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Best way to get flats

iwols

By iwols
in Getting Started With Imaging

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RayD

RayD

Posted
Posted
10 minutes ago, alacant said:

 

Hi. What's the difference between those and a white laptop screen?

 

Better diffused lighting giving a more even light.

I have one also, and the only issue I have is getting a consistent brightness.  If you hold the button the brightness changes linearly, so doesn't have preset levels or a settable knob as some of the AP specific ones do, but they are pretty good :thumbright:

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Oddsocks

Oddsocks

Posted
Posted
1 hour ago, alacant said:

Hi. What's the difference between those and a white laptop screen?

 

1 hour ago, RayD said:

Better diffused lighting giving a more even light.

I have one also, and the only issue I have is getting a consistent brightness.  If you hold the button the brightness changes linearly, so doesn't have preset levels or a settable knob as some of the AP specific ones do, but they are pretty good :thumbright:

 

21 hours ago, iwols said:

as title says guys just wondered what people used and are the  electroluminescent panel any good thanks

 

Just as important, the light from a LED backlit TFT laptop or iPad etc is polarised and is refreshed many times per second.

Polarisation doesn't affect pure CCD imaging cameras so badly but causes problems with unmodified DSLR cameras that still have the antialiasing filter over the sensor where the interaction between polarised light source and antialiasing filter creates an uneven field that mimics vignetting, when such affected flats are applied they often result in a dark centre and bright borders to the calibrated image as the flat overcompensates for the inherent optical vignetting.

The refresh rate of a TFT laptop display, or iPad will cause unsynchronised dark bars at varying angles and widths to appear in flats depending on the length of exposure time and the type of shutter fitted to the sensor, rolling electronic shutters seem to be more likely to suffer this defect than global electronic or mechanical shutters.

In both cases it is easy to avoid such problems when using a TFT laptop or iPad screen source by adding one or two sheets of plain white 80 to 90 gsm printer paper in front of the screen and moving the paper/laptop/iPad slightly between each flat exposure, the paper diffuses and de-polarises the light and the reduced light output forces the camera to use a longer time exposure which helps prevent the appearance of interference (hum) bars. For flats with a TFT LED backlit  laptop/iPad etc then enough paper absorbers should be added so that the flat is correctly exposed using an exposure time of around two seconds to avoid interference bars being captured. Moving the paper and laptop/iPad between flats helps even out the small variations in illumination inherent in the paper making process and hot or cold spots on the laptop/iPad screen.

Older laptops that are edge illuminated with fluorescent tubes are practically useless as flat light sources since they suffer from uneven illumination, fluorescent strobing and TFT refresh period all contributing to messy unusable flats though I doubt there can still be that many of these old laptops still in use.

The problem with some of the LED draughting panels is the light coverage is not always that even and not well regulated, often the output light will vary quite markedly with battery condition and temperature though variation with temperature can also apply to laptops and iPads too. 

Most of the large non-astro LED panels I have seen suffer from illumination fall-off towards the edges of the frame, not so much a problem with a small objective but can become problematic when the OTA is getting close to the same diameter as the LED panel is wide. You can not always see the fall-off towards the panel edges visually but with a photometer it can be easily measured and it can be demonstrated as a visible difference between flats taken as sky flats against those taken with a draughting LED panel of poor design, the size of the camera sensor comes into play as well. a small sensor sitting in the middle of a fairly wide image circle and a using a poor LED panel as the source will be less affected than a big sensor occupying all of the image circle so that what works with one persons setup can not be guaranteed to work with somebody else!

The neutral density gel-acetate sheets used in photo studio and theatre lighting rigs work well as absorbers/diffusers with backlit LED TFT laptops or iPad screens showing a blank white word document and are not expensive. They also simplify the use of the brightness setting on LED draughting screens by allowing you to set maximum brightness all the time and just add a sheet or two of ND acetate to reduce the light levels down to a manageable level. Link to a sample gel-acetate sheet supplier: http://www.sblite.co.uk/67-neutral-density a 1.2ND absorber sheet reduces the light level by the equivalent of four (DSLR) camera stops, single sheets costing around £7.00, excluding delivery, are sized 1.22m x 0.53m and can be cut to suit with scissors or a knife.

Electroluminescent panels are not without their issues either, the light from an electroluminescent panel is not always broadband and can have large chunks of the spectrum missing which means very different exposure times between narrowband flats. Flats with my Aurora electroluminescent panel need eight times longer exposures for the 5nm Ha filter than for the 5nm OIII and three times longer for 5nm SII which means additional darks need to be taken.

For an automated observatory I still get the best flats using dawn or twilight sky flats, but then I don't have to stay up late or get up early to take them, the automation takes care of that, and once the observatory is set up I don't need to keep taking flats, usually a good set of flats will last many months or even years in a fixed closed refractor based system where the camera is never removed to allow fresh dust in.

Finally, the oft quoted white-tee-shirt over the objective can be a bit hit-and-miss, the weave and material has a big influence over the quality of the flats. A coarse weave over a short focal length instrument produces a weave pattern in the flats, the same material over a long focal length instrument produces no obvious pattern. For short focal length instruments a very fine white silk material (or good quality paper) will give better flats than a course white cotton tee shirt.

Probably getting a bit boring now so a good place to end.

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Thalestris24

Thalestris24

Posted
Posted

I have a Huion A4 LED panel powered via usb. It's ok, but... you have to be careful to set the camera exposure so as to not pick up (subtle) dark bars. I presume this has something to do with the refresh rate of the panel. I've had some such problems with it recently resulting in messed up stacks! Certainly I use a clean A4 white paper sheet over it but the refreshing is intrinsic so adjusting the brightness doesn't cure it. If all else fails, reflected cloudy-day illumination of an A4 sheet is usually ok :)

Louise

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StuartJPP

StuartJPP

Posted
Posted
15 hours ago, Thalestris24 said:

I have a Huion A4 LED panel powered via usb. It's ok, but... you have to be careful to set the camera exposure so as to not pick up (subtle) dark bars. I presume this has something to do with the refresh rate of the panel. I've had some such problems with it recently resulting in messed up stacks! Certainly I use a clean A4 white paper sheet over it but the refreshing is intrinsic so adjusting the brightness doesn't cure it. If all else fails, reflected cloudy-day illumination of an A4 sheet is usually ok :)

Louise

I rotate and move the OTA (or panel) slightly after each shot so if there are any dark spots in a panel it will average out.

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Thalestris24

Thalestris24

Posted
Posted
12 minutes ago, StuartJPP said:

I rotate and move the OTA (or panel) slightly after each shot so if there are any dark spots in a panel it will average out.

That's an idea though maybe a bit tedious! There's probably a long enough exposure time that will do the job but might be difficult to achieve in practice - even with the brightness turned down.

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Tiny Small

Tiny Small

Posted
Posted
1 minute ago, Thalestris24 said:

It's ok-ish... It was quite cheap

 

This is the important bit I think. Unless you are prepared to spend the relatively large amount of money on a panel that is specifically designed for flats, there will always be a compromise. It's up to individuals how much of a compromise they can put up with for a given price.

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iwols

iwols

Posted
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Posted
1 minute ago, Thalestris24 said:

It's ok-ish but see my comments above :) It was quite cheap, as I recall

 

thanks i can see your not 100% happy with that so the  Electroluminescent panel may be slightly in front,unless anyone else has comments,thanks thalestris

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ChrisLX200

ChrisLX200

Posted
Posted
51 minutes ago, Thalestris24 said:

I don't know anyone who's tried both - I haven't!

I've got both - I use an LED panel now. The EL panel was too 'blue' needing long exposures for anything in the Red end of the spectrum (Ha, SII, etc) and extremely short exposures for those in the Blue end. The LED panel is white, and it's more controllable for brightness.

ChrisH

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Thalestris24

Thalestris24

Posted
Posted
1 minute ago, ChrisLX200 said:

I've got both - I use an LED panel now. The EL panel was too 'blue' needing long exposures for anything in the Red end of the spectrum (Ha, SII, etc) and extremely short exposures for those in the Blue end. The LED panel is white, and it's more controllable for brightness.

ChrisH

Hi Chris

I don't think the colour balance or temperature of the panel should matter for flats - out of curiosity, how long is 'long' for your EL panel?

Louise

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