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Celestron C11 Starbright + Celestron 0.63x Reducer/Corrector

The Celestron C11 Starbright SCT (pre-XLT coatings), weighing nearly 30 lbs and large enough to old a basketball inside it, is a large telescope. With a native focal length of 2800mm and 280mm (11 inches) of aperture (F/10), it’s a long, slow scope and requires a substantial mount. It’s field of view is too narrow for a supermoon to fit inside an APS-C sized sensor, even when reduced to F/6.3 (1760mm-equivalent), so it’s best-suited for shooting globular clusters, galaxies further than Andromeda, and planetary imaging. Below is a summary of tests I’ve performed on this telescope in conjunction with Celestron’s F/6.3 Reducer Corrector.

Corner/Edge Performance

The following 100%-crop mosaic was made with a Canon EOS70D. No filters were used for this test, although it’s possible to attach 2-inch filters using a Blue Fireball SCT Eyepiece Holder with any 2-inch nosepiece adapter threaded for filters, or in conjunction with the Orion Thin Off-Axis Guider.

Figure 1: Single exposure of Messier 13 taken with a Canon EOS 70D (unmodified), 120 seconds, ISO 400. Several dozen exposures, including this one, were stacked to create the image of the Great Cluster in Hercules in Figure 3 below.

Some color fringing is present, but it isn’t anything that can’t be dealt with in post. What’s disappointing is that the corners and edges exhibit elongation of the stars, so it appears that the field curvature native to the telescope itself is not well-corrected throughout the entire APS-C frame.

Vignetting

Below is a contoured image created from flats produced by a Canon EOS 70D.

Figure 2: Vignetting profile for the Canon EOS 70D.

In the extreme corners and edges, more than 50% of light gathered in the center is lost. The end result is that these extreme edges must be cropped out in most final versions of processed images. The odd shape of the contour in the upper left is caused by the prism of the Orion TOAG just barely sticking into the path of light reaching the 70D’s sensor, between it and the Reducer/Corrector, so this is not a deficiency of the Reducer/Corrector.

Thoughts

All telescopes of Schmidt-Cassegrain design project a curved field of sharp focus, so a field flattener must be used to get decent image quality when using larger sensors. The Celestron F/6.3 Reducer/Corrector provides some level of flattening, but not all the way to the edge of an APS-C sensor. Micro Four Thirds sensors will fare much better where flat fields are concerned. This corrector also may be somewhat soft, although it’s hard to tell if the softness in my images is coming from the corrector, bad seeing, or bad focus (I did use a Bahtinov mask). Perhaps a comparison with either of the available Starizon SCT Correctors is warranted.

Sample Images

Figure 3: Messier 13, taken with a Canon EOS 70D.

 

Figure 4: Messier 101, taken with a Canon EOS 60Da.

 

Figure 5: Messier 81, taken with a Canon EOS 60Da.

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Explore Scientific ED127CF Triplet APO + 0.7x Reducer/Corrector

The Explore Scientific ED127CF Triplet APO is the first refractor telescope I have ever owned. With a native focal length of 952mm and 127mm (5 inches) of aperture (F/7.5), it lives somewhere between the “wide field” 80mm refractors and the longer SCTs used for planetary imaging, both in terms of field of view and imaging “speed”. When coupled with Explore Scientific’s 0.7x Reducer/Corrector, the ED127CF widens to an equivalent focal length of 666mm and faster focal ratio of F/5.25. Here are some star quality tests shot with this scope/reducer combo.

Corner/Edge Performance

The following 100%-crop mosaics were made using a Canon 6D and a Canon 60Da to show the differences in corner/edge performance between Full-Frame and APS-C sensor sizes. No filters were used as the 0.7x Reducer/Corrector does not provide any means of attaching filters (although some have made modifications to do so).

Figure 1: Single exposure taken with a Canon EOS 6D (unmodified), 180 seconds, ISO400. Several dozen exposures, including this one, were stacked to create the star field background in the 2017 Total Solar Eclipse photo in Figure 5 below. The eclipse itself was also shot on the same ES ED127CF + 0.7x Reducer/Corrector with the same EOS 6D body.

 

Figure 2: Single 4-minute exposure taken with a Canon EOS 60Da, 240 seconds, ISO800. Several dozen 4-minute exposures, including this one, were stacked to create the image of the Pleiades in Figure 6 below.

In both of these images, some color fringing is present in high-contrast areas, but the amount of Chromatic Aberration present is nothing that can’t be dealt with in post (see sample images below to see how it can still be removed after heavy processing). What’s great about both of these mosaics is that there is very little elongation of stars at the corners, both for APS-C and Full Frame sensors. Coma and astigmatism are both very well-controlled, all the way to the edges.

Vignetting

Below are two contoured images created from flats produced with both the Canon EOS 6D and EOS 60Da. This is where the similarities in performance for both sensor sizes end.

Figure 3: Vignetting profile for the Canon EOS 6D.

 

Figure 4: Vignetting profile for the Canon EOS 60Da.

As you can see, the vignetting on the EOS 6D is severe with less than 30% of the light coming in at the center reaching the corners. This can result in unwanted processing artifacts appearing in the corners of your images as soon as you calibrate your subs. On the other hand, vignetting on the EOS 60Da is almost non-existent with nearly 90% of the light present in the center still reaching the corners.

Thoughts

The Explore Scientific ED127CF is a great piece of hardware for the money, yielding fantastic images given the right sensor size. While it’s unfortunate that the vignetting ends up being so severe for the extreme corners of full-frame sensors, APS-C sensors fare much better. Even so, great images can still be made with full-frame sensors, given the right observing conditions. One of my favorite full-frame images captured with this setup is my shot of the 2017 Solar Eclipse, shown below. Barring another Solar Eclipse outing, however, I think I’ll stick to APS-C sensors on this particular combo.

Sample Images

Figure 5: 2017 Total Solar Eclipse, taken near Douglas, WY, with a Canon EOS 6D. Prints available.

 

Figure 6: The Pleiades, captured with a Canon EOS 60Da. Prints available.

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Hoya Intensifier (Red Enhancer)

Sodium vapor lamps, which are used in street lights, cause much of the light pollution produced by cities. These lamps emit yellowish light with a wavelength around 589nm. Didymium is particularly good at blocking this wavelength and finds its way into many light pollution filters, including the Hoya Intensifier (also known as the Hoya Red Enhancer). Here are a few test results showing what the Hoya Intensifier can do to help reduce the effect of light pollution in your astro images.

Continue reading Hoya Intensifier (Red Enhancer)

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Canon EF 35mm F/1.4L USM + Hoya Intensifier

The Canon EF 35mm F/1.4L USM lens (first generation) has been around since 1998, but is still highly acclaimed for its overall image quality despite its age. This lens was my first prime with L glass and is probably my most used. Here are some star quality tests I shot with it using a Canon EOS 6D body and a Hoya Intensifier to mitigate the effects of light pollution where I live.

Corner/Edge Performance

The following mosaics were made from 100% crops using light frames shot at ISO1600 from a red zone. Exposure times started at 4 seconds at F/1.4 and doubled with each successive aperture stop, all the way through F/5.6. A Hoya Intensifier LPS filter was used to reduce the effects of light pollution, as well as some very light curves work in post. Use the gallery below to cycle through light frames gathered at different apertures (click to view full-sized images). Coma and Chromatic Aberration remain strong at apertures wider than F/2.8, then all but disappears at F/2.8 and narrower. Some is still present in the extreme FF corners, but slight cropping of images is enough to get rid of it.

Vignetting

Vignetting is fairly strong wide open on a full-frame sensor, but improves dramatically by F/2.0 and is mostly gone by F/4.0. APS-C users will see great vignetting performance from F/2.0 and narrower.

Thoughts

There seems to be a sweet spot at F/2.8 for this lens, as far as astrophotography is concerned. Vignetting is well-controlled, as well as Comatic and Chromatic Aberration. It’s a great focal length for stitching wide landscape shots. It’s also possible to shoot deeper stacks with this lens, but you’ll need wide-open skies; plan to deal with gradients in post, as this lens covers a lot of sky!

Sample Images

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Canon EF 50mm F/1.8 II STM + Hoya Intensifier

Cheaper than many fully-manual budget lenses, the Canon EF 50mm F/1.8 II STM (and its variants) is fairly ubiquitous. Below are some evaluation images I shot using this lens on a Canon EOS 6D. A Hoya Intensifier filter was also used to help lessen the light pollution effects from the red zone I live in.

Corner/Edge Performance

The following mosaics were made from 100% crops using light frames shot on a Canon EOS 6D body (unmodified) at ISO1600 from a red zone. Exposure times started at 3.2 seconds at F/1.8, 4 seconds at F/2.0, and then doubled with each successive aperture stop after that. A Hoya Intensifier LPS filter was used to reduce the effects of light pollution. Use the gallery below to cycle through light frames gathered at different apertures (click to view full-sized images).

Vignetting

Full-frame users may be disappointed by the vignetting performance of this lens. Vignetting remains strong even through f/2.8 and doesn’t much better unil f/4.0. APS-C users will be much happier, with decent vignetting around f/2.0 and becoming very good by f/2.8.

 

Sample Images

Figure 1: Taurus and the Pleiades, Canon EOS 70D, 80x180sec stack, ISO1600, f/3.2, Prints Available. Read more about how this image was created here.

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Rokinon 85mm F/1.4 AS IF UMC + Hoya Intensifier

Falling well within the “budget lens” category is the Rokinon 85mm F/1.4 AS IF UMC lens ($299 new at the time of this writing). Below are some evaluation images I shot using this lens on a Canon EOS 6D. A Hoya Intensifier filter was also used to help lessen the light pollution effects of the red zone I live in.

Corner/Edge Performance

The following mosaics were made from 100% crops using light frames shot on a Canon EOS 6D body (unmodified) at ISO1600 from a red zone. Exposure times started at 2 seconds at F/1.4 and doubled with each successive aperture stop. A Hoya Intensifier LPS filter was used to reduce the effects of light pollution. Use the gallery below to cycle through light frames gathered at different apertures (click to view full-sized images).

The Rokinon 85mm F/1.4 AS IF USM lens exhibits reasonably good corner/edge performance at f/2.8 and above. Coma is still an issue at F/2.8 in the Full-Frame edges, and astigmatism is always present in the far Full-Frame corners. Chromatic aberration is pretty harsh until F/4.0 and mostly gone by F/5.6.  It’s probably best if this lens is stopped down to F/4.0, and perhaps better on an APS-C body.

Vignetting

Vignetting is very strong at F/1.4 but improves at F/2.0 and is much better at F/2.8 and higher. These images were shot using the same body and settings as the light frames above.

Sample Images

Figure 1: Deneb shines left of center in front of several emission nebulae, including Gamma Cygni, North American, and the Pelican Nebula. Unmodified Canon EOS 6D, ISO1600, F/4, 60x60sec calibrated stack. Prints available.

 

Figure 2: Orion and Monoceros region, Canon EOS 6D, ISO1600, f/2.8, 29x120sec calibrated stack. Prints available.