Field of View Between Two Telescopes

I have two telescopes, a Skywatcher 80ED (identical to the Orion 80ED – 600mm focal length at F7.5) and a Williams Optics Gran Turismo 71 APO with 420mm focal length at F5.9. Just looking at the numbers it’s easy to see that the GT71 is a smaller and faster telescope, and because of the shorter focal length it should have a larger field of view.

Comparing size with Skywatcher 80ED

Comparing size with Skywatcher 80ED

Now I’ve photographed the same part of the sky with both telescopes, and can now overlap the images to see exactly what is the difference between the field of view between these two telescopes.

First I need to say that that GT71 NEEDS a field flattener when imaging with DSLR. The distortions off-center are terrible.  Don’t get me wrong, as a three objective lens telescope (including 1 fluorite for color correction), it has provided me with the best lunar photos, however it has issues when using the large DSLR sensor. The SW80ED provides a much flatter field of view for photography out of the box.

The flattener for GT71 is in the plans…

So how does both telescope compare?  Below is a photo of open star cluster Messier 38 taken with my GT71 and I’ve overlapped as a brighter box an image taken with the SW80. For those wondering, I used IRIS to register and align both photos using the coregister command.

Messier 38 - Field of view with William Option GT71 and Skywatcher 80ED (brighter box)

Messier 38 – Field of view with William Option GT71 and Skywatcher 80ED (brighter box)

Both telescopes deliver just about the same field of view with the GT71 providing 1 degree more of horizontal field. But the difference is much less on the vertical.

What did surprise me is how much light the GT71 gathers. Inspecting the photos showed me that even with the smaller setup, the GT71 has great light gathering capabilities.  I got down into magnitude 12 with only 15 seconds of exposure, which is nearly similar to the SW80ED at 30 seconds.

WO GT71 vs SW80ED Optics

WO GT71 vs SW80ED Optics

In conclusion I would say the GT71 has good photographic potential, but requires a field flattener if it will be used with DSLR.  Stay tuned…

Solar Eclipse – Post Processing

With the eclipse behind us, and all the gear put away it’s time to transfer and process the images to create something memorable.  I decided to make a mosaic with some of the photos of the eclipse, as well as the visible sun spots. Click on the image below for a high-resolution version.

August 21, 2017 Solar Eclipse

August 21, 2017 Solar Eclipse

The weather cooperated and I had the right gear to get some decent photos. Before the start of the eclipse, the sun presented two observable active sun spot regions: 2671 and 2672. This helped in achieving a proper focus and gave something to observe prior to the start of the eclipse.

Sunspot Region 2671 (right) and 2672 (left)

Sunspot Region 2671 (right) and 2672 (left)

As I had installed and aligned my Vixen equatorial mount the night before, once I had proper focus with the camera, it was child’s play to start an automatic sequence of images every 60 seconds. Hence for the entire solar eclipse, it was hands-off and automated. I could simply glance once in a while at the screen or grab one of the hand-held solar viewers to look up.

58% Cover from the Montreal, Canada Location.

58% Cover from the Montreal, Canada Location.

While the effect was nowhere near that of those in the path of totality, the light level and heat did drop at the peak of the eclipse. The brightness was lower, not like when there are high altitude clouds as the shadows were still sharp and well-defined. And the sun’s rays did feel cooler, a welcomed relief from standing under the sun for the last hour.

In the end, it was a fun experience, especially with the kids. And with over 150 images taken I decided to compile them into two formats. A time-lapse video and a mosaic as seen above.

The video was actually the quickest thing done. With Microsoft Movie Maker, it takes the Canon CR2 RAW files directly and stitches them together into a video. It actually took me longer to find a suitable soundtrack to the clip.

With that experience under my belt, I’m looking forward to April 8th 2024 total solar eclipse that will pass close to home.

Telescope: Skywatcher 80ED with Thousand Oaks R-G solar film
Camera: Canon Rebel XTi (450D)
Setting: 1/1000s at ISO 100

William Optics GT71 f/5.9 Triplet Refractor

My last telescope purchase goes back to about 11 years when I upgraded from a beginner 130mm Newtonian to the 80ED bargain APO refractors launched by Syntha (Orion/Skywatcher) that everyone was raving about. I got one of the light metallic blue Skywatcher (SW80ED) and have been happy with its good optics and versatility for both visual and photographic use. Well actually, I upgraded the focuser as the stock unit didn’t do well with the weight of DSLR, often sliding out of focus or shifting when the tension was adjusted.

Over the past few years I’m been evaluating what should be my next move. From the 80ED there are many possibilities in the $800-$1200 CDN price range:

  • An 8in or 10in fast newton; a good bargain when it comes to pure light gathering power, and the fast ratio is great for photography. However collimation needs to be spot on, and will require frequent adjustments.
  • A 100-110mm doublet refractor will also gather more light and retain the easy of use like the 80ED.
  • Some entry-level catadiaoptric like Ritchey-Chrétien or Schmidt-Cassegrain are interesting with longer focal lengths for planets and galaxies

But with any of these options, the weight of the optic increases, and my current Vixen GP will start to struggle. Changing both telescope and mount was out of the question. I wanted something that could go well with my current gear. Hence a 70-80mm APO triplet started to look interesting, especially the small packages offered by some of the fast ones. After a week under dark skies without my telescope, I decided I needed something portable.  That’s when I jumped on an occasion to grab one of the star party demo units from William Optics: the Gran Turismo 71mm APO Triplet Refractor.

davIt may be a demo, but it looks brand new. Not a scratch on the powder-coat finished white optical tube or even the gold-colored dove-tail. Everything feels solid and the focuser looks like it can easily handle the heaviest DSLR. The SW80ED focuser is screwed to the back of the optical tub while the GT71 is one CNC machined unit, eliminating mis-alignment. As this is designed to be a travel scope, the soft-case that comes with it is very nice, much more portable than my current Orion hard-case. And at 2.2kgs, the weight is well within the range for the Vixen GP mount.

To compare the size, I’ve set it up next to my SW80ED, where it comes to almost half its length. The 80ED shares the same tube as the 100mm version, hence it’s bigger than it needs to be and the dew shield does not retract. Overall, the GT71 is more compact and will pack just about the same viewing power as the SW80ED.

hdr

Below is a view of the optics.  While the SW80ED only has about 5 baffles within the tube, the GT71 has a good 30 of them to keep any stray light from ruining the view.  While both use FPL-53 glass, the SW80ED only has it in the rear element, while the GT71 is used for all three optical elements.  The SW80ED provides views free of chromatic aberration, however it’s designed to perform well in the blue and green part of the spectrum.  Anything is the deep red was falling out of focus, especially if a focal reducer was used.  The GT71 will perform better over a wide range of spectrum.

dav

I didn’t travel with my SW80ED, but now with a more compact telescope, I’ll be inclined to simply grab it during for my outings.  I just now need a light alt-az tripod to go with it.

2017 Product – Meade EclipseView

With the total solar eclipse scheduled for August 21st, expect to see new lines of products catering to the novice observer wanting to get up close with the event.  Meade has recently announced the EclipseView product line for April 2017 which includes a binocular, a small refractor and three small reflector telescopes specifically for those wanting to experience the eclipse but with a limited budget and beginner experience.

Meade EclipseView [Meade]

Meade EclipseView [Meade]

The product line offers the following models, all equipped with removable white-light solar filter required to view the sun at all times.

  • 10×50 Binoculars
  • 60mm f/13.3 Refractor (includes 12.5 and 4mm eyepieces and 2x barlow) with an AZ mount
  • 76mm f/9 Reflector  (includes 26 and 6.3mm eyepieces and 2x barlow) with an AZ mount.
  • 82mm f/3.7 Reflector (includes 26 and 9mm eyepieces and 2x barlow) in a compact table-top mount
  • 114mm f3.95 Reflector  (includes 26 and 9mm eyepieces and 2x barlow) in a compact table-top mount

Of the bunch, only the 114mm has the better parabolic mirror, the others opting instead for the simpler spherical mirror.  Therefore the 114mm will provide a sharper view edge to edge, especially a high magnification.

These aren’t new telescopes from Meade, but existing models from their Infinity, Polaris and LightBridge Mini Series kitted for solar observation.  While Meade advertises that these telescopes can also be used at night to view the Moon, planets and the stars, you’ll want to get a red-dot view finder to replace the existing solar pin-hole finder.

Of course you can also add the appropriate solar filter to any telescope, no need to limit yourself to the above gear.

[Meade]

 

2017 Event : Total Solar Eclipse for North America

Every given year there are between two and five solar eclipses, this upcoming one for August 21st will be special.  The last total solar eclipse for North America goes back to 2008.  As Earth is largely covered by water, many of the eclipses are over the ocean where the number of viewers are limited.  But this one will pass over millions of people, all with access to equipment and social media to share their experience.  Hence this one has lots of people planning and getting ready.  The eclipse is most impressive when you’re located in the path of totality; where the Moon completely blocks out the Sun.  Hence if you are able to travel to such a location along its path, it will be worth it.  I also suggest finding a local astronomy group or association as they will most-likely have telescopes and other special observing gear out for everyone to use.

August 2017 Total Solar Eclipse.

The total solar eclipse will only be viewed in the narrow path crossing the middle of the USA. North and south of that will get a partial eclipse. The green vertical lines indicate the time of maximum eclipse. Courtesy Michael Zeiler, GreatAmericanEclipse.com.

Observing the solar eclipse requires protective eye-wear and solar filters for any observing or photographic equipment.  For my telescope it’s a film solar filter, now branded SolarLite by Thousand Oaks Optical.  These can be purchased already mounted in an aluminium cell or in sheets for your own custom application.

Thousand Oaks Optical R-G Solar Filter

Thousand Oaks Optical R-G Solar Filter

The American Astronomical Society has created a web site just for the event with plenty of information on safe observation and suppliers of necessary optical filters.

Stay tuned…

December 25th – No Sunspots

The sun has been without sunspots for two days, but that is expected as we are heading to a minimum in the 11-year cycle.

Cycle 24 Sunspot Number

Cycle 24 Sunspot Number (NASA)

Nevertheless as it was a nice afternoon grabbed the scope and did some observation of the sun.  A little of a challenge to focus when there is no contrasting details to base yourself on.

December 25th 2016 - No Sunspots

December 25th 2016 – No Sunspots

Skywatcher 80ED
Canon XTi (450D) ISO 100 – 1/800sec
Thousand Oaks R-G Solar Film

Apollo 11 and 17 Landing Sites

Image

What is the smallest detail of the lunar surface can I get with a 80mm telescope (600mm focal length) and Canon 10.1Mpixel camera?  Matching some of the smaller craters in a Moon atlas gives me roughly 6-8km/pixel.  But with image processing anything below 10km doesn’t really show or will blur in the noise.

I tagged a few geological features and dimensioned two craters for reference.  At the same time identified the approximate Apollo 11 and 17 landing sites.  The Apollo Lunar module is only 9.4m wide, hence it is impossible for any Earth bound telescope can possibly pick them up (even Hubble).  However the Lunar Reconnaissance Orbiter (LRO) did manage to pull it off by lowering its orbit as low as 50km above the lunar surface.

Apollo 11 and 17 landing sites and other features

Apollo 11 and 17 landing sites and other features. Moon (October 6th, 2016) – Benoit Guertin

My original photo of the Moon.

Autumn – Darker Skies and No Mosquitos

I hadn’t taken the telescope out since April. With other projects and hobbies I just didn’t bother setting up the equipment.  But a few nights ago looking at a dark blue evening sky I noticed a nice crescent Moon and a triangular star formation over the horizon. The kids weren’t in bed yet so I grabbed my gear and made a quick set-up for observation with the 80ED telescope.

After an observation of the Moon, spending time examining the lights and shadows across the lunar craters and valleys I looked at the triangular star formation and suspected that at least one was a planet.  Slewed the telescope over and discovered Mars. Tried different eye pieces and settled for an Orion Edge-On 9mm planetary with Televue 2x Barlow.

With the kids off to bed I changed the set-up for webcam imaging before Mars could dip below the horizon. Follow up processing that evening wasn’t very rewarding.  Mars is some distance with Earth therefore appeared rather small compared with other times I pointed the telescope and with the heavy atmospheric turbulence imaging at such a low altitude it blurred the scant details I could have captured.

But that evening I broke the ice and got the gear out. And since I’ve been enjoying the night sky when weather permits. With the galactic plane crossing the sky it’s a great time for wide angle shots. It’s also much faster to set-up and more forgiving to an incorrect polar alignment. I got three photo sessions to analyze and hope to have some good shots to show in the next few days.

Autumn,  with cooler nights, dark skies and no mosquitos it’s prime time to enjoy the night sky.

Closest Comet in 246 Years

Status

There’s a good article in Sky & Telescope on comets 252P/LINEAR and the smaller fragment P/2016 BA14, explaining observation opportunities.  A comet hasn’t passed this close to Earth in 246 years.   And as it does the wonderful green halo around 252P/LINEAR  is sure to grow but will probably remain around magnitude 6.

As the comet flies by Earth it will sweep through the constellations quickly and then fade back to below magnitude 12 in short order.  Therefore try not to miss it.

Observing Pluto – What Does it Take?

With New Horizons‘ flyby of Pluto and all the great images the spacecraft has been returning I’ve wondered: Is it possible to observe Pluto from one’s backyard?  Personally I’ve never bothered trying for Neptune and beyond as I knew my small 80mm aperture telescope would not be up to the task.  Nevertheless I looked up Pluto’s apparent magnitude and found that it varies between 13.6 and 16.3 due to its elliptic orbit around the Sun.  It’s last closest approach (perihelion) was September 1989, and unfortunately Pluto is currently distancing itself for its 248 year journey around the Sun therefore slowly dimming, sitting right now at apparent magnitude 14.

Pluto as viewed by New Horizons during flyby (14 July 2015) - NASA

Pluto as viewed by New Horizons during flyby (14 July 2015) – NASA

What size of telescope does it take to observe an apparent magnitude 14 object?  Based on the theoretical limits it should be possible to make visual observation with a 10in aperture telescope, but most would say you need a 12in if you plan to observe with an ocular.  Of course, equipped long exposure cameras you can have a smaller telescope, but a high focal length would be preferred to reduce to better pick it out from the background of stars.  And I’ve managed to pick up mag 14 stars in my photos with the Skywatcher 80ED with 60sec exposures.  Therefore Pluto should be accessible to backyard astronomy.  Note that at Pluto’s size and distance it shows up as a light point and not a sphere like the other planets.

Up to the challenge?  Middle of November will be a great opportunity to locate Pluto as it will swing within 1deg of Ksi 2 Sagittarius, a magnitude 3.5 star.  In the June edition, Sky & Telescope created a great star-chart to locate Pluto until December 2015.  Good Luck!

Click on image for Sky & Telescope Pluto 2015 Sky Chart

Click on image for Sky & Telescope Pluto 2015 Sky Chart

Reference: Sky & Telescope