Flats and things that can mess them up.

[ollypenrice 28,425]

Some of you may remember that, try as I might, I could not get workable flats from Yves' setup based at my place. I have now get them working better but not perfectly. Why I've realized the improvement is not clear but Yves has just sent me this link. It's quite long and involved but concludes that stray light inside the optical path is a big player. There is also a detailed discussion of the spectrum affecting flats. One rather alarming discovery is that a surface you thought was black might, in fact, be highly reflective in some wavelengths and might be pouring light onto your chip! All of this strikes me as a powerful explanation of why we were having our problems and why I was not getting them in other setups using similar techniques.

What I'd love to know is how black my black flocking really is...

http://www.sbig.com/...the-ugly-truth/

Olly

Edited April 4, 2013 by ollypenrice

[michael.h.f.wilkinson 20,757]

Apart from reflection in the tube, the spectral dependence of flats should be greatest in refractors (even APO triplets). A pure reflector like an RC should be better. An ODK has a corrector which might introduce problems.

[ollypenrice 28,425]

Apart from reflection in the tube, the spectral dependence of flats should be greatest in refractors (even APO triplets). A pure reflector like an RC should be better. An ODK has a corrector which might introduce problems.

True, though in reality there are hardly any pure reflectors in amateur circles because of the need to flatten the field and/or speed up the F ratio. Many RC users add the reducer, though not all. I wonder how the Riccardi-Honders fares since, in theory, the chromatic aberration should be self cancelling...

Olly

[MartinB 2,497]

Given that the light is so "defocused" won't chromatic aberration cease to be an issue?

[michael.h.f.wilkinson 20,757]

Given that the light is so "defocused" won't chromatic aberration cease to be an issue?

Less of an issue, certainly, but not "no issue". An IR-UV blocking filter will remove most of the unwanted spectrum for which the optics are typically not corrected.

[Ant 4,089]

That's quite a scary read actually. Thanks for finding and posting it. Shows that even taking the time to do everything properly you can still be held back...

It's one of those things that unique to each setup - even those that are identical. I guess the answer would be to look through the focuser during the day and see what reflections you have and try and deal with the worst of them...

Will fully read the link again later.

Cheers

Ant

[ollypenrice 28,425]

The daytime test looks like a 'must.' In fact when Ralf Ottow looked through Yves' 14 inch ODK he instantly said, 'You will have internal reflections with this,' and he was dead right, we were getting some nasty ones. He designed a simple baffle to slip over the outside of the baffle tube, fitted it, and the problem dsappeared. I'm now going to flock that baffle, though.

Olly

[opticalpath 278]

Thanks for drawing attention to that article, Olly. Very interesting indeed ..... and quite scary! My own flats are not perfect these days and this article brings to focus a couple of things I've been wondering about:

1. The aurora flat-field foil. It's supposed to have a wide spectrum, but it's also definitely much brighter at the blue/green end - could that be an issue?

2. I take my flats with the panel perched on top of the dew shield (keeping to the imaging setup) but now I wonder what reflections from the inside of the dew shield might be doing to my flats? Should I perhaps remove the dew shield and put the panel right on top of the OTA, I wonder?

Adrian

[ollypenrice 28,425]

Thanks for drawing attention to that article, Olly. Very interesting indeed ..... and quite scary! My own flats are not perfect these days and this article brings to focus a couple of things I've been wondering about:

1. The aurora flat-field foil. It's supposed to have a wide spectrum, but it's also definitely much brighter at the blue/green end - could that be an issue?

2. I take my flats with the panel perched on top of the dew shield (keeping to the imaging setup) but now I wonder what reflections from the inside of the dew shield might be doing to my flats? Should I perhaps remove the dew shield and put the panel right on top of the OTA, I wonder?

Adrian

Alistair Howie sent me a reply to the first question from GN himself, arguing that since the natural spectrum of the daytime sky is blue dominated the foil has to mirror this. I'm not sure that the night time spectrum is going to be the same as the day time one but I've no means of knowing and I dare say GN has gone into this. Maybe the sky is blue at night, though faint, as well. Not a clue!

Your second point also has to be worth a think. If internal tube reflections are an issue why not minimise them and keep the foil close to the objective? My instinct (for no very rational reason) has been the reverse. I worried about glare being generated by close proximity between foil and lens. I think I'll try it dewsheild down, though, because I can see your point.

Generally my flats work fine on the refractors. It's the ODK that's sometimes the killer. These big reflectors do get internal reflections and glare, though. Lots of people told us that they were having problems with RCs etc. We were getting inverse vignetting exactly as described in the article.

Olly

[opticalpath 278]

....... Maybe the sky is blue at night, though faint, as well. Not a clue! .........

Well yours is certainly pretty black (or very dark blue at least) and faint, Olly. Mine on the other hand is dirty orange and a few magnitudes brighter! Perhaps a warm light would be a better match for my flats.

What do the pros do - real sky flats?

Adrian

[ollypenrice 28,425]

Well yours is certainly pretty black (or very dark blue at least) and faint, Olly. Mine on the other hand is dirty orange and a few magnitudes brighter! Perhaps a warm light would be a better match for my flats.

What do the pros do - real sky flats?

Adrian

Good question. I'll see what I can find out.

Olly

[Cath 1,106]

http://www.sbig.com/...the-ugly-truth/

Interesting read, thanks.

I wonder how black board paint responds to IR and across the spectrum? .. I guess a simple IR led and IR sensitive camera would soon tell.

Also, (although I've never taken flats) I would have though that it would be important to move your scope around while taking a sky flat exposure (rotate/twist it as well if you can) - so as to try to overcome the sky light gradient?

Some pro's do take sky flats and some use a light box/image on the inside of the dome ..

Skip to 5:44 .. http://youtu.be/A0Tyrh5VqPc?t=5m44s

Skip to 1:38 .. http://youtu.be/K0Jm53B3zd0?t=1m38s

Edited April 7, 2013 by Cath

[ollypenrice 28,425]

The key thing, I think, is to let the sky drift past between subs because you're likely to pick up faint stars.

Personally I can never get sky flats to work, though some swear by them. Nor can I take flats in any significant ambient light. They nearly always give a gradient. I've stopped beating myself up over the illogicality of this and just accept it these days.

Olly

[opticalpath 278]

Before I had an EL panel I used to do twilight flats but leave the mount tracking, nudging it slightly between exposures. Any faint stars would be sharp in individual frames, but a median or sigma combine eliminated them. But I never felt confident that the bit of twilight sky I was pointing at was really free from a gradient.

Adrian

[RobH 681]

Interesting article. I'm not sure I agree with some of the conclusions though, although the science is obviously right.

Reflections......you'll be getting the same reflections when you're imaging as when you shoot your flats, just fainter, so there shouldn't be an issue here.

Different spectrums......your flats are mono (obviously with an OSC things are a bit different). As long as you have enough illumination it doesb't matter at all what the colour spectrum is....I certainly found that it didn't make a jot of difference when using an EL panel (very blue spectrum) with my refractors even when shooting flats for an Ha filter.

I have no issues getting good flats with my RC. I don't use the EL panel as I foolishly ruined it trying something clever with a dimmer switch so I use skyflats at dusk or dawn. I use skyflats assistant and put a perspex sheet which I've gone over with a sander to make translucent over the end of the scope thus getting rid of any faint stars.

Cheers

Rob

[opticalpath 278]

How did you frost the perspex, Rob: orbital sander and fine abrasive?

Adrian

[RobH 681]

How did you frost the perspex, Rob: orbital sander and fine abrasive?

Adrian

Exactly that Adrian.....I'm sure that it's not absolutely even, but it looks it, and I always run Gradient Xterminator and Pixinsight DBE anyway so any minor issues tend to get cleared up once this is done.

Rob

[Cath 1,106]

Been eating stars again Rob?

I'm going to try the perspex sheet method a go I think. I guess this might be ok? ...

http://www.sheetplastics.co.uk/Acrylic_Perspex_Sheet/Frosted_Acrylic_Sheet/White_Frosted_Perspex_Sheet/3mm_White_Frosted_Perspex_Sheet?product_id=2333

[IanL 986]

So let me see if I understand what they are saying?:

- The problem isn't internal/stray reflections per se. Although one should aim to minimise these to increase contrast, it should be possible to remove any residual effects by means of flat fielding.

- The problem is a mis-match between the spectrum of the night sky and the spectrum of whatever you are using to create flats. If your flat-fielding device emits a lot more IR than the night sky, you may end up with a flat field image that doesn't match the image to be corrected.

- Several ways to fix this:

- Use a flat field source that properly matches the night sky source (hard to achieve in practice unless you can analyse the spectrum being emitted).

- Sky flats, (True sky flats or maybe twilight flats - harder than it sounds from the comments above).

- Covering the internal surfaces with anti-reflective paint or flocking across a wide spectrum so that it doesn't reflect IR. I'd guess it is pretty easy to check paint/flocking samples since all you'd need is your current camera, an IR LED light source (1.5V AA battery, 3.6 ohm resistor http://www.maplin.co.uk/metal-film-0.6w-resistor-2162, IR LED http://www.maplin.co.uk/high-power-infrared-emitting-diode-2253#overview), a shield to block direct light from the LED, a painted/flocked sample and a darkened room.

- Using an IR blocking filter for luminance/R/G/B imaging and for corresponding flats, thus cutting off the unwanted IR spectrum.

- Narrowband filters shouldn't suffer from the problem (but clearly the flat light source needs to have sufficient illumination in the relevant area of spectrum).

My LED lightbox wasn't giving me a flat field with my DSLR (and definitely isn't optimised to give a full spectrum!) but then I started using my IDAS LPS2 filter and it has definitely improved in that area, since it (and the CLS) appear to be good IR blockers: http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5743518/page/0/view/collapsed/sb/5/o/all

If you're looking for a good diffuser, search fleabay for "Opal Acrylic Sheet"; I got a couple of offcuts for not much more than the postage for my LED lightbox. I guess you could sand a clear sheet if you have one, but if you're going to have to buy one anyway, saves hassle!

[opticalpath 278]

I just bought one of these to experiment with - a handy square size:

http://www.ebay.co.uk/itm/3mm-Opal-Plastic-Sheet-Perspex-Cast-Extruded-Acrylic-Plexiglas-278-x-278mm-/171011694481?pt=LH_DefaultDomain_3&hash=item27d1175f91

Adrian

[RobH 681]

Been eating stars again Rob?

I know....they're bad for you....long term star eating can give you hydrogen poisoning, but they taste great!

[ollypenrice 28,425]

Interesting. We got very much the inverse vignetting predicted by the reflections theory, not that this proves the theory to be correct.

What a lot of this hangs on is whether the hypothetical reflections remain linearly proportional to the beam across a range of beam brightnesses. I simply don't know whether they would or not. Could they become realitively more significant as the beam brightness increases? In which case they would cause the inverse vignetting since flats are taken in a brighter beam than lights.

Or could the issue be that there is a particularly reflection prone part of the spectrum in the light sources used for flats but not found in the lights?

The crux is just whether or not the reflections really do remain proportional to the true signal landing on the chip between flats and lights. Rob, you reckon they must. As I say, I'm not confident that I know one way or the other. (Not an unfamiliar situation for me!!)

Olly

[opticalpath 278]

It's a good question!

Just a thought: Suppose we take a completely dark frame. It would make no sense to apply flat correction to that - right? OK, so now go to a very dark place and take a very short exposure of the sky. Imagine just a handful of photons above true dark level get through. Should we flat-field correct that image, and if so why, since it's very nearly a dark frame? How bright would the image have to be before flat-frame correction IS appropriate?

I think the answer is that flat calibration is a multiplicative correction: the calibration calculation includes a factor that takes account of the brightness of the light frame. In other words the calculated pixel value adjustments should be proportionate to the brightness of the light frame. In the case of the almost-dark light frame, the (much brighter) flat frame still only applies a proportional change to the pixel values - in this case a tiny correction - so it seems like the difference in illumination levels between flat and light frames should not be a problem (... should it??).

I can see the potential for trouble if the light spectrum is significantly different, though. It seems from the article that a flat taken with a source rich in IR may be affected by unexpected reflections and would look different to a flat taken with visible-only light that does not suffer such reflections (unless the IR is fully filtered out by the imaging train). Two different looking flats can't both be correct. Even with IR filtered out, who knows what other subtle differences there might be in unwanted reflections even between blue and red visible light?

Maybe twilight flats with a decent diffuser IS a better match than artificial light sources. Perhaps the final answer is to construct pseudo flats from exposures of the actual sky taken around the time and place of the light frames, and at the full light-frame exposure time. Then we could just subtract the pseudo-flat and be sure it was a pretty accurate match. Bit time-consuming though!

Adrian

Edited April 8, 2013 by opticalpath

[RobH 681]

The crux is just whether or not the reflections really do remain proportional to the true signal landing on the chip between flats and lights. Rob, you reckon they must. As I say, I'm not confident that I know one way or the other. (Not an unfamiliar situation for me!!)

Olly

I'm not certain that they will Olly, but I haven't had an issues like this so will carry on doing what I do normally! If the reflections are non-linear, then one would assume that shooting flats at a lower ADU would help alleviate the problem. I do tend to shoot my flats at a quite low ADU, rather than 1/2-2/3 of full well depth, so maybe that's why I'm not seeing these effects.

Cheers

Rob

[ollypenrice 28,425]

Adrian, the flat calibration is performed by division so it should be self-adjusting, as it were. I don't know the mathematical term but I have a mathematical genius coming soon and will bend his ear...

Rob, I think that our ODK flats started to work at least fairly well when I dropped the ADU as you are suggesting. I now go for about 15000. In fact I think you suggested this, for which our thanks. Why this is important I don't know. It doesn't affect the refractors, so I do wonder if it's something in the optical path - which is why I liked the reflections theory.

Olly