Home > Uncategorized > Vignetting measurements for every scope I’ve owned

Vignetting measurements for every scope I’ve owned

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.

 

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  1. June 22, 2014 at 5:37 pm

    Hi,
    I like to compare astro-equipment among itself, it brings surprising results quite often, but this time I have to defend the Borg77EDII at F/4.3. Where can you see 50-60% of vignetting on such a tiny sensor KAF-8300? do you have some evidence, image, raw fits???
    I do get the same bad number, but on a KAI-11000 (36x24mm) full frame format!
    http://blog.astrofotky.cz/pavelpech/?p=552
    sincerely
    Pavel

    • June 28, 2014 at 12:07 pm

      I’ve posted an important correction to the data. I think the new numbers will agree with Pavel’s results with the Borg: 18% vignetting for an 8300 chip.

      The issue was that I was using a mix of CR2 and FITS data imported into Photoshop, but I didn’t realize that FITSLiberator was still applying a gamma curve during import, despite selecting “Linear” for the transformation and resetting the sliders. So I’ve gone back and re-done the analysis using only raw ADU counts in CCDSoft and excluding all of the CR2 data, since I can’t confidently normalize those to an equivalent ADU count.

      My apologies for the confusion.

  2. June 22, 2014 at 6:38 pm

    [Correction now noted in above comment.]

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