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Review: Samyang 135 mm f/2 Astrograph

March 6, 2016 9 comments

IMG_2663

By calling the Samyang 135 mm f/2 lens an astrograph in the title, I’m giving away a bit of the conclusion, so let me just state the conclusions up front: I found this lens to be sharp from corner to corner with a reasonably flat field across a full-frame sensor. This is better performance than all but the best prime telephoto lenses, and also better than many telescopes I’ve owned claiming to be astrographs. Even more impressive is the fact that it accomplishes this at a focal ratio four to eight times faster than “fast” refractors. Finally, it’s hard to beat the price: currently just over $500.

While the performance was adequate at f/2, I found that stopping down the lens one full stop to f/2.8 improved sharpness. This is true of any lens, and even stopped to f/2.8, that’s still four times the light gathered per sensor area than an f/5.6 telescope. It’s easy to forget how fast this is, but my first night using the lens reminded me. My usual telescopes are two f/5 William Optics Star 71s and a Takahashi FSQ-106ED, also f/5. I usually shoot narrowband exposures of 20 minutes with these. So the combination of broadband and f/2.8 put me in the realm of 30 to 120 second exposures—anything longer at ISO 800 overexposed the stars.

The infinity focus point is about 2 mm left of the mark on the lens barrel, so you’ll have to carefully dial in focus. There isn’t much tolerance for error at such fast focal ratios, as the zone of focus is very narrow.

Vignetting is substantial in the corners, but it is more reasonable if you move slightly inward.  For very fast optics, this is typical, though it does lead to lower SNR toward the corners. I was able to keep the full frame images without cropping by using good flat frames, but this is essential.

Master Flat created from 33 pictures (Average)

Master Flat created from 33 pictures (Average)

The quick 99×1 minute image below of the Rosette Nebula area gives you a sense of how wide the view is with a Canon 6D. In the center is the Christmas Tree/Foxfur/Cone Nebula area, with huge dark nebula Barnard 37 prominent. This was taken without a CLS filter, so light pollution prevented me from adequately revealing the Foxfur nebula well. The Rosette Nebula shines brightly to the left, though. (Note that this image is reduced to 25% of the actual image resolution.)

Samyang 135 Rosette_widefield 25 percent size

As you can see from the full resolution close-ups below, the lens is impressively sharp across the Canon 6D’s entire field of view, with very minimal distortion even in the extreme corners.

Samyang 135 corner performance

The 9-blade diaphragm of the Samyang results in a pleasing radiant around bright stars, but the lens does exhibit some internal reflections.

Samyang 135 reflection

I look forward to using this lens as my (very) widefield astrograph. Depending on your sensor size, the ideal targets for this lens will vary, but I’m looking forward to shooting:

  • The Orion Molecular Cloud Complex
  • IC2177 and Thor’s Helmet area
  • The California Nebula to Pleiades area
  • Orion’s Head/ Meissa Nebula
  • Heart and Soul Nebulae area
  • The Rho Ophiuchi area
  • Cygnus
  • The IC405/IC410 area
  • Taurus Molecular Cloud
  • The Milky Way’s Pipe nebula region
  • Sagittarius

After complex mosaics and multi-night narrowband CCD projects, it’s a joy to throw a simple setup like this onto the mount to grab bright widefield images in a few hours.

Pros:

  • Fast focal ratio
  • Sharpness
  • Flatness of field
  • Price (currently ~$529 USD)

Cons:

  • Some internal reflections
  • Limited targets available for this focal length
  • Will require adapter to fit CCD cameras

Evaluating the full-frame performance of the William Optics Star71

January 23, 2016 1 comment

I’m excited to start imaging with my new Canon EOS 6D.  Having a full-frame chip will allow some very widefield shots that would require mosaics with a 4/3 sensor, like the KAF-8300 cameras I use most of the time.  Since conditions were less than ideal, I used the first couple of nights out to run some tests.  First up is a test of the 6D with the Star71.

The obvious target:  M42.  Below is a very brief exposure (17 min total, in 15s subexposures).

M42 17 min

That is a very wide field.  5.8 x 3.8 degrees.  I didn’t even intend to include the Horsehead Nebula when I pointed the mount at M42, but the field is so wide, I accidentally captured most of it.

Are the stars sharp out to the corners?  Yes.  The image below is a crop of 100 pixel squares from each corner of the above image.  No star reduction was done in any of these images. The performance is really good.  A little distortion on the right side, but quite tolerable.

Corners

What about vignetting?  I estimate less than 10% light falloff between the center and the corners from the flat frame analysis below (the image is highly stretched to reveal vignetting).  Note that there is a dark band across the bottom.  This was consistent across my images, and I’m not sure of the source, but I suspect something to do with dcraw (this image was imported into PixInsight, which calls dcraw for conversion).  DeepSkyStacker seemed to have trouble with some of the 6D’s images too.

vignetting

The 6D and Star71 are a good pair, and it’s nice to have a DSLR again for simple one-shot color imaging, especially for wide fields.  Once I get some adapters, I look forward to running the same test with the Takahashi FSQ-106ED.

Vignetting measurements for every scope I’ve owned

June 22, 2014 3 comments

In my recent review of the WO Star 71, I stated that the measured light falloff from center to corner of an APS-C sensor was 3-4%.  That got me thinking… how does that compare to other scopes?

I’ve owned and imaged with many telescopes over the past few years, thanks to the active used market for scopes.  That means I’ve taken flats with all of them, so on my hard drive, I have measurements to quantify vignetting for all of them.  Since that might be useful to people, and since there might be some lessons to be learned, I spent the morning going back through my files calculating vignetting.

Methodology

  • UPDATED:  Only FITS files were used, with raw ADU counts.  (Previous methodology was unable to account for differences in file types due to gamma curve applied by FITSLiberatoror and Photoshop’s RAW import routine).  Camera-scope pairing where I had only CR2 files have now been excluded.
  • I used a 51×51 pixel average, measured in the center of the image, and in each corner.  I averaged the corners, then compared that to the center.
  • I tried to use flats where the peak value was still in the linear range of the sensor (the middle 20-80% of full well capacity).
  • Where the data were variable, I looked at multiple flats from different days and took an average.
  • All flats were taken using a t-shirt over the objective end of the telescope.
  • This is not a bench test; these are values taken from flats used in practical (read: sometimes imperfect) situations.  There could have been a wrinkle in the fabric or non-orthogonality (sensor not perpendicular to the optical axis) in the system, though I tried to exclude any flats like this in the set used for consideration.

Results

  Light falloff from center to corner for:
Telescope KAF-8300

(18 x 13.5 mm)

Full frame (STL-11000)

(36 x 24 mm)

Takahashi FSQ-106ED at native f/5.0 11%
Takahashi FSQ-106ED at f/3.65 with 0.73x Reducer QE 35%
Televue NP101 at native f/5.4 20% 45%
Televue NP101 at f/4.3 with NPR-2073 0.8x reducer 35%
Borg 77EDII at f/4.3 with 7704 reducer 18%
Orion EON 120 at native f/7.5 22%
AstroTech AT8RC at native f/8 with AT2FF flattener * *
William Optics Star 71 at native f/4.9 12%

So have you ever looked at your flats?  I mean really looked at your flats?  I skimmed through four years of my own, and I was very surprised by some of these results.

With faster focal ratios, it gets challenging to fully illuminate the larger sensors.  The effect of mechanical vignetting on the NP101 is apparent on the full frame STL-11000M, and this is exactly what Televue addressed in creating their “is” series scopes, so I’m guessing an NP101is would perform much better here.

I’ve shown the light fall-off between the center and the worst corner I could find.  When the vignetting gets significant, the slightest bit of tilt between the sensor and the objective is revealed, so one corner or side was usually a few percent worse than the best.

It’s also clear that the Takahashi lives up to its stellar reputation.  At the native f/5, it easily covers the full frame sensor.  At a crazy f/3.65, it’s probably pushing the limit of acceptable vignetting for the full frame chip.

The new WO Star 71 holds up very well in terms of vignetting, at least with the ST-8300 chip.  I previously tested it with an APS-C DSLR, but those data are not comparable to the FITS data here.

I’ve long since sold the Orion EON120, but I was surprised it didn’t perform better due to its higher focal ratio.  I didn’t have good data on this scope with reducers.

Finally, the AT8RC is an interesting case, and I’ve excluded the data because it was harder to make sense of.  I’ve always taken it as gospel that the AT2FF was the flattener to use with this scope.  Taking a good look at my own flats, I saw two things:  1) it looks like there is a bit of a “ring” shape in the brightness, which may be introduced by the flattener’s  (which was, after all, designed for refractors) interaction with the RC optics;  2) there seems to be some non-orthogonality in my system that is making one side brighter than the other.  Or maybe the mirror need to be re-aligned a little.  In some cases one corner was slightly brighter than the center.  Either way, this made it harder to state a simple center-to-corner ratio, so I’ve left the data out.

Lessons Learned

This was an exercise I should have done a long time ago.  I took away several important lessons from it:

  • Look at your flats.  Why put all of the effort into taking good lights, then undermine it by introducing gradients due to poor flat fielding?  I got more careful over time, but some of my early flats left me shaking my head, thinking, “rookie.”   Some days I was taking flats where the peak value was uncomfortably high or low, which experience has since taught me to avoid.
  • It gets harder to maintain even illumination with faster focal ratios.
  • There really is a difference with quality scopes.  The Tak easily upholds it reputation here, which makes me feel better about its price.
  • With smaller sensors (I’m looking at you, Sony 694’s), you can save a lot of money by buying a scope that would be otherwise less acceptable for larger sensors.
  • Conversely, when you buy a larger sensor, you need to support it with better optics.
  • With significant vignetting, non-orthogonality is exaggerated in the flat.

Call for Data

Does this represent your experience with your scopes?  Am I crazy?  Have I missed something here?  I’d love to hear what other people have found looking at their own flats.

 

Review: William Optics Star 71 Imaging Refractor

June 19, 2014 21 comments

I can’t speak for everyone, but my criteria for evaluating a wide-field imaging scope are:

  • Fast optics:  since the goal is typically to capture large, diffuse, and dim objects (usually nebulae), focal ratio is key.
  • Quality optics:  stars should be sharp to the corners, it should be truly apochromatic, and the field is evenly illuminated.
  • A good focuser:  the zone of critical focus is very narrow at fast focal ratios, so any focus shift is unacceptable.

Living under light-polluted New Jersey skies, my preference is to shoot narrowband objects, as that negates most of the effect of the light pollution.  I have an affection for small refractors, and I particularly enjoyed having a Borg 77EDII, which was a super-fast f/4.3, until I recently sold it.  There’s something nice about the simplicity of small refractors.  They are light, easy to balance, easy to align.  Give me a fall evening, a wide-field scope, and a camera set up for narrowband imaging, and I’m happy.

The William Optics Star 71 recently caught my eye as a replacement for my old Borg 77.  I signed up for the pre-order, and Agena AstroProducts delivered my scope this week.  The WO Star 71 has a stated focal ration of f/4.9 and a focal length of about 354 mm* (though this is not explicitly stated anywhere I can find), which nicely frames many large showpiece objects on common sensors. For an APS-C sized sensor, the field of view is 3.6 x 2.4 degrees. For a 4/3 format sensor like the KAF-8300, the field of view is about 2.9 x 2.2 degrees.  I plan to use it mostly with the ST-8300, and this field of view nicely frames a lot of narrowband targets I’m after this year.

(* Based on the field of view calculated by astrometry.net (3.61 degrees) and Canon’s stated 22.3 mm sensor measurement on the long side for their APS-C sensor, I calculated a focal length of 354 mm, which leads to a focal ratio of 4.98 if the aperture is truly 71 mm.  If they fudged a bit, and it’s 72 mm, the ratio would be 4.91, which would match the stated 4.9.)

The evening I got the scope, it was actually clear.  Now, it was a lousy night for imaging because it was over 80 degrees, the winds were gusting, and high humidity made for terrible transparency.  Worse, the waning gibbous moon would only allow for about two hours of dark skies.  So it was a terrible night for imaging… but perfect for taking a few test shots.

Look and Feel

Before it gets dark, let’s have a look at the scope.

The WO Star 71

The WO Star 71

This is classic William Optics:  white and gold.  The focuser has a thermometer–neat, but I’m not sure how much I’ll use that.  The scope comes with a nice pair of rings, a Vixen-style dovetail, and a M48 to Canon adapter.  So even visually, it’s clear that this is an imaging scope.  The focuser terminates in M48 threads, not a 2″ eyepiece holder.  (Note, that’s M48, not the standard T-thread of M42.)  If you want to use this scope for visual use, you have to buy a special adapter, which is fine with me, because imagers prefer to keep all the connections threaded.  For scale, you can see the Orion 50 mm guidescope mounted on top, and the Canon 450D/XSi DSLR.  For a 71 mm scope, it’s actually pretty big.  I don’t have my Borg 77EDII anymore, but I’m almost certain this is bigger.

What you can’t see in the photo is how heavy the scope is.  WO claims it’s 5.3 lbs with the rings, and that seems about right, but until you handle the scope, you don’t realize how dense that is.  There is clearly a lot of glass in there, and not just at the objective end.  It’s the same impression I got the first time I picked up my Tak 106ED.  This thing is solid.  WO notes that FPL-53 glass is used, and that there are a total of five elements in three groups.  People seem to attribute almost mystical powers to FPL-53, and while it is found in the best optics in the world, the glass type doesn’t matter if the elements aren’t well matched or poorly figured.  But between the weight and glass type, we’re off to a good start.

Imaging Performance

Let’s get down to brass tacks.  How did it perform for imaging?

Bear in mind that this was just a chance to take some quick test shots, so hopefully I’ll be able to do more extensive imaging work with it later, but first let’s check the overall field of view with a DSLR.

Field of view around M81/82

Field of view around M81/82

This is a single 30-second exposure of the area around M81 and M82, just to give a sense of scale.  Yeah, that’s a wide field.

“Great,” you say.  “But is the field of view flat?”  Let’s check.  Here are extreme close-ups of stars at the corners and center.  (These are from a different, 15-second exposure, since wind gusts kept streaking the stars a little.  The color was dropped to help reduce noise.)

Corner sharpness

Corner sharpness

These are 200×200 pixel areas from the edges and center.  The stars are sharp, and that’s a pretty flat field, especially for f/4.9.  This substantially outperforms my old Borg 77EDII (though to be fair, that scope was f/4.3 and not known for exceptional flatness of field or perfect apochromatism).  It’s probably not as good as the Tak 106ED at f/5.0, but it’s slightly better than the Tak at f/3.7.

What about vignetting?  WO claims the scope will deliver a 45 mm usable imaging circle. That would be large enough for a full-frame 36 x 24 mm sensor.  That’s always a bit of a judgement call, because the manufacturers never seem to state what level of light falloff is acceptable in determining the size of the usable imaging circle, but okay.  An APS-C sensor needs about a 27 mm circle, so that’s all I can test for now.  I took a flat frame, and then I measured the 8-bit brightness level at the corners and in the center.  I’m sure someone with CCDInspector could provide a pretty 3D map, but this method will tell you what you need to know.

Vignetting

Vignetting

The flat field image was exposed to stay in the linear range of the sensor, and we can see that there is only about a 3-4% falloff from the center to the edge.  That’s really good.  (NOTE:  see next post above.  Photoshop applies a gamma curve when importing CR2 raw data, so the vignetting is actually higher than this, but the performance still holds up very well.)

All right, final test… what about apochromatism?  Do all the colors come into focus at the same plane?  Sorry, I’ll have to update this review later with that information once I get a full night out with this scope and my ST-8300.

And what about the focuser?  I monitored focus through a Bahtinov mask, and as expected, focus is really touchy at f/4.9.  The critical moment came once I had lined up focus and turned the focus lock knob.  This is where cheap focusers will suddenly throw the image out of focus, leading to an annoying game of “How far out of focus do I need to start so that it will be in focus after tightening?”  (Oh, you’ve played that game, too I see.)  In fact, I used to have an early Zenithstar II from William Optics whose focuser was… well, it kinda sucked.  And judging from comments on forums, WO has had some challenges with their focusers.

The Star 71’s rack and pinion focuser performed well.  The image stayed in focus after tightening the locking knob.  I have to say that the focuser does not feel as tight as a Moonlight or Feathertouch, but it did the job admirably.

Summary and Final Thoughts

My initial impressions of this scope are very good.  I’m really pleased so far, and I hope to provide a fuller review with images at a later date.

In my view, here are the WO Star 71’s strengths:

  • A sharp, flat field
  • Exceptional fit and finish
  • Nice focuser
  • Threaded fittings
  • Solid tube rings and a Canon adapter are included
  • Fast enough focal ratio

I would have been thrilled with f/4 or even f/4.5.  An 80 mm objective, but still at around 350 mm focal length would have been amazing, but I’ll take it at f/4.9.  It’s substantially better than some similar scopes.  (I never bought the AT65EDQ because no matter how sharp it is, at f/6.5 I’d never get a narrowband image finished.)

If I had to name a few “opportunities for improvement,” I’d say:

  • I would rather have had an M48 to M42 T-thread adapter included
  • No case is included
  • The dew shield only extends 1.5 inches past the objective lens cell.  This is too short, especially for those of us in humid climates.

And I’m not sure how I feel about the price of $998.  ($898 for the first few sold.)  I know this is a new, patented optical design.  And the optical performance looks really good.  And it’s f/4.9.  But it’s also only 71 mm of aperture.  The price compares well to the Borg 77ED or 71FL, and the Televue 76.  But WO sells their own Zenithstar 71 + 0.8x focal reducer for $568, and that would be an f/4.9 system as well.  Granted, I don’t know if the field is as flat, the optics are as sharp, or the focuser as good, but it makes for an interesting comparison at over 40% less.  To be fair, WO explicitly notes that the Star 71 is a  “[n]ew, unique, patented design – not a conventional objective/flattener/reducer design.”  But having another 71 mm f/4.9 scope out there plants a little seed of doubt.

I have a unique consideration when reviewing this scope.  I originally ordered two with the plan to create a dual-scope narrowband imaging set-up.  I reasoned that I only get a few nights a month of decent weather, so why not double up on the exposure time with two scopes?  But I’ve had a hard time finding another used ST-8300, and once I started pricing everything I’d need, I started to chicken out and reduced my order size to one scope.  (Oh, so that’s why no one seems to be imaging with a dual-scope setup…)  So the question for me now is not whether I’m keeping the Star 71 (I am), it’s whether I’m going to buy another one.

A quick evaluation of the NP-101 for use with the SBIG STL-11000

November 10, 2012 Leave a comment

I recently purchased an SBIG STL-11000, and my biggest concern was whether buying a camera with such a large sensor was going to require me to buy a new telescope to cover that sensor.  My main scope is a Televue NP-101 (the non-is version), which means I have to shoot through the 2″ focuser.  While I’d trade up to the NP101-is or even a Tak FSQ-106 at the right price, let’s face it:  those are very expensive upgrades.

Last night was first light with the new camera, and I’m posting this information to help others make a similar decision.  Two important caveats here:

  1. Focus was a little off.  I think I need to recalibrate FocusMax, and this was just meant to be a “first light” test of the camera. So compare the relative sharpness of the corners, not the overall sharpness.
  2. No polar alignment was done, so there is a little field rotation evident.

First is the question of whether the NP-101 can deliver sharp stars all the way out to the corners.  Let’s have a look at the full image first.

This is 18 10-minute exposures taken through a luminance filter, synthesized in DeepSkyStacker.  When I loaded up the files in Photoshop, it was impressive to see their scale:  4000 pixels across!  Now, let’s look at the corners:

This is sharp enough for me, especially with the field rotation evident.  I am really impressed with the edge performance of the NP-101.  At this point, I’m not seeing a need to upgrade to the FSQ or 101-is.

Second, we have to consider the light fall-off.  I braced myself for considerable vignetting.  Here is the master flat with levels on an 8-bit scale marked in green:

Again, to me this is acceptable performance, though less than ideal.  There is about a third reduction in light at the corners versus the center.  That’s a lot, but it’s not nearly as bad when you move just a little bit inward.  I figure with the usual cropping and overlapping of frames that happens, this won’t be much of an issue.  It’s almost the same levels of vignetting I’ve seen on the ST-8300’s chip when using this scope at f/4.3 via the reducer.  Careful processing there proved that the vignetting wasn’t a problem.

So what’s the verdict?  The NP-101 is perfectly acceptable for use with the STL-11000.  My guess is that the -is version with its larger focuser would perform better, but until I see a deal on one of those, I think I can happily image with this combination.  If anyone has similar information for the STL-11000 with either of the Televue -is scopes or the Tak FSQ scopes, please post a comment for comparison.

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