Astrophotography with Newtonians- A Review of the Celestron Omni XLT 150

For many years, I have used small refractors to photograph the night sky. Refractors are great if they are well corrected and up to 80mm diameter they are quite affordable. With the right reducer/flattener and a decent camera it’s possible to create wonderful images: if tracking is done properly, stars will be nice and small pinpoints. Even without guiding, exposures up to 2 minutes are possible with a medium sized equatorial mount like the Celestron AVX. Polar alignment is very important and I usually keep the focal length under 750mm.

I have often been asked what is the best starter scope if you want to observe but also start learning astrophotography. Typically I would recommend an 80mm ED or APO refractor with a motor driven equatorial mount. There is no arguing that they offer great portability and deliver very satisfying results. However, there is always the issue of light gathering. Aperture rules – to see the fainter galaxies and nebulae, you need a large opening. Even 4 inches is still small. Well corrected apochromatic refractors 5 inches and over are expensive and out of reach for most people, certainly beginners.

This is where the Newtonian design deserves serious consideration. Over the past year, I have tested and worked with several Newtonian reflectors and I came to really like them. They are fast (meaning that the f-stop ratio allows for fairly short exposures), affordable and very capable instruments for moon, planets as well as Deep Sky Objects. My first Newtonian was a big 10″ Dobsonian – the views blew me away, but except for some memorable lunar photographs, I was not able to use it for imaging because of the simple mount. John Dobson, who spent his life bringing astronomy to a larger audience, designed this particular Newtonian as simply and inexpensively as possible, and for anybody who starts out there is simply no other telescope for visual observations that gives you more bang for your buck.

Here is a diagram of a Newtonian showing the mirror arrangement and light path:

NewtonianReflectorDiagram

Initial Results Using a Newtonian for Astrophotography

The first serious Deep Sky photographs I took with a Newtonian were done last winter with a Vixen R200SS. What a great scope! Combining an 8-inch opening with an F/4 ratio allowed me to go deeper than I had ever gone before, still keeping my (unguided) exposures under one minute. Orion produces an 8-inch F/3.9 Newtonian astrograph that is less expensive and very comparable in performance. Once you get used to the unavoidable drawbacks of the Newtonian design, they are simply wonderful all-around telescopes. Here is an example of my work with the Vixen R200SS.

Astrophotography with Newtonians

Tech Data for Image Above: The Horsehead Nebula (Barnard 33) stands out like a dark cloud against the bright emission nebula IC 434. Below center is the bluish reflection nebula NGC 2023. The bright star to the left is Alnitak, the easternmost of the three stars that form Orion’s belt. Underneath Alnitak is the Flame Nebula (NGC 2024) at the center of which scientists have discovered a cluster of newly formed stars. Shot with a fast f/4 Newtonian telescope (800mm focal length) from the hills above Malibu, California on December 30th, 2015. Distance: 1500 light years Constellation: Orion Telescope: Vixen R200SS Camera: Nikon D5100 (astro modified) 16 subs of 45 seconds each at ISO1600, stacked in Deep Sky Stacker and processed in Photoshop CS6.

Newtonian Collimation

Let’s focus on those drawbacks for a moment. One of the most common complaints is that you need to regularly align (collimate) the mirrors, otherwise the views are far from optimal. Very true, but it’s really not that complex. There are many online tutorials that show you step by step how to use a Cheshire and/or laser collimator and most reflectors keep their collimation quite well. Even the fast Vixen R200SS only needs to be re-aligned every once in a while. It becomes a simple routine that only takes a few minutes, so I recommend a quick collimation before every photographic session.

Here is a helpful (and entertaining) video by Ralph Bell and Robert J Dalby showing you how to quickly collimate a Newtonian reflector telescope using a laser collimator.

Eyepiece / Camera Positon

The second drawback is the position of the eyepiece or camera: with a German Equatorial Mount (GEM) which is the best for astrophotography, it can be a real nuisance when you point the scope to a different place it the sky, suddenly the eyepiece ends up at the bottom (or the top) of the tube and you need to loosen the tube rings and rotate the tube in order to see through it. One solution is getting rotating parallax rings – quite expensive, but they completely solve the problem. My cheap solution is to plan most of my imaging in one area of the sky and once I move to another part of the sky I take the extra few minutes to loosen the rings and rotate the tube. It’s also possible to make your own “Wilcox” rotating rings, they work well and keep the balance of the scope intact.

Instructions for Making Your Own Wilcox Rotating Rings

Coma

The third drawback is optical and becomes a problem especially with the faster Newtonians: coma. Coma shows itself at the edges of the frame (both visually and photographically) when stars become streaks instead of nice pinpoints. The universal solution is a coma corrector and there are several good ones out there that are not too costly. I recommend the Baader MPCC (Multi Purpose Coma Corrector) as well as the GSO (or Astro Tech) Coma Corrector, designed by Roger Ceraglio:

GSO Coma Corrector

Using this corrector with a DSLR (I use Nikons as well as Canons) you will need a spacer ring to create the proper distance for the back focus, see further down the article.

Tube Balance

The final issue that needs to be addressed is balance. With a refractor, the camera adds weight at the end of the tube, which is fairly simple to compensate either by moving the scope forward in its rings or moving the dovetail bar that connects to the scope forward in its mount. With a reflector, the camera adds weight at the SIDE of the tube, causing a torque that will change the balance of the tube considerably depending on its position. The remedy for this is to turn the tube in such a way that the focuser is at the bottom of the tube, right over the mount.

Of course, this can’t be done if you want to look through an eyepiece, but since the weight of your eyepiece is far less than the camera it’s not a problem for visual observations.

Diffraction Spikes

Bright stars shot with a Newtonian will typically show four diffraction spikes, which is a result of the spider vanes that hold the secondary mirror in place. It can be used as a creative effect – for the image of the Pleiades I actually like it. It is possible to buy special spiders with curved arms, to eliminate this “star filter” effect, but I personally don’t find it too objectionable.

Destiny Curved Vane Newtonian Spiders

Example of diffraction spikes in M45 I imaged with a Newtonian.

Astrophotography with Newtonians

Tech Data for Image above: The Pleiades or the Seven Sisters (Messier 45) is a cluster of very hot blue stars, clearly visible with the naked eye as a tiny group that resembles a miniature of the Big Dipper. The blue nebulosity is caused by interstellar dust the stars are passing through, creating a reflection nebula around the brightest stars. The diffraction spikes are a result of the vanes that hold the secondary mirror in place –  common with Newtonian telescopes. Shot with a fast f/4 Newtonian (800mm focal length) from the hills above Malibu, California on December 30th, 2015. Distance: 434 to 446 light years  Constellation: Taurus  Telescope: Vixen R200SS   Camera: Nikon D610 15 subs of 45 seconds each at ISO1600, stacked in Deep Sky Stacker and processed in Photoshop CS6.

Newtonian Advantages

In spite of what may seem like a long list of extra problems to deal with, in reality these are all minor obstacles that are easily overcome. And the advantages are substantial: more light gathering, faster f-stop ratios (meaning shorter exposures so less tracking errors) and for the large Newtonians amazing sharpness and clarity. Some of the best lunar and planetary imagers prefer a reflector over an apochromatic refractor that may cost three times as much – see for example the outstanding work of Wes Higgins and his sons:

High Resolution Lunar and Planetary Photography by Wes Higgins

Beginner Newtonian for Astrophotography

One of the great starter kits available on the telescope market today is actually a 6-inch Newtonian on a motor driven CG-4 equatorial mount. It is entirely capable of simple astrophotography (moon, planets as well as Deep Sky Objects) and the larger opening will show far more faint objects than an 80mm APO refractor. No false color on moon and planets and very sharp images. And yes, you will need a coma corrector and from time to time the mirrors need to be collimated. For only $750 or so, you will have everything you need to begin this wonderful journey.

When my good friend Larry Bergl expressed his desire to learn more about the stars and buy a nice telescope that would enable him to observe as well as take astro photographs, I recommended the Celestron Omni XLT 150 with dual axis motor drive. For a very reasonable $560 you get the mount, motors and gears, tripod and 6-inch Newtonian with finder and one 25mm eyepiece.

Click on photo for more info:

Astrophotography with Newtonians

All you need extra is a polar alignment scope (about $40), the coma corrector, and some more eyepieces: I’d get some nice used Plossls on Astromart or Cloudy Nights in 5mm and 10mm focal lengths to start with. Investing in some carrying cases is not a bad idea either.

The scope arrived from High Point Scientific (excellent service, I will buy from them again) well packaged in two large boxes, with manuals and everything. Set-up was pretty straightforward and took less than thirty minutes (since I have done this many times before, it may take twice as long if it’s your first time).

First light with the telescope under semi-dark skies in the Santa Monica mountains (red zone, half an hour drive from my studio) was a wonderful experience, especially for Larry who was entirely stoked about his new toy. We watched a lovely sunset and then we polar aligned the scope, which took about ten minutes.

Photograph by Larry Bergl

Astrophotography with Newtonians

Larry Bergl, proud owner of his brand new Celestron Omni XLT 150

Astrophotography with Newtonians

Soon we were ready to go and find out what the scope had to offer. The moon was a beautiful crescent, just over three days old. Jupiter was shining in the west, still perfectly visible while Mars and Saturn were bright and clear in the southern part of the sky in constellation Scorpius, near Antares.

Visually, the moon was breathtaking, Mare Crisium was standing out like a dark eye with the craters Peirce and Picard easily discerned as well as the small crater Swift. Jupiter clearly showed its equatorial belts and the four Iovian moons; Saturn was tack sharp with a 5mm planetary eyepiece (150x magnification) and even some of the Cassini division was visible in the outer rings (not in front of the planet). The mount was more than sturdy enough and tracked very well, even through the slewing controls did not always respond immediately: sometimes you had to press the buttons for a couple of seconds before the scope started to move. However, our polar alignment must have been quite good because typically a planet would stay in the center of the eyepiece for at least ten, fifteen minutes.

Astrophotography with Newtonians

After a good forty-five minutes of observing, we were ready to try some photographs to see what the scope and mount were capable of delivering. Certainly, we were not disappointed here.

The Omni XLT 150 has a smooth 2-inch focuser that will easily hold a DSLR. Without a coma corrector only the center of the field will render pinpoint stars, so the first step is to connect your camera to the coma corrector. If you use the GSO coma corrector mentioned earlier in the article (same one is sold under the Astro Tech brand), then you unscrew the top part of the corrector that holds your eyepiece for visual use. The bottom part which contains the actual optics has T-threads at the top (M42 x 0.75mm) and will connect to your camera specific T-ring. Here is one for the Canon EOS line.

Unfortunately, the distance between the closest lens element and the sensor of your camera needs to be about 75mm for the coma corrector to work properly. Since the back focus (FFD or Flange Focal Distance) of your Canon EOS is 44mm you need to purchase extra T-thread spacers to make it work. The best deal I found on Ebay is this one.

I would recommend using the 15mm or the 20mm spacer, since the T-ring adds at least another 10mm to the distance. For Nikons, the FFD is 46.5mm, so there the 20mm ring will probably be the right choice. It’s good to experiment with the spacers to see what gives the best pinpoint stars in the corners. I ended up using two 10mm spacers that I already owned and that worked quite well with the Canon EOS T2i, see photographs.

Astrophotography with Newtonians

Note how the focuser with camera is pointing down, which is the best position for optimal balance. Balancing the telescope well is crucial for good tracking: neither the declination nor the RA axis should have any torque on them. With both clutches unlocked, the scope needs to remain in its position – if it is heavier on one side, move the counterweights or the position of the tube in the tube rings.

Astrophotography with Newtonians

We started off photographing the crescent moon, taking single exposures in prime focus. ISO was set to 400 – since the scope is fast, we could have used ISO 200 without a problem. Exposures varied between 1/100 and 1/200 sec. and the best image showed a lot of detail at 1/160 second. Using a good Barlow lens to extend the focal length would have been worth a try, that way exposures would have been longer but the image larger. Since the mount was tracking so well, longer exposures should not be a problem. Here is our “first light” photograph of the moon:

Astrophotography with Newtonians

For deep sky photography, an interval timer needs to be connected to the camera. The CG-4 mount does not provide a connection for an autoguider, but unguided exposures up to 90 seconds are quite feasible with this mount. Here is a link to the inexpensive interval timer we used.

Our target was the Lagoon Nebula (Messier 8). After locating it with some wide field 7 x 35 binoculars, we pointed the telescope at the nebula and with a 25mm eyepiece and a Lumicon deep sky filter, the view was quite stunning in spite of being in a red zone. Light pollution was strong enough that the Milky Way was not visible, yet the 6-inch mirror pulled in a fair amount of faint stars and nebulosity.

Using live view of the Canon EOS T2i, we focused on one of the brighter stars in the area. The focuser can be locked down, which is absolutely necessary after attaining focus. Getting the nebula centered was cumbersome, because of the delay with the slewing controls: it required ten minutes of messing around with test exposures. Being used to slicker and more expensive GoTo mounts, it can be a bit challenging to get everything set up correctly with a CG-4 but it still works!

Finally we got to start the series of sub-exposures: we took twenty subs of 68 seconds each at ISO 1600. Out of the twenty subs, eight were showing pinpoint stars and those were stacked in Deep Sky Stacker with a dozen of dark frames. The final result is not bad at all for an economy telescope:

Astrophotography with Newtonians
Lagoon Nebula (Messier 8)

Note how well the coma corrector is working: even in the corners, the stars are nice and small. To increase the contrast of the image, a CLS-CCD clip-in filter was used inside the camera body. Without it, it would have been very hard to get a good image of the nebula because of the light pollution. Photoshop processing further brought out the color as well as the nebulosity.

In conclusion, the inexpensive Newtonian design is certainly capable of great astrophotography. It takes many months or even years of shooting and processing experience to get the most out of your instrument, but the bottom line is that without having to spend a fortune you can enjoy both visual and photographic work.

Check out Pocketrubbish website for many more excellent examples of what can be achieved with the Omni XLT 150.

Clear skies!!

Martin Cohen


Martin Cohen is a professional photographer and amateur astronomer in Santa Monica, California.

Martin Cohen’s Astrophotography Website

Martin Cohen Photography

Photos of the Moon

Photos of Havana, Cuba

See Martin’s other tutorials on AstroPhotography Tonight:

Astrophotography on a Budget

Astrophotography with Small Refractors

Telescopes or Telephoto Lenses?

Creating Seamless Mosaics in Photoshop


 

Comments

  1. Nice website. I use an Omni 150 on a AVX mount. I do autoguide and use a GSO coma corrector like yours. I’ve had more expensive gear, but these days I really enjoy getting the most out of more modest gear. It’s amazing what can be accomplished for hundreds, not thousands of dollars these days.

    Thanks!

    Randy

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