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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.

 

Testing the William Optics AFR-IV (aka PFLAT4) focal reducer/field flattener with EON120 and TMB80SS

December 11, 2011 Leave a comment

Since it’s a full moon, and I’m not well placed for the total lunar eclipse tonight, I took advantage of the clear skies to test out a new piece of equipment: the WO AFR-IV.  I already have an excellent field flattener, the AstroTech AT2FF, but I was looking for a focal reducer, especially for the EON 120, which at f7.5 is a little slower than I’d like.

The AFR-IV bills itself as an adjustable flattener reducer (0.8x) designed for refractors between 500-1000mm. It also claims to cover the full DSLR sensor area with a 50mm image circle.This claim is interesting because the adjustable back focus settings range from 66-86mm.  A Canon DSLR has a flange to sensor distance of 44mm.  Add on 5mm or so for the t-ring, and you’ve got around 50mm for a standard DSLR, well below the 66mm minimum setting on the AFR-IV.  As we’ll see, this causes some issues.

Here, I’ve tested it with two refractors:  the Orion EON120 (900mm FL, 120mm aperture = f7.5) and the TMB 80 Signature Series triplet (500mm FL, 80mm aperture = f6.3).  The camera is a Canon EOS XSi DSLR.  I shot the region around Deneb in order to get on the other side of the sky from the moon.  Let’s go right to the results. All images are 15 seconds, uncalibrated, with only minimal processing (blackpoint was set).

First up, the EON120.  As a baseline, I took a frame with the AT2FF.  Here are 400 pixel squares from each corner.  You may have to click it to get a full resolution view.  As you can see, the AT2FF works really well with this scope.  (Actually, it works well with all three of my scopes!)

EON120 with AT2FF

I was really excited to see how the AFR-IV worked with the EON.  With the AFR-IV, it would be around 720mm and f6, which is a very exciting prospect.  Unfortunately, the AFR-IV fails here.  Or maybe the EON is the failure.  Either way, they don’t work together.  The EON needs about 2mm more in-focus to work with the AFR-IV at its lowest setting, at least on a DSLR.  2 freakin’ millimeters!  In fact, you can see how close it is in this view through the Bahtinov mask with the focuser racked all the way in.

EON120 with the AFR-IV (so close!)

It should work with a focuser that has more in-focus, perhaps a Moonlite?  Alternately, it probably just requires more back focus, in which case a $17 spacer ring from ScopeStuff will solve the problem.  With a CCD (+filter wheel, +OAG) that naturally has more back focus than a DSLR, the AFR-IV’s 86mm maximum setting is probably a blessing.  I’ll hopefully get the chance to find out soon.  But for now, let it be known that the EON120 does not work with the AFR-IV attached to a DSLR, at least with the stock focuser.  I’ll order a spacer and see if this remedies the problem.

Next up is the smaller refractor, the Thomas M. Back 80mm Signature Series triplet.  It’s a great little scope at 500mm and f6.3, but at 400 mm and f5, I’m even more excited!  Again, let’s start with the AT2FF, which works beautifully with this scope.  Here are the 400 pixel squares from each corner:

TMB with the AT2FFNice, right?  I was focusing by eye here, since I’ve lost my smaller Bahtinov mask, so it’s not perfect, but you can see that the stars are not stretched on the corners.  The full image is 4200 pixels across, so these crops are just the extreme edges.

Now, cross your fingers for the AFR-IV… it works!  There’s not a lot of in-focus left, but it works.  At the minimal setting of 66mm, you can see that it doesn’t fully correct the field:

TMB AFR-IV Corners at 66

TMB AFR-IV Corners at 66

But somewhere around 76mm, it works very nicely:

TMB with AFR-IV

Because of the in-focus issue, the TMB was racked in all the way when the AFR was around 78mm, but it corrects well at that point, so it’s not a problem.

It lives up to its reducer billing too.  The field of view is obviously wider, and the moonlit background sky is brighter as well.  Using the ruler in Photoshop, I calculate that at this setting on this scope with the DSLR, it operates as an 0.82x focal reducer, right in line with spec.

The adjustability of the AFR-IV is a nice feature no one else offers, but the required in-focus or back focus may be a problem for some people.  I really wanted it for the EON, so it’s disappointing that it won’t work there.  I’ll try the spacer ring to lengthen the back focus and see if that helps.

The AT2FF remains amazingly sharp and versatile as a flattener (it even works on my AstroTech AT8RC), but it doesn’t have any reduction power to it.  I’ll be keeping both for now.

M63, The Sunflower Galaxy

July 3, 2011 Leave a comment

While this is only 5h 8m of total exposure time, I was able to get a decent amount of detail out of it.  When it clears up again, I’ll get some more time on it to reduce the noise level, but this is a nice start.  The exposures were taken on two nights a month apart:  Jun 2 and Jul 2.  It’s a total of 77 4-minute shots through an Astro-Tech AT8RC on a Celestron CGEM using a Canon Rebel XSi. The nights are short and hot this time of year, which makes it difficult on astroimaging!

M63, The Sunflower Galaxy

M63, The Sunflower Galaxy

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