corner: The War and Peace Nebula

So, first things first – I have absolutely no idea how in the universe this particular nebula got its name. It bears absolutely no resemblance to the novel by Tolstoy (except being ridiculously huge, hur hur), and I just can’t see how the boffins at the Midcourse Space Experiment managed to see a dove on one side and a skull on the other. Oh wait, they were imaging it in infra-red, which is pretty bloody useless for visual spectrum imagers like me!

I prefer to refer to it by its New General Catalogue number  – NGC 6357. It removes confusion on what you’re actually meant to see, apart from a cool-looking nebula.

Interesting fact – at centre frame of this picture is star cluster Pismis 24, which contains some of the most massive stars known to science.

NGC 6357 HaLRGB

This was a 10-hour composite image – 5 hours hydrogen-alpha, 2 hours luminance, 1 hour red, green and blue.

 

Clear skies!

EbolaBooze’s Astro Corner: 47 Tucanae

Whew, it’s been a while since I posted one of these!

Blame inclement and uncooperative weather for the lack of pictures.

This here is 47 Tucanae, one of the largest, and the second brightest globular cluster in our night sky. Easily visible to the naked eye from dark skies, and very easily seen through even a modestly-sized telescope.

47 Tucanae

Unfortunately, this object is only visible to observers in the Southern Hemisphere. It’s quite a large cluster in terms of area covered as well, the the outer halo of stars coving an area around the same size as the full moon.

The image is a composite of 113 sub-frames in LRGB, averaging 11 minutes of exposures per filter.

Clear skies!

EbolaBooze’s Fortnightly Astro Corner: A Guide on Astrophotography Purchases to Destroy Budgets and Ruin Relationships

Or: Burning absurd amounts of cash on space-looky-glass.

 

So, you’ve decided to start photographing space! (I assume you’ve decided to start photographing space, else you wouldn’t be reading this.) You want to take pictures of stars, and star clusters, and nebulae, and galaxies and SPAAAAAACE.

Now, you probably want to rush out and get your hands on a camera and a telescope, and bam, astrophotography, right?

Nope!

There are a few questions you need to ask yourself before you go out and lay down some cash money on expensive glass.

 

Okay, first question: On what image scale do you want to photograph?

This is a pretty damn important one, as it will form the basis for your entire purchase.

Do you want to image constellations? The Milky Way in its entirety? Any sort of landscape plus stars? You’ll want a decent DSLR or DSLR equivalent, plus a nice, fast f-ratio lens.

 

Do you want to image large nebulae, or take star field pictures? You’ll need a motorised tracking mount plus a telescope of focal length 600-1000mm, and a camera with a large sensor to make the most of the field of view.

 

Want to do really deep-space imaging, of distant galaxies, planetary nebula and some smaller nebulae? You’ll need a very good motorised tracking mount, and a telescope with a minimum focal length of 1200mm.

 

If you want to image the planets, get a tracking mount, a telescope with focal length 2000mm or more, a 2x-5x teleconverter or Barlow lens to bump your focal length to truly absurd levels, and a camera capable of taking video.

 

Made a decision yet? Let’s move onto the second question.

Do you plan on making this a long-term hobby?

Simply, you don’t want to be laying down thousands of dollars to find that you just don’t enjoy astrophotography. Telescopes and equipment do tend to retain their value very well over time though, as much as 80% of their value for certain brands.

 

Question the third:

How strong are you?

Surprisingly relevant! If you don’t plan on having a permanent setup like an observatory, you’ll need to move your equipment out of the elements once you’re finished imaging!

To put this into perspective, I have what I like to call a “portable” setup. The lightest component of this is my telescope and camera, which weighs in at a svelte 25 kilograms (55lb). Next heaviest is my tripod, battery pack and mount head at 45kg (99lb) a piece, and various counterweights and sundries. All up, 125kg (275lb) worth of equipment.

So, if you’re planning on getting a heavy-duty setup that you can’t mount permanently, start lifting!

 

Now for the most important question!

What is your budget?

Once you get past the stage of very wide-field imaging with a DSLR and lens, the cost of equipment starts to jump up dramatically. Here are some price points, and what you can buy for that amount that won’t have you hating astrophotography and all associated with it forever.

 

$500 and below

DSLR imaging

It’s pretty much your only option at this price point. Get a sturdy tripod, and a nice lens.

Recommended focal lengths for this sort of photography are 22mm and below, with as low an f-ratio as you can afford to buy, since the lower the f-ratio, the more light can reach the camera sensor. The shorter focal length you go, the wider the field of view.

The glass and the operator are the two most important things here; camera sensors are all very similar these days. Depending on the lens and what model of camera, an appropriate fast wide-angle lens will set you back between $200 and $700.

Deep-space imaging

Pthlblblblblt, jog on mate.

Oh, wait, you’re actually looking for equipment at this price point? Seriously, skip ahead a few hundred dollars.

 

$500 to $1000

DSLR imaging

A very nice tripod, and a very nice lens. What? Not like you can get a Hasselblad until further down the list!

Deep-space imaging

Here is where a lot of second-hand beginner equipment can be found, shop around! One caveat though is the assumption that you already have some form of DSLR, or imaging unit available to you.

A decent computerised, motorised mount is the Celestron Advanced VX Equatorial mount. Brand-new, around $700USD, can be found for a lot cheaper second-hand. It’s got an adequate payload capacity too, at 13.5kg (~30lb). A good telescope to match this mount would be a 152mm (6”) Newtonian telescope. They should weigh in at around 7kg plus camera. The telescope should set you back around $300USD, total package cost ~$1000 all told.

The focal length will be in the ~600mm range, so pretty good for wide field work when coupled to anything larger than a micro four thirds (22.5mm diagonal) sensor.

Remember, good polar alignment is key to good astrophotography!

 

$1000 to $2000

DSLR imaging

Go wild, splurge, etc. I think it’s pretty obvious by now that I don’t know DSLRs terribly well. A full frame or larger sensor here with good glass becomes an option at this price range and above.

Deep-space imaging

Here’s where the fun starts.

You can keep the mount from the previous price point, and add a 200mm (8”) aperture Newtonian to it (~$500), for a total cost of around $1200. This is a good telescope for deeper-field imaging.

However, even though the mount shouldn’t be overloaded, you don’t really want to go over ¾ of the mount’s weight capacity when doing astrophotography. Ideally you would only mount a telescope weighing ½ of the mount’s weight capacity.

Here’s where a larger variety of computerised mounts of similar quality become available to you. The Celestron CGEM is a popular choice here, as are the iOptron iEQ30, Losmandy GM8, Skywatcher NEQ-5 (Orion Sirius EQ-G) and Skywatcher NEQ-6 (Orion Atlas EQ-G). All these are between $1100 and $1500, with payload capacities up to 18kg (~40lb).

Other choices for telescopes to put on the mounts here are a Schmidt-Cassegrain telescope of around 152mm (6”) for deep-field and planetary work, or an apochromatic refractor telescope for wider views.

 

$2000 to $5000

We’re getting into second-hand car territory here, but still less than I’ve spent in total. Ahem.

For a mount, the lowest payload capacity you’d want to use is an 18kg (~40lb) capacity one. The SkyWatcher EQ6 or Celestron CGEM are the cheapest mounts I’d recommend here, but you wouldn’t want to put more than 15kg (33lb) worth of equipment onto them.

Higher payload capacity mounts would be the iOptron iEQ45E or CEM60, Losmandy GM-8 or G11, or Celestron CGEM DX. You’ll want to spend between $1500 and $3000 on a good quality mount at this price point.

Here, the choice of telescopes gets blown wide open; there are some very nice options available.

For deep-field imaging, you can’t go past a 200mm aperture Ritchey-Chretien telescope with a carbon-fibre tube from GSO, or one of the re-brands (~$1400). Want a bit more aperture? How about a 279mm (11”) aperture Schmidt-Cassegrain? That’ll set you back around $2000 from Celestron. Very good for planetary imaging.

Want a wider field? A 306mm (12”) imaging Newtonian will only cost ~$800, add a corrector lens for around $300, and you have a wonderful instrument for imaging very faint objects. Want wide-field starscapes? You can’t go wrong with a 100mm (4”) f/7 apochromatic refractor.

Also, you’ll want to buy and learn how to use autoguiding equipment as well if you have this sort of budget. What autoguiding does is compensate for mechanical error in your mount’s tracking machinery, and errors in polar alignment. Very handy, and it will allow sub-exposures up to 20 minutes is done right.

LittleScope

$5000 to $20000

Well, this is the price bracket where I’m sitting at the moment.

I have an Skywatcher EQ8 (Orion HDX110) mount (50kg (110lb) payload capacity, oooh yeah), a customised SkyWatcher 254mm (10”) aperture imaging telescope, and a very, very shiny KAF-8300 based CCD camera. All told, a little over $10000.

BigScope

This price point is interesting, as you can get some very nice equipment for not terribly much, compared to more expensive brand names. A Chinese-made 406mm (16”) aperture Ritchey-Chretien telescope with carbon fibre truss structure, quartz mirrors and all will set you back ~$6500, as compared to $42000 for a telescope of the same optical design and equivalent aperture from Officina Stellare.

If you’re aiming to get really incredible images, you’ll need a good CCD camera, and I’m of the school that you should buy equipment that you should be able to grow into, and only replace it when it’s holding you back. Decently-sized monochrome CCD cameras for astrophotography start around the $5000 mark, with the cost of a set of good luminance, red, green and blue filters included. You can buy colour CCDs, but if you do, you’re a heretic and deserve to have your entire family line purged from the face of the earth.

Honestly, learning how to use a good CCD and autoguider combo will open up deeper vistas for you all at once.

For a deep field telescope, a 406mm Ritchey-Chretien from TPO or Astro-Tech would be a dream scope, coupled to a SkyWatcher EQ8 mount, and a CCD camera with a KAF-16803 sensor.

Wide field, you can’t go past either a Takahashi FSQ-106 or a Officina Stellare Veloce RH200, same mount, same sensor.

 

$20000 and above

Hey big spender~

Spend a little (imaging) time with me.

Astro-Physics, Software Bisque, Astro Systeme Austria, Takahashi, Parallax, Mathis, Planewave, Officina Stellare; These big names start their big scopes and mounts at this price bracket.

Mounts with periodic error +/- 2 arc seconds, and payload capacities in the hundreds of kilos; telescopes with mirrors milled to nanometre tolerances by ion beams, adaptive optics, this is professional territory, and the prices still keep going up.

Space is the limit, and by limit I mean limiting magnitudes in the twenties.

Just send from of that money my way, eh?

 

Clear skies!

300 Megapixels Worth of Heartache

So the DNA sequencing facility I work at moved to a different hospital campus a couple weeks back, and the boss finally got rid of a few deprecated sequencing machines that were taking up valuable storage space. Gave them a good clean-out so no nasty chemicals were left in them, and sent them off for disposal.

Two SOLiD 5500XLs, two SOLiD 5500s and a single SOLiD 4 sequencer. Five sequencers in total. These sequencers use fluorescently-tagged DNA fragments to sequence stands of DNA bound to microscopic beads immobilised on a glass plate; in short, they take pictures of glowing DNA, cleave off the glowy bit, attach a new, different glowy bit, rinse and repeat. Then, through the magic of SCIENCE and COMPUTING POWER, we get a DNA sequence.

Solid5500xl_SMALL_100410

 

That’s not important at the moment.

What’s important is, that to get the sorts of resolution needed to tell each of these microscopic beads apart, you need a pretty damn good camera and optics assembly. I don’t care about the optics assembly, I have narrower bandpass filters in my camera already. The camera sensor though….. There were five of these beasts that I could have acquired.

The sensors inside those machines were sixty, that’s right, six-zero, 60 megapixel CCD-type sensors. That’s not the biggest sensor in the world, but it’s certainly up there. As a comparison, the sensor I used for all of the astrophotos I’ve put up on this blog is a TrueSense Imaging KAF-8300, 22.5mm diagonal 8.3 megapixel sensor.

A little bit of searching brought up the model of sensor in the sequencing machines.

They were FTF9168M sensors from Teledyne DALSA, the glorious bastards that brought into the world the 111MP single-chip sensor for the Astrometry Department of the U.S. Naval Observatory (http://www.dpreview.com/articles/1319797632/dalsa100mp), and the 570MP FermiLab Dark Energy Survey camera (https://www.darkenergysurvey.org/DECam/DECam_add_tech.shtml).

dalsa100mp

 

Long whitepaper short: these imaging assemblies had larger sensors with a higher resolution than any commercially available CCD for astrophotography, quantum efficiency that puts a lot of other sensors to shame, and double the dynamic range of my current sensor.

I only learnt of this AFTER the machines had been disposed of.

 

No chance of salvage, no chance at making an array of sensors, no chance at selling three off and making obscene amounts of money. I was in physical pain when I heard that. The expression on my boss’s face when he realised what he had done was an amazing amount of shock and horror. I will be hurting about this to the end of my days.

 

300 megapixels worth of heartache.

 

Stay angry my friends, I certainly will.

EbolaBooze’s Fortnightly Astro Corner: The Tarantula Nebula

So, I’ve missed a couple of these recently! Clouds and rain are not conducive to astrophotography.

Getting this completed is a bit of a personal goal reached for me, as I’ve been trying to get a good picture of the Tarantula Nebula (also known as 30 Doradus, or NGC 2070), for the better part of my astrophotography career.

 

The Tarantula Nebula is a massive star-forming region in the Large Magellanic Cloud, one of the Milky Way’s satellite galaxies. It contains some of the heaviest stars known to astronomers, and was the site of the closest supernova to Earth (SN1987A). It’s also bright enough that if it were as close to us as the Orion Nebula, it would cast shadows at night.

Image04_ABE_ABE

This picture is a narrowband bi-colour composition of 4 hours of hydrogen-alpha data plus 2 hours of oxygen-III exposures. It’s a pretty difficult target for my at this time of year, as it’s pretty low on the horizon already when I can start imaging, and the shopping centre to my south doesn’t help with the light pollution levels. Still, good processing can work miracles. Ideally I’d get another hour worth of hydrogen-alpha data, and another three of oxygen-III, but beggars can’t be choosers.

 

Clear skies!