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A flatbox that's...flat!

ollypenrice

By ollypenrice
in Discussions - Cameras

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ollypenrice

ollypenrice

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Hi all,

One of my French guests appeared with a sheet of luminescent plastic for taking flats and I knew straight away that I was in love. (with the sheet.) They are not cheap - this cost 80 euros for A4 size - but, boy, are they ever the business. If you are lucky enough to have a really big scope then the cost soars but this will do all our instruments up to the 5 inch.

You can use them 'as is' but I wanted to protect mine and avoid generating cool spots where the sheet is in contact with the scope so I sandwiched it between a sheet of hardboard and a sheet of the translucid perspex I used for the original Cumbersome Box I made.

It gives exemplary flats. You just sit it on the zenith-ward scope and shoot. There is no back light or even stray light to annoy other imagers (very important here, obviously) and it just is super convenient. It's fine for narrowband.

I had a hard time getting daytime flats because of light getting past the filterwheel and in at a tangent past T shirts etc. (It rreally is bright here by day, really a killer for flats.) This is fine at night so problem solved.

Mine came from Selectronic in France so I don't know about the UK but here is the link;

SELECTRONIC ::: L'univers électronique :::

Olly

post-15040-133877402977_thumb.jpg

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ollypenrice

ollypenrice

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No. However, my fastest focal ratio is just f3.9 and it's fine with that. If you had a hypersensitive camera that was saturating on your shortest exposure all you'd do would be add another light absorbing diffuser; another panel of perspex or a couple of layers of white T shirt. If you want more light you just give it a longer exposure. I usually find I need less than a second so there is plenty of room to collect more light.

Olly

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RobH

RobH

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I got mine from Earlsmann after seeing Steves.

It's great. No time to set up and predictable results, so you can keep a record of exposure times for different optical combinations and filters.

This means that you can change configuration during a session easily....just point the scope straight up, put the ELP on it, punch in your settings and shoot a bunch of flats.

With luminance filters, I have to dim it down more, and just use a couple of sheets of paper to do this.

Cheers

Rob

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TakMan

TakMan

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Thanks for the heads up on this guys - just placed my order from Earlsmann - also for the A4 one. Any idea on a cheap place to get a couple of sheets of A4 perspex...?

Should make things a whole lot simpler and consistant when sorting in the garage!!!

Regards

Damian

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ollypenrice

ollypenrice

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Certainly! But first, for Damian, you need to look for a graphics shop and/or a sign maker to find perspex.

Now, why do we shoot flats? In a perfect world, if you photographed a perfectly smooth, white, evenly-illuminated surface your photo would show exactly that. In reality, though, it won't for two main reasons. 1) There will be specs of dust on various optical surfaces, some close enough to the chip to produce slightly darker doughnuts and circles to mar the 'white radiance of eternity.' (Shelley).

2) The said radiance will be further marred by the fact that your telescope, even if it is a flatfield astrograph of serene quality, will more fully illuminate the middle of the picture than the edges. If your chip really is too big for the image circle of the telescope then you will get vignetting.

All these artificially induced variations in brightness will be present in your image. You may not see them but once you try to drag out all the faint stuff - the whole point of imaging - these unwanted artifacts will also be dragged out into view.

By telling the image software where the artifacts are, by putting a 'flat field image' in an appropriate box, it will re-calibrate the light values in accordance with that image. You now have a level playing field. To take a picture of something perfectly even and featureless we use these odd boxes.

All good fun,

Olly

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Sunwind

Sunwind

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Certainly! But first, for Damian, you need to look for a graphics shop and/or a sign maker to find perspex.

Now, why do we shoot flats? In a perfect world, if you photographed a perfectly smooth, white, evenly-illuminated surface your photo would show exactly that. In reality, though, it won't for two main reasons. 1) There will be specs of dust on various optical surfaces, some close enough to the chip to produce slightly darker doughnuts and circles to mar the 'white radiance of eternity.' (Shelley).

2) The said radiance will be further marred by the fact that your telescope, even if it is a flatfield astrograph of serene quality, will more fully illuminate the middle of the picture than the edges. If your chip really is too big for the image circle of the telescope then you will get vignetting.

All these artificially induced variations in brightness will be present in your image. You may not see them but once you try to drag out all the faint stuff - the whole point of imaging - these unwanted artifacts will also be dragged out into view.

By telling the image software where the artifacts are, by putting a 'flat field image' in an appropriate box, it will re-calibrate the light values in accordance with that image. You now have a level playing field. To take a picture of something perfectly even and featureless we use these odd boxes.

All good fun,

Olly

so you shoot these things to kind of "calibrate" your camera's before taking a picture of the space-stuff? if that's it then I think I do something similar when simply taking pictures with my webcam (non-astronomical things, I don't do astrophotography or even have my scope yet), where I flood the camera with light to 'fix' the picture (am I way off or?)

thanks

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ollypenrice

ollypenrice

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Yes, you are calibrating the camera-telescope system. If you took a flat field image with one of these light boxes it would have artifacts as I mentioned - dust bunnies and a brighter middle. If you then applied an earlier flatfield image to the new one it would come out perfectly flat and featureless. The errors would be calibrated out.

Someone else will have to explain about 'fixing' a webcam, though, because I have never heard of this.

Olly

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  • 3 months later...
anthony

anthony

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I had a look at these on Gerd Neumann's site where there was a bit of info about powering them and reducing the lifespan with the wrong inverter etc.

How have you guys who are already using them supplied power? Ian King sells them with the mains (or 12v) power supply as an extra. In reality can any old 12v supply work? Or is the purchase of a specific power supply a definite need?

Cheers

Anthony

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Ford Prefect

Ford Prefect

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But no reason why you couldn't run from a 12v source jon?

to be honest, im not sure. These things can be a bit twichy if they are not handelled correctly and have a lot of power flowing though them. I always think better safe than sorry and have used the suppliers power supplies.

Jon

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