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Flat Field Panels

onj's picture
onj
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Hi,

I find it difficult to schedule twilight flats, more so as I use the same telescope for visual observation.

I wonder if anyone has experience with flatfield panels?

In particular the electroluminescence Flatfield Panels adverised by Gerd Neumann:

http://www.gerdneumann.net/english/astrofotografie-parts-astrophotography/aurora-flatfield-panels/uebersicht-aurora-flatfield-panels-overview.html

I use a 28cm SCT (C11) and a 106mm Tak refractor.

Any suggestions appreciated.

John

 

 

 

flat panels
MJB's picture
MJB
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This is what I am using:

http://www.spike-a.com/flatfielders/

Mounted on the wall directly in front of my c11 in the park position.  Seems to work fine.  

Flat issue
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Roger Pieri
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Hi John,

I would not specifically speak about those panels but more generally about making flats with diffuser/light source at or at short distance of the pupil of the optics. At this level of the light path what determines the pixel being illuminated is not the position of the rays in the aperture but the direction of them, the angle. Then most other rays emitted by such light source (or diffuser) don't address the imager pixels but are diffused inside the optical tubes. Such lost flux is much stronger than the useful one. The resulting light diffused at lens surfaces, tube walls... eventually reachs the sensor and affects the shape of the flat. The result is very depending of the optical design, I have seen cases where the error was only a couple of percents but others showing several tens of percents. The comparison was made against a flat made using collimated light (like the stars). 

This problem is a serious one for us in photometry, I was suspecting it for long time, but a recent remark from Arne pushed me to restart experiments about that. Now I understand the problem but regretfully I have no solution to propose ! I can just say: be carreful, check the flat made at pupil against twilight ones. And, as recommanded by Arne, put the critical stars at/near the center of the field ! 

Clear Skies !

Roger (PROC)

Flat Panel
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WBY
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Roger, You are correct about collimated vs uncollimated light, but there are a couple of things you can do to reduce the affect from a flat field source to make them less than you get with sky flats. 

1. Locate the source a few feet from the end of the optical tube but not so far that it subtends less angle than your FOV.  

2. Blank off the emitting area so that it is only slightly larger than the diameter of the OTA. For example, if like me, you have a 10" scope and a 4", and the panel is sized for the 10" you want to restrict the emitting radius when used with the 4".

That reduces the angles of rays into the tube. You actually end up with light entering the OTA over a smaller range of angles than with sky flats because the sky is a radiator over essentially a 180 degree angle and is not collimated like light from stars. 

The most important things are that the light source be really flat, and have an appropriate continuous flux over the spectrum of interest. I am a bit concerned about sufficient illumination at the extreme ends of the spectrum,  Ic in my case since I don't have a U filter, but I will soon find out since I just ordered one of these gizmos. I will also find out if I need one of the neutral density filters they mention. It wouldn't be good for an automated observatory if the intensity varies over the spectrum sufficiently that you need a neutral density filter for some passbands and not for others.

By the way, there is even Question about the need for continuous spectrum flux provided you are not using elemental filters, as long as emissions are present near the peak of each filter bandpass. See attached article. 

Brad Walter, WBY

Flat Panels
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MJB
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Great article, thanks.  But, it would seem that the problem with the sky flat was not related to the lack of collimated light but the unavoidable gradient in sky illumination?  In any event, the allure of the panels such as inquired about, for a robotic set up, is they provide a dust cover function.  Based on this article, one advantage of these flip flap designs is it would seem to assure perpendicularity.  Especially for a mirrored instrument where the pupil is necessarily somewhat removed from the panel, one wonders whether any error from imperfect collimation at the focal plane would outweight the potential advantages of the system and/or be material in the context of other inherent errors. A refractor system would seem more vulnerable but even there one wonders whether a well baffled system would not substantially deminish the effects.  I opted for the wall mounted system out of fear for windage that the flap system would involve.  

Flat Panels
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WBY
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Oh absolutely. I wasn't trying to claim that the major problem with sky flats is the lack of collimation. I was only trying to point out that even sky flats are uncollimated and that this effect should not stop anyone from using flat panels. 

I asked Gerd Neumann about the spectrum of his Aurora device and he sent me the attached with the caveat that outside 400-700 nm his test equipment lacked sensitivity. So althought it looks like it has essentially no output in the Ic band, which I expected, I simply will have to try ii through my Ic filter so see if it is useful in Ic. It looks pretty good in B, V and R and potentially has the same problem in U as in Ic. Also attached are a set of Astrodon UBVRI passband curves, for comparison.

I got interested in this when I discovered that a light box I had used with alacrity for several months was about as flat as Little Round Top and the lights had to be completely repositioned to make it acceptably flat (less than 1% variation).

Brad Walter 

Flat Target
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Roger Pieri
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Hi Brad, 

Yes, agree, I also use this solution. It provides a valuable improvement in case of instrument showing a serious problem but some errors remain (at a few percent level at edges). It also needs some optical consideration to determine the right size of the target, large enough to provide all rays normally reaching the sensor and tacking in account the strong defocus effect (as the instrument as to be set at infinity focus). 

It is easy to apply to telescopes but not so to shorter focal length lenses. 

Use of LEDs is also somewhat difficult as they have a non-smooth pattern. It needs a good additional diffuser to provide a uniform illumination of the target.

Clear Skies !

Roger (PROC)

LEDs for Flats
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WBY
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If used in a light box I assumed they woule reflect of the back wall of the box and go through two diffusers similar to the common types of halogen or incandescent light boxes. In dome flats I assumed they would be on the outside of the scope baffled from the aperture and reflecting off the dome screen. Both ways seem to work. 

Flat Target
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Roger Pieri
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Brad, if the illumination from LED is indirect yes, agree. But I have seen guys directly illuminating the flat target with LEDs ! 

Anyway it's difficult to ensure the target is uniformly illuminated. I usually check it with a photometer and I can say it's not straightforward to get it below 1%. Then the Lambertian condition of the target is also an issue (most are not at % level), including under "pupil" condition (for a telescope a couple of meters is in). 

CS  Roger 

The real issue ...
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wel
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Folks,


I am very intrigued by this thread and by the realization of an EL panel that is full aperture for this particular purpose.

As far as I can tell, the issue is not whether it is perfect or not but whether it is a significant improvement over existing non-sky-based practices. While people can express their opinions and doubts, the proof of the pudding is in the eating. When someone puts it to the test and tells us the  level the residuals are, we will know the answer.

I have to say that my expectation is that such an apparently-uniformly emissive, broadband surface sounds very promising relative to essentially any scattered light target or small number of illuminants target in the near field.

We shall see!

Doug

Flat Panel
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WBY
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Doug,

I'll let everyone know in about a month. I figure it will be about that long before it arrives and I have chance to test it.  Mew Mewlon with a 6303 CCD chip camera is a good testing ground since it vignettes rather drastically. I am particularly interested to find out if it is useful in the Ic band. I don't have high expectations, but we'll find out. 

Brad Walter

Flat Target
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WBY
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Yes targets are a problem. The company in Canada that made the lambertian screens is no longer making them. About the best you can do is use a projector screen if you are doing dome flats. I have tried to find intensity vs. angle specs for them but have had no success. The only specs you seem to get are size and weight and for some of the better ones viewing angle. Some have viewing angles as wide as 160 degrees ( = 80 degrees on each side of the normal) so it seems they would be relatively constant over a small angle of say 15 degrees from the normal . Spectral resonse info is even more scarce. 

Brad Walter

Flat Experiments
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Roger Pieri
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Hi Doug,

When I am saying there are issues it is not just opinion or doubt, this comes from experiments and measurement results. I am a guy from the visualization technologies R&D and I think I got some experience in optics experiment and design after many years in the field.

There are problems with flats !

Electroluminescent panels are only one case,  all of the techniques being used for flat have problems on various optical aspects, the most critical being the vignetting compensation (that strongly affect our photometry). 

Attached are three curves. It relates to a classical 80 mm refractor. The yellow curve is the flat profile on the horizontal axis of the sensor made with a collimated light flux (like the flux of a star, similar to what is shown in the paper distributed by Brad). The blue is the same H profile of a classical flat made with a strong, near Lambertian, transmissive diffuser near the pupil of the refractor (like an electrolum panel). The green curve is also a classical flat but made with a white target limited to the field of view of the refractor and the sensor. The target is surrounded with a black material to eliminate as much as possible the unwanted diffuse light. The target is set at two meters from the refractor pupil. The targets uniformity has been checked with a photometer.  

No problem ?

As a visu tech guy I would also mention NO screen: projection, LCD, plasma, CRT... are Lambertian (one of our usual problem) They are all VERY FAR to be Lambertian. 

I didn't mention the limited target solution at start as I consider it's not perfect and needs more work. 

Clear Skies !

Roger (PROC)

Flat Experiments
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WBY
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Thanks Roger. the curves are informative. I expected some variation but not as much as between the yellow line and the Blue curve. So It looks like the best thing for the EL screen is to move it away as far as you can within the confines of your set up and still have it cover the FOV and mask off any excell illuminated area with non reflecting black material. In my little observatory that is only half a meter, but it will have to do. 

I have found that black matte foam sheet material made for kids arts and crafts (It goes by a bunch of names like funny foam, crazy foam or just foam sheet at Walmart) works very well as a mask. I used it to cover the outside of my light box made of white foam board so that it wouldn't disturb others if used it at a star party and I have used it to mask around flat field reflecting surface. It is very black and it reflects very little light. It is flexible, cuts easily with scissors or a knife,  glues well (but certinly not to the EL screen) and does not absorb water. So dew is no problem and it keeps dew out my light box foam board. 

Brad Walter

Proof of the Pudding_First Course
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WGR
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Hello Doug;

I was hoping someone was going to post their raster results, comparing panels.  No one has so far.  As Doug points out, its not an opinion of which flats are better, there is an objective test that measures how good one's flat field process is.  It is the Raster test as described in the paper that Brad posted. 

I have attached my raster test from last week.  I made at least 2 mistakes.  I was trying to make the test short, so as to have uniform transparancy throughout the test.  I used 1 second exposures, and then realized that scintillation was probably interferring.

The second, was that I had to use month old flats.  I will revise this. 

Posted below is the result.  The repeatability of 4 raster runs was .014 mag one sigma.  Throw in the lack of flatness, brings this to .04 mag one sigma.  Looks by eye that 3 sigma is about 0.1 mag.  I plan to use 20 second exposures and fresh flats and will repost. 

So lets see your Rasters.

 

Gary

 

 

 

 

As it is already said in this
Tonisee's picture
Tonisee
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As it is already said in this thread, for (ultimate?) scattered light correction in the flatfield frames, one can use stars. Those stars doesn't need to have well defined standard magnitudes,  just constant brightness. For such schemes see e.g. Manfroid 1995 (http://adsabs.harvard.edu/abs/1995A%26AS..113..587M) or more recent Ubercalibration data processing scheme (http://arxiv.org/pdf/1111.2058v2.pdf). Both of them include "rastering". I'd also suggest to read Manfroid's paper just for background - well written and besides "complex-looking-but-easy-math", has numerous important points.

It is known, that quite a good substance to use for lamp-illuminated flat field panels is barium sulfate. It's white and it's spectral reflectance is very flat across whole visible (and near-infrared?) spectrum:
From: http://www-kpno.kpno.noao.edu/glaspey/KP/FFLamps.html
---------------------------------------------------------------------
Here is the recipe for the high reflectance white paint, as taken from the paper by Wu et al. (1972, AAS Photobulletin 1, 9).

"To make one pint, heat 50ml of distilled water to 52-66 C (125-150 F) and slowly add 2.25 grams of elvanol polyvinyl alcohol (Grade 72-60), mixing until all the alcohol dissolves.� To this mixture add 150 ml of distilled water and 200 ml of 200 proof anhydrous ethyl alcohol.� Then slowly add (stirring continuously) 227 grams of USP grade barium sulfate to the mixture.

In practice it seems best to mix this paint just before use. A coating of Krylon 1502 flat white paint was sprayed as a primer.
-------------------------------------------------

And of course, from AAVSO site: http://www.aavso.org/files/ch8_0.pdf

Manfroid and UberCal Papers
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WGR
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Hello Tonisee

Thanks for pointing me to these papers.  Much to absorb.  However, 

  "Adopting the global linear least-squares (\ubercal") approach developed for the Sloan Digital Sky Survey (SDSS), we derive corrections for all observing runs, which indicate that the original sky flats were nonuniform by up to 0.13 mag peak to valley in z band, and by up to half that amount in BVR." from the Ubecal paper.

I guess my Raster is not so bad.  If Sloan can be off by 0.13 mags.  Of course, these papers describe how to get much better.

 

Gary

Raster
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WBY
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Gary, Are your 5 series 5 different X positions at 5 different Y positions, in otherwords is this a 5x5 matrix, or the same 5 positions repeated 5 times? Do you do this every evening or just just periodically, like transformation coefficients, and whenever you change anything? I guess if you are doing sky flats you would do it every night. I would think dome flats or light box flats would be very repeatable, as long as you hadn't changed anything in the image train, the light source or the relative positioning of the two. 

Brad Walter, WBY

Raster
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WGR
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Hello Brad

The first plot was a 5x5 raster, ie 25 positions on the chip.  I repeated the test 4 times, so there are 100 data points total.  I calculated a std dev of the 4 data points at each position.  Typical std dev was 0.014 mags.

I do sky flats every clear night.  Rarely do the raster test.  Mostly do Raster to track down stray light, etc.  I don;t change things very often, most frequent change is about 6 months to change filters in the filter wheel. 

As for dome flats, light box flats, and ELU panels, been there, done that and have settled with Twilight flats as the best solution.  I can use twilight flats for a week or so with no problem, at 4 weeks, they show signs of some dust donut leak thru--ie the dust donuts are not completely eliminated.  Sometimes weather forces to use old flats.  I the weather is good, I take then everytime I observer.

 

Gary

AAVSO Files Chapter 8
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Richard Wagner
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Hi Tonisee:
This is an interesting document.  Can you tell us what it is, is there any more of it, is it current (out of date, under construction?), and where can we get more of it?

Thanks

Richard

Mysterious document :-)
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Tonisee
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I believe, you pointed to "AAVSO document". I don't have slightest idea! But I suspect that it is written by Arne. Hopefully it was not a wrong decision to link it here. After all - I found that link using Google.

flat fielding chapter
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HQA
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That chapter is an early release copy from the CCD book, and was used for the CHOICE course on image calibration.  A later version spins on my disk, but you won't find it through Google. :)

My understanding of the flip flat is that you turn it on and then use the camera shutter to adjust the intensity.  In that case, any nonuniform deviation for short exposures is caused by the shutter, not the EL panel.  This was being discussed on the CBA list.

Flat-fielding is an art.  There are many ways of doing it; most work; you just need to understand what is going on and be prepared to test your assumptions.

Arne

Brightness of Flip Flat/ELU
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WGR
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Hello

Just a minor correction here.  The flip flat does, according to their web page and op manual, have a means of adjusting the intensity of the panel to accomodate narrowband, normal band and wide band filters.  

I also have an early one that I picked up on Amart, of unknown manufacturer, that has a 10 setting output from the power supply to do the same thing.  I suspect that most do this.

This relieves one of the need to adjust the exposure as the only means of setting the level of the flats.  

 

Gary

 

PS:  I am still waiting for someone to post their raster results with an ELU or sky or dome flats.  

AAVSO 49 Bay State Rd. Cambridge, MA 02138 aavso@aavso.org 617-354-0484