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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.

IC 2177, The Seagull Nebula

April 16, 2012 Leave a comment

This is a narrowband image of the nebula IC 2177, known as the Seagull Nebula.  Frankly, I think it looks a lot more like a parrot, even a phoenix (which would be a more dramatic name), but it’s hard to deny that it looks like a bird of some kind. It’s just a terrifically photogenic deep-sky object.

Also going for it is the fact that it was discovered by an amateur, Isaac Roberts, who published what Wikipedia calls ” the first popular account of celestial photography of the deep sky” in 1893.  I was going to complain that it took me four nights to collect the data for this image, but I’m sure Mr. Roberts had it a lot tougher than me back in the day.

Click to enlarge

Image data:

Exposures:  28 x 600s Ha , 23 x 900s O-III, 17 x 900s S-II (14h 40m total), all binned 2×2

Software:  guiding by PHD, stacking in DeepSkyStacker, processing in Photoshop CS3

Telescope:  Borg 77EDII 330mm f/4.3

Camera:  SBIG ST-8300M with Baader standard narrowband filters

Mount:  CGEM

Taken March 17, 18, 19, and 22, 2012 from Whitehouse Station, NJ.

IC405 and IC410 in narrowband

March 17, 2012 Leave a comment

This image represents four nights of exposures, including plenty of human errors and adjustments along the way.  Ultimately, 11 hours of exposures went into the final image, though I took about 20.  This became more of a project than I thought it would!

IC405 (right) is known as the Flaming Star Nebula.  I don’t know if IC410 has a nickname, but people call the two little gas squiggles near the top The Tadpoles (not to be confused with the interacting galaxies with the same nickname).  I wanted to capture both in one frame, which is just barely doable at 330 mm with the Borg 77EDII.  In H-alpha, these are both reasonably bright, but the O-III and S-II data are very dim.  In fact, I took two nights of exposures, split equally among the three filters, before I realized that 10 minutes binned 2×2 wasn’t giving me enough O-III or S-II to stretch.  The histogram was so narrow, the nebulosity pulled into a few discrete levels, even at 16 bits.  So I went back and took two more nights of just O-III and S-II, but binned 3×3.  This sacrifice in resolution was less than ideal (it’s a ridiculous 10″ per pixel), but I drizzled the resulting frames to pull a little more detail out, then combined it back with the H-alpha at the original resolution.  I don’t even want to talk about the night of data I lost because I forgot to check the “autosave” box in CCDSoft.  Then I processed the heck out of it, and though I’m less than thrilled with the final result, it’s time to let this one go until next year.

Image data:

Exposures:  23 x 600s Ha binned 2×2, 23 x 600s O-III binned 3×3, 20 x 600s S-II binned 3×3, a total of 11h 0m

Software:  guiding by PHD, stacking in DeepSkyStacker, processing in Photoshop CS3

Telescope:  Borg 77EDII 330mm f/4.3

Camera:  SBIG ST-8300M with Baader standard narrowband filters

Mount:  CGEM

Taken March 5-6 and 13-14, 2012 from Whitehouse Station, NJ.

The Horsehead Nebula in H-alpha

February 27, 2012 Leave a comment

The 1983 National Geographic cover featuring this nebula really captured my imagination as a child.  I’ve always thought it was one of the coolest things in the sky, and it’s amazing that amateurs can now take images from their backyard that rival the best professional observatory pictures then.

This grayscale image represents 27 ten-minute exposures through a Hydrogen-alpha narrowband filter. This filter captures only deep-red light produced by ionized Hydrogen (and Nitrogen) in the nebula.  Consider that the visible light we see ranges from about 400 to 700 nm in wavelength.  Here, you are only seeing the light from a tiny sliver of the spectrum from 653-659 nm which, fortunately for us, is where nearly all of the light from this object is emitted.

The Horsehead Nebula in H-alpha

Image data:

Exposures:  27 x 600s Ha, a total of 4h 30m, taken 17 Feb 2012.

Software:  guiding by PHD, stacking in DeepSkyStacker, processing in Photoshop CS3

Telescope:  Borg 77EDII w/ f/4.3 reducer

Camera:  SBIG ST-8300M with Baader filters

Mount:  CGEM

NGC 1499, The California Nebula, in Narrowband

February 25, 2012 Leave a comment

This Borg 77EDII is just too much fun with the large narrowband objects.  Combining the f/4.3 focal ratio with 2×2 binning on the ST-8300, I can get a ton of data in a short time.  This image was taken over a single night (2/9).

NGC 1499 is also known as the California Nebula.  I’ve chosen to orient the image differently, but you can see the California shape if you turn your head to the side.  It’s a huge object at over 2 degrees across.  That’s more than the width of four full moons.

NGC 1499, The California Nebula

Image data:

Exposures:  15 x 600s Ha, 10 x 600s O-III, 10 x 600s S-II, a total of 5h 50m

Software:  guiding by PHD, stacking in DeepSkyStacker, processing in Photoshop CS3

Telescope:  Borg 77EDII w/ f/4.3 reducer

Camera:  SBIG ST-8300M with Baader narrowband filters

Mount:  CGEM

The Rosette Nebula in Narrowband

February 10, 2012 Leave a comment

This is my second narrowband image, and the debut of my new Borg 77EDII.  The best thing about this image is that it was gathered over three nights when the moon was over 85% full.  I could never have done that before moving to narrowband!

It could definitely use more S-II data, but I’m pleased with this first attempt.  I didn’t realize how dim the S-II portions of the Rosette would be, otherwise I would have used more of the total exposure time there.

This image is processed in a modified version of the teal-gold motif that Bob Franke describes on his website (http://bf-astro.com/hubbleP.htm).

The Rosette Nebula in Narrowband

Image data:

Exposures:  28 x 600s Ha, 20 x 600s O-III, 20 x 600s S-II

Software:  guiding by PHD, stacking in DeepSkyStacker, processing in Photoshop CS3

Telescope:  Borg 77EDII w/ f/4.3 reducer

Camera:  SBIG ST-8300M with Baader narrowband filters

Mount:  CGEM

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